Michael Sorensen
“"Hypoxia tolerance of the invasive zooplankton Bythotrephes longimanus and the native
zooplankton Leptodora kindtii"
ABSTRACT
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 Bythotrephes (n=690) exhibited a pooled hypoxia tolerance limit (LC50) of 1.65
mg L-1 D.O. (95% confidence interval: 1.59, 1.71), while that of Leptodora (n=380) was 0.60 mg
L-1 D.O. (0.53, 0.68). Both species were more tolerant of hypoxia than zooplanktivorous fish,
which cannot generally withstand prolonged exposure to D.O. concentrations lower than 2 mg L1
. However, Bythotrephes is significantly less tolerant than Leptodora, 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 Bythotrephes
successfully invades primarily mesotrophic and oligotrophic lakes while Leptodora 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 Bythotrephes invasion of North American lakes.