Description: Highlights: • Ocean acidification increases phytoplankton standing stock. • This increase is more pronounced in smaller-sized taxa. • Primary consumers reac differently depending on nutrient availability. • Bacteria and micro-heterotrophs benefited under limiting conditions. • In general, heterotrophs are negatively affected at nutrient replete periods. Abstract: In situ mesocosm experiments on the effect of ocean acidification (OA) are an important tool for investigating potential OA-induced changes in natural plankton communities. In this study we combined results from various in-situ mesocosm studies in two different ocean regions (Arctic and temperate waters) to reveal general patterns of plankton community shifts in response to OA and how these changes are modulated by inorganic nutrient availability. Overall, simulated OA caused an increase in phytoplankton standing stock, which was more pronounced in smaller-sized taxa. This effect on primary producers was channelled differently into heterotroph primary consumers depending on the inorganic nutrient availability. Under limiting conditions, bacteria and micro-heterotrophs benefited with inconsistent responses of larger heterotrophs. During nutrient replete periods, heterotrophs were in general negatively affected, although there was an increase of some mesozooplankton developmental stages (i.e. copepodites). We hypothesize that changes in phytoplankton size distribution and community composition could be responsible for these food web responses.

Description: A two-dimensional biomarker approach including fatty acids and stable isotopes of seston and copepods was applied to examine how the variability at the base of the food web affects trophic interactions between primary producers and copepod consumers over a sampling period of two years. We investigated how the composition of the seston affected feeding behaviour by analysing the fatty acid and stable isotope signals of the copepods Calanus helgolandicus, Acartia spp., Centropages spp. and Temora longicornis at Helgoland Roads, North Sea. Our results indicate that the relative contributions of autotrophic and heterotrophic fractions in the seston determined the stable isotope signal of the seston and hence the δ15N of copepods. Our findings show that the combination of stable isotope and fatty acid analyses provides an ideal tool to address the complexity of trophic relations in planktonic food-webs and to define relative trophic position and feeding preferences of e.g. copepods. Defining accurate baselines from bulk seston samples containing a mixture of auto- and heterotroph protist communities still remains a challenge when defining lower food-web dynamics in natural plankton communities

Description: 1. Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), were conducted to investigate feeding selectivity and interspecific competition between the grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Conventional methods, such as cell counts and estimated biovolume, were used first to detect feeding preferences within the different grazer treatments. 2. The results revealed a significant decline in algal biovolume in all grazer treatments and no indications of active selectivity were observed. In contrast to conventional methods, measurements based on isotope signatures showed strong differences in tracer uptake, thus indicating different degrees of assimilation and digestion by the two grazers. 3. The selectivity index Q, which provides information on the uptake ratio of 13C to 15N, showed a significant time effect for both grazer species and a significant difference between single- and mixed-grazer treatments for P. antipodarum. Thus, this technique enabled the direct quantification of the uptake by grazers and, therefore, served as an ideal tool for the detection of passive selectivity. 4. Our results indicate a shift in feeding preferences related to between-species competition and a potential divergence of trophic niches when species coexist.

Description: It is a well accepted fact that nutrient limitation of plants affects the growth and survival of herbivores, generally leading to lower performance of herbivores feeding on nutrient stressed plants. The effect of plants' growing conditions on predatory organisms, feeding one trophic level up, has been much less studied, and there is a general consensus that such effects would be small as herbivores often show relatively strong homeostasis with respect to their nutrient content. Here, we challenge this view, and show from several examples that despite the fact that herbivores buffer much of the variance in nutrient stoichiometry of their food, effects of growing conditions of the primary producers can travel up the food chain. We discuss the implications of these findings, and argue that phosphorus limitation of secondary consumers might be more common in marine than in freshwater systems.

Description: To improve our mechanistic understanding and predictive capacities with respect to climate change effects on the spring phytoplankton bloom in temperate marine systems, we used a process‐driven dynamical model to disentangle the impact of potentially relevant factors which are often correlated in the field. The model was based on comprehensive indoor mesocosm experiments run at four temperature and three light regimes. It was driven by time‐series of water temperature and irradiance, considered edible and less edible phytoplankton separately, and accounted for density‐dependent grazing losses. It successfully reproduced the observed dynamics of well edible phytoplankton in the different temperature and light treatments. Four major factors influenced spring phytoplankton dynamics: temperature, light (cloudiness), grazing, and the success of overwintering phyto‐ and zooplankton providing the starting biomasses for spring growth. Our study predicts that increasing cloudiness as anticipated for warmer winters for the Baltic Sea region will retard phytoplankton net growth and reduce peak heights. Light had a strong direct effect in contrast to temperature. However, edible phytoplankton was indirectly strongly temperature‐sensitive via grazing which was already important in early spring at moderately high algal biomasses and counter‐intuitively provoked lower and later algal peaks at higher temperatures. Initial phyto‐ and zooplankton composition and biomass also had a strong effect on spring algal dynamics indicating a memory effect via the broadly under‐sampled overwintering plankton community. Unexpectedly, increased initial phytoplankton biomass did not necessarily lead to earlier or higher spring blooms since the effect was counteracted by subsequently enhanced grazing. Increasing temperature will likely exhibit complex indirect effects via changes in overwintering phytoplankton and grazer biomasses and current grazing pressure. Additionally, effects on the phytoplankton composition due to the species‐specific susceptibility to grazing are expected. Hence, we need to consider not only direct but also indirect effects, e.g. biotic interactions, when addressing climate change impacts.