Seasonal and long-term trends in the diatom and phytoplankton community composition of Lake Simcoe, Ontario

Lake Simcoe and the Laurentian Great Lakes are among the freshwaters of the world that have collectively experienced accelerated rates of qualitative and quantitative degradation from multiple anthropogenic influences. Recent stressors include the introduction of invasive species, eutrophication and climate change. This study is twofold: 1) a short-term study (2009-2011) monitoring the diatom relative abundance during the ice-free and ice-covered periods using sediment traps and pelagic sampling methods and 2) a long-term study (1980-2011) monitoring trends in the total phytoplankton and diatom spring blooms. Seasonal phytoplankton relative abundances were analyzed for changes in dominant species and correlated to environmental variables.

In the short term study diatom community structures of the ice-free and ice-covered sediment traps were statistically different (p = 0.0003) as were the ice-free pelagic and adjacent sediment traps (p = 0.0001): the diatom assemblages of the four sites, however, were not statistically different. Overall, the pelagic samples were dominated by Stephanodiscus minutulus/parvus and Fragilaria crotonensis. The ice-free sediment traps were dominated by Stephanodiscus binderanus and Fragilaria crotonensis and the ice-covered sediment traps were dominated by Stephanodiscus minutulus/parvus. Although total phosphorus (TP) was significantly related to the ice-free pelagic diatom community, this was not the case in the sediment traps. Silica (Si) concentrations were significantly 'drivers' in all sampling methodologies and chloride concentrations were also important in shaping the ice-free pelagic and ice-covered sediment communities.

In the long-term study, there were no significant changes in the total phytoplankton or diatom biovolumes in the spring between the 1980s and the 2000s. There was however, evidence of a shift from larger Stephanodiscus species to Cyclotella, and smaller Stephanodiscus species. There is also evidence that the spring bloom may begin under the ice. A shorter ice-covered period lengthens the ice-free season which could cause earlier spring blooms.