Diel vertical migrations and trophic interactions of freshwater organisms

Abstract

Abiotic and biotic conditions interact to cause dynamic diel vertical migrations (DVM) of organisms in the pelagic area of freshwater lakes. This dissertation includes four unique studies to improve our knowledge of how and why DVM patterns change between species, ecosystems, seasons, and years. The first study documented a normal DVM of the endangered Hovsgol grayling (Thymallus nigrescens) in Lake Hovsgol, Mongolia, during the summer season. This migration was driven by the distribution of prey because there were no pelagic predators. Next, I examined how different abiotic and biotic conditions between 11 inland lakes in Minnesota and northern Wisconsin caused differences in DVM patterns of cisco (Coregonus artedi) during the summer season. I found cisco populations that performed normal DVM, no DVM, and reverse DVM, resulting from differences in temperature, oxygen, prey density, growth potential, and predation risk between lakes. To determine differences in DVM patterns between seasons and years, I followed up this study by analyzing reverse DVM patterns of cisco in Ten Mile Lake, Minnesota, during the spring, summer, fall, and winter over multiple years. I observed a small reverse DVM of cisco during the spring and fall, a large reverse DVM during summer, with no DVM during winter. These differences were attributable to seasonal changes in the abiotic and biotic conditions of the lake. Lastly, I examined seasonal and yearly changes in normal DVM patterns of four pelagic species in Lake Superior. DVM patterns changed between seasons, but were more consistent among years, and were driven by changes in prey and predator densities and distributions.