Search

Marine zooplankton, i.e., heterotrophic marine plankton, serve as trophic links between primary producers and higher trophic levels, and as recyclers for nutrients and carbon in the pelagic ecosystem. In addition, they play a major role for the carbon export flux due to fecal pellet production and fragmentation of particles. They are distributed all over the ocean and constitute a large variety of organisms. Because of large uncertainties in the estimation of parameters and the forms of equations, zooplankton are often parameterized in strongly simplified forms in ocean biogeochemical models. Nowadays, however, increasing data availability from experiments and observations makes it possible to implement different zooplankton functional types in models. This thesis presents the implementation of new zooplankton functional types into an ocean biogeochemical model. Subsequently, the sensitivity of net primary production, carbon export and nutrients to the implementation of these new zooplankton functional types was analyzed. In my thesis, I use a global setup of the biogeochemical model Regulated Ocean Ecosystem Model (REcoM) coupled with the Finite Element Sea-Ice Ocean Model (FESOM). I implemented an explicit parametrization of micro-, meso-, and polar macrozooplankton based on process rates and biomass observations from the literature, as well as a representation of fast-sinking detritus. This extended version of REcoM was used to analyze the role of zooplankton for carbon export, nutrient recycling, and phytoplankton bloom phenology. In a second step, a new sinking routine that considers the roles of mineral ballasting and seawater viscosity on the particle sinking speed and the effect of oxygen on remineralization rates was added to the model. This set-up was used to assess the role of each factor (ballast minerals, seawater viscosity, and oxygen concentration) for the export and transfer efficiencies of carbon, i.e. the amount of particulate organic carbon that is exported across the euphotic depth and reaches the deep ocean. The implementation of the new zooplankton groups changes the carbon transfer efficiency and net primary production in the model. Publication I and III highlight the influence of zooplankton on the transfer efficiency of carbon. Publication I shows that the transfer efficiency of carbon reaches up to 50% due to the high biomass of polar macrozooplankton in the Southern Ocean. Similarly, it was illustrated in Publication III that the high mesozooplankton biomass increases the transfer efficiency of carbon to 80% in the Equatorial Pacific. In addition, the model results presented in Publication I and II show the stimulation of net primary production due to the fast recycling of nutrients. After the parametrization of three zooplankton functional types, the new state of the model leads to a 25% increase in annual mean net primary production. In addition to the effects on annual mean bulk fluxes, the more complex representation of zooplankton also affects the timing of phytoplankton blooms and biogeochemical fluxes. Zooplankton fecal pellets constitute an important share of sinking particulate organic carbon depending on the season in the Southern Ocean. In Publication I, it is shown that the typical shift from a dominance of phytodetrital aggregates in spring to zooplankton fecal pellets later in the year is now reasonably reproduced by the model after the implementation of polar macrozooplankton. Zooplankton grazing can play a decisive role in phytoplankton bloom phenology since it is a loss mechanism for phytoplankton. In Publication II, it is shown that the increased loss rates of phytoplankton due to stronger zooplankton grazing lead to the later start of the spring bloom. In addition, nutrient recycling by zooplankton prevents the fast exhaustion of nutrients by phytoplankton and consequently leads to a later end date of the bloom. In the end, the more complex parametrization of zooplankton provides a modeled phytoplankton bloom phenology closer to observations. The results also indicate that the explaining mechanism behind the bloom phenology changes. While the start of the spring bloom is explained better with the ’Critical Depth Hypothesis’ in the low grazing scenario, the system aligns with the ’Dilution-Recoupling Hypothesis’ in the high grazing loss simulation. Finally, the global spatial distribution of export and transfer efficiencies are analyzed in Publication III. In particular, I examined the impact of ballast minerals, seawater viscosity, and oxygen-dependent remineralization on export and transfer efficiencies. These three processes are often not considered in biogeochemical models. My results show that the global mean of export efficiency across the euphotic zone stays similar ( 13%) when the effects of mineral ballasting, seawater viscosity, and oxygen-dependent remineralization are added to the model. However, the global mean carbon transfer efficiency is more sensitive to these processes and varies between 25% and 32% in different simulations dependent on the representation of these processes. The magnitude of the effect of each process varies spatially. While the effect of ballast minerals can increase the transfer efficiency by a factor of nine in high latitudes and subtropical gyres, including oxygen-dependent remineralization can increase the transfer efficiency by 28% in low latitudes. The influence of seawater viscosity on the transfer efficiency is smaller compared to the other effects, and it increases the transfer efficiency 8% in subtropical gyres. The thesis highlights that the zooplankton compartment in biogeochemical models should not only be treated as a closure term, and zooplankton functional types should be implemented in the global ocean biogeochemical models by using available datasets from the literature. It further underscores that missing out process representations of mechanisms that underlie carbon export has considerable effects on estimated carbon transfer efficiencies in biogeochemical models. Thus, further attention should be paid on the representation of missing processes related to particle formation and sinking.

Marine phytoplankton constitutes about half of the primary production on Earth. It forms the base of the marine food web and is a pivotal player in the marine biological carbon pump. The primary environmental drivers that control phytoplankton growth are temperature, nutrient availability, light, and the concentration of inorganic carbon species. Ongoing climate change modifies these drivers, leading to a warming, high-CO2 ocean with altered nutrient availabilities and light regimes. Changes in phytoplankton productivity and community composition resulting from these newly emerging environmental states in the ocean have important implications for the marine ecosystem and carbon cycling. Biogeochemical ocean models are used to investigate how marine primary production may be affected by future climate change under different emission scenarios. Phytoplankton growth rates in models are typically determined by functions describing growth dependencies on temperature, light, and nutrients. However, a large body of laboratory studies on phytoplankton responses to environmental drivers reveals two points that are usually not considered in current biogeochemical models. Firstly, phytoplankton growth can be considerably modified by the state of the carbonate system. Changes in inorganic carbon species concentrations can be either growth-enhancing (CO2(aq) and bicarbonate are substrates for photosynthesis), or growth-dampening (increasing CO2(aq) levels lead to a shift in the carbonate equilibria and result in a pH decrease, a process which is called ocean acidification). Functions describing this growth dependence of phytoplankton on the carbonate system have not been implemented in large-scale ocean biogeochemical models so far. Secondly, growth responses towards one driver can be modified if the level of another driver is changing. Functions including these so-called interactive driver effects partly exist in models (e.g. the response to varying light levels may depend on the nutrient limitation term). However, the large number of laboratory studies on multiple driver effects has never been used to constrain driver interactions in large-scale ocean biogeochemical models. This holds especially true for the findings of growth responses to driver interactions that include ocean acidification, which make up the largest share of laboratory experiments. This thesis aims to investigate sensitivities of marine phytoplankton to changing CO2(aq) levels as well as to interactive effects between CO2 and other environmental drivers. A comprehensive and reproducible literature search in combination with a statistical analysis (Publication I) reveals that increasing CO2(aq) levels robustly dampen the growth-increasing effects of warming and improving light conditions. In addition, the results show that the calcifying phytoplankton group of coccolithophores experiences the strongest negative effects by ocean acidification compared to other phytoplankton groups. A second study (Publication II) examines the effects of mechanistically described carbonate system dependencies on primary production and community composition in a model. To this end, carbonate system dependencies of phytoplankton growth and and coccolithophore calcification are implemented into the global biogeochemical ocean model REcoM. The study shows that responses to ocean acidification cascade on growth responses to other drivers, which partly balance or counteract the direct impact of the carbonate system on growth rates. In addition, warming is identified as the main driver of the observed recent increase of coccolithophore biomass in the North Atlantic. A final study (Publication III) investigates the interactive effects between CO2 and temperature as well as between CO2 and light on phytoplankton biomass and community composition in a high emission scenario. For the parametrization in REcoM, growth responses to interacting drivers as synthesized in Publication I are used. The decrease of global future phytoplankton biomass and net community production by the end of the century is similar in simulations with and without driver interactions (-6% and -8%, respectively). However, phytoplankton responses to future climate conditions are considerably modified on a regional scale and the share of individual phytoplankton groups in the community changes both globally and regionally when accounting for multiple driver effects. Globally, diatoms and coccolithophores are impacted more and small phytoplankton less severely by future oceanic conditions when accounting for driver interactions. Future projections of the Southern Ocean phytoplankton community are modified most dramatically with the new interactive growth formulation, as diatoms and coccolithophores become less and small phytoplankton more abundant, while it is the other way round in simulations without driver interactions. The thesis highlights 1) that the carbonate system is a critical growth-modifying driver for phytoplankton in a high-CO2 ocean, which can furthermore modify growth responses to other drivers substantially, and 2) that driver interactions have considerable effects on climate-change induced alterations in the phytoplankton community as well as on regional biomass changes in a future ocean.

As the worldwide metal demand is drastically increasing, and the metal supply are limited, countries and industries have prospected the deep-sea for their precious resources. Polymetallic nodules form on the seafloor and are rich in metals such as manganese, cobalt, and copper. Due to their economic potential, polymetallic nodules have been targeted for mining during the past three decades. The highest nodule’s abundance so far has been found in the Clarion Clipperton Zone (CCZ) where currently 16 license areas have been issued for exploration only. Very limited information is available to assess the anthropogenic impact of mining activities on this fragile ecosystem. The main objective of this dissertation is to evaluate the impact of sediment plumes released during mining activities on the benthic and pelagic environment. The oceanographic description of the eastern German License area (CCZ) indicated that surface waters were influenced by nutrient-enriched water coming from the Mexican coastal area. During winter a depleted transitional zone appeared between these water masses and the highly productive water of the equatorial region. The vertical particle distribution was investigated using a combination of high definition camera images and CTD sensors. Specific water characteristics such as the oxygen minimum zone (OMZ) was identified and appeared to be strongly correlated to particulate matter alteration, distribution and transfer to greater depths. Overall, median particle sizes are small (77μm), and bigger particles (>300 μm) were rarely encountered. Polymetallic nodules occur in the CCZ abyssal plains which are made of fine siliceous ooze sediments (d50= ± 20 μm). The anticipated impact of mining activities was tested by reproducing variable sediment plumes concentration (35–500 mg⋅L-1) and turbulence rate (0–10.4 s-1). The most rapid sediment plume flocculation was found using a high sediment discharge (500 mg⋅L-1) combined with a turbulent shear rate of ≥ 2.4 s-1. The modeling results simulating four days of plume release suggested that mining under “normal” flow conditions (± 4 cm⋅s-1) would result in a relatively fast deposition of particles, thus restricting the blanketing effect to a relatively small fall-out area (up to 4 km downstream). Setting such sediment plumes requirements (sediment concentration and turbulence rate) should, therefore, be considered for the design of the mining collector exhaust pipe. To date, the work in this thesis represents the most advanced and complete dataset freely available to everyone. The bacterial community associated with the sediment plume was investigated using a cultivation-based approach. In total, 40 fast-growing bacterial strains belonging to 13 fast-growing bacterial species were identified. Based on their phenotypic characterization and heavy metals resistance, three potential organisms were found to be potential model systems for future ecological studies. Intra-species variabilities were not only found in phenotypic profiles but also in heavy metal tolerance although the taxonomic marker 16s rRNA sequences were almost identical. We proposed the use of API strips and partial-enrichment media that can easily be implemented on-board for rapid and cost-effective monitoring of deep-sea mining plume dispersion using microbial dissemination analyses. In this context, heavy-metal resistance analysis provides new scope for future research on Mn2+ resistance pathways and their role in microbial dispersion from anthropogenic impacts on deep-sea environments. The exploitation of deep-sea resources is developing faster than policies are established and still requires extensive exploration. A precautionary approach based on environmental awareness and the history of similar activities suggest that deep-sea mining will have an impact on the marine environment. However, if mining is approved, the studied impacts will play a crucial role in regulating the mining activities. Overall, this thesis complements the scientific efforts made by describing the deep-sea environment and provide valuable knowledge on how to reduce the environmental impacts from sediment plumes released during mining activities in the CCZ.

The economic interest in deep-sea mineral resources has grown in recent years. Minerals of interest include polymetallic nodules, which occur in extensive deposits at more than 4000 m water depth in the Clarion-Clipperton Zone (CCZ), NE Pacific Ocean. In light of recent technological advances in mining equipment, the CCZ represents a prospective area for commercial deep-sea mining. In the framework of this PhD thesis, sediments from four European contract areas for the exploration of polymetallic nodules in the CCZ and one of the protected Areas of Particular Environmental Interest (APEI) were investigated. Comprehensive pore-water and solid phase data were produced for undisturbed sediments and sediments from disturbance tracks of small-scale deep sea mining experiments. The presented studies provide new insights into the spatial and temporal variation of redox zonation, biogeochemical processes and element fluxes in the sediments and the impact of potential mining activities in the CCZ.