Ecological and evolutionary responses of zooplankton communities to changes in lake chemical environments

One of the consequences of the development of landscapes for human uses is the release and accumulation of chemicals in the environment. The long-term effects of multigenerational exposure to this chemical pollution in wild populations are poorly understood. Both ecological and rapid evolutionary responses are likely, as both species and populations are known to vary in sensitivity to toxicant exposure. While we have observed frequent rapid evolutionary changes in wild populations, particularly in response to human impacts, we are only beginning to understand how important rapid evolution might be in shaping long-term ecological and evolutionary responses to environmental stressors such as chemical pollution. My dissertation uses freshwater zooplankton as a model to contribute to this knowledge gap, examining both ecological and evolutionary consequences of exposure to pollution stress across a variety of spatial and temporal scales. In chapter one I surveyed 51 small lakes in Connecticut, US to evaluate the relative importance of the lake physicochemical environment, habitat connectivity and broader spatial properties in shaping pelagic crustacean zooplankton communities. I found that the chemical environment, particularly dissolved ions, was far more important than space and connectivity in predicting zooplankton species distributions. This evidence suggests that for the most part in this system zooplankton dispersal is not limited and environmental filtering is playing a key role in the distribution of zooplankton species across the landscape. Chapters 2 through 4 examined long-term ecological and evolutionary changes in daphniid zooplankton taxa in four Connecticut lakes that have experienced differing degrees of pollution over the past century. Using paleolimnological techniques I reconstructed changes in eutrophication and heavy metal contamination in these lakes over time. Examination of daphniid diapausing egg banks deposited in sediments of these lakes uncovered evidence of taxonomic homogenization of the daphniid species over time in the three eutrophied lakes. I also found that eutrophication may have been more influential than metals in shaping species compositional patterns (chapter 2). Chapters 3 and 4 investigated phenotypic responses of Daphnia ambigua populations to heavy metal contamination. I found that Daphnia diapausing eggs from time periods when metal contamination was elevated were less likely to hatch and that those animals that did hatch had a higher rate of juvenile mortality (chapter 3). Daphnia hatched and successfully cultured from high copper and high cadmium time periods were more sensitive to exposure of these metals (chapter 4), a pattern consistent with rapid maladaptation to metals over multi-decadal timescales. Overall, my dissertation research uncovers widespread long-term effects of changes in lake chemical environments on both ecological and evolutionary trajectories of lake zooplankton communities. Future research into the drivers and consequences of these trends, particularly those observed in chapters 3 and 4, is warranted. It is important to understand, both for basic scientific and conservation purposes, whether exposure to widely distributed toxicants such as heavy metals is disrupting the evolutionary capacity of lake zooplankton, an important component to lake communities worldwide.