Long-term change in the phenology of spring phytoplankton: species-specific responses to nutrient enrichment and climatic change

1. A number of studies have shown that spring biological events have advanced in recent decades, and concluded that these changes in phenology are driven by climatic change. Freshwater lakes are sensitive indicators of climate change, where direct effects of climate on physical processes can affect the seasonal timing of planktonic communities. However, many lake ecosystems have also experienced long-term changes in other ecological pressures that could affect phenology. 2. In this study, long-term (1955-2003) physical, chemical and biological data from Windermere (UK) were analysed in order to assess the relative effects of a number of coincident pressures on the phenology of two spring diatom taxa. The analysis provides a detailed case study, highlighting the species-specific drivers that affect the phenology of dominant members of the phytoplankton community. 3. The results showed that, whilst the spring peak biomass of one taxon (Cyclotella) appeared to be advancing as a result of earlier thermal stratification, the advancement of the other (Asterionella) was closely linked with both progressive nutrient enrichment and lake warming. Furthermore, nutrient enrichment explained more variation in phenology than water temperature. Both taxa also reached their peak abundance earlier when the over wintering biomass at the end of the previous year was higher. 4. Patterns of change in phenology and ecological pressures were markedly nonlinear in time, as were the effects of some drivers of seasonal timing. This highlighted a need to relax the restriction of linearity in our analyses of biological seasonality. 5. Synthesis. Phenological shifts may be brought about by local processes, such as eutrophication, as well as by climate change. Even in the same ecosystem different mechanisms may alter the phenology of different species.