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Berichte zur Polarforschung = Reports on Polar Research

The thesis covers the topics of bioinvasions and climate change in the context of global seaweed distributions under the application of niche identification tools such as ecological niche identification and species distribution models. In the first chapter, I identify the fundamental and realized niches of the invasive red alga Capreolia implexa in Chile, and use these to identify its suitable habitat range and project changes in suitability under global warming. I conclude that C. implexa’s suitable range in South America is much larger than currently occupied, which might imply a huge potential for further spread. Under global warming, the suitable range will dramatically be reduced. In the second chapter, I assess the invasive potential of non-polar seaweeds to invade Antarctica, and conclude that such species might pose an invasion risk. This challenges the climate matching hypothesis which assumes that rather polar species pose a risk. As such, the risk of invasion for Antarctica is probably much larger than previously assumed. In the third chapter, I compare fundamental and realized niches of 126 seaweed species and assess how well distributions can be predicted based on thermal tolerance limits. I report that especially cold-tolerances are poor predictors of distributional patterns, while warm-tolerances are better predictors. I conclude that many seaweed species have cold-tolerances which are not discernable from their distributions. Further, approximately 60% of global seaweed species will face major challenges under global warming, as their distributional limits closely match their warm-tolerance limits, or lie even beyond. In conclusion, I point out challenges in thermal niche identification of seaweeds and illustrate and discuss consequences for forecasting of bioinvasions and climate change effects. Further, I highlight that global seaweed distributions will face major threat under warming conditions.

Natural hard bottom ecosystems worldwide provide unique ecological functions and services within their respective environments. They protect coastlines by dampening large waves, reducing flooding, and preventing erosion. They act as nursery grounds for the associated fauna and play an important role in benthic-pelagic coupling, nutrient cycling and water purification. The rapid growth of the coastal infrastructure due to an ever-increasing population in coastal regions, as well as the growing challenge of global climate change (sea level rise, ocean acidification), are a serious threat to these ecosystems. The management focus on supporting local biodiversity in order to maintain natural ecosystem functioning and services is essential to safeguard a sustainable coastal development. Hence, while there is a strong need in protecting the still remaining natural hard bottom ecosystems, there is also an upcoming demand for reconsidering the way new coastal infrastructure is to be built. Engineering solutions need to focus on the objective to maintain local species richness, while also reducing the demand for natural resources in the production process of new building materials. The main objective of this doctoral project was to define drivers, which influence benthic community establishment in marine natural and artificial hard bottom systems. The influence of different physical environmental conditions and substrate characteristics in the settlement processes and community establishment was evaluated. Risks of invasion by neobiota in natural and artificial structures and options to implement natural ecosystem services in artificial environments as protection against invasion were discussed. The first part of this dissertation focusses on habitat characteristics of natural hard bottom systems of the southern North Sea, Germany. The southern North Sea is a marine environment with relatively low proportion of natural hard bottom ecosystems. Observation and monitoring of these protected grounds is often difficult due to the heterogeneous habitats and overall low visibility. Chapter 2 shows how video sampling methods in combination with environmental distribution maps on sediments, currents, and depth can help to model the status quo of biotic and abiotic conditions within a German nature conservation area, the “Helgoländer Steingrund” (HSG; 54°14.00N and 8°03.00W). Within this study, a new approach using species distribution models was tested on presence/absence data of nine benthic species (Echinus esculentus, Metridium senile, Cancer pagurus, Phymatolithon spp., Axinella polypoides, Homarus gammarus, Flustra foliacea, Alcyonidium diaphanum, Alcyonium digitatum). The species distribution models revealed good evaluation measures (true skill statistic >0.7; area under the receiver operation characteristic curve >0.90), implying that the model shows a good predictive performance. The outcome of this study is a clear recommendation on SDM application in further environmental monitoring programs on the HSG and other protected hard ground areas. Chapter 3 compares different hydroacoustic and hydrodynamic measurement tools to improve the assessments of the environmental conditions in the study area. Sonar systems, underwater videos, and bottom samples were used for mapping and classifying the abiotic and biotic components of the habitat. Based on acoustic backscatter data three main seabed types (sand, gravel, and hard substrate) were identified. The additional information from underwater videos and sediment samples lead to an expansion to six seabed types with different abiotic and biotic components. The flanks of the ridge of the HSG and their transition to the surrounding soft-ground areas were characterized by a distinct dominance of the bryozoa Flustra foliacea and Alcyonidium diaphanum. Acoustic Doppler Current Profiler data showed a uniform flow pattern across the ridge, and even resolved the local variability of current patterns, dependent of the tidal stage and bottom relief. Flow patterns are likely responsible for the zonation of the two benthic species. The second part of this dissertation focusses on habitat characteristics and succession on artificial substrates and environments. Those were in the focus of one-year succession studies presented in chapter 4 and chapter 5. The experiments described the establishment of benthic communities on concrete cubes (15 x 15 x 15 cm) made from five different concrete mixtures. The concrete mixtures contained various cements (Portland cement and blast furnace cements) and aggregates (sand, gravel, iron ore and blast furnace slag). Depending on their mixture, natural resources were saved, and CO2-emissions were reduced. This makes the materials “environmentally friendly”. All cubes were deployed in April 2017. After 12 months, they were examined regarding species composition and coverage, followed by statistical analysis (PERMANOVA, SIMPER, DIVERSE). One succession study was carried out in a natural hard bottom ecosystem near the island of Helgoland (chapter 4). The second study was conducted at the JadeWeserPort, Wilhelmshaven, as an example of a recently erected artificial habitat with high anthropogenic impact (chapter 5). Chapter 4 showed differences in settlement communities for different surface orientation of the cubes. Significant differences in settlement communities of the Front/Back side were present depending on the used concrete mixtures. Chapter 5 indicated marked differences in settled communities at the Port site compared to the natural environments of Helgoland. At the Port site community composition did not differ between the concrete mixtures. However, surface orientation of the cubes again revealed significant differences in species abundances and compositions. Cubes hold more neobiota in the Port site than in natural hard ground environments. Recommendations for the usage of “environmentally friendly” produced concrete mixtures are given. Regarding new coastal constructions, a sustainable production process of the required building materials should always be considered. As long as no significant difference in succession patterns and establishment of benthic communities between the “environmentally friendly” produced concrete mixtures and those that are commonly provided is present, “environmentally friendly” produced mixtures should be used. Anyhow, this is not sufficient to help maintaining ecosystem services under future scenarios of climate change. On the one hand, a protection of natural hard bottom systems is essential to maintain natural ecosystem service and functioning. On the other hand, the potential of artificial structures in restoring ecosystem services should not be underestimated. Artificial structures, if they fulfill certain criteria, as the use of environmentally friendly produced materials and combine those for instance with an enhanced habitat complexity, can, to certain extent compensate for natural habitat losses. If they host high biodiversity of native species, they can also be used to protect natural coasts from invasion by neobiota.

Seagrasses are marine flowering plants that inhabit the coastal area forming important ecosystems due to a number of ecosystem services they provide. However, they are subjected to both global and local impacts, including warming water temperatures and eutrophication, which threaten their survival. Despite the fact that the most diverse seagrass meadows are found in the tropical Indo-Pacific Region, there is less information about tropical species than their temperate counterparts. There are, therefore, knowledge gaps in the response of tropical seagrass meadows to environmental drivers and their links to ecosystem functions and services. In the last three decades, trait-based frameworks (TBFs) have advanced different fields of ecological research through establishing novel links between functional traits, environmental drivers and ecosystem functions. A number of concepts have been proposed in order to answer different ecological questions using a functional trait-based perspective. This field of research has been widely developed in terrestrial plants. However, the use of TBFs in seagrass research is currently in its infancy. The goal of this dissertation is the incorporation of TBFs into seagrass ecological research, by establishing novel links between seagrass traits, environmental drivers and ecosystem functions and services. The study site chosen for this work was Unguja Island (Zanzibar Archipelago, Tanzania). Unguja Island is located in the tropical Indo-Pacific region, and is considered one of the hotspots of seagrass biodiversity worldwide. The seagrass meadows in Unguja Island are subjected to a wide range of conditions, from pristine and oligotrophic to heavily impacted and eutrophic. Due to its high seagrass diversity and the variety of conditions under which seagrass survive, Unguja Island is a perfect laboratory for the study of seagrass communities using a TBF. The research questions selected for this dissertation have the goal of understanding the importance of traits at different organizational levels, from their individual responses to environmental drivers, to the effect of traits on the interspecific competition of seagrass species and, lastly, their effect on ecosystem functioning. First, to assess the knowledge gaps in seagrass trait-based research, I carried out a systematic review of the seagrass literature. The analysis showed that seagrass trait research has mostly focused on the effect of environmental drivers on traits (65%), whereas links between traits and functions are less common (33%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (7% of studies). These knowledge gaps in seagrasses indicate ample potential for further research. In order to address these knowledge gaps, I propose a TBF that can help guide future seagrass research. Secondly, the responses of traits of individual seagrass plants of tropical seagrass species (Halophila stipulacea, Cymodocea serrulata, Thalassia hemprichii and seedlings of Enhalus acoroides) were assessed to two environmental drivers: temperature (global) and nutrient enrichment (local). To achieve this aim, a 1-month experiment under laboratory conditions combining two temperature (maximum ambient temperature and current average temperature) and two nutrient (high and low nitrogen and phosphorus concentrations) treatments was conducted. The results of this experiment showed that trait responses are species-specific, and that temperature was a much more significant driver than nutrient enrichment. In the case of the seedlings of E. acoroides, they rely energetically in the reserves within the seedling and increasing temperature resulted in faster seedling development. T. hemprichii and C. serrulata showed an enhanced morphology, while the contrary was true for H. stipulacea. These results highlight the different effects and strategies that co-inhabiting seagrasses have in response to environmental changes. Thirdly, an experiment was developed in the field to test the effects of light shading and trampling due to the farming of Euchema denticulatum on seagrass meadows, an environmental driver endemic to the tropical region. Areas covered by T. hemprichii, H. stipulacea were selected for the building of seaweed farms for 3 months. Light was reduced in the seaweed farm plots by 75 to 90% by the end of a seaweed growth cycle. The responses of seagrass were, again, species-specific. H. stipulacea, despite its capacity for rapid growth, was significantly affected by the combination of shading and trampling under the seaweed farm treatment, while the climax seagrass species T. hemprichii was unaffected. Fourthly, to link individual plant traits to seagrass community level processes, I carried out an observational study in Unguja Island. The goal was to understand how seagrass traits linked to light and nutrient competition affected space preemption among seagrass species under different trophic scenarios. Traits determining the functional strategy of the seagrass showed that there was a size gradient in the seagrass species. When tested the effect of the difference in the functional strategy of species pairs, the probability of preemption was highest for the bigger species, increased when their size difference was higher and was not affected by the eutrophication. This indicated that the competitive interactions among seagrass species were asymmetrical, i.e. a species had a negative effect on another species, while the effect was not reciprocal and the driver behind space preemption was determined by traits related to the size of the seagrass plants. Fifthly, to study the link between seagrass traits and ecosystem functions, sediment cores were collected and compared within seagrass meadows of varying communities across sites of Unguja Island. The goal was to find out which seagrass traits are relevant indicators of carbon storage, and which environmental conditions constrain the storage of carbon in the sediments. Very fine sediments (

In coastal ecosystems, large brown macroalgae are important foundation species, growing on rocky shores from temperate to polar regions. They form marine forests, also known as kelp forests, which are among the most productive coastal marine ecosystems in the world. Kelps are of high ecological and economic value, however, climate change poses a great threat to these ecosystems. Warming has vast implications on marine forests and their global biogeographic patterns, but also other drivers affect kelps and strong interactive effects are observed. The aim of this study was to gain a deeper understanding of the inter- and intraspecific acclimation processes of kelp species to abiotic conditions along large spatial and environmental gradients. The thermal tolerance of the broadly distributed kelp Saccharina latissima towards marine summer heatwaves across latitudes and its biochemical and morphological variability across its entire distribution range in Europe was investigated. Seasonal and inter-annual differences in the susceptibility of S. latissima to marine heatwaves in summer were revealed and the impact of potential interactions of marine heatwaves in summer paired with hyposalinity or enhanced nutrient availability was analyzed. The combined effect of different temperatures and salinities on physiological and biochemical response variables was also investigated in young Laminaria solidungula sporophytes. In conclusion, this thesis presents important information on the functional variability of two kelp species with different distributional ranges. Both studied species exhibit different sensitivity towards drivers related to environmental change. Furthermore, intraspecific variation must not be underestimated. The observed changes in the kelp populations of Europe, thus, cannot exclusively be ascribed to temperature variation but to an interplay of various abiotic and biotic factors. These findings contribute further knowledge to support future approaches to the conservation of marine forests.

Along temperate to polar rocky shorelines, large brown algae known as kelps form marine forests which provide a three-dimensional habitat for many associated species. Ocean warming is posing an increasing threat to kelps at their warm distributional edges and first range shifts have been recorded. For these sessile species unable to migrate, trait variability due to phenotypic plasticity and genetic variation is an important mechanism of response to environmental change. The aim of this thesis was to produce a comprehensive assessment of the variation and plasticity of thermal traits across populations and life cycle stages of a keystone marine forest species, the cold-temperate to Arctic kelp Laminaria digitata. Using physiological response parameters, population genetics and transcriptomics, I present evidence for four levels of thermal trait variability in a marine forest key species. I describe (1) genetic and physiological differentiation along the species' distribution range, (2) genetic variation for phenotypic plasticity among genotypes, (3) carry-over effects over reproduction and individual ontogeny, and (4) the production of new phenotypes by outbreeding among distant lineages. Integrating these responses into a framework of seasonal temperature variation and predictions of ocean warming showed that L. digitata, as a species with a cold-temperate thermal profile, is adapted well to the current conditions along its distributional range, but may not be equipped to respond to rapid climate change at its warm range edges. The concepts investigated in this thesis provide further insight into trait variability as a mechanism of marine forest resilience and offer intriguing features for mariculture and conservation efforts.

Arctic phytoplankton is the base of the foodweb in the most rapidly changing region on the planet. Within this thesis I show that Arctic coastal diatom communities can sustain stable productivity under future climate scenarios by several mechanisms: though large phenotypic plasticity, sorting between highly diverse genotypes and species shifts. The specific focus of this work lies on the role and functioning of intraspecific selection and population dynamics of the diatom Thalassiosira hyalina. Through application of new population genetic methodologies, I was able to resolve detailed selection dynamics in simplified diatom populations and found that their outcomes are not predictable from responses recorded in single-strain cultures, hinting towards interactions between conspecifics. Furthermore, my experiments and field studies on natural populations suggest that dynamics within populations are likely driven by more complex processes than clonal dominance. Forecasts on the future functioning of phytoplankton populations are therefore bound to include intraspecific diversity and evolutionary adaptation as crucial processes.

Species invasions in marine habitats have dramatically increased in the last decades. This thesis revealed characteristics of the invasive kelp Undaria pinnatifida that support its invasive potential along various phases of the invasion process. Experiments, including morphological, physiological and biochemical analyses, explored traits of different life history stages. The kelp displayed broad physiological tolerance to the tested abiotic factors (desiccation, temperature, salinity). Based on its capability to survive extended periods of air exposure, overland transport is proposed as an invasion vector for U. pinnatifida. With regard to elevated temperatures the invader outperformed native kelps, indicating a competitive advantage in a warming ocean. Observations of simultaneously developing gametophytes revealed the occurrence of inter-specific interactions at microscopic stages. Interactions of U. pinnatifida and Macrocystis pyrifera resulted in an enhancement of the invader's performance.

Seagrasses are the prevailing macrophytes along soft sediment coasts. One of the most important anthropogenic stressors for benthic macrophytes is increased suspended sediment loads and the attenuation of underwater light. Along many temperate coasts seagrasses occur predominantly in the intertidal but knowledge on the photophysiology of intertidal seagrasses is scarce. Studies focussing on the photoecophysiology of intertidal seagrasses are therefore urgently needed and the aim of this thesis was to fill this gap and provide some in-depth information on light acclimation mechanisms in intertidal seagrasses over diurnal and spatial scales. In a second step the impacts of suspended sediments on the underwater light regime and thus seagrass photosynthesis were examined. The first study assessed light adaptive strategies in a comparative analysis of the congeneric seagrass species Zostera muelleri and Zostera marina at two case study areas in New Zealand and Germany. The results showed marked fluctuations of photophysiology (maximum and effective quantum yield, non-photochemical quenching, cycling of xanthophyll cycle (XC) pigments) over daily and tidal cycles with a full xanthophyll cycle at both locations. At the New Zealand site we also observed significantly larger XC-pigment pool sizes in seagrass leaves sampled in a week when low tide coincided with noon. This very dynamic adjustment of xanthophyll pool sizes has not been previously reported for intertidal seagrasses and highlights their ability to adjust to strongly fluctuating irradiances in the intertidal. The high physiological plasticity of Zostera in light-saturated environments was also illustrated by the second study, which evaluated spatial differences in seagrass cover related to environmental conditions (light, temperature, sediment grain size distribution, and porewater nutrients) and differences in seagrass photosynthetic pigment composition and morphometry (above and below ground biomass, shoot length and leaf width, and percentage cover) at four sites in the meso-tidal estuarine lagoon of Tauranga Harbour, North Island, New Zealand. There were marked differences in pigment content and composition (as a marker of physiological plasticity), seagrass metrics as well as in environmental conditions. Our findings emphasize the high physiological plasticity and revealed also morphological plasticity of Zostera muelleri. Both studies underlined the general high light adaptation of intertidal seagrasses and their ability to thrive in fluctuating light environments. However, strong reductions in light conditions, e.g., through increased sediment loads entering coastal systems, may negatively affect seagrasses. Hence, the third study elucidated to what extent different sediment types (marine vs terrestrial) in different concentrations changed the underwater light regime in terms of overall photosynthetically active radiation (PAR) and the attenuation coefficient of downward irradiance of PAR (Kd) and quality (spectral composition). In a tank experiment the influence of different sediment types and concentrations were determined. In situ measurements of light quality and quantity, and suspended sediments (total suspended solids, TSS) at 12 sites in and outside of Tauranga Harbour, demonstrated that the lowest Kd and TSS values were found outside, and the highest within the harbour. Different scenarios for sediment loads, how they influence the underwater light regime and the potential for the vertical distribution of seagrass meadows are discussed. Relatively low Kd values of ~1 (as observed e.g., at the inner site of the lagoon) may already lead to maximum colonization depths of less than ~2 m, i.e., a limitation of seagrasses to intertidal habitats. This shows that current sediment loads and the resulting light conditions impair the growth of seagrasses beyond the intertidal at most sites. The (re)colonization of subtidal habitats may require significantly lower sediment loads entering the coastal zone. Therefore, authorities should consider an effective management on land and at the coast to keep anthropogenic sediment inputs at a minimum.

During the construction of a deep-water port (JadeWeserPort), bathymetry, sediment distribution, and macrofauna community structure were studied in the Inner Jade, a tidal channel located in the southern North Sea. The relationships between macrofauna community structure and natural as well as anthropogenic environmental variables were investigated in this very heterogeneous study area. The manual expert hydroacoustic classification of the backscatter image derived by side scan sonar was successful to detect the different dredging activities and the natural bedforms in the undisturbed areas. The sediment distribution was very patchy and no significant congruence with the hydroacoustic classification could be identified. In contrast, low, but significant relationships between the hydroacoustic classification and the macrofauna community structure as well as the sediment distribution and the macrofauna communities were found. The most important impact on the spatial community structure was the number of days after the last dredging/dumping activity for the JadeWeserPort (JWP), followed by the sediment characteristics explained by grey values of the backscatter image. This study stresses the problems of benthic habitat mapping in such a heterogeneous area. In order to assess the effects of physical disturbance by dredging activities, macrofaunal community compositions between 2002 (before the construction work had begun) and 2010 (during the final construction phase) were compared. The sand extraction for land reclamation and the redirection of the navigation channel changed the bathymetry markedly. While the old navigation channel in the centre of the study area remained mud dominated, a general increase in coarse sediments was detected in 2010. The dynamic nature of the study area in combination with the direct and indirect effects of dredging increased the bathymetric heterogeneity (measured by singlebeam (2002) and multibeam (2010) echo-sounder). In 2010, the macrofauna community structure roughly resembled the different categories of dredging activities. A general increase in macrofaunal abundance and taxa number was observed in 2010, with the exception of the recently dredged area. The structure of the macrofauna community during the port construction phase seemed to be determined by secondary dispersal of the dominant taxa and recolonisation by highly mobile and opportunistic species. The effects of the presence of two adult bivalve species with different feeding modes on the post-settlement dispersal of their juveniles were examined in a flume experiment.

One of the primary challenges in ecology is to understand how environmental disturbance affects diversity and community structure, and what are the subsequent consequences on ecosystem functioning. Coral reefs are some of the most diverse ecosystems on the planet resulting in complex sets of interactions between benthic, habitat-forming constituents and mobile fish consumers. However, scleractinian corals, the primary habitat engineers, are dependent on high-light, low-nutrient water conditions and thus are highly responsive when the environment varies from this status. In Southeast Asia, an increase in human coastal populations centred around urban areas has resulted in extensive changes to the coastal environment such as degraded water quality and removal of fish consumers. This has resulted in highly varied abiotic and biotic conditions in relation with distance from the shore. Often, coral reefs closer to shore are much lower in benthic and fish diversity than those further from anthropogenic influences, with direct impacts on ecosystem functioning. Therefore the aim of this thesis was to explore coral reef ecosystem functioning with respect to changes in benthic community structure and fish diversity in relation to varying environmental conditions in the Spermonde Archipelago, Indonesia. A combination of observational, experimental and theoretical analyses were conducted on the functioning of coral reefs using eight islands on a transect of increasing distance from the mainland, varying from 1 to 55 km. At these eight sites, benthic and pelagic surveys identified variation in the status of coral reef communities, while recruitment and feeding assays identified variation in important ecological processes. Lastly, experimental observations were further qualified with stable isotope analysis and the application of contemporary indices of functional diversity. It was found that indeed, the coral reefs varied along a continuum of structure, assemblage and processes. Increasing distance from shore was associated with greater live coral cover and structural complexity, while sites closer to shore were dominated by turf algae and rubble. Furthermore, turf algae was observed as playing a particularly important role, as this group was dominant during recruitment and subsequent development of open benthic space as supplied by terracotta tiles. Fish diversity, along with redundancy in the important herbivore group, also increased with distance from shore, resulting in an increasingly diverse response to Sargassum and Padina assays. The functional composition of the fish assemblages became increasingly variable with loss in coral cover and structural complexity, suggesting communities become destabilised under habitat degradation. Furthermore, stable isotope analysis indicated that the trophic niche of a fish species can increase at sites with more degradation suggesting varying functional utility. However, functioning is not determined only by exposure to chronic, abiotic conditions. Outbreaks of the crown-of-thorns starfish (Acanthaster planci) and mechanical destruction (bomb fishing) resulted in extreme loss of live coral. At these sites, biological and functional diversity displayed some of the lowest values among all sites. Coral reefs can exist in systems with altered water condition if physiological and ecological capacity of the organisms allow for their continuation. Nevertheless, degraded water condition will select against many species, resulting not only in the observed lower biological diversity, but also in less species taking part in functional roles as reflected in higher functional variability. Combined, these results show that the functioning of coral reefs does not exist in discrete states; rather, their functioning is a result of abiotic stressors and biological feedbacks. It is becoming increasingly clear that pristine coral reefs are not a reality in many cases around the world. Thus understanding coral reef functioning at all stages of degradation will help with future management. This thesis adds to the ever-growing knowledge about disturbed coral reefs, but more importantly, it describes the changing relationship between diversity and functioning of coral reefs in relation to disturbance.

Anthropogenically induced greenhouse gas emissions result in two major environmental changes on the global scale for coral reefs: ocean acidification (OA) and ocean warming (OW). Additionally, increasing levels of terrestrial runoff, that introduce fertilizer, sewage, sediments and other contaminants into coastal areas, can decrease water quality on the local scale. Consequently, photosynthesizing and calcifying coral reef organisms are affected by OA, OW and coastal runoff, but knowledge about the interactive effects of these stressors on key coral reef organisms is scarce. The aim of this thesis was to investigate how OA individually, and in combination with OW or local stressors (i.e. decreased light availability and inorganic eutrophication), affects important calcifying coral reef organisms. A series of field- and laboratory-based experiments were conducted on the Great Barrier Reef and at natural volcanic carbon dioxide seeps in Papua New Guinea. A range of response parameters, including growth, calcification and photosynthesis, were investigated at the species and community level. OA showed no negative impact on three large benthic foraminiferal species in the short term (Chapter 2) and several calcifying green algae species of the genus Halimeda in the long term (Chapter 3). OA combined with decreased light availability resulted in additive negative effects on the coral Acropora millepora in the short term (Chapter 4), while OA combined with inorganic eutrophication did not exhibit any significant effects on the corals Acropora tenuis and Seriatopora hystrix and on the calcifying green alga Halimeda opuntia in the short term (Chapter 5). In the long term, the combination of OA and OW resulted in decreased growth and calcification of epilithic algal communities, particularly in crustose coralline red algae (Chapter 6). The different sensitivity of the species investigated to global and local stressors, suggests that changes will occur in coral reef community structures in the near future. Reduced light availability may amplify negative effects of OA on corals and thereby contribute to shifts from coral to algae dominated communities on reefs affected by coastal runoff. The scientific findings of the present thesis indicate that OA in combination with other stressors may reduce the abundance of calcifying organisms and thus lower the calcium carbonate production on coral reefs in the future. This may lead to reduced reef growth, increased brittleness and reduced recovery potential after acute disturbances. Ultimately, declining reef habitat will likely lead to a reduction in biodiversity and may thus have implications on the people, who are dependent on coral reefs for their livelihood.

Tropical coral reefs are among the most productive ecosystems on this planet, despite being surrounded by very oligotrophic waters. Effective recycling processes of the limiting nutrients, particularly nitrogen (N), and input of new bioavailable N via dinitrogen (N2) fixation are essential to sustain such high gross primary production. In fact, several benthic reef organisms and substrates are associated with diverse communities of N2 fixing microbes (diazotrophs), but the respective contribution of the different benthic groups to total benthic N2 fixation and the effect of changing environmental conditions on N2 fixation have not been investigated yet. Therefore, this thesis, through a series of interconnected studies carried out in a seasonally dynamic coral reef system, the Northern Red Sea, and in a number of manipulative experiments, contributes to the understanding of benthic N2 fixation in coral reefs by answering the following key questions: 1) How much N2 is fixed by the dominant benthic groups in the Northern Red Sea? 2) What is the relative contribution of the benthic groups to total benthic N2 fixation within the reef? 3) What is the effect of seasonally changing environmental conditions and of single environmental factors on key metabolic processes, particularly N2 fixation, associated with the different benthic groups? Findings revealed that all investigated benthic groups showed N2 fixation activity, whereof bare coral rock, turf algae, carbonate sands and living hard corals were the main N2 fixing components contributing ~ 90% to benthic N2 fixation in investigated Red Sea coral reefs. Soft corals revealed the lowest N2 fixation activity among all investigated groups and released significantly less organic matter to the surrounding water compared to hard corals indicating that N2 fixation may also influence their role as allogenic ecosystem engineers. N2 fixation by most benthic groups was usually highest when nutrient availability was lowest, and water temperature as well as light intensity highest suggesting that the N2 fixation products fuel the metabolic N requirements of reef organisms, particularly during nutrient-depleted conditions (i.e. summer). The manipulation of single environmental factors revealed a stimulation of N2 fixation activities under global warming conditions and a reduction under ocean acidification scenarios indicating that global climate change will affect the nutrient status of reef organisms. In summary, this thesis underlines the ubiquity of N2 fixation associated with different benthic coral reef organisms and substrates, and highlights its importance in sustaining coral reef productivity. Both, the benthic community structure and the prevailing environmental conditions appear to be important in controlling the amount of N2 fixation in coral reefs. Finally, as coral reefs are increasingly and simultaneously exposed to global and local stressors, it is necessary to understand how important metabolic processes on coral reefs (i.e. organic matter fluxes and N2 fixation activity) will be affected, and this thesis provides first quantitative insights into these processes from physiological, biogeochemical and ecological points of view.

Tropical subtidal rocky reef communities are less prominent than coral reefs and characterized by macroalgae, mobile invertebrates, reef fish and diverse communities of epibenthic invertebrates. In the Galápagos Marine Reserve, more than 80% of the shallow benthic habitats are rocky lava reefs. In order to examine the effects of small-scale differences in oceanographic regimes, two rocky reef study sites, an upwelling and a non-upwelling site, were chosen off Floreana Island. The benthic community structure of the sites was studied for two years by subtidal monitoring. Simultaneously, benthic succession studies and grazer exclusion experiments were conducted at both sites. Further, the food web structure for Floreana Island was analyzed by using the stable isotope approach.Results revealed significant differences between both study sites. These findings reflect the particular oceanographic setting of the Galápagos Archipelago and that Floreana Island may also represent a model system to investigate ecological responses along abiotic gradients.

Temporal variations in induced anti-herbivory traits and underlying cellular mechanisms have been largely neglected in the study of seaweed-herbivore interactions. Therefore, fluctuations in the induced defense of the brown seaweed Fucus vesiculosus as well as changes in gene expression occurring in response to isopod (Idotea baltica) and periwinkle (Littorina obtusata) attack were assessed and compared. This thesis provides evidence for a finely adjusted response of F. vesiculosus to isopod and periwinkle grazing based upon a temporally variable defense pattern, which is accompanied by extensive transcriptomic changes. The induction of defenses involved a functional reorganization from anabolic to catabolic processes. Furthermore, the expression of several genes with putative defensive functions was increased in response to isopod attack. Moreover, this thesis tested whether F. vesiculosus responds to water-borne cues from grazed neighbors and constitutes strong evidence against defense induction after receiving info-chemicals in North Sea F. vesiculosus and indicates that the ability to communicate may vary among seaweed habitats.

Anthropogenic carbon dioxide (CO2) emissions from fossil carbon storages to atmosphere are causing major impacts on the Earth s climate. The increase of atmospheric temperatures due to a strengthening of the natural greenhouse effect is a well known phenomenon. Moreover, a large fraction of the atmospheric CO2 that has been released by mankind enters the ocean and induces, besides other effects, the acidification of surface waters ( Ocean Acidification ). The effects of increasing partial pressures of atmospheric CO2 on parameters that describe the abiotic environment have been examined extensively. It is now of particular interest to gain a detailed comprehension of the complex interactions between the changing environment and the metabolism of marine organisms. This knowledge is essential when aiming to understand the ongoing changes in their entirety. In the spotlight of this thesis are coccolithophores, unicellular algae that significantly impact the marine carbon cycle. Coccolithophores perform photosynthesis and calcification. Both processes rely on different carbon species and thus have a different effect on the carbonate system of the surface ocean. In connection with the formation of calcite particles, they further impact the sinking rates of particulate carbon from the photic zone into the ocean. The ratio of photosynthesis to calcite precipitation rate varies with the composition of the carbonate system. In order to understand the dependencies of both individual processes on the complex cabonate system, a detailed understanding of cellular carbon fluxes is essential. Until now, it is impossible to measure these fluxes directly. Therefore, mathematical models are used in this thesis to examine these fluxes. All presented models describe Emiliania huxleyi, one of the globally most abundant and important coccolithophores. The first part of this PhD thesis summarises current knowledge concerning the intracellular formation of coccoliths, i.e. the calcite platelets that surround the cells. Former propositions about the provision of carbon and calcium ions (Ca2 ) towards the site of calcite precipitation are reviewed and discussed. Furthermore, abiotic prerequisites on initiation and continuation of calcite precipitation, as well as the biological control of precipitation onset and crystal growth are of particular interest. Gaps of knowledge are discussed. In the second part, different hypotheses concerning the pathway of Ca2 are discussed on basis of analytical calculations. It turns out that a continuous trans- port across the membrane of the compartment, in which calcite precipitation takes place, i.e. the coccolith vesicle, is more likely than is a vesicle-based transport. The latter possibility constitutes the hypothesis currently favoured in literature. A transport via molecules that exhibit high Ca2 binding capacities, as found in other coccolithophores, is thought to be unlikely for E. huxleyi, because there is currently no evidence hinting at comparable molecules. Based on the assumption that Ca2 is transported continuously into the coccolith vesicle, a numerical model is developed, which examines the influence of different membrane transporters on the precipitation rate of calcite. These transporters convey varying substrate stoichiometries and are inserted into the in silico membrane of the coccolith vesicle. The influence of these transporters on the calcite precipitation rate is examined. Since the precipitation rate is known from literature, it is possible to exclude some of the tested transport stoichiometries. Two potential substrate stoichiometries are detected that serve as basic assumptions in the next step of modelling calcite precipitation. In the third part, different numeric models are developed that examine carbon fluxes through cytosol, coccolith vesicle, and chloroplast. Based on current knowledge of external carbon sources for organic matter production and calcite precipitation, it is concluded that both processes are supplied independently of each other with carbon under replete conditions. When external CO2 becomes limiting, how- ever, external bicarbonate ions that usually constitute the substrate for calcite precipitation are used by photosynthesis also. Moreover, an energy-efficient possibility is proposed how CO2 could be accumulated around Ribulose-1,5-bisphosphat (RubisCO), the CO2 fixing enzyme, without being transported against concentration gradients. The general discussion summarises the findings of this thesis and picks up gaps that were not discussed in detail previously. Moreover, measurements and models are proposed that might fill gaps of knowledge. Finally, implications of this work on physiological, ecological, and biogeochemical aspects of coccolithophore research are given.

To improve the understanding of population dynamics of organisms, it is necessary to examine organisms under natural conditions. Only at such conditions, knowledge about their in situ use of resources, predator-prey interactions and loss rates can be obtained. It is possible to examine and understand these processes with controlled ocean iron fertilization experiments, which stimulate the growth of unicellular algae. This thesis provides a comprehensive description of the reactions and vertical distribution of the main components of the pelagic ecosystem and highlights the food-web interactions of individual organisms, which characterize this habitat. The response of the plankton community to iron addition was successfully observed during the Indo-German iron fertilisation experiment LOHAFEX (LOHA is hindi and means iron, FEX stands for fertilisation experiment) carried out in the Southern Ocean from January to March in 2009 lasting for 38 days. The iron-induced bloom was achieved in the closed core of a mesoscale eddy. The aim of the experiment was to study the growth and demise of the phytoplankton bloom and to examine whether the biomass is retained in the surface layers through recycling processes or whether biomass sinks out to the deep ocean. The fertilized patch was characterized by low silicic acid concentrations, which is an essential nutrient for diatoms, hence a flagellate-bloom developed with cells < 20 µm in size. The bloom remained stable over the course of the experiment. This was verified by microscopic analysis and molecular methods. The main reason for the lack of large scale biomass increase, was the strong grazing pressure by the large copepod population, consisting primarily of Calanus simillimus and Oithona similis. Incubation experiments proved that copepods increased their grazing rates and fecal pellet production within the patch. Neutrally buoyant PELAGRA traps were deployed to quantify the export fluxes. These contained a few diatoms and fecal pellets and were dominated by unicellular plankton like dinoflagellates, flagellates and coccoid cells. Hence fertilizing this type of plankton community did not increase the vertical flux. This lead to the hypothesis that the system was influenced by recycling processes in surface layers. The protozoan community and its vertical distribution was an important part of this work and was studied by focussing on key heterotrophic organisms. These included thekate dinoflagellates, loricate and aloricate ciliates, foraminifera, radiolarians, and juvenile and adult copepods. Again, the high grazing pressure by the copepods controlled the development of the protozoan community, and the copepod fecal pellets contained large amounts of damaged and empty loricae, but also foraminifera, which are usually not a preferred food item for copepods. This indicated that the copepods were food limited and resorted to large, armoured protozoans which they might have otherwise avoided. Dinoflagellates were mainly abundant at depths below 100 m, possibly this distribution pattern was caused by an escape response in order to avoid predators and to feed on the flux of larger particles which generally peaks at those depths. In conclusion, the plankton community during LOHAFEX was top-down controlled whereby a highly efficient retention type system prevailed, which resulted in low particle export fluxes

Anthropogenic carbon dioxide emissions cause a chemical phenomenon known as Ocean Acidification (OA). The associated changes in seawater chemistry are believed to have significant impact especially on coccolithophores, unicellular calcifying primary producers that take an outstanding role in the regulation of the marine carbon pumps. This thesis investigated the calcifying diploid and the non-calcifying haploid life-cycle stages of the globally dominant coccolithophore Emiliania huxleyi, and their responses to OA. Emphasis was put on investigating the role of energy-availability (i.e., irradiance) in the manifestation of OA-responses. A suite of methods was applied to resolve the effects on the phenomenological level (growth, elemental quotas and production), the physiological level (photosynthesis, carbon acquisition) and the level of gene expression (transcriptomics). In publication I, haploid and diploid cells were compared using microarray-based transcriptome profiling to assess stage-specific gene expression. The study identified genes related to distinct cell-biological traits, such as calcification in the diplont as well as flagellae and lipid respiration in the haplont. It further revealed that the diploid stage needs to make more regulatory efforts to epigenetically administrate its double amount of DNA, and therefore strongly controls its gene expression on the basis of transcription. The haplont in turn, possessing only a single sized genome, does not require these administrative efforts and seems to drive a more unrestricted gene expression. The proteome is apparently regulated on the basis of rapid turnover, i.e., post-translational. The haploid and diploid genomes may therefore be regarded as cellular operating systems that streamline the life-cycle stages to occupy distinct ecological niches. Publication II investigated the responses of the life-cycle stages to OA under limiting and saturating light intensities. Growth rates as well as quotas and production rates of carbon (C) and nitrogen (N) were measured. In addition, inorganic C acquisition and photosynthesis were determined with a 14C-tracer technique and mass spectrometry-based gas-flux measurements. Under OA, the diploid stage shunted resources from calcification towards biomass production, yet keeping the production of total particulate carbon constant. In the haploid stage, elemental composition and production rates were more or less unaffected although major physiological acclimations were evident, pointing towards efforts to maintain homeostasis. Apparently, both life-cycle stages pursue distinct strategies to deal with OA. As a general pattern, OA-responses were strongly modulated by energy availability and typically most pronounced under low light. A concept explaining the energy-dependence of responses was proposed. In publication III, microarray-based transcriptome profiling was used to screen for cellular processes that underlie the observed phenomenological and physiological responses observed in the life-cycle stages (publication II). In the diplont, the increased biomass production under OA seems to be caused by production of glycoconjugates and lipids. The lowered calcification may be attributed to impaired signal-transduction and ion-transport mechanisms. The haplont utilized genes and metabolic pathways distinct from the diploid stage, reflecting the stage-specific usage of certain portions of the genome. With respect to functionality and energy-dependence, however, the transcriptomic OA-responses resembled those of the diplont. In both stages, signal transduction and ion-homeostasis were equally OA-sensitive under all light intensities. The effects on carbon metabolism and photophysiology, however, were clearly modulated by light availability. These interactive effects can be explained with the influence of both OA and light on the cellular redox hub , a major sensory system controlling the network of metabolic sources and sinks of reductive energy. In the general discussion, the newly gained views on the life-cycle stages are synthesized and biogeochemical implications of light-dependent OA-effects on coccolithophore calcification are considered. Furthermore, emerging physiological response patterns are identified to develop unifying concepts that can explain the energy-dependence of physiological effects. Finally, the critical role of redox regulation in the responses to changing environmental parameters is argued and research perspectives are given how to further resolve effects of the changing environment on marine phytoplankton.

Since the industrial revolution, atmospheric carbon dioxide (CO2) concentrations have been increasing, and the surface waters of the global oceans have absorbed 30% of the anthropogenic CO2 released into the atmosphere. A higher CO2 concentration in surface ocean waters shifts the carbon chemistry, resulting in higher concentrations of bicarbonate ions (HCO3-) and protons (H ) and lower concentrations of carbonate ions (CO32-). Such a shift in ocean carbon chemistry decreases the pH and the saturation state of the seawater with respect to CO32- thereby making the precipitation of CaCO3 less kinetically favorable. These changes in ocean chemistry termed ocean acidification) are expected to have negative impacts on marine calcifying organisms, which deposit CaCO3 in the form of aragonite, calcite and high-magnesium calcite into their shells and skeletons. Because calcifying marine primary producers are very important to the carbon cycle and for rocky shore habitat structure and stability, investigating how they will respond to future oceanic CO2 levels is a relevant and important topic of research. Therefore, two calcifying marine macroalgae were chosen as the central organisms for investigation in this thesis. I investigated the physiological responses of the temperate calcifying coralline rhodophyte alga Corallina officinalis (L.) and the tropical calcifying chlorophyte alga Halimeda opuntia (L.) J. V. Lamouroux to elevated CO2 concentrations which are expected to occur by the end of this century. Furthermore, the effect of elevated CO2 on the competitive interactions between these two calcifiers and their noncalcifying counterparts was investigated in order to predict how macroalgal communities will respond to future surface ocean CO2 levels in both temperate and tropical environments. Because CO2 concentrations are increasing in surface ocean in parallel with other abiotic stressors, I also chose to investigate the response of H. opuntia to the combined effect of elevated CO2 and inorganic nutrients, which replicates a likely scenario for the condition of some eutrophied tropical coral reefs at the end of this century. The studies carried out during this thesis revealed that there are differences in the physiological responses of calcifying macroalgae to elevated CO2, but similar patterns of competitive interactions between calcifiers and noncalcifiers occur under elevated CO2 regardless of species and latitude. I found that the temperate coralline alga C. officinalis was highly sensitive to elevated CO2, as shown by lower growth and 3 photosynthetic rates and less calcified cell walls than under normal conditions. On the other hand, the tropical calcifying chlorophyte alga H. opuntia was only moderately sensitive to elevated CO2 concentrations, as this species had lower growth rates but maintained normal calcification rates and increased electron transport rates. Enzyme activity (external carbonic anhydrase and in situ nitrate reductase) in both species was affected by CO2 indicating that external carbonic anhydrase plays an important role in calcification by regulating the speciation of inorganic carbon, and that nitrogen assimilation in these species is affected by elevated CO2. The effect of CO2 on energy balance in these two species is also discussed. The different calcification mechanisms utilized by these two species is likely to account for some of the observed differences in physiological responses, and is discussed in detail below. While these two species showed different susceptibilities to elevated CO2 in isolation, they both showed similar sensitivity to overgrowth and outcompetition by noncalcifying algae when grown with their natural communities under elevated CO2 conditions. This trend was amplified under conditions of inorganic nutrients. The results of this thesis indicate that calcifying macroalgae show differences in their susceptibility to ocean acidification, but regardless of their sensitivity in isolation, both temperate and tropical species are likely to be outcompeted by noncalcifying macroalgae under elevated CO2 conditions. Tropical systems are especially susceptible to a shift in community composition (from calcifier- to noncalcifier-dominated) when eutrophication and ocean acidification occur simultaneously.

Reducing the net release of nutrients into the coastal environment has become an important issue of ecological and societal relevance. Efforts to remove excess nutrients have been made to utilize chemical or physical treatments, but they have not been successfully implemented, due to negative affect to the environment. To utilize abundant macroalgae may become a sustainable alternative to eliminate eutrophic conditions, and additionally produce a valuable by-product. However, it is necessary to deepen the sustainable concept into focused studies on macroalgal ecophysiological response to obtain a better understanding on macroalgal performance under environmental alterations. Numerous studies have examined ammonium utilization by marine macroalgae, with the most current research focusing on low ammonium concentrations. There is comparatively few information on macroalgal resistance to high ammonium concentrations, a condition which is usually found in aquaculture systems. Thus, studies in three different experimental series under laboratory conditions have been conducted to investigate the maximum uptake and physiological tolerance to high ammonium concentration treatments, including ammonium toxicity symptoms and protein content, to test the feasibilities as nutrient bioabsorber. Dictyota bartayresiana J.V. Lamoroux and Gracilaria verrucosa (Hudson) Papenfuss were compared in relation to their ammonium uptake capacities in the first study. A series of perturbation experiments was conducted in a 360 minute experimental period. Ammonium concentrations used were 0, 50, 100, 200, and 400 µM under 70 µmol photons/m²/s irradiation. In general, G. verrucosa showed higher nutrient uptake and photosynthetic performance. Final Fv/Fm of G. verrucosa remained high in all ammonium treatments, in contrast with D. bartayresiana. G. verrucosa showed no saturation point over various time intervals. In contrast, D. bartayresiana showed a saturation point and discoloration after the first 15 minutes. A high ammonium supply and light availability might support photosynthesis and growth of G. verrucosa, but on the contrary, indicated ammonium toxicity in D. bartayresiana. To investigate the maximum physiological tolerance of G. verrucosa under a very high ammonium concentrations and different irradiances was the focus of the second study. In the first experiment with 70 µmol photons/m²/s irradiation, uptake rate of G. verrucosa showed a surge phase in the first two days, and was saturated after day 4, while in the experiment with 300 µmol photons/m²/s irradiation, uptake saturation was shown from day 2 of the experimental period. The final Fv/Fm of both irradiances decreased significantly with increasing ammonium concentration treatments and irradiation. Tissue loss started from day 7 when exposed to high ammonium concentration treatment for both irradiances. Ammonium toxicity symptoms were found in G. verrucosa with ammonium addition greater than 800 µM for both irradiations. High ammonium concentration treatments and different irradiance significantly affected accumulative biomass, total growth rate, pigments, and nutrient in tissue. The third study was conducted to obtain information on biochemical levels (e.g., protein content) to high irradiance and ammonium stress. A series of experiments under irradiation of 70 and 300 µmol photons/m²/s with 1200 µM ammonium concentration treatment had been conducted in a climate room for 7 days. In general, G. verrucosa showed low performance in most observed parameters, earlier than previously found. Massive discoloration as a symptom of infectious disease was found by day 4 in the macroalgae under 300 µmol photons/m²/s irradiation. Protein content in all treatments was lower than 0.5 µg/µL extract. Overall, the most relevant findings of these studies were that 1) G. verrucosa exhibited a high performance in ammonium uptake and storage capacity, 2) unsaturated ammonium uptake shown by G. verrucosa was not a passive diffusion process, but actively transported due to charged ions of ammonium and huge storage capacity in macroalgal tissue, and 3) a very high ammonium concentration interacted with high irradiance to affect nutrient uptake, growth rate, and photosynthetic performance of G. verrucosa, and lead to toxicity symptoms. The overall outcome of these studies advanced a different perspective on relevance of nutrient abundance and macroalgae, although in some points illustrate that high ammonium concentrations may contribute to substantial negative consequences to the macroalgae. To involve G. verrucosa in an integrated applied system to control the nutrients excess and to achieve an appropriate balance of uptake and growth ratio would be the next important step.

This thesis describes the importance of microgradients around organic tissue of calcifiers in comparison to bulk seawater conditions. It is shown that microenvironmental acidification around calcifiers can result from various causes, such as calcification (Chapter 1), microbial mat or sediment exposure (Chapter 2), respiration (Chapter 2) or ocean acidification conditions (Chapter 3). Some calcifiers, like sediment dwelling foraminifera and bivalves, are naturally adapted to low pH and hypoxic conditions (Chapter 1, 2). Yet, microenvironmental low pH, hypoxia and high levels of sulphide resulted in tissue necrosis of corals, but only if those conditions were trapped close to the tissue of corals for extended periods of time (Chapter 2). The performance of some calcifiers under low pH and hypoxic conditions thus depends upon the duration of the exposure, as well as the diffusional resistance between the bulk seawater and their tissues. If diffusivity around their organic tissues is significantly reduced, some calcifiers may not be able to maintain pH homeostasis and thus severely suffer from extended exposure to low pH, hypoxia and high levels of sulphide (Chapter 2).

Marine macroalgae are globally distributed on rocky coastal shores, from tropical to polar regions. They are important marine coastal primary producers, and of particular importance to the function of many ecosystems. Kelps, brown algae of the order Laminariales, dominate rocky shores of cold-temperate regions. There, they help to structure the biodiversity of coastal ecosystems by forming huge forests, which provide habitats and nurseries for various marine organisms. The distribution of kelps is constrained by abiotic factors like light including UV radiation and temperature. Future global environmental changes could therefore have a potentially significant impact on geographic distribution patterns, vertical zonation, and primary productivity of kelp. The basic physiological and ecophysiological characteristics of kelps are well studied. Several physiological studies have been performed on kelp, primarily on the effects of single abiotic stressors, e.g. UV radiation and temperature. Only a few projects have focused on the interactive effects of multiple stresses. So far, no study is available on the molecular processes underlying physiological acclimation to abiotic stress factors in these important organisms. This thesis represents the first large-scale transcriptomic study of acclimation to abiotic stress in a kelp species, and aims on investigating molecular mechanisms underlying physiological acclimation to multiple abiotic stresses in Saccharina latissima from the Arctic (Spitsbergen). Young sporophytes of Saccharina latissima were exposed in multifactorial experiments to different combinations of photosynthetically active radiation, UV radiation and temperature for durations of 8h, 24h and 2 weeks. In order to observe the degree of photoinhibition in response to different exposure conditions, maximum quantum yield of PS II (Fv/Fm) was measured at the beginning and at the end of the experiments. A cDNA library from RNA sampled under various light and temperature regimes was constructed for subsequent functional genomic studies on the mechanisms and pathways involved in stress acclimation to multiple stressors. Gene expression profiles under abiotic stress were assessed by microarray hybridizations. Thereby two different stress exposure durations, 24hours and 2 weeks, were applied to distinguish molecular mechanisms of short-term versus long-term acclimation to stress. Finally, a comparative approach investigating gene expression profiles in both laboratory and field grown sporophytes was carried out to elucidate interactive effects of UVR, temperature and growth conditions. The established cDNA library consists of 400,503 ESTs, which were assembled to 28,803 contigs. Sequence comparison by BLASTx, Interpro protein-motif annotation, and Gene Ontology (GO) yielded in putative functions or orthology relationships for over 10,000 contigs. Comparative analysis with the genome of E. siliculosus revealed high functional genomic coverage of 70% of the cDNA library. The cDNA library is representative of the S. latissima transcriptome under the tested conditions and displays a rather complete gene catalogue of the species. It therefore constitutes an excellent basis for subsequent functional genomic studies on molecular acclimation to multiple stresses in Saccharina latissima. S. latissima responds to abiotic stress with a multitude of transcriptional changes. Temperature had a smaller influence on metabolic processes than light. Two main temperature effects were observed. On the one hand, induction of genes associated with the glycine, serine and threonine metabolism in response to low temperature, and on the other hand repression of transcripts encoding carbohydrate biosynthetic and catabolic processes at high temperature. General stress responses observed in sporophytes subjected to high PAR include induction of catabolic processes for energy supply, heat shock proteins and antioxidant enzymes. The combination of the stress factors high PAR, UVR and temperature caused interactive effects on photosynthesis and gene expression. Thereby excessive light at 17°C was the most destructive stress condition for S. latissima, resulting in a strong repression of several crucial metabolic processes, e.g. photosynthesis, carbohydrate metabolism and amino acid metabolism. Acclimation to high irradiance at low temperatures includes enhanced induction of glycine, serine and threonine metabolism, potentially as a consequence of a higher demand of glutathione (GSH), a reducing co-factor for several enzymes involved in reactive oxygen species (ROS) detoxification. Reactive oxygen species formation (ROS) displays a central element of abiotic stress response. The observed regulation of various ROS scavenging enzymes in response to temperature, high PAR and UVR stress demonstrates the crucial role of ROS metabolism in acclimation to abiotic stress in S. latissma. Interestingly, gene expression data bear evidence for the existence of compartment specific ROS scavenging mechanisms in S. latissma. Furthermore, sophisticated regulation of Hsps was observed, which is involved in acclimation not only to temperature but also to combined environmental stresses such as high PAR in combination with high temperature. Short- and long-term acclimation to UVR includes enhanced regulation of photosynthetic components, e.g. light harvesting complex proteins, thylakoid proteins and proteins associated with both photosystems. Gene expression analysis showed that photosystem II exhibits a higher susceptibility towards UV radiation than photosystem I. Furthermore, repair of UV damaged PS II reaction centre seems to function by increasing the transcript pool for transcripts associated with PS II. The observed induction of vitamin B6 biosynthesis after all short- and long-term UVR treatments seems to be a crucial component of UVR acclimation in Saccharin latissima. Only short acclimation to UVR caused enhanced regulation of DNA replication and DNA repair. Three different DNA repair processes, photoreactivation, homologous recombination, and nucleotide excision repair were detected, indicating a sophisticated regulation of different DNA repair processes. As no enhanced regulation of DNA metabolism was detected after the 2 weeks UVR exposure experiments, S. latissima seems to be able to acclimate to UVR radiation and to overcome the negative effects of UV radiation on DNA. Finally, comparisons of gene expression profiles in field and cultivated sporophytes were conducted. Large differences in gene expression between cultured and field material were observed. Principal effects of UVR, targeting mostly photosynthesis and DNA, were similar in cultured and field sporophytes, demonstrating laboratory experiments being well suited to investigate basic molecular mechanisms of acclimation to abiotic stresses. The study revealed that field sporophytes exhibit a higher susceptibility to UVR and a higher oxidative stress level at 12°C, whereas cultivated sporophytes in contrast must make stronger efforts to acclimate to UVR at 2°C. These findings are mostly due to the different growth temperatures of between -3°C and 1°C for field sporophytes versus 10°C for cultivated sporophytes. However, the results indicate that cold acclimation of S. latissima from the field caused metabolic alterations to increase stress performance at low temperatures, which concurrently led to higher susceptibility at 12°C. This thesis presents an initial idea on the complexity of molecular acclimation to abiotic stress in Saccharina latissima. The molecular data obtained by this study improve our understanding on stress response in a kelp species and provide a useful platform for future research regarding molecular approaches in kelp. This project furthermore demonstrates the importance of research on interactions of abiotic stresses on both the physiological as well as on the molecular level. The results of the comparative gene expression study in cultured and field sporophytes highlight the influence of growth conditions on molecular acclimation to stress and underscore the importance of conducting experiments with field material, when aiming to predict effects of changing abiotic factors in the field.

Coral bleaching involves the breakdown of the symbiosis between corals and dinoflagellates of the diverse genus Symbiodinium (also referred to as zooxanthellae), where the zooxanthellae and/or algal pigments are lost from the coral-algae symbiosis. Elevated temperatures and high solar irradiances are considered to be the primary factors causing mass coral bleaching. Yet, in spite of extensive research in this area, the physiological processes underlying the bleaching response, specifically the large inter- and intraspecific variability of bleaching susceptibility, are not unequivocally resolved. The major objective of this study was to provide further insights into the mechanistic understanding of the inter- and intraspecific variability of bleaching susceptibility by addressing potential causes for this variability. The study focussed on photophysiological responses of Symbiodinium to elevated temperatures and high light (HL), with a particular emphasis on the engagement of photoprotection, and on the role of the coral host in modifying the response of the algal symbionts to HL. The capacity of corals to change their mode of nutrition towards heterotrophy under light deprivation in order to maintain the integrity of the symbiosis was also evaluated. In a bi-factorial experiment, the combined effects of enhanced temperatures and increased irradiances on photosynthetic performance and photoprotective mechanisms of two Symbiodinium clade A phylotypes in culture were investigated over a period of three weeks. Both phylotypes clearly differed in their thermal sensitivity, and HL exposure generally amplified the temperature effect. Symbiodinium Ax was highly sensitive to elevated temperatures greater than 30°C, showing strongly impaired growth and a sharp decline in photochemical efficiency of photosystem II (PSII) indicative of severe chronic photoinhibition. By contrast, Symbiodinium A1 was thermally tolerant and maintained high photochemical efficiency even at 32°C and HL. This could be attributed to an enhanced photoprotective capacity associated with higher cellular concentrations of xanthophyll cycle pigments and the low-molecular antioxidant as well as the dynamic regulation of these photoprotective pathways in response to experimental conditions. Glutathione plays a central role in the cellular antioxidative system due to its involvement in many antioxidative reactions. This study presents the first results on the role of glutathione as a proxy for oxidative stress in cultured Symbiodinium under thermal and HL stress. The findings of the differential temperature sensitivity prove that the generalization of physiological attributes to specific Symbiodinium clades is not justified. To evaluate the contribution of the coral host in modifying the bleaching response, the photophysiological response of in hospite Symbiodinium to short-term HL exposure was studied in two coexisting corals, both associated with the same Symbiodinium phylotype. The photophysiological responses towards high levels of solar radiation clearly differed between both coral species: The rapid synthesis of photoprotective pigments, including xanthophyll cycle pigments as well as a high xanthophyll cycling activity, was solely observed in symbionts of the thin-tissued Pocillopora damicornis. Nonetheless, this up-regulation of photoprotection was insufficient to prevent photoinhibition and bleaching in P. damicornis. By contrast, the thick-tissued Pavona decussata was able to withstand HL exposure without experiencing a sustained decline in photochemical efficiency or symbiont density and, most notably, without a pronounced engagement of photoprotective pigments. Our results suggest species-specific differences in the light levels reaching Symbiodinium within the coral tissue, presumably due to disparate inherent optical properties of the coral tissue in both species. The importance of phototrophic vs. heterotrophic mode of nutrition for the functionality of the photosynthetic apparatus of algal symbionts and the integrity of the entire coral-algae symbiosis was investigated in P. damicornis by exposing corals to artificial light deprivation in situ at two sites in a (natural) reef. Both sites are located around Heron Island (Great Barrier Reef, Australia), but differ in their hydrodynamic characteristics. Site-specific differences in the response of P. damicornis to light deprivation were detected: light-deprived corals at North Wistari Reef (low flow-site) showed a partial decline in photochemical efficiency of PSII and a dramatic loss of symbionts after two weeks, while corals at Coral Gardens (high flow-site) maintained a functional symbiosis. Hence, the higher water flow may have enabled corals at Coral Gardens to shift from photo- to heterotrophic nutrition more efficiently in order to meet the energy needs of the symbiosis compared to those at North Wistari reef due to a higher availability of food.

Bivalve shells are often used to reconstruct past environmental conditions at the time of carbonate formation. The aim of this thesis is to optimize the process of reconstructing environmental history of marine ecosystems from bivalve shells (Arctica islandica) and to contribute to a better understanding of the correlations between shell chemistry and environmental parameters. In two chapters I examine how sample preparation and data collection may affect the outcome of subsequent trace element analyses in A. islandica shells. The other two chapters of the thesis focus on the applicability of specific trace element to calcium ratios in A. islandica shells to reconstruct the recent environmental history of the German Bight (North Sea). I analyzed Pb/Ca ratios as a tracer of anthropogenic lead pollution as well as Ba/Ca and Mn/Ca ratios as indicators of the pelagic primary production of the German Bight.

Antifreeze proteins (AFPs) are an important adaptation mechanism for organisms subjected to subzero temperatures. The motivation of this thesis was to elucidate the distribution of AFPs in sea ice diatoms and to study their function. The findings were used to deduce the mechanisms of action and relevance in vivo. Diatom isolates were tested in culture experiments for the presence of recrystallisation inhibition activity, as a measure for AFP activity. Seven Arctic and four Antarctic diatom isolates were subjected to a temperature decrease and salinity shift resembling the inclusion of the diatoms into sea ice. All tested polar diatom species showed AFP activity, ten thereof even without being stressed. In three species, AFP activity was furthermore up-regulated by the temperature and salinity shift. Cell numbers and photosynthetic quantum yield indicated that cellular damage caused by the stress was more severe for water column isolates than for isolates from the ice column. The correlation of recrystallisation to the protein concentration for a Fragilariopsis nana Ant cell extract and a recombinant AFP allowed the calculation of AFP concentrations as AFP equivalents. Intracellular concentrations of 0.3 µM to 68.5 µM AFP equivalents confirmed the function of AFP as recrystallisation inhibitor and even come close to the concentrations required for thermal hysteresis. As AFP equivalents made up 0.1% to 5% of the total cell protein, diatoms invest a large amount of energy to produce AFPs. This indicates that AFPs are an important factor for diatom success in sea ice. Further, two isoforms of AFPs from Fragilariopsis nana were heterologously expressed in Escherichia coli, to study their molecular function. Recombinant proteins were deposited in inclusion bodies, but successfully refolded to functionally active proteins with respect to crystal deformation, recrystallisation inhibition and thermal hysteresis. The two isoforms showed different characteristics. One isoform produced a thermal hysteresis of up to 1.53 °C ± 0.53 °C and modified ice crystal growth to formation of hexagonal bi-pyramidal shapes, whereas the other isoform produced a thermal hysteresis of up to 2.34 °C ± 0.25 °C and ice crystals in the form of hexagonal columns. Thermal hysteresis activity of both proteins was positively correlated with protein concentration. The AFP activity increased with increasing buffer salinity in a linear correlation. High AFP concentrations or buffer salinities caused radial dendritic burst patterns of ice crystals in AFP solution. Thermal hysteresis potentials, crystal deformation habits and burst patterns led to the classification of both AFPs as hyperactive AFPs. In addition, the two isoforms differed with respect to the signal for subcellular localisation. Preceding the antifreeze domain, the gene of the first isoform carries a signal peptide indicating the secretion into the extracellular space, whereas the second isoform has a long N-terminal sequence of unknown function. We thus propose that AFPs have different functions in vivo, with distinct localisations of the proteins inside or outside the cell. To investigate AFP activity in situ, five cDNA libraries of eukaryotic communities from Arctic and Antarctic sea ice were analysed for abundance and phylogenetic relationship of AFP sequences. AFP transcripts were found in all sea ice cDNA libraries from the Arctic (Kongsfjord) and Antarctic (Weddell Sea and Dumont d´Urville Sea). Abundances of AFPs ranged from 115 to 1824 AFP transcripts per 100.000 reads. The different abundances did not correlate with taxonomic distribution or environmental parameters of the respective samples. Phylogenetic placement assigned 90% of the sequences to a clade of AFPs from the diatoms Navicula glacei, Chaetoceros neogracile and the crustacean Stephos longipes. The remaining 10% were placed into a separate clade of Fragilariopsis AFP sequences. In the Arctic sample all AFP sequences were assigned to the Fragilariopsis clade, whereas in only one Antarctic sample were some sequences found in this clade. These findings indicate that AFPs play an important role for eukaryotic sea ice organisms in their natural habitat and that the Navicula/Chaetoceros clade is more prominent than the Fragilariopsis clade under the conditions investigated.

Not much information is available about the genetic background of growth and toxicity- related processes in toxic Haptophyta species. The aim of my thesis was to contribute to better understanding of these issues using functional and comparative genomic approaches with the ichthyotoxic prymnesiophytes Chrysochromulina polylepis and Prymnesium parvum. In particular, I explored different gene-expression profiling methods in order to monitor the transcriptomic responses in these species to different environmental conditions. Through the sequencing of a cDNA library, a transcriptomic database (Expressed Sequence Tag library) was established for both prymnesiophyte species. Approximately 2900 and 6300 contigs were found in the Chrysochromulina polylepis and Prymnesium parvum datasets, respectively. The sequences were annotated and compared to similar data sets available from other Haptophyta species (Pavlova lutherii, Isochrysis galbana and Emiliania huxleyi). This analysis revealed a `core set` of approx. 1500 genes which were found in all Haptophyta species investigated in this study. Moreover, 67 and 362 genes were present only in C. polylepis and P. parvum, respectively. The physiological background and cellular regulation of synthesis and liberation of Chrysochromulina and Prymnesium toxin(s) is still poorly understood, but the involvement of PKS genes in the biosynthesis of certain compounds is likely. The presence of the conserved ketosynthase (KS) domains - an obligatory part of PKS genes were shown in both species, represented by fourteen and four copies in C. polylepis and P. parvum, respectively. In order to indirectly test the hypothesis invoking a role of PKS genes in toxin biosynthesis, the correlation between toxicity and PKS gene expression was monitored in both species. The observed positive correlation strengthens the hypothesis on the involvement of PKS genes in toxin production C. polylepis as well as in P. parvum. A gene expression microarray was generated based on the EST data originating from P. parvum, and this tool was used to monitor gene-expression changes during growth in nutrient replete and phosphorus (P)- or nitrogen (N)-deprived P. parvum cells. In accord with previously published data, elevated intracellular toxicity was observed in P-deprived cells, whereas it did not change in N-depleted or nutrient replete cells. As a response to P limitation, the upregulation of different genes related to transport and acquisition of phosphate could be observed. On the other hand, N limitation did not lead to such a clear effect on the gene expression level, since most genes likely involved in the uptake, storage and transport of N sources were not upregulated. Utilizing the tools of ecophysiology and functional genomics we identified gene-expression patterns indicative of physiological (nutrient, toxicity) and growth status of C. polylepis and P. parvum. With reference to this data set, knowledge about cellular processes in toxic Prymnesiophyceae species was expanded considerably, and pointed the way forward for incorporation of functional genomic approaches to determining regulatory factors involved in prymnesiophyte bloom dynamics through gene expression studies.

Sugar cane cultivation has become the main land use in northeast and southeast Brazil and it is still increasing because of a growing national and international demand of sugar and biofuel. Shallow coastal lagoons and estuaries are the linkage between terrestrial and marine environments and are susceptible to anthropogenic modifications. Little is know about the impact of sugar cane cultivation in these systems. Aim of this study was to gain knowledge on the impact of effluents from sugar cane cultivation on the biogeochemistry and phytoplankton dynamics in two shallow coastal systems affected by sugar cane monoculture in their watersheds, a coastal lagoon and an estuary in Brazil. In the Mundau Manguaba Lagoon (MML), water samples were collected at the beginning and end of the dry season during five expeditions between September 2007 and February 2009. Additionally, sediment cores were taken in March 2007. In the Paraiba do Sul River (PSR), samples were taken during the rainy (January) and dry (September) season 2008. The main elements of this study were the determination and characterization of nutrients (nitrate, nitrite, ammonium, silicate, phosphate) and total suspended matter (TSM) as well as phytoplankton communities in the MML and the PSR. Particulate organic matter (Corg, PN) and stable carbon and nitrogen isotopes (13C, 15N) were determined in TSM of surface water in the MML and PSR and sediments of the MML. Inorganic nutrients were measured in the water samples. Phytoplankton abundance, biomass and community composition were identified in the MML. Effects of nutrient enrichment caused by fertilizer runoff from sugar cane fields on phytoplankton communities in the MML were experimentally simulated. Chlorophyll a (chl a) was used to estimate phytoplankton biomass in the PSR. Nutrients and organic matter derived from fertilizer runoff in the catchment area of the PSR were transported from the river and its tributaries to the estuary and subsequently to adjacent coastal waters during the rainy season because of high river discharge. Reduced river discharge and long residence time during the dry season led to an increase in chl a amount and retention of allochthonous and autochthonous material inside the system. Nutrient concentrations in the MML were mainly governed by the rivers which drain the sugar cane dominated catchment area. A seaward nutrient decrease coincided with high phytoplankton biomass inside the lagoons and was the result of fast nutrient uptake by algae. Parts of the organic matter were removed from the water column due to sedimentation and were recycled or stored in the sediments of the lagoons. Other parts of the organic matter were exported from the Manguaba and Mundau lagoons into adjacent coastal waters due to tidal currents. Phytoplankton community and diversity were highly variable in both lagoons because of rapidly changing salinity and nutrient concentrations as well as residence time. Thus, temporal and spatial phytoplankton distributions and shifts between cyanobacteria and diatom dominated communities were observed in the MML. Nutrient enrichment experiment conducted in order to simulate high nutrient input from fertilizer runoff showed a direct response of phytoplankton communities. Diatoms were the main profiteers due to permanently high silicate concentrations in tropical aquatic ecosystems. Alterations in the nutrient ratios affected by high nitrogen and phosphate inputs from sugar cane cultivation lead to a shift in phytoplankton communities and consequently influence the entire food web dynamics in the lagoons. It can be concluded that lagoons and estuaries play an important role for nutrient and organic matter cycles in the coastal ecosystems affected by sugar cane cultivation in the northeast and southeast regions of Brazil. Seasonal variations in fertilizer runoff, geomorphology of the system and biological processes are the main factors that control the material distribution, composition and finally the retention of nutrients and organic matter inside the system or the outwelling from the system into adjacent coastal waters.

The Segara Anakan Lagoon on Java, Indonesia, is mainly threatened by sedimentation and mangrove tree logging. The lagoon size decreased by >50% since the 1970´s due to high sedimentation loads from the Citanduy River and therefore the agriculture-dominated hinterland. The nutrient concentrations were significantly higher during the rainy season and mainly derived from the Citanduy River. Also mangrove leaves leached high amounts of nutrients into the system. However, the nutrient concentrations were low to moderate on a global scale. Nutrient sinks in the SAL were outwelling, assimilation by mangrove and shrub species, consumption by microbes and phytoplankton. A further shift from true mangrove tree species to shrub species due to logging can alter the carbon and nutrient inventory in the lagoon. Shrub species leached significantly more nutrients than true mangrove tree species which might accelerate the nutrient turnover rates in the lagoon and therefore affect the whole food web.

The overall aim of this study was to investigate the influence and importance of benthic communities on the carbon- and nitrogen- flow through the food web in the Segara Anakan Lagoon. I used the method of stable isotope analysis to resolve trophic structures of benthic food webs in several stations within the lagoon. Carbon isotopic compositions were used to trace carbon sources through the food web, whereas nitrogen isotopic compositions help to distinguish between trophic levels. A high spatial variability was found on a small scale, traced back to food availability and possibly also anthropogenic impacts, such as sewage wastes by the city and villages within the lagoon and effluents of the oil refinery. Furthermore, a seasonal area dependent variation was observed, which can be explained mainly by the impact of increased precipitation, and therewith an increased river runoff. As underlying processes of isotopic fractionation are still unclear today, several experiments were conducted to estimate the importance of benthic decapod species on the turnover of carbon and nitrogen in the mangrove ecosystem of the SAL.

This thesis represents a study of the ecophysiology and toxicity of the prymnesiophyte Prymnesium parvum. The first aim was to investigate changes in the relative toxicity of P. parvum following a series of physiological shock treatments, meant to simulate environmental conditions under which harmful blooms of this species have been observed. As blooms of this haptophyte often occur in dynamic coastal brackish water systems, Prymnesium parvum is noted for its physiological flexibility, which may contribute to providing a competitive advantage over other coexisting species. Due to the unconfirmed nature of the compounds involved in toxigenic processes, two bioassays were employed to characterize changes in lytic capacity (extracellular vs. intracellular). These bioassays are considered physiologically relevant, as observed icthyotoxicity occurs through lysis of the gill cell membranes, rendering the fish unable to perform gas-exchange processes and obtain oxygen. Additionally, the gene expression of three polyketide synthase genes (PKS) were analyzed via quantitative PCR (qPCR), based on current chemical characterizations of toxic compounds produced by P. parvum. Low salinity and high irradiance were observed to alter the lytic effects of P. parvum on the sensitive cryptophyte Rhodomonas salina and erythrocytes. Furthermore, these two shock treatments were found to increase the transcript copy number in selected PKS genes, suggesting a possible correlation between toxicity and the PKS biosynthetic pathway. Allelochemical mediation has been suggested to affect competition and predatory relationships associated with formation of P. parvum blooms. As interactions between species are an integral part of understanding plankton ecology, interspecific interactions between P. parvum and three coexisting species were accordingly investigated. Combining bioassays with a functional genomic approach allowed differential characterization of cell-cell contact vs. waterborne cues depending on the organism with which incubated. A unique response on both the levels of toxicity, gene expression profile as well as PKS transcript copy number to the potential predator Oxhyrris marina suggest a fundamentally different type of interaction between the two species. Additionally, a dose-response time series experiment showed that changes in gene expression and toxicity did not occur immediately in P. parvum, rather after 60-90 minutes. Such a response by P. parvum may in fact signify a co-evolutionarily adaptive defense. Finally, examination of the effects of phosphorous limitation and low salinity stress on the gene expression profile and lytic capacity showed that the combination of these two stressors induces secretion or extracellular transport of toxic substances to a much higher degree than either stressor individually. Whether this observation is due to changes in membrane integrity due to homeostatic processes needs further research. The pattern of gene expression, however, revealed regulation of among others genes associated with active cellular transport processes, suggesting that maintenance of intracellular-extracellular homeostasis may play a role in the observed toxicity. In summary, these studies integrate the concepts of ecophysiology and functional genomics, providing a useful platform for further research regarding environmental factors associated with the toxicity of P. parvum. As functional genomic methods become more accessible, such approaches illustrate their potential application within the field of harmful algal research.

In scope of worldwide reef degradation due to anthropogenic environmental changes there is an urgent need to understand the different factors driving the processes within the coral reef ecosystem. Thus, the investigation of macroalgae becomes important, as they are observed to spread over the degraded reefs. Next to some conspicuous species, like Lobophora variegata and Dictyota spp., other smaller fialemntous algae, commonly known as algal turf, are observed. Algal turf has hardly been investigated to species level and has mainly been treated as functional group in many ecological studies. The need for studies at the species level is crucial, as certain species specific algal-coral interactions are reported to play an important role in the maintenance of the coral reefs. In order to increase the knowledge of turf algal communities in coral reefs ecosystems, the aim of the present study was to identify the species composition and to investigate the environmental factors potentially shaping the turf algal communities in a Carribean coral reef in the SW coast of Curacao. There were 101 different species identified, including 15 species of Chlorophyta, 10 species of Phaeophyceae, 61 species of Rhodophyta and 15 species of Cyanobacteria. To ease the investigation of these diverse assemblages, these species were grouped according to the lowest taxonomic level possible (preferably at genus level) for an adequate investigation under the dissection microscope (12 times magnification). Thus a total of 39 different taxa were distinguished. To understand the dynamics shaping these assemblages, the algal turf succession was followed over a period of 500 days, and the distribution of several turf algal species, growing on settlement tiles and as epiphytes on the dominant macrophyte L. variegata was investigated over a depth range from 2 to 40 m. Furthermore, in order to investigate the responses of turf algal communities towards changing environmental conditions, different successional stages were exposed in six multifactorial experimental studies to different environmental factors, including UV radiation, substrate quality, change in depth, eutrophication, annual variation, and grazing pressure. The findings of the present study confirmed that algal turf is not only comprises a diverse assemblage, but a highly complex community that is shaped and driven by specific responses to different environmental factors and interspecific relations. Furthermore, turf algal communities serve as habitat for different benthic and demersal fauna, provide a refuge for unicellular organisms, and may play a role as a bank of microstages for several bigger macroalgae. The communities responded to different environmental conditions, whereas the response depended on the species present and the different succesional stages. While the early stages were more affected by certain environmental stressors, e.g. grazing pressure, the later stages showed to be more tolerant, which were partly linked to the high observed tolerance of certain Cyanobacteria taxa, e.g. Dichothrix, which forms dense tufts, and might provide shelter for more sensitive organisms. The present study not only provides information about the composition and distribution of turf algal species in the research area, but also about community and species/taxa specific responses to different environmental stressors. This information is crucial as it allows for the development of a turf algal based assessment to predict environmental conditions, which can be used as monitoring device for the reef health. Overall the present study contributes to increase the knowledge of a long time understudied but important integrative part of the coral reef ecosystem. In the process of anthropogenic driven environmental change, further investigation of these complex communities will become crucial as they continue to spread over the reefs in the future.

This dissertation highlights the ability of macroalgae to protect themselves by inducing their palatability changes in the face of consumer attacks. In particular, I examined how macroalgae change their chemical traits in response to herbivore properties classified by herbivore specificity, temporal variation of the herbivores, and feeding cues. Further, I tested macroalgae can predict the herbivory and induce defenses by the cues from an attacked conspecific and a heterospecific species. Finally, I investigated the effective magnitude of the induced responses using non-native macroalgae as well as species-specificity of herbivores. These approaches support that herbivore-induced defenses are not accidental reactions but adaptive options of plants through coevolved history with specific herbivores.

Communities of macro- and microalgae present a dominant group of coastal benthic ecosystems. Macroalgae exhibit a complex developmental cycle involving microscopic life stages which are fully exposed to environmental factors such as UV radiation (UV) and water temperatures displaying the most sensitive stage of the entire macroalgal life-cycle. In the last 50 years, the ozone content in the atmosphere has significantly diminished due to growing emissions of synthetic chlorofluorocarbon molecules. At the same time, global mean temperature increased due to the so-called green house effect. These global environmental changes may affect ecosystems to a hitherto unknown extent. The aims of the present study therefore involve the depiction of acclimation to UVR and PAR in the protective and metabolic mechanisms of brown algal juvenile life stages of the species Alaria esculenta, Laminaria digitata, Saccharina latissima and Saccorhiza dermatodea as well as the species' adaptive potential to enhanced temperatures. Including a comparative approach of field and laboratory experiments, the present study integrates various radiation regimes and intensities, temperature experiments and exposure times. For a broader understanding of effects, a variety of methods investigating e.g. intra- and extracellular levels of phlorotannins by the Folin-Ciocalteu method, fatty acid determination by gas chromatography, determination of reactive oxygen species and electron microscopy was applied to several juveniles of various Arctic macroalgae from Spitsbergen (Kongsfjorden). Results reveal that the sensitivity to visible and UVR of various macroalgal species is determined by their radiation tolerance and the protective potential of their parental tissue. As still only few data are available on interactive effects of PAR, enhanced UVR and temperatures, predictions of future consequences for algal recruitment and survival as well as on the community level remain difficult. While enhanced UVR in combination with low PAR in the laboratory was not affecting algal propagules, field experiments under high PAR showed detrimental effects leading to a decrease of spore germination. Most studies conducted in the past have focused on UVR effects while the present study reveals that high PAR was always underestimated and might play a more pronounced role than usually expected. Ozone depletion might be of greater importance for smaller organisms such as spores and bacteria which are physically less protected from UVR damage, especially in marine environments where concentrations of chromophoric dissolved organic matter are low and UVR transparency is high. Stratospheric ozone depletion over the Arctic in combination with rising temperatures is therefore expected to have a strong impact on the survival of macroalgal zoospores and finally the vertical zonation of Arctic and cold temperate macroalgal species around Spitsbergen.

The main motivation for this thesis was to describe the responses of the N2 fixing cyanobacterium Trichodesmium to elevated pCO2 and to provide a detailed understanding of underlying processes. The focus was hereby to characterize inorganic carbon acquisition and its interaction with photosynthesis and N2 fixation. Based on these findings, the potential influence of Trichodesmium on the ecosystem and elemental cycles in the future oceans was assessed.

The results of this work are a contribution to the understanding of the influence of dramatic natural variations of the physico-chemical environment in coral reef areas caused by Large Amplitude Internal Waves (LAIW).LAIW are a worldwide oceanographic phenomenon generated in areas with a rich underwater topography, strong density stratification and tidal currents. They deliver sub-thermocline water upslope into shallow shelf regions but their possible biological impact especially on benthic communities and ecosystems such as coral reefs is so far largely unknown. In this work the impact of LAIW on coral reefs was investigated at the Similan Island chain in the Andaman Sea. The unusual coral reef distribution with reefs flourishing only on the sheltered eastern island sides and no true reef framework in the west was shown to be consistent among the islands and related to LAIW. Temperature records revealed that LAIW have their strongest impact on deeper west slopes where they cause frequent and abrupt drops in temperature with peak activity during the north-east monsoon (January through April). LAIW advect deep cold, nutrient rich, suboxic and low-pH waters coming with strong currents into shallow near shore areas. In contrast to low numbers of frame-building species in east, the west slopes harbour loose yet more divers communities of scattered corals. This might be due to the alternating impact of south-west monsoon (May to October) and LAIW from above and below maintaining the coral community in a non-equilibrium state of intermediate disturbances which enhances species diversity.Coral growth of and bioerosion on Porites lutea revealed both, lowest rates at the site of highest LAIW-impact in west deep areas and highest rates within the dense coral reef of the sheltered east shallow. A calculated carbonate budget confirmed that carbonate accretion by reduced coral growth and carbonate erosion by alike reduced grazing and boring organisms are balanced but only barely positive under highly variable LAIW conditions in west deep. Coral recruitment on natural reef substrate revealed significantly higher abundances in shallow west than in east. However with increasing exposure period of experimental settlement tiles, the proportion of juvenile corals increased in east but decreased in west, indicating a failure of recruits to establish permanently on the LAIW-exposed sides in spite of the higher initial spat fall. Early stages of settlement and growth seemed to occur independently of LAIW exposure while the subsequent establishment of juvenile corals appeared to be related to LAIW intensity, reflecting the lack of true reefs on the LAIW-exposed west sides of the islands.

The environment of Antarctica represents one of the most challenging and harshest ecosystems, characterized by very low temperatures and a strong seasonality in light availability. Due to the abiotic conditions, inhabiting organisms are highly adapted to their habitat and possess the ability to cope flexibly with changing environmental factors. In the context of global climate change, the Antarctic and especially the Antarctic Peninsula undergo the most rapid and significantly changing regions worldwide. The combination of low temperatures and high light intensities are challenging conditions for photosynthetic organisms, as low temperatures reduce for instance enzymatic processes and the turn-over of the D1 centre protein of photosystem II. This protein plays a crucial role in photosynthetic function.Low water temperatures also decrease membrane fluidity, resulting in an impairment of transfer processes for instance through the thylakoid membrane of chloroplasts. In addition, photosynthetic activity is particularly sensitive to low temperatures, as enzymatic secondary reactions are strictly temperature-dependent, while primary reactions are not. The present thesis investigated the physiological performance, the acclimation potential and tolerance limits of the endemic Antarctic rhodophyte Palmaria decipiens. In various experiments conducted under laboratory and field conditions, the alga was exposed to changing light and temperature levels. It was hypothesized that in particular the combination of photosynthetically active radiation (PAR, 400-700 nm) and low temperatures lead to stress responses in the organism. To estimate the responses of P. decipiens, the alga was exposed over different periods of time to natural and artificial radiation. The macroalgae investigated in the present study revealed a relatively high acclimation potential within their species-specific tolerance ranges. Obviously, this tolerance is limited and particularly rapid and pronounced changes can impact seaweeds negatively. To obtain realistic predictions in a more ecological context, investigations should involve interactive effects of the abiotic and biotic conditions. In summary, this study demonstrates that Antarctic rhodophytes are very well adapted to their environment and are, in spite of this, still able to acclimate and cope with certain changes in the abiotic conditions. The results obtained during the present study also indicate that the combined effects of environmental conditions is an important issue that has to be considered when adressing ecological questions.

Coral reef ecosystems sustain their high productivity in an oligotrophic environment by the efficient recycling of nutrients and tight benthic-pelagic coupling. The basis for productivity within coral reefs is the primary production, which is accomplished by diverse benthic photosynthetic organisms and, to a minor contribution, by phytoplankton in the waters aloft. Photosynthesis is determined by the availability of light, influenced by water depth and turbidity, and the availability of inorganic nutrients, essential for the organic synthesis of vital substances like amino acids. Benthic photosynthesis in coral reefs is characterised by the high abundance of manifold zooxanthellae bearing hosts, in particular scleractinian corals. Findings of the following chapters are presented from the organism to the ecosystem level (chapter 1-5), concerning organic matter recycling (nutrient availability) and energy conversion (light availability and photosynthesis) within a coral reef ecosystem. Nutrient and light availability vary likewise between ecosystems and are comparatively investigated at the Similan Islands in the Andaman Sea of the Indian Ocean (chapter 6-8), concerning environmental conditions, benthic community composition, primary production and heterotrophy.

Picoplanktonic organisms are ubiquitous around the world and are able to dominate biomass, production, and metabolic activity in diverse regions of the oceans. It is suggested that they perform important and basic functional roles in the marine ecosystem, although many of their ecological functions and characteristics remain unknown. For example, the reaction of picoplankton to anthropogenic influences, such as eutrophication, pollution and climatic change, or the spatial and temporal differences among clades and ecotypes have not been determined yet. This thesis examined the identity and ecological abundance of picoeukaryotes in the coastal waters of the German Bight on a weekly frequency over a 17-months period by using diverse currently existing methods. Special emphasis was placed on the comparison of these methods and one species, Micromonas pusilla (Mamiellales, Prasinophyceae), which is the best-studied and possibly most abundant phototrophic picoeukaryote.

In this thesis, broad phenotypic variation in the genus Alexandrium with respect to allelochemical properties targeting a wide range of other planktonic protists was observed and indications of co-evolutionary processes shaping the respective allelopathic phenotype were found. At the intra- and inter-population level in the species A. tamarense large genotypic variation was observed. A conceptual population genetic model on how this variation is preserved in the long run - despite evidence of clonal selection and frequency shifts of clonal lineages during vegetative planktonic growth - is presented. Phenotypic characters such as allelochemical properties and content and relative composition of paralytic shellfish poisoning toxins (PSP) may be such characters that are under directional selection during planktonic population growth and harmful algal bloom (HAB) development. However, isolates from the populations studied varied widely with respect to these quantitative genetic characters.

In order to understand the present, environmental knowledge from the past is important. Observational records of environmental data are spatiotemporally incomplete and extremely scarce prior to AD 1860. Large scale environmental variability can be well represented by integrating data from a limited number of geographically scattered indicators or 'proxies' of past climate. This is partly achieved by the analysis of tree rings and of marine sediments cores. Long term environmental reconstruction with high resolution can be achieved by the calcareous skeleton of long lived invertebrates. In the last decade the ocean quahog Arctica islandica has attracted increasing interest among marine biologists owing to its extremely long life span approaching 400 y, and occurrence along the entire North Atlantic Ocean margins. But even more important is that its calcareous shell archives information on past environmental conditions in the morphological growth patterns and biogeochemical properties. The Ocean quahog A. islandica may become an important proxy in environmental change research, although until now the full capabilities of this species have not yet been fully realised.This thesis is an attempt to achieve deeper insight of the environmental proxy A. islandica, in terms of its environment, biology as well as population dynamics and evolutionary history. In order to achieve this goal, a number of approaches targeting six different sites of the North East Atlantic Ocean (i.e. Norwegian coast, Kattegat, Kiel Bay (Baltic Sea), White Sea, German Bight (North Sea) and off North East Iceland) were developed: (i) A general metabolic model which allows calibration of individual metabolic rates of A. islandica from five populations to body mass, water temperature, age and site. (ii) Growth and energy budget models of A. islandica at six different sites allowed a better understanding of the ecological role of the bivalve in its environment and to enable a linkage of the dynamics of A. islandica populations to environmental drivers (iii) Morphological and genetic comparison confirm differences between populations of A. islandica and revealed whether the morphological differences are driven by the environment or underlying genetic differences.Between different populations, environmental temperature is found to be the prime driving force that exerts distinct site specific effects on respiration rate. Respiration of A. islandica is significantly below the average of 59 bivalve species when temperature and mass are taken into account. In addition, growth rates of A. islandica are very low. A. islandica populations differ distinctly in maximum lifespan (40 y Kiel Bay, to 197 y Iceland). Lifespan is the principal determinant of the relationship between energy budget parameters, whereas temperature affects net growth efficiency only. Genetic results indicate a fairly mixed assemblage over the North East Atlantic area. Morphometric and genetic patterns appear to be unrelated and confirm morphological differences between populations of A. islandica, morphometrics appear to be driven by the environment (e.g. temperature and salinity) rather than underlying genetic differences between populations. Therefore this study enhances our understanding of the relationship between environmental conditions and the population biology such as growth, metabolism as well as population dynamics and evolutionary history of A. islandica. In addition, different models may enable the coupling of regional oceanographic temperature models as an environmental proxy of past environmental variabilities. Further, a geographically more extensive data set is necessary to apply A. islandica as a proxy and eliminate uncertainties related to its biological and evolutionary history.

In Arctic coastal ecosystems, marine macroalgae are exposed to a multitude of abiotic and biotic stressful conditions, including naturally seasonal variations and additionally potential effects of climate changes. Although the different abiotic variables are acting in combination and interdependently in the natural environment, the responses of macroalgae to interactive effects of multiple abiotic stressors are relatively unknown. UV-induced effects and growth temperatures are well studied on different macroalgal species, but mainly in single-factor experiments. Accordingly, the present thesis aims to detect interactive effects of combined abiotic environmental factors on Arctic marine macroalgae from the Kongsfjord (Spitsbergen) for a better understanding of their stress physiology and to estimate potential ecological implications. The study focused on physiological responses of different macroalgal species exposed to impacts of temperature stress in combination with radiation as well as salinity conditions.

Worldwide about 80% of the warm-water coral reefs have been harmed, and about 25% irreversibly damaged from direct human pressures. The effects of sedimentation on reef-building corals are well documented, as previous studies focused mostly on the coral response. The processes of how sediment actually damages corals, and the role of contrasting sediment properties have remained poorly understood. In this study we therefore focused on the sediments and investigated the harming processes. Our central hypothesis was that bacteria play a crucial role in the damage that sediment causes to corals. This thesis presents the advancement of two existing methods, data from mesocosm experiments, as well as field data, obtained with the new submersible microsensor system DOMS. This work revealed that harmful effects of sediment exposure on reef-building warm-water corals are tightly linked to sediment properties, primarily the percentage of silt grains, the organic matter content and the microbial activity, demonstrating that the exposure to organic-rich fine sediment is particularly dangerous for coral reefs, and that the demise of sediment-covered corals is mediated by microbial activity.

Declines in biodiversity have caused concern because of ethical and aesthetic reasons, but also because of the consequences for the goods and services provided by natural ecosystems. Consequently, ecologists have focused for decades on testing the idea that systems with more species are more stable. The results, however, have been complex and inconsistent. In particular, it is still unclear whether high stability in species-rich communities is due to the number of species per se (species richness) or to the increased likelihood of including particular species or functional types (species composition). In this thesis, I evaluated the contribution of species richness and species identity to the stability of marine hard-bottom communities. Combining observational and manipulative experimental methods, I conducted three field studies in intertidal and shallow subtidal habitats of Helgoland Island, NE Atlantic. First, I conducted an observational study to test whether intertidal communities containing many species are more stable (i.e. do vary less over time) than communities containing fewer species. Species covers were estimated every 6 months for 24 months and an index of stability was calculated for total community cover across time (S = mean SD-1). Second, I conducted a synthetic-assemblage experiment - in which I increased the diversity of field-grown sessile suspension-feeding invertebrates - to determinate whether assemblages containing several functional groups consume a greater fraction of resources than is caught by any of the functional types grown alone. (A functional group is a group of species with the same effect on an ecosystem property.) Finally, I conducted a removal experiment to test whether the loss of the canopy-forming alga Fucus serratus and mechanical disturbances that provide free substratum affect the temporal variability in cover of intertidal communities. In the removal experiment, species covers were estimated every 3 months for 18 months and the temporal variance was analysed.In general, the effects of the number of species and functional groups on ecosystem stability were weaker than those of species composition. In the observational study, stability was a negative and curvilinear function of species richness, which probably resulted from the dominance of few species. In accordance, the synthetic-assemblage experiment showed that there was no relationship between resource consumption and functional group diversity per se, but that different functional groups had idiosyncratic effects. On the other hand, the removal of Fucus changed the physical environment by increasing temperature, irradiance, and amount of sediment, which depressed the abundance of sensitive species like encrusting algae and small sessile invertebrates, but raised the abundance of more tolerant species like ephemeral green algae. This resulted in a significant increase in the variability of species abundances, but not in that of communities. The negative covariances resulting from the compensation between sensitive and tolerant species buffered the community stability against the environmental disturbances. These patterns were consistent across two sites, suggesting a consistent effect of canopies across the spatial variability of this system.Species composition appears to be more important for ecosystem stability than taxonomic and functional richness. Yet, the occurrence of compensatory dynamics in the face of environmental changes (i.e. the removal of Fucus) suggests that a variety of species with differing environmental tolerances is needed to maintain the functioning of this ecosystem. Therefore, predicting the consequences of species loss requires a detailed knowledge about the effects of species on ecosystem functioning and their responses to the environment. Conservational managers should strive (i) in identifying species with disproportional effects on ecosystem functioning, and (ii) in maintaining a redundancy of species with similar effects on ecosystem functioning and a diversity of species with different sensitivities to a suite of environmental conditions.

This dissertation thesis deals with temporal and spatial variations in bacterial activities and community structure in coastal sandy sediments of the temperate German Wadden Sea. The activities of the heterotrophic bacterial community were found to be strongly coupled to microphytobenthic primary productivity in this habitat and as such varied strongly with space and time. The vertical and temporal changes in bacterial activities were accompanied by significant shifts in bacterial community structure. While the spatial shifts could mainly be related to lateral variations and vertical gradients in microphytobenthos-associated labile organic carbon availability and redox potentials, temporal shifts were mainly stochastic and could not be related to any of the environmental parameters included in this study. The bacterial community was highly diverse and extremely dynamic with a replacement of 47% of operational taxonomic units over 2 years. The bacterial community structure did not reflect the strong seasonal patterns in bacterial activities, indicating a high redundancy in general ecosystem functions.

To test the effect of food on the thermal tolerance of the symbiotic association, two feeding experiments were carried out at the Hasanuddin University Marine Field Station in southwest Sulawesi, Indonesia between June and October 2005. In both experiments corals were either starved, i.e. deprived of organic particles >0.5 μm or fed daily with freshly hatched Artemia salina nauplii. In Experiment 1 the effect of zooplankton feeding versus starvation on the bleaching susceptibility, photosynthetic activity of photosystem II (PSII) and non-photochemical quenching (NPQ) of zooxanthellae in the temperature sensitive species Stylophora pistillata under elevated temperature was assessed over a period of 10 days employing pulse-amplitude-modulated (PAM) chlorophyll fluorometry. Experiment 2 was run for 15 days in order to investigate the potential interactions between zooplankton feeding, basal metabolic functions (photosynthesis and respiration), energy status (lipid concentrations), total protein concentrations, and the bleaching susceptibility of S. pistillata and the more temperature tolerant species Galaxea fascicularis.The effects of mineral accretion on the physiology and rates of mortality of coral transplants were investigated in North Sulawesi, Indonesia between April and July 2006, using the congeneric species Acropora yongei and A. pulchra. In order to discriminate between the effects of electrochemical processes in the vicinity of a cathode and those of an electric field on coral performance, coral transplants were grown under 3 treatment conditions: 1) on a cathode in the presence of mineral accretion, 2) on bamboo inside an electric field, or 3) on bamboo outside the experimental matrix (control), for 4 months. At the end of the treatment phase coral survival, growth, zooxanthellae population characteristics and chlorophyll fluorescence responses of the two species were determined.The overall results of the two feeding experiments show that the thermal stress tolerance in terms of photosynthetic activity and pigmentation of both species was coupled to food availability. Zooxanthellae in fed corals in experiment 1 maintained high photosynthetic activity while starved corals displayed strong signs of chronic photoinhibition which was reflected by a progressive decline in the daily maximum quantum yield (Fv/Fm) of PSII and nocturnal recovery rates of PSII, which was manifest by a significant decrease in Fv/Fm, effective quantum yield (∆F/Fmà ¯Ã ¿Ã ½) and relative electron transport rate (rETR). This was paralleled by the progressive inability to dissipate excess excitation energy via non-photochemical quenching (NPQ). In addition, both gross (Pg) and net (Pn) primary production of starved S. pistillata and G. fascicularis in experiment 2 decreased significantly over the course of 15 days. Fed corals by comparison maintained higher levels of NPQ, which coincided with significantly higher rates of nocturnal PSII recovery. Sustained photosynthesis of fed corals was accompanied by a reduced loss in pigmentation compared to starved corals. After 10 and 15 days of exposure to elevated temperature the tissue of starved corals appeared pale relative to fed corals. The three most relevant findings of the investigation evaluating the proposed mechanisms of enhanced survival and skeletal growth rates of coral transplants in the presence of mineral accretion show that 1) the electric field and not the cathode resulted in the highest growth rates, 2) corals grown under the presence of mineral accretion can exhibit low growth and reduced health , and 3) the suitability of the mineral accretion technology for coral transplantation can vary greatly between closely related species. Contrary to previous observations, the presence of mineral accretion had either no or even a negative effect on coral survival over a period of 4 months. Mortality rates of A. yongei after 4 months were significantly higher for corals grown on the cathode than for those inside the electric field and on the control. By contrast there were no significant differences in mortality between treatments for A. pulchra.

For this doctorate study several experiments with seagrasses were planned and conducted on the German North Sea island Sylt from 2002 till 2004. The results show that seedlings from both seagrass species grew better at weaker current velocities than at stronger velocities, with Zostera noltii being able to withstand higher velocities longer than Z. marina. The lugworm Arenicola marina negatively influenced Z. noltii, but did not influence Z. marina. The combination of higher current velocities and lugworm presence was particularly bad for Z. noltii. Both seagrass species grew better in the upper eulittoral compared with the lower eulittoral. In the donor population, Z. marina seedlings emerged 3-4 weeks earlier at the sediment surface than Z. noltii seedlings. Despite this Z. noltii seedlings had nearly the same shoot length by late June 2004. Neither seagrass species transferred carbon or nitrogen from the leaves to the roots/rhizomes with the onset of winter. The phosphate concentration in the porewater of the seagrass meadows might be limiting seagrass growth but ammonium and silicate concentrations are not limiting.

Coastal upwelling areas are highly productive systems and were initially characterized by having a short food chain, being ecologically efficient in the trophic transfer. Large micro-phytoplankton, predominating under high availability of nutrients in the mixed layer, are grazed by large herbivorous zooplankton, and they, in turn, are consumed by planktivorous fishes. Under this scheme, little attention was paid to the role of micro-organisms in these areas. This thesis provides an assessment of the temporal variability in the structure of micro-organism assemblages and of the trophic interactions in microbial food webs in the Humboldt Current System (HCS) off Concepcion, central Chile, as a basis to understand the relevance of the carbon flow through the microbial food web in this coastal upwelling area.

First, the technical developments of sensors, camera systems and applications of the imaging techniques were reviewed. Since the first more primitive approaches, more sophisticated systems have been developed, in which both fluorescence intensity and fluorescence lifetime imaging was possible in one as well as two dimensions (planar optodes). The oxygen distributions were visualized by the use of planar sensor foils with an oxygen sensitive fluorophore layer, containing a photostable ruthenium complex, that was reversibly quenched by oxygen. The planar optodes were fabricated of organically modified sol-gel (ORMOSIL). All sensor foils were based on the use of ruthenium(II)-tris-(4,7-diphenyl-1,10-phenantrolin)-perchlorate, which was a fluorescent dye quenched dynamically by oxygen. Sensors made with different sol-gel immobilization matrices, different concentrations of precursors and indicator dye, as well as different types of scattering particles co-immobilized in the sensor foil were investigated systematically. Optimal sensor performance was obtained with dye concentrations of 2-10 mmol/kg in an immobilization matrix made of diphenyldiethoxy-silan and phenyltriethoxy-silan precursors with addition of organically coated TiO2 particles. The sensors exhibited a good mechanical stability and a high sensitivity from 0 to 100% oxygen, which remained constant over at least 36 days. The recording system was optimized for modular luminescence lifetime imaging (MOLLI). The central parts of the system were a CCD-camera with a fast electronic shutter and gated LED (light emitting diode) or Xe excitation light sources. A personal computer controlled the gating and image acquisition via a pulse delay generator. The planar optodes were used to investigate 2-dimensional O2 dynamics in burrow systems inhabited by Hediste diversicolor. Natural burrows often deviated from the classic U-shape and changed over time. This complicated quantitative assessment of the O2 dynamics of the burrow. Therefore, a stable U-shaped burrow was established surrounded by agar-solidified sediment. Visual inspection and measured ventilation patterns indicated similar behavior of polychaetes in natural and artificial systems. The volume specific O2 consumption, RP, along the primary interface in the two systems were identical. Planar optode and microelectrode measurements gave similar results along the primary sediment-water interface. The radial diffusion of O2 around a burrow was disturbed by the presence of the planar optode directly attached to the burrow wall causing an enhanced oxygen penetration depth compared to the undisturbed system. With the known geometry of the burrow system the effect could be corrected. Microbial colonization along the burrow lining increased the volume specific O2 consumption, RB, from 3.8 0.5 to 13.7 2.1 mol m-3 d-1 within two days reflecting that the burrow lining of H. diversicolor acts as a zone of intensified diagenetic activity. Extrapolated to natural densities in coastal sediments (600 m-2) the findings indicate that 88 % of the benthic O2 uptake is associated to worm burrows and that the major fraction hereof (66 %) is related to microbial activity in the burrow lining, while 22 % can be ascribed to fauna respiration.