Seasonal and Long-term Dynamics of Phytoplankton Community Composition, Function and Diversity in Lake Constance

Abstract

Phytoplankton have long been classified according to the stage of seasonal succession or nutrient status of systems where they are found. However, a mechanistic and predictive theory of the response of phytoplankton community composition and diversity to changes in the environment requires rigorous examination of how these changes are mediated by the functional traits of a community’s members. Using high frequency and long-term monitoring data from Lake Constance, seasonal and long-term changes in the composition and diversity of phytoplankton were investigated in terms average function and functional diversity. Annually, an increase in predation pressure from filter- feeding zooplankton triggered the transition from a predation susceptible winter-spring community to a defended summer bloom community. This transition came with both an increase in diversity of predation susceptibilities and an increase in species diversity, suggesting that the filter-feeding strategy of their predators facilitated coexistence of both predation susceptible and defended phytoplankton. Remarkably, a strong tradeoff between the resource acquisition traits phosphate and light affinity was uncovered at two temporal scales: once in the response of the predation susceptible winter-spring community to seasonal changes in light and phosphate availability, and once in the response of the defended summer bloom community to long-term changes in the lake’s nutrient status. Evidence for a tradeoff between these resource acquisition traits was only weak in the species pool, but emerged as strong when traits were weighted by their biomass. Suggesting that long-term changes in nutrient status and seasonal changes in phosphate and light availability selected phytoplankton along this tradeoff line. In addition, diversity in phosphate affinity depended also on the nutrient status of the lake. Under the severely phosphate limiting conditions of the oligotrophic summer bloom, diversity in phosphate affinity was lowest, suggesting that phosphate limitation strongly selected for high phosphate affinity. Furthermore, this lower diversity in phosphate affinity corresponded to lower species diversity, indicating that strong competition for phosphate led to a reduction in species coexistence. The findings presented in this dissertation were largely enabled by a novel approach to the analysis of community composition, function and diversity. Trait-based approaches can be powerful because they suggest mechanisms responsible for changes in the community. However, the link with taxonomic composition is often lost. Efforts to unify these approaches led to the linking of ecological metrics describing the average function, species diversity and functional diversity to principal component axes of community composition. By enabling the examination of these metrics in principal communities separately, this novel approach has the potential to paint a nuanced and convincing picture of the mechanisms regulating the composition, function and diversity of ecological communities.