Sorensen, Michael2016-02-122016-02-122015-12https://hdl.handle.net/11299/177054University of Minnesota M.S. thesis. December 2015. Major: Biology. Advisor: Donn Branstrator. 1 computer file (PDF); viii, 68 pages.A predatory species of Eurasian zooplankton, Bythotrephes longimanus (spiny water flea), has been invading lakes and damaging food webs across the central part of North America since the early 1980s. In an effort to understand its niche, I investigated its short term survival under low dissolved oxygen (D.O.) concentrations in the laboratory and compared it to the native predatory zooplankton Leptodora kindtii. Both species are large-bodied, strongly selected by zooplanktivorous fish, and known to commonly engage in diel vertical migration. Trial conditions were chosen to mimic the hypolimnion of a eutrophic temperate zone lake during late summer. Tested B. longimanus (n=690) exhibited a pooled hypoxia tolerance limit (LC50) of 1.65 mg/L D.O. (95% confidence interval: 1.59, 1.72), while that of L. kindtii (n=380) was 0.58 mg/L D.O. (0.51, 0.65). Both species were more tolerant of hypoxia than zooplanktivorous fish, which cannot generally withstand prolonged exposure to D.O. concentrations lower than 2 mg/L. However, B. longimanus was significantly less tolerant than L. kindtii, and may exhibit greater spatial overlap with zooplanktivorous fish in seasonally-hypoxic lakes. These lab-based physiological results are consistent with landscape-scale observations that B. longimanus successfully invades primarily mesotrophic and oligotrophic lakes while L. kindtii thrives across a wider range of lakes, including eutrophic ones. The combined effects of climate change and cultural eutrophication throughout the 21st Century will likely increase the occurrence of temperate lake hypoxia, and provide a barrier to B. longimanus invasion of North American lakes.enBythotrephesHypoxiaLeptodoraSpiny Water FleaZooplanktonThesis or Dissertation