Hypoxia tolerance of Bythotrephes longimanus and Leptodora kindtii
Range expansions by species may be dependent on the availability of suitable habitat where vulnerability to predation is reduced. For freshwater zooplankton, fish avoidance through Diel Vertical Migration (DVM) can be critical for species survival but can also potentially subject individuals to hypolimnetic environments with low dissolved oxygen concentrations. The ability of zooplankton to withstand fish-based hypoxia (<2 mg/L) for up to 12 hours per day may be crucial to their range expansion into productive (e.g., eutrophic) lake basins with zooplanktivorous fish. We investigated short term survival of two predatory cladocerans, the invasive Bythotrephes longimanus (spiny water flea) and the native Leptodora kindtii under low dissolved oxygen concentrations. Both species are large-bodied, strongly selected by zooplanktivorous fish, and known to commonly engage in DVM. Trial conditions were chosen to mimic the hypolimnion of a temperate zone lake during late summer. Live zooplankton and filtered water were collected from Island Lake Reservoir, Minnesota and transported to the University of Minnesota Duluth. Zooplankton were individually placed in 300 mL BOD bottles with nitrogen-sparged lake water. Survivorship was noted at the end of a 12 hour exposure period and an LC50 value was calculated for each trial.
Preliminary results indicate that Bythotrephes longimanus was less tolerant of fish-based hypoxia than Leptodora kindtii. The 12 hour LC50 values of Bythotrephes longimanus were approximately 1.75-2.25 mg/L, while those of Leptodora kindtii were approximately 0.75-1.25 mg/L. Hypoxia intolerance may influence the ability of Bythotrephes longimanus to invade eutrophic lakes of North America. This is in agreement with the observation that Bythotrephes longimanus is typically detected in deep, cold, oligotrophic North American lakes. Further trials, to be conducted during the summer of 2015, will analyze among-lake and seasonal variation in hypoxia tolerance of Bythotrephes longimanus and Leptodora kindtii.