B1

State of the art

Seasonal plankton succession depends on the seasonal course of abiotic conditions and changes with trophic state and climate warming. According to the conceptual model of the Plankton Ecology Group (PEG) a distinct clear water phase separates a spring and a summer phytoplankton bloom under more eutrophic conditions, whereas under oligotrophic conditions a single peak develops because of grazing and nutrient limitation in summer (Sommer et al. 1986, 2012). Daphnia is considered to be the most important grazer contributing to the development of the clear-water phase. Climate warming may shift bimodal to unimodal phytoplankton seasonality and may lead to trophic uncoupling (de Senerpont Domis et al. 2013). In shallow stratifying lakes phytoplankton can bloom during winter mixing (e.g. Thackeray et al. 2008), suggesting that the patterns of plankton seasonality and their response to trophic and climatic change may differ substantially from those in deep lakes. The effects of climate warming for spring plankton phenology and the shift from top-down to bottom up control of spring phytoplankton during oligotrophication have been discussed intensively (e.g. Thackeray et al. 2008; Kerimoglu et al. 2013; Straile et al. 2015b). Typical plankton seasonality has been described in simplified zero dimensional models or in water quality models applied to specific lakes. Most theoretical studies investigating plankton dynamics in vertical water columns assumed constant mixing conditions and/or did not include grazing (e.g. Huisman et al. 2002; Jager et al. 2010; Peeters et al. 2013; Ryabov and Blasius 2014).

Preparatory work

The experiences of the applicants cover the development and application of hydrodynamically driven 1-dimensional phytoplankton (Peeters et al. 2007b, 2013; Kerimoglu et al. 2012, 2013; Frassl et al. 2014) and coupled phytoplankton-zooplankton models (Kerimoglu et al. 2014) as well as physiologically structured models of Daphnia (Schalau et al. 2008). The applicants have investigated competition in phytoplankton theoretically (Kerimoglu et al. 2012), simulated the interaction between copepods and ciliates during spring in Lake Constance (Kerimoglu et al. 2014), and analysed field data from Lake Constance with respect to vertical migration of Daphnia (Huber et al. 2011) and the effect of exceptionally warm winters on plankton succession (Straile et al. 2010). Modelling studies focusing on the impact of changing environmental conditions on phyto- and zooplankton included investigations of the consequences of climate warming for the phenology of plankton in spring (Peeters et al. 2007a; Straile et al. 2015b), the implications of seasonal mixing for the seasonality of nutrient and light limited phytoplankton in vertical water columns (Peeters et al. 2013), the role of temperature for the development of Daphnia populations (Schalau et al. 2008), and the shift from top-down versus bottom up control of spring phytoplankton during oligotrophication (Kerimoglu et al. 2013).

Proposed project and role within the RTG

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.

The project is linked to project A4 benefiting from the analysis of field data from Lower Lake Constance and collaborating in the modelling of plankton distributions. Simulation results will be compared to seasonal dynamics of long-term data. Detailed information on seasonal plankton development (A2) and on the response of copepods to trophic change (B3) will allow for an incorporation of additional plankton groups into the model (copepods, mixotroph-bacteria combinations). Results from A3 will stimulate model extensions with respect to competition between the two Daphnia species.