Phytoplankton that form harmful algal blooms (HABs) can foul water with unpleasant odors and tastes, accumulate as visible surface scums, and produce potentially dangerous toxins. The cyanobacterium Microcystis aeruginosa is the most widespread of the freshwater HAB-forming species, and this dissertation explores the influences of variation in environmental drivers (biotic, abiotic) and variation in biological traits (colony size, growth rate) on its ecology—in particular, the interaction with a facilitator species, the invasive zebra mussel (Dreissena polymorpha). Chapter 1 quantifies the vulnerability of Microcystis to grazing by zebra mussels as a function of large variation in both Microcystis colony size (5-88 μm equivalent diameter) and zebra mussel body size (8-28 mm shell length), and relates the findings to their size distributions in the primary study lake, Gull Lake, Michigan. Based on colony size alone, the Microcystis population in Gull Lake can vary widely in its vulnerability to grazing within single growing seasons, and the range of ingestible colonies (≤ 80 μm equivalent diameter) is greater for zebra mussels than published ranges for other dominant filter-feeding grazers (e.g., Daphnia). Following a mass mortality event (~100%) of zebra mussels on epilimnetic sediments in Gull Lake during a relatively warm summer, evidence from a combination of in situ monitoring and experiments presented in Chapter 2 demonstrates a causal relationship between chronic, accumulated heat exposure (> 25 °C) and elevated zebra mussel mortality. Though these temperatures are lethal to zebra mussels, they are within the optimal range for Microcystis. Results from a long-term (13-summer) study of the Gull Lake Microcystis population are presented in Chapter 3, the first long-term analysis of Microcystis dynamics in a low-nutrient lake—an uncharacteristic niche for this species strongly facilitated by zebra mussels. Microcystis biomass and microcystin toxin were significantly higher and peak biomass occurred significantly earlier in warmer summers, consistent with climate change projections. However, the heat-induced mass mortality event of zebra mussels (Chapter 2) resulted in a 2-year collapse of the Microcystis population during the warmest period in the time series, highlighting the need to understand how these two strongly interacting species will respond together to climate warming. Lastly, Chapter 4 returns to the importance of large intraspecific trait variation for the ecology of Microcystis, to further understand its niche expansion into low-nutrient lakes. Laboratory growth assays of 18 colonial strains, recently isolated from 11 Michigan inland lakes spanning the entire productivity gradient (7.6-196 μg L-1 total phosphorus), show that Microcystis strains from high-nutrient lakes grow significantly faster (up to ~7 fold) than those from low-nutrient lakes, which may indicate the presence of an ecological trade-off enabling local adaptation to these widely disparate habitats. Possibly as a result of their faster growth rates, strains from high-nutrient lakes are also more likely to cease colony formation in culture sooner, which has implications for the design and interpretation of lab studies of Microcystis.Taken together, the chapters within this dissertation demonstrate important ecological consequences of the biological trait variation inherent within and among populations, and illustrate how that diversity might interact with other biotic and abiotic factors, improving our understanding of species’ responses to complex global change.
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