The ecological stoichiometry theory has been successful in enhancing our understanding of trophic interactions between consumer and prey species. However, despite the considerable progress made in explaining the stoichiometric interplay between consumers and resources, major tenets of this theory remain poorly or not tested. The experimental evidence for the validity of stoichiometric predictions has mainly been assembled focussing on linear food chains, with single species on different trophic levels. These studies often report direct links between consumer nutrient demand, plant nutrient composition and the effects the consumer have on the prey nutrient content. However, most natural ecosystems harbour a variety of producer and consumer species, which may substantially alter the link between consumer presence and prey nutrient content. It thus remains unclear whether grazer or producer diversity dampens or exaggerates the effects of material imbalances in trophic interactions. Our project aims at filling these gaps in our knowledge by testing the importance of diversity in stoichiometric interactions and the relative importance of limitation and diversity on transfer of resource imbalances between trophic levels. In order to test the hypothesis that the number of species in a trophic group alters the nutrient dynamics and resource ratios in a pelagic community, we conducted two experiments, which build up on each other. All experiments were conducted with co-occurring species of marine phytoplankton and zooplankton in laboratory microcosms (1000 ml) under semi-continuous regime and under stable temperature and light conditions. In the first experiment, we tested the effect of producer diversity on the stoichiometry of the trophic interaction, while we varied the consumer diversity in the second experiment.

Results/Conclusions

From the data obtained, we first show the effect of resource supply on phytoplankton stoichiometry and biomass. According to our expectations, decreased light intensity and nitrogen supply led to lower phytoplankton biomass and abundance compared to non-limited (+N) cultures. Preliminary results revealed that copepod biomass was significantly (p = 0.0008) higher in +N cultures. As expected, mixed algae cultures had a significant positive effect (p = 0.016) on copepod biomass under light and nitrogen limitation.

These analysis allow disentangling the effect of algal and consumer diversity on trophic transfer, and the effect of the consumer on the stoichiometry of the algae. These enhancements of conceptual understanding are highly needed in order to predict consequences of altered nutrient availability and nutrient ratios in coastal ecosystems in the face of human alterations of global biogeochemistry.