1996 Volume 57 Issue 1 Pages 27-37
Community grazing rate of zooplankton larger than 98 μm in body size were examined at the north and south basins of Lake Biwa in late summer, 1993. The lake seston labeled with 14C was divided into different size fractions (<2, 2-8, 8-20 and 20-70 μm), and the grazing rate for each size fraction was determined under ambient food condition. During the study period, Eodiaptomus japonicus dominated, followed by Daphnia galeata at both sites. In all experiments, the grazing rate was higher for middle size fractions (2-8 and 8-20μm) than the smallest and largest size fractions, indicating that the zooplankton communities selectively remove seston in the size range of 2-20 μm. Community ingestion rate estimated from the grazing rate and abundance of ambient seston revealed that seston in the middle size fractions was main food of zooplankton. When abundance of seston <20 μm was low relative to those >20 μm, however, the largest size fraction contributed 57% of the ingested carbon, suggesting that the food source of zooplankton is not necessarily restricted to the small sestonic particles, even if feeding efficiency was low for large sestonic particles. Based on grazing rate, 6 to 10% of total seston was estimated to be removed by the zooplankton community within a day.