Assessing the seasonal onset of daily horizontal migration behavior of Daphnia dentifera in Sites Lake, OH

Background/Question/Methods

Daily migration patterns of zooplankton offer insights into the ecology of these small planktonic animals. Sites Lake, Richland Co., OH is a small, eutrophic, plant-rich kettle lake (A₀=3.7 ha, Z_max=12.8 m). Sites Lake is known to stratify throughout the growing season (~Apr-Nov). Previous studies in Sites Lake found that Daphnia dentifera, unlike similar taxa, moves away from the central open-water of the lake in daytime in the fall. This behavior has never been observed in a relatively deep lake, but daily horizontal movement is now commonly found in more shallow, plant-rich lakes. One hypothesis for why D. dentifera is behaving as if Sites is a shallow lake is that lower oxygen levels force the Daphnia from the refuge of the hypolimnion. Understanding the timing of seasonal onset of horizontal migration during the summer-fall transition and comparison to environmental data should allow us to support or refine this hypothesis. Triplicate whole-water column net-tows were taken from the center of the lake at mid-day (~1300 hours) and mid-night (~0100 hours).  Lake environment was assessed with profiles of temperature, dissolved oxygen, and Secchi depth. Samples were collected at ca. 2-4 week intervals throughout the growing season.

Results/Conclusions

This study found significant differences in D. dentifera abundance between mid-day and mid-night in September and October (P=0.0313 and P=0.0028). There was a 225% to 130% increase in abundance at night. These results were consistent with earlier studies of portions of other years, and add evidence that D. dentifera is moving away from the center of the lake in the daytime.  Preliminary results for July suggest that Daphnia are migrating in mid-summer as well (~71% increase). However, spring populations showed no evidence of daily horizontal migration (Apr-Jun 2008). Environmental data showed a decrease in oxygen levels in the hypolimnion throughout the summer, with rapid loss between mid-June and late July.  From August through mixis, anoxic depths (<1 mg L^-1) extended all the way up to the thermocline (~5 m).  Further results should help us see if summer development of the anoxic layer corresponds to the onset of daily horizontal migration behavior in D. dentifera in Sites Lake.  If so, limited oxygen in the hypolimnion would no longer provide an optimal habitat for D. dentifera, thus forcing them to migrate horizontally for safety.