Combined effects of warming and nutrient loading on an experimental phytoplankton community: Stoichiometric implications

Background/Question/Methods:

The impact of climate change on biological systems is already apparent through changes in the phenology and the distribution of individual species. However, other stressors of ecosystem functioning, such as habitat fragmentation, eutrophication and pollution may act in concert with climate change. Possible non-additive impacts of these multiple ‘stressors’ make it difficult to predict effects based on single-stressor studies. Earlier studies suggest that nutrient loading and climate warming may have non-additive synergistic effects on lake ecosystems. To obtain a more mechanistic understanding of the combined effect of nutrient loading and temperature warming on the planktonic food web base, we exposed experimental phytoplankton communities to different nutrient loading and warming scenarios using a full-factorial design. The starting community in our experimental cosms was a typical spring community, as bottom-up processes are postulated to be the most important drivers of phytoplankton succession in spring. In our analyses we focused on community properties of the part of the phytoplankton edible to zooplankton grazers, to elucidate potential effects on trophic transfer efficiency.

Results/Conclusions:

Our results indicated that the combined effects of nutrient loading and temperature warming on phytoplankton biomass were additive. Interestingly, further logistic growth model analysis showed that carrying capacity of the phytoplankton was not only affected by nutrient loading but also by temperature. A likely mechanism behind the effect of temperature warming on phytoplankton carrying capacity is a more efficient use of nutrients at warmer temperatures, resulting in higher carbon-nutrient ratios. This mechanism is backed-up by our results on seston carbon-nutrient ratios, which showed non-additive synergistic effects of nutrient loading and temperature warming on carbon-nutrient ratios. At higher temperatures, nutrient content of our experimental phytoplankton community decreased as indicated by higher carbon-nutrient ratios of the seston. By lowering the stoichiometric food quality of the planktonic food web base, this interaction between nutrient loading and temperature warming can potentially lead to stoichiometric bottlenecks in aquatic food webs, even more so under low nutrient supply. At present, we are testing to what extent these stoichiometric constraints affect trophic transfer in tri-trophic food webs.