Phytoplankton functional composition determines limitation by nutrients and grazers across a lake productivity gradient

Functional trade-offs among ecologically important traits govern the diversity of communities and changes in species composition along environmental gradients. A trade-off between predator defense and resource competitive ability has been invoked as a mechanism that may maintain diversity in lake phytoplankton. Trade-offs may promote diversity in communities where grazing- and resource-limited taxa coexist, which determines the extent to which communities are resource- or consumer-controlled. In addition, changes in temperature may alter nutrient demands and grazing pressure, changing the balance between the two regulating factors. Our study aims to understand whether a trade-off between grazer vulnerability and nutrient limitation promotes coexistence of phytoplankton functional groups in communities that differ in trophic status, and how this trade-off may shift with warming. We conducted multifactorial experiments manipulating grazing, nutrients, and temperature in phytoplankton communities from three Dutch lakes varying in trophic status, and used a trait-based approach to classify functional groups based on grazing susceptibility. We found no associations between susceptibility to grazing and response to nutrient additions in any of the communities or temperature regimes, indicating that a competition-defense trade-off is unlikely to explain diversity within the tested communities. Instead, we observed a tendency toward both a higher grazing resistance and weaker nutrient limitation along with a shift in the functional composition of phytoplankton in communities across a gradient from low to high productivity.