Phytoplankton response to climate warming modified by trophic state

We investigated the combined effect of reduced phosphorus supply and warmer winter and spring conditions on the diatom spring bloom of a shallow lake. Simulations with a simple dynamic model indicated that reduced ice cover and increasing water temperatures resulted in a more intense and earlier bloom independently of phosphorous concentrations. However, whereas the collapse of the bloom was caused by silicate limitation under high phosphorus supply, it was caused by Daphnia grazing under reduced phosphorus supply. This switch from a bottom-up to a top-down driven collapse of the diatom spring bloom explains why, despite similarly mild winters, the bloom was observed earlier under high than under reduced phosphorus supply in the lake studied. Thus, an assessment of possible changes in nutrient loading is crucial when anticipating how phytoplankton could evolve under future climate warming. Increasing anthropogenic pressure requires a better understanding of how ecosystems react to multiple environmental stressors. During the last decades many freshwater systems were subject to both a changing climate and changes in trophic state due to reduced nutrient supply (Jeppesen et al. 2005). Yet, most analyses of long-term data of these systems focused either on the effect of climate change or on the effect of changes in trophic state. Few studies have tried to disentangle the combined effect of rising temperatures and changing nutrient supply on freshwater ecosystems (e.g., Horn 2003; Elliott et al. 2006). Abiotic factors, influenced by climatic conditions and trophic state, are the primary drivers of phytoplankton succession in spring (Sommer et al. 1986). In lakes of the temperate zone, the phytoplankton spring bloom is predominantly initiated by increasing light availability (Sommer 1994), which is directly determined by solar radiation and day length and also indirectly depends on specific lake features such as water transparency and depth. In deep lakes, phytoplankton starts growing once strong mixing ceases and phytoplankton is no longer constantly transported out of the euphotic zone (Peeters et al. 2007). In many shallow lakes, the phytoplankton spring bloom is initiated once the ice cover melts, inducing a change in the underwater regime of light and turbulence (Adrian et al. 1999; Weyhenmeyer et al. 1999). With the exception of very nutrient-rich lakes where grazer-resistant algae dominate early in the year, the phytoplankton spring bloom then collapses leading to a biomass minimum in late spring/early summer called the clear water phase (Sommer et al. 1986). The collapse of the phytoplankton spring bloom is attributed to different environmental factors. First, many studies have shown that zooplankton grazing rates (mainly by Daphnia) often exceed algal production rates in early summer, thus producing the clear water phase (Lampert et al. 1986). Second, nutrient limitation (potentially combined with increasing sinking losses) can induce a collapse of the phytoplankton bloom before grazing becomes important (Lund 1950; Smayda 1971). And third, a sharp increase in sinking losses due to the onset of stratification can also cause the collapse of the algal bloom if stable summer stratification develops in moderately deep lakes (Winder and Schindler 2004b). 1 Corresponding author ([email protected]). Acknowledgments We thank Sebastian Diehl for his helpful comments during the model construction process and on the method section of the manuscript, Ursula Gaedke for advising us on many modelrelated questions, Andreas Nicklisch and Jan Köhler for their advice on phytoplankton dynamics in Müggelsee, and all scientists and technicians involved in the collection and compilation of the long-term data set. We are also grateful to Don Scavia and one anonymous referee for giving valuable comments on the first version of the manuscript. The German Research Foundation (DFG) supported Veronika Huber within the program AQUASHIFT (SPP 1162).