RTG B1: Modelling the implications of changes in abiotic conditions for the plankton dynamics in seasonally stratified lakes
The project is a theoretical investigation aiming at a process based understanding of the response of seasonal plankton dynamics in freshwater lakes of different water depths to trophic change and climate warming. A major goal is to quantify predictions of the PEG-model utilizing numerical experiments with a vertically resolved dynamic model that includes the most important plankton groups and forcing factors for plankton succession in seasonally stratified lakes. The model will be applied to test the hypothesis that patterns of seasonal plankton succession (i.e. similar phytoplankton biomass during spring and summer blooms versus a dominance of the spring phytoplankton bloom) are resilient with respect to trophic state. Specifically, we will test whether the nutrients introduced in summer from the catchment during eutrophication are a major source for phytoplankton summer blooms and whether the lack of this source during oligotrophication results in a seasonal pattern dominated by spring phytoplankton blooms that grow on internal nutrients made available during spring mixing. The patterns of algal seasonality may persist during eutrophication and oligotrophication, respectively, over a wide range of trophic states if they are linked to the seasonal course of nutrient loads. In a second step, we will investigate the consequences of changes in algal seasonality for herbivore interactions. Using Daphnia as an example, we will study competition between D. galeata that immigrated to Lake Constance in the 1960s and native D. longispina.
Utilizing the model we will test the hypothesis that the persistence of the pattern of seasonal plankton succession leads to a resilience of the abundance of D. galeata with respect to trophic change. Specifically, we will investigate whether the non-migrating D. galeata recruiting in spring from resting stages benefits from strong phytoplankton spring blooms during re-oligotrophication, whereas D. longispina with its strategy of vertical migration and recruitment from the overwintering population benefits from phytoplankton blooms in summer. In addition to the theoretical investigations, the plankton succession model will be applied to the three basins of Lower Lake Constance and compared to data and results from three dimensional simulations conducted in A4.