Zooplankton Grazing in a Eutrophic Lake: Implications of Diel Vertical Migration

During summer and fall, depth profiles of zooplankton community grazing were determined in situ during day and night in the Schohsee, a small eutrophic lake. Labeled algae of two different sizes were mixed with the natural suspension of phytoplankton in a grazing chamber. A small blue—green alga (Synechococcus, I μm) was labeled with ³ ²P; a larger green alga (Scenedesmus, 4—15 μm)was labeled with ¹ ⁴C. Grazing and vertical migration of zooplankton were strongly coupled. During summer, grazing in the upper 5 m was negligible during day but strong at night. Hence, algae grow relatively unimpeded by grazing during daytime but are harvested at night. Vertical and diel differences in grazing rates disappeared when the vertical migration ceased in fall. Selectivity of grazing was controlled by the zooplankton species composition. Eudiaptomus showed a strong preference for Scenedesmus (electivity index D = 0.9). Daphnia showed a slight preference for Scenedesmus, but Ceriodaphnia preferred Synechococcus. Cyclopoid copepodites did not ingest the small blue—green. Because Daphnia and Eudiaptomus were dominant, grazing rates on larger cells were usually higher than grazing rates on the small cells. Negative electivity indices for Scenedesmus occurred only when the biomass of large crustaceans was extremely low (near the surface, during day). Zooplankton biomass was the main factor controlling both vertical and seasonal variations in grazing. Highest grazing rates (65%/d) were measured during fall when zooplankton abundance was high. Differences between day and night in the grazing activity per unit biomass were not statistically significant. Laboratory experiments showed considerable losses of tracer from preserved animals, and loss rates differed for the two radioisotopes. Because such differential losses can produce substantial errors in the results, it was necessary to process the samples on the boat immediately after collection, without preservation.