Escaping the pelagic zone: Diel horizontal migration of zooplankton in a small, eutrophic kettle lake (Sites Lake, OH)

Sites Lake, OH (40^oN, 82^oW) is a small, deep kettle lake (2.6 ha, Z_max>11 m) with ~20-30% littoral area.  Sites is eutrophic and hypolimnetic anoxia establishes ca. June.  We have been investigating the possibility that pelagic zooplankton use littoral areas as predation refugia.  This hypothesis was suggested by researchers who found evidence for diel horizontal migration (DHM) by zooplankton of shallow, unstratified lakes with relatively abundant aquatic macrophytes.  They observed DHM instead of the more typical predator-avoidance behavior of diel vertical migration (DVM).  Previous studies of Sites Lake have found strong support for DHM during fall by Daphnia parvula, but no evidence for DVM.  Larger Daphnia (0.5-1.1 mm) accounted for 75% of the night-time increase in population size and expanded from 45 to 62% of the population.  This study focused on the spring zooplankton, using a combination of net-tow and 30-L Schindler-Patalas samples to compare mid-day and mid-night abundances.  Data so far suggest that, unlike in fall, spring Daphnia are not migrating horizontally in spring.  Onset of DHM may be related to the development of anoxic depths.  Overall, work on Sites Lake shows differences in DHM among species and within species that are consistent with the predation refuge hypothesis.