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Abstract
Eutrophication, the increase of nutrients into a system, can lead to management challenges in shallow lakes. My dissertation investigates two potential challenges posed by eutrophication. First, harmful algal blooms of nuisance cyanobacteria that produce toxins dangerous to aquatic species and humans increase in frequency and size with eutrophication. The first chapter seeks to identify if duckweeds are suitable bioremediators of cyanotoxins by testing for intraspecific variation in tolerance. I tested the impact of sublethal microcystin, the most common toxin produced by harmful algal blooms, on fifteen genetic lineages of two species of duckweed, Lemna minor and Spirodela polyrhiza. I found general tolerance with some intraspecific variation. Then, in the second chapter, I tested the indirect ecological and evolutionary impacts of microcystin on duckweed herbivores, Rhopalosiphum nymphaeae, the water lily aphid. Microcystin directly suppressed duckweed populations, but had positive indirect ecological effects on aphids, leading to highest populations on microcystin-stressed plants. Microcystin also drove indirect evolutionary effects on aphids, showing the potential for toxins to act as selection pressures beyond their interactions. Lastly, the third chapter addresses how regime shifts may be impacted by evolution and stochasticity. I used a quantitative genetics framework to simulate populations of competing submerged and floating macrophytes. I found that stochasticity in nutrient input only had slight impacts on the timing of regime shifts, and noise-induced tipping was not seen. When evolution was present, long transients emerged where floating plants dominated. Submerged plants abruptly exited the transient through evolutionary rescue and reached densities higher than floating plants. My dissertation results highlight how evolution impacts species interactions to lead to unique ecological outcomes. Management actions may benefit from considering such eco-evolutionary interplay.
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