Nutrient loading and climate warming: Two synergetic drivers of freshwater algal dynamics?

Changes in the phenology and the distribution of individual species are evidence of the impact of climate change on biological systems.  However, the observed responses of individual plant and animal species are just the starting point of tackling the ecological complexity of ecosystem responses to climate change. Other drivers of ecosystem functioning, such as habitat fragmentation, eutrophication and pollution may act in concert with climate change. Non-additive impacts of these multiple ‘stressors’ make it difficult to predict effects based on single-stressor studies. Analysis of the full-ecosystem model PCLake suggested that nutrient loading and climate warming may have non-additive synergistic effects on lake systems. In a series of microcosm experiments, we examined whether climate warming and nutrient loading act as synergistic or antagonistic drivers of freshwater plankton community dynamics. By exposing natural plankton communities to different spring warming scenarios and nutrient loading scenarios in a full-factorial experimental design we tested the following hypotheses: 1. The combined effect of eutrophication and warming on a planktonic community equals the effect of the single worst stressors, i.e. nutrient loading (simple comparative effect). 2. As temperature affects different biological properties than nutrient availability, the effect of nutrient loading and climate warming will accumulate additively. 3. The combined effect of nutrient loading and climate warming will be multiplicative, as the tolerance of plankton species to nutrient availability and temperature are negatively related. The outcome of these experiments indicates that nutrient loading and climate warming may act as synergistic drivers of algal dynamics.