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In temperate lakes, it is generally assumed that light rather than temperature constrains phytoplankton growth in winter. Rapid winter warming and increasing observations of winter blooms warrant more investigation of these controls. We investigated the mechanisms regulating a massive winter diatom bloom in a temperate lake. High frequency data and process-based lake modeling demonstrated that phytoplankton growth in winter was dually controlled by light and temperature, rather than by light alone. Water temperature played a further indirect role in initiating the bloom through ice-thaw, which increased light exposure. The bloom was ultimately terminated by silicon limitation and sedimentation. These mechanisms differ from those typically responsible for spring diatom blooms and contributed to the high peak biomass. Our findings show that phytoplankton growth in winter is more sensitive to temperature, and consequently to climate change, than previously assumed. This has implications for nutrient cycling and seasonal succession of lake phytoplankton communities. The present study exemplifies the strength in integrating data analysis with different temporal resolutions and lake modeling. The new lake ecological model serves as an effective tool in analyzing and predicting winter phytoplankton dynamics for temperate lakes.

To counteract the severe consequences of eutrophication on water quality and ecosystem health, nutrient inputs have been reduced in many lakes and reservoirs during the last decades. Contrary to expectations, in some lakes phytoplankton biomass did not decrease in response to oligotrophication (nutrient reduction). The underlying mechanisms preventing a decrease in biomass in these lakes are the subject of ongoing discussion. We used a hitherto unpublished long‐term data set ranging from 1961 until 2016 from a German drinking water reservoir (Rappbode Reservoir) to investigate the underlying mechanisms preventing a decrease in biomass. Total phosphorus (TP) concentrations in the Rappbode Reservoir dropped abruptly in 1990 from 0.163 to 0.027 mg/L within three consecutive years, as a result of banning phosphate‐containing detergents. Despite substantial reductions in TP, total annual phytoplankton biomass did not decline in the long‐run, and therefore, the yield of total phytoplankton biomass per unit phosphorus largely increased. Regression analysis revealed a positive association between the yield and potentially phagotrophic mixotrophs (R² = .465, p

The invasive clam Corbicula fluminea was first recorded in Lake Constance (Germany) in 2003 and subsequently established high population densities at various locations in the lake. In 2010, four standardized field experiments were conducted in Lake Constance, one in each season, to investigate seasonal changes in somatic growth and survival of C. fluminea at different water depths. Somatic growth and survival of the clams were related to seasonal variations in water temperature and seston characteristics, that is, phytoplankton species composition and essential nutrient concentrations, and to the elemental and biochemical composition of clam tissues using principal component analyses and regression models. The field experiments revealed that somatic growth and survival of C. fluminea in Lake Constance were primarily determined by water temperature. In winter, low temperatures (

Many lakes exhibit seasonal stratification, during which they develop strong thermal and chemical gradients. An expansion of depth-integrated monitoring programs has provided insight into the importance of organic carbon processing that occurs below the upper mixed layer. However, the chemical and physical drivers of metabolism and metabolic coupling remain unresolved, especially in the metalimnion. In this depth zone, sharp gradients in key resources such as light and temperature co-occur with dynamic physical conditions that influence metabolic processes directly and simultaneously hamper the accurate tracing of biological activity. We evaluated the drivers of metalimnetic metabolism and its associated uncertainty across 10 stratified lakes in Europe and North America. We hypothesized that the metalimnion would contribute highly to whole-lake functioning in clear oligotrophic lakes, and that metabolic rates would be highly variable in unstable polymictic lakes. Depth-integrated rates of gross primary production (GPP) and ecosystem respiration (ER) were modelled from diel dissolved oxygen curves using a Bayesian approach. Metabolic estimates were more uncertain below the epilimnion, but uncertainty was not consistently related to lake morphology or mixing regime. Metalimnetic rates exhibited high day-to-day variability in all trophic states, with the metalimnetic contribution to daily whole-lake GPP and ER ranging from 0% to 87% and

Optical sensors for fluorescence of chlorophyll a (f-Chl a) and phycocyanin (f-PC) are increasingly used as a proxy for biomass of algae and cyanobacteria, respectively. They provide measurements at high-frequency and modest cost. These sensors require site-specific calibration due to a range of interferences. Light intensity affects the fluorescence yield of cyanobacteria and algae through light harvesting regulation mechanisms, but is often neglected as a potential source of error for in-situ f-Chl a and f-PC measurements. We hypothesised that diel light variations would induce significant f-Chl a and f-PC suppression when compared to dark periods. We tested this hypothesis in a controlled experiment using three commercial fluorescence probes which continuously measured f-Chl a and f-PC from a culture of the cyanobacterium Dolichospermum variabilis as well as f-Chl a from a culture of the green alga Ankistrodesmus gracilis in a simulated natural light regime. Under light, all devices showed a significant (p

Although the treatment of eutrophied lakes with aluminium (Al) compounds has been established for more than 40 years, publications reporting on long-term consequences for phytoplankton are rare. Here we present observations from Lake Barleber for the period 1985–2016. The lake was treated in autumn 1986 because of high phosphorus (P) concentrations and cyanobacteria blooms, which limited the lake's recreational use. Within six weeks 480 t of Al sulphate solution (37 t of Al) were applied to the lake. This was equivalent to a dose of 36 g Al3+ m−2 or 5.7 mg Al3+ L−1. Already after having applied half of the Al sulphate, the concentration of soluble reactive phosphorus (SRP) reached its analytical limit of quantification (3 μg L−1). Removal rates calculated after completion of the treatment were 98% for SRP and 90% for total phosphorus (TP). In the following 13 years from 1987 to 1999, cyanobacteria were almost absent. In the years 2000 to 2003 as well as in 2005 and 2014 they appeared in low abundances. In the period 1987–2014, almost complete absence of cyanobacteria and high transparency provided good conditions for recreational use of Lake Barleber. Compared to pre-treatment conditions, phytoplankton biomass increased temporarily from 1987 to 2016. This increase in biomass did not interfere with the use for bathing and swimming, because phytoplankton community composition changed towards a dominance of chlorophytes and dinophytes. In 2016, however, P concentration and cyanobacterial biomass rose again to the level of the last pre-treatment years (TP 134 μg L−1, cyanobacterial biomass 1 mg L−1; averages for the period May–October). We conclude that Al treatment is effective and can last for decades. For recreational lake use, the effects of the alum treatment on phytoplankton community composition showed to be more important than its effects on total phytoplankton biomass.

We applied a coupled hydrodynamic-ecological lake model to Lake Constance and investigated the consequences of luxury uptake of mineral resources (phosphate) by phytoplankton on the distribution of this resource in the environment. We compared two different resource limitation models, a static P model with a fixed cell stoichiometry disabling luxury uptake (the Monod approach) and a dynamic P model with a flexible cell stoichiometry enabling luxury uptake (the Droop approach). Our research was based on the finding that a model simulation using the static P model showed good results for phytoplankton dynamics but was unable to simulate the vertical distribution of the algal resource properly. In this model, the resource was only depleted in the euphotic zone (ca. 0–20 m) while in Lake Constance observed phosphate depletion was about twice as deep (down to ca. 40–50 m). A simulation using a dynamic P model reproduced the vertical extension of resource depletion. The driving process behind the deeper resource depletion was luxury uptake of phosphate by algae sedimenting out of the productive zone. Even though the spatial extent of phosphate depletion strongly differed, the difference in phytoplankton dynamics between the two resource limitation models was minor. It is shown that a process acting at the cellular level has wide implications at the ecosystem level. Thereby, the inclusion of a flexible cell stoichiometry of phytoplankton into a complex lake model is important for predicting spatial nutrient gradients within the lake.

Global warming is already causing salinization of freshwater ecosystems located in semi-arid regions, including Turkey. Daphnids, which are important grazers on phytoplankton and a major food source for fish and invertebrates, are sensitive to not only changes in salinity levels, but also presence of predators. In this study, the interactive effect of salinity toxicity (abiotic factor) with predation pressure mimicked by the fish-exuded kairomone (biotic factor) and the effect of salt acclimation on daphnids were investigated. Impacts of these stressors on daphnid survival, life history and molecular profile were observed. The presence of the kairomone antagonistically alters the effect of salinity, as observed from the 24- and 48-h LC(50) values and survival results. Molecular findings provided solid evidence to this antagonism at even lower salt concentrations, for which antagonism was not evident with organismal data. Fish predation counterbalances the negative effect of salinity in terms of reserve energy density. Therefore, it is important to investigate multiple stressor effects in ecotoxicological bioassays complemented with molecular techniques. The single effect of increasing salinity resulted in increased mortality, decreased fecundity, and slower somatic growth in Daphnia, despite their acclimation to salinity. This insignificance of acclimation indicates that Daphnia do not have any physiological mechanisms to buffer the adverse effects of salinity, making it a very crucial factor. Salinity-induced reduction in population growth rate of freshwater keystone species Daphnia-despite acclimation-indicates that global warming-induced salinity may cascade through the food web and lead to dramatic environmental consequences in the structure of lake ecosystems.

Summary 1. Leaf litter breakdown by shredders in the field is affected by leaf toughness, nutritional value and the presence of secondary compounds such as polyphenols. However, experiments involving the use of single fungal strains have not supported the assumption that leaf parameters determine food selection by shredders perhaps because of a failure to test for high consumption prior to isolation of fungal strains, overrepresentation of hyphomycetes or the potential effects of accompanying bacteria. In this study, we used bacteria-free, actively growing fungi and oomycetes isolated from conditioned leaf litter for which a shredder had already shown high consumption rates. 2. Black alder (Alnus glutinosa) leaf litter was exposed to the littoral zone of Lake Constance in autumn, and subsamples were analysed for leaf parameters and consumption by Gammarus roeselii under standard conditions at regular intervals. On dates with a high consumption rate of the exposed leaves, 14 single strains of fungi and oomycetes were isolated, freed of bacteria and grown on autoclaved leaves. 3. Six of eight measured leaf parameters of exposed leaves were significantly correlated with Gammarus consumption rates, with high colinearity among leaf parameters hampering the identification of causal relations between leaf parameters and feeding activity. 4. When single strains of fungi and oomycetes were grown on autoclaved leaf litter, toughness of colonised leaves was always lower than in the control and the content of protein, N and P were increased. There were pronounced strain-specific effects on leaf parameters. Consumption rates also differed significantly, with nine of fourteen isolates consumed at higher rates than controls and none proving to be a deterrent. Protein and polyphenol content were significantly correlated with consumption rates. Oomycete-colonised leaves were consumed at similar rates but were of lower food quality than fungi-colonised leaves. 5. We argue that direct strain-specific attractant or repellent effects of fungi and oomycetes on consumption by G. roeselii are not important. However, we found indirect strain-specific role operating via effects on leaf parameters.

The interaction of microbial colonization with leaf litter breakdown by the shredder Gammarus roeselii was studied in the littoral of large Lake Constance. In a first step we studied selective feeding of gammarids on leaf litter from three different treatments. Littoral exposed, tap water exposed, and autoc1aved leaves were offered to G. roeselii in standard food-choice assays under laboratory conditions. We found highly selective feeding on littoral exposed leaf litter indicating that microbial conditioning is 'affecting the shredders feeding behaviour. Neither C, N, and P nor protein nor ergosterol content were positively correlated with the relative consumption rates of littoral exposed leaves. For a more detailed study of the microbial conditioning of the leaves oomycetes and fungi were iso­ lated from the littoral exposed leaves. Based on the sequence of internal transcribed spacer (ITS) regions of rDNA, we identified single strains of Fusarium sporotrichoides, Microdochium sp., Ascomycete sp., and Cylindrocladiella parva; two strains of Cylindrocarpon sp.; and three strains of Pythium sp. Subsequently, food choice assays were conducted using the isolated strains by offering autoclaved leaves and leaves colonized with a single strain. Three strains (c. parva, Cylindrocarpon sp. 94-2057 and Ascomycete sp.) were preferred by G. roeselii, while leaves with F. sporotrichoides were avoided. Leaves with each of the three oomycete isolates (Pythium sp.) were neither pre­ ferred nor rejected by G. roeselii. Our results suggest that the selective feeding behaviour of G. roeselii is affected by the relative abundance of specific fungal strains. Thus, whether microbial conditioning acts as a repellent or as an attractor for shredders depends on the microbial community on the leaves.

We studied three-dimensional distribution patterns of temperature, phyto- and zooplankton, and fish in the large, prealpine Lake Constance during spring 2007. A strong westerly wind induced an intense eastward displacement of epilimnetic water and upwelling of hypolimnetic water in the western part of the lake. This led to the formation of an internal front separating cold, hypolimnetic water depleted of chlorophyll in the western part from epilimnetic, warm water with high chlorophyll concentrations in the eastern part. Hydroacoustic detection of zooplankton (by Acoustic Doppler Current Profiler) and juvenile fish (by echosounding) revealed both to be passively transported by the wind. Consequently, zooplankton and fish showed comparable horizontal distributions as temperature and chlorophyll. During periods of low wind velocities (, 6ms 21), water temperature was more evenly distributed, whereas phytoplankton distribution was still heterogeneous, probably because of local differences in resource supply. The relative influence of biotic factors for the distribution of organisms increased when external forcing was low. At periods with weak wind forcing, phytoplankton typically showed highest concentrations in the metalimnion, where zooplankton also aggregated in thin layers. In conclusion, we found spatial distributions of temperature and organisms to be strongly controlled by wind forcing when wind velocities were sufficiently high, whereas the importance of internal biotic factors for distribution of organisms increased when wind velocities were less strong. Abiotic factors appeared to act over relatively large spatial scales and affected distributions within the entire ecosystem, whereas biotic factors affected distributions of algae, zooplankton, and fish on a more local scale. Abiotic and biotic processes commonly cause lake-wide distributions of plankton to be highly variable and heterogeneous, resulting in considerable patchiness (Folt and Burns 1999). The ecological consequences of patchy distributions of organisms are manifold; metapopulation ecology has shown that spatial heterogeneity is important for species persistence and population dynamics (Hanski 1981; Bascompte and Sole´ 1995), rendering the phenomenon of patchiness a key factor for mediating species coexistence and maintenance of high species diversity (Hastings 1988, 2001). Patchy distribution of prey organisms affects the intensity and dynamics of predator–prey interactions (Eggers 1976; Hastings 2001) and enhances ecosystem productivity (Rovinsky et al. 1997; Brentnall et al. 2003). For a thorough understanding of ecosystem functioning, the aspect of spatial distribution patterns and their consequences therefore need to be taken into account. Model applications have demonstrated that it is an intrinsic property of plankton distributions to be spatially heterogeneous because of the interaction of physical and biological processes that work on different spatial and temporal scales (Turing 1952; Abraham 1998; Brentnall

Diel vertical migration (DVM) of Daphnia is a well-studied inducible defence mechanism against predation by fish. Our study is anchored in constructing an individual-based model of DVM in order to bring established knowledge about essential key processes into a synthesis. For that purpose, we combined information about both proximate and ultimate factors of DVM with the intention to unify published results from these historically separated lines of research. The model consists of three submodels: (i) movements, (ii) growth and reproduction, and (iii) mortality. The submodel ‘‘movements’’ includes algorithms for light-dependent migration behaviour of Daphnia that were able to reproduce spatiotemporal distribution patterns of DVM. By means of scenario analyses, we tested the predator avoidance hypothesis by comparing population growth rates of migrating and non-migrating populations over a range of fish biomasses in the habitat. This enabled us to quantify the adaptive value of DVM under various environmental settings. Simulation results supported the predator avoidance hypothesis and showed a particularly high adaptive value of DVM if fish predation is intense. However, since DVM is associated with costs, a certain predation pressure in the habitat has to be prevailing in order to turn DVM into an adaptive strategy. Otherwise, if fish predation is weak, migrating populations realize lower population growth rates than non-migrating populations. In a second scenario, we tested the influence of vertical gradients of temperature and food on the adaptive value of DVM. We found a great potential to maximize the adaptive value of DVM if daphnids are able to modify their migration amplitude in dependence of the vertical structure of their habitat. For example, a deep chlorophyll maximum (DCM) can be a strong attractor for Daphnia to modify the migration amplitude in such a way that the daytime depth corresponds to the depth of the DCM. However, flexibility of the migration amplitude is only advantageous if the predation intensity is moderate – if predation is intense, only maximum migration amplitudes maximize fitness. r 2008 Elsevier GmbH. All rights reserved.

Here, we present a community perspective on how to explore, exploit and evolve the diversity in aquatic ecosystem models. These models play an important role in understanding the functioning of aquatic ecosystems, filling in observation gaps and developing effective strategies for water quality management. In this spirit, numerous models have been developed since the 1970s. We set off to explore model diversity by making an inventory among 42 aquatic ecosystem modellers, by categorizing the resulting set of models and by analysing them for diversity. We then focus on how to exploit model diversity by comparing and combining different aspects of existing models. Finally, we discuss how model diversity came about in the past and could evolve in the future. Throughout our study, we use analogies from biodiversity research to analyse and interpret model diversity. We recommend to make models publicly available through open-source policies, to standardize documentation and technical implementation of models, and to compare models through ensemble modelling and interdisciplinary approaches. We end with our perspective on how the field of aquatic ecosystem modelling might develop in the next 5-10 years. To strive for clarity and to improve readability for non-modellers, we include a glossary.

We present a model framework for the simulation of growth and reproduction of Daphnia at varying conditions of food concentration and temperature. The core of our framework consists of an individual level model that simulates allocation of assimilated carbon into somatic growth, maintenance costs, and reproduction on the basis of a closed carbon budget. A fixed percentage of assimilated carbon is allocated into somatic growth and maintenance costs. Special physiological adaptations in energy acquisition and usage allow realistic model performance even at very low food concentrations close to minimal food requirements. All model parameters are based on physiological measures taken from the literature. Model outputs were thoroughly validated on data from a life-table experiment with Daphnia galeata. For the first time, a successful model validation was performed at such low food concentrations. The escalator boxcar train (EBT) was used to integrate this individual level model into a stage-structured population model. In advance to previous applications of the EBT to Daphnia we included an additional clutch compartment into the model structure that accounts for the characteristic time delay between egg deposition and hatching in cladocerans. By linking two levels of biological organisation, this model approach represents a comprehensive framework for studying Daphnia both at laboratory conditions and in the field. We compared outputs of our stage-structured model with predictions by two other models having analogous parameterisation: (i) another individual level Daphnia model (Kooijman–Metz model) and (ii) a classical unstructured population model. In contrast to our Daphnia model, the Kooijman–Metz model lacks the structure to account for the optimisation of energy acquisition and maintenance requirements by individual daphnids. The unstructured population model showed different patterns of population dynamics that were not in concordance with typical patterns observed in the field. Thus, we conclude our model provides a comprehensive tool for the simulation of growth and reproduction of Daphnia and corresponding population dynamics. [KEYWORDS: Daphnia ; Structured populations ; Energy allocation ; Individual level ; Physiology ; Resource limitation ; Growth; Reproduction]

According to the size-efficiency hypothesis (SEH) larger bodied cladocerans are better competitors for food than small bodied species. In environments with fish, however, the higher losses of the large bodied species due to size-selective predation may shift the balance in favor of the small bodied species. Here we present a theoretical framework for the analysis of the competitive abilities of zooplankton species that takes both competition and predation into account in one coherent analysis. By applying the conceptually well-understood framework of physiologically structured population models we were able to predict the relative difference in predation rates necessary to cause a shift in dominance of the large-bodied species (Daphnia pulicaria) to the small-bodied species (D. galeata). These predictions depend only on seven easily interpretable parameters per species: size at birth, size at maturity and maximum size, age at maturity, maximal clutch size, egg development time and finally the half-saturation constant for food. The critical equilibrium mortality of D. pulicaria was 0.16 d1 at food concentrations close to the critical food concentration of D. galeata, i.e. D. pulicaria will win the competition as long as its mortality rate is below 0.16 d1. At higher food concentrations the differential mortality curve (plotting equilibrium mortalities of both species against each other) approached a linear function with a slope of one and an intercept equal to the difference in maximal population birth rates. The prediction of critical predation rates was independent of the ingestion rate of the cladocerans and the algal carrying capacity and food regeneration rate of the environment although the mechanism works through competition for a shared algal food resource. We interpret these findings in terms of the relative predation risk large and small-bodied cladocerans will face in various freshwater ecosystems.

Individual based simulations of population dynamics require the availability of growth models with adequate complexity. For this purpose a simple-to-use model (non-linear multiple regression approach) is presented describing somatic growth and reproduction of Daphnia as a function of time, temperature and food quantity. The model showed a good agreement with published observations of somatic growth (r 2 = 0.954, n = 88) and egg production (r 2 = 0.898, n = 35). Temperature is the main determinant of initial somatic growth and food concentration is the main determinant of maximal body length and clutch size. An individual based simulation was used to demonstrate the simultaneous effects of food and temperature on the population level. Evidently, both temperature and food supply affected the population growth rate but at food concentrations above approximately 0.4 mg Cl −1 Scenedesmus acutus temperature appeared as the main determinant of population growth. Four simulation examples are given to show the wide applicability of the model: (1) analysis of the correlation between population birth rate and somatic growth rate, (2) contribution of egg development time and delayed somatic growth to temperature-effects on population growth, (3) comparison of population birth rate in simulations with constant vs. decreasing size at maturity with declining food concentrations and (4) costs of diel vertical migration. Due to its plausible behaviour over a broad range of temperature (2–20 °C) and food conditions (0.1–4 mg Cl −1 ) the model can be used as a module for more detailed simulations of Daphnia population dynamics under realistic environmental conditions.

Here, we communicate a point of departure in the development of aquatic ecosystem models, namely a new community-based framework, which supports an enhanced and transparent union between the collective expertise that exists in the communities of traditional ecologists and model developers. Through a literature survey, we document the growing importance of numerical aquatic ecosystem models while also noting the difficulties, up until now, of the aquatic scientific community to make significant advances in these models during the past two decades. Through a common forum for aquatic ecosystem modellers we aim to (i) advance collaboration within the aquatic ecosystem modelling community, (ii) enable increased use of models for research, policy and ecosystem-based management, (iii) facilitate a collective framework using common (standardised) code to ensure that model development is incremental, (iv) increase the transparency of model structure, assumptions and techniques, (v) achieve a greater understanding of aquatic ecosystem functioning, (vi) increase the reliability of predictions by aquatic ecosystem models, (vii) stimulate model inter-comparisons including differing model approaches, and (viii) avoid 're-inventing the wheel', thus accelerating improvements to aquatic ecosystem models. We intend to achieve this as a community that fosters interactions amongst ecologists and model developers. Further, we outline scientific topics recently articulated by the scientific community, which lend themselves well to being addressed by integrative modelling approaches and serve to motivate the progress and implementation of an open source model framework. © 2011 Springer Science+Business Media B.V.

1. We studied the effect of size-selective predation on the outcome of competition between two differently sized prey species in a homogenous environment.2. Using a physiologically structured population model, we calculated equilibrium food concentrationsfor a range of predation scenarios defined by varying maximum predation intensity and size-selectivity, the latter being characterized by the neutral selection length (NSL), where negativeselection of small prey size-classes turns into positive selection of larger prey. We parameterized the model according to the well-studied example of fish predation on two differently sizedcladoceran species, Daphnia pulicaria and Daphnia galeata.3. Although the larger D. pulicaria was principally the better competitor for food, competitive superiority shifted to the smaller D. galeata under certain predation scenarios. The lowest predationintensity needed to induce a shift from D. pulicaria to D. galeata was found at NSL values in between the sizes at maturity of both species.4. Analysing the per capita mortality rates as a result of the underlying parameters of the mortality model, NSL and maximum predation intensity, revealed that the tolerable mortality rate of daphnids decreased as NSL increases towards values close to the size at maturity. This effect was most pronounced in D. pulicaria, thus explaining a higher vulnerability of the larger speciesto size-selective predation.5. A reduction of the size at maturity in the smaller species (D. galeata) as a phenotypic response to the presence of fish resulted not only in an increased capability to withstand predation, butalso in competitive dominance shifts over the larger D. pulicaria at lower predation intensities and a much wider range of NSL as a trait-mediated indirect effect.6. Overall, our results demonstrate that shifts in dominance of differently sized herbivores under size-selective predation regimes may be facilitated by the demonstrated alteration in competitivecapacities and thus might not be exclusively caused by direct predation effects.

1. The zebra mussel (Dreissena polymorpha) is one of the most successful invasive species; it has colonised many aquatic systems in Europe and North America with strong impacts on various ecosystem processes. The effect of D. polymorpha filtration on pelagic seston concentrations has been quantified in several studies, but the magnitude and stoichiometry of the transfer of sestonic biomass into benthic detritus by D. polymorpha and the accompanying enrichment of the benthic habitat is still under-investigated.2. We studied biodeposition by zebra mussels in two series of laboratory experiments with the food algae Cryptomonas erosa and Scenedesmus obliquus. We also measured the year-round biodeposition rate under natural conditions in the oligotrophic Lake Constance.3. In all experiments, zebra mussel biodeposition was linearly related to seston concentration. In the field, the relationship changed with a seasonal shift in algal composition and lower biodeposition rates during the spring algal bloom.4. For both algal species in laboratory experiments, biodeposited material was depleted in phosphorous at an algal concentration ≤0.6 mg ash-free dry mass L-1, but not at higher concentrations. This effect was not observed in the field, probably because of high variation in C : N : P stoichiometry.5. By mediating the transfer of pelagic resources into the benthos zebra mussels provide a sufficient amount of detritus for benthic invertebrates, especially during summer. Thus, material biodeposited by the mussels might increase benthic secondary production from pelagic resources, and zebra mussels are important mediators of this flux of organic matter from the pelagic zone into the benthos.

We studied costs and benefits of life history shifts of water fleas (genus Daphnia) in response to infochemicals from planktivorous fish. We applied a dynamic energy budget model to investigate the resource allocation patterns underlying the observed life history shifts and their adaptive value under size selective predation in one coherent analysis. Using a published data set of life history shifts in response to fish infochemicals we show that Daphnia invests less energy in somatic growth in the fish treatment. This observation complies with theoretical predictions on optimal resource allocation. However, the observed patterns of phenotypic plasticity cannot be explained by changes in resource allocation patterns alone because our model-based analysis of the empirical data clearly identified additional bioenergetic costs in the fish treatments. Consequently, the response to fish kairomone only becomes adaptive if the intensity of size selective predation surpasses a certain critical level. We believe that this is the first study that puts resource allocation, energetic costs, and adaptive value of predator induced life-history shifts – using empirical data – into one theoretical framework.

Plankton succession during spring/early summer in temperate lakes is characterised by a highly predictable pattern: a phytoplankton bloom is grazed down by zooplankton (Daphnia) inducing a clear-water phase. This sequence of events is commonly understood as a cycle of consumer-resource dynamics, i.e. zooplankton growth is driven by food availability. Here we suggest, using a modelling study based on a size-structured Daphnia population model, that temperature and not food is the dominant factor driving interannual variability of Daphnia population dynamics during spring. Simply forcing this model with a seasonal temperature regime typical for temperate lakes is sufficient for generating the distinctive seasonal trajectory of Daphnia abundances observed in meso-eutrophic temperate lakes. According to a scenario analysis, a forward shift of the vernal temperature increase by 60 days will advance the timing of the Daphnia maximum on average by 54 days, while a forward shift in the start of the spring bloom by 60 days will advance the Daphnia maximum only by less than a third (17 days). Hence, the timing of temperature increase was more important for the timing of Daphnia development than the timing of the onset of algal growth. The effect of temperature is also large compared to the effect of applying different Daphnia mortality rates (0.055 or 0.1 day-1, 38 days), an almost tenfold variation in phytoplankton carrying capacity (25 days) and a tenfold variation in Daphnia overwintering abundance (3 days). However, the standing stock of Daphnia at its peak was almost exclusively controlled by the phytoplankton carrying capacity of the habitat and seems to be essentially independent of temperature. Hence, whereas food availability determines the standing stock of Daphnia at its spring maximum, temperature appears to be the most important factor driving the timing of the Daphnia maximum and the clear-water phase in spring.

1. We present data with a high spatio-temporal resolution from a 72-h field survey in Bautzen Reservoir (Saxony, Germany). The aims of this survey were to observe hydrophysical processes during a period of unstable stratification in spring and investigate the effect of wind-induced internal waves on the vertical distribution of zooplankton.2. Wind velocities up to 10 m s)1 caused a strong downwelling event of warm water at the sampling site and led to the generation of internal waves with an amplitude of 4 m.3. The zooplankton community, which was dominated by Daphnia galeata, inhabitedepilimnetic waters. Downwelling enlarged the thickness of the epilimnetic layer and, hence, led to high zooplankton abundances down to relatively deep water strata indicating lateral transport of zooplankton. As a consequence, area-specific zooplankton abundances increased considerably (max. fourfold) during downwelling.4. We conclude that classical limnological field sampling, such as for monitoring purposes, can lead to severely biased estimates of zooplankton abundance due to the interfering effects of hydrophysical processes like internal waves.5. Backscattering strengths measured by a simultaneously deployed Acoustic Doppler Current Profiler (600 kHz) were found to be correlated with estimated zooplankton abundances based on plankton samples.