Grazing and food size selection of zooplankton community in Lake Biwa during BITEX '93

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 fraction (<2,2-8,8-20 and 20-70 μm), and the grazing rate of each size fraction was determined under ambient food condition. During the study period, Eodioptomas japonicus dominated followed by Daphnia galeata at both sites. In all experiments, the grazing rate was higher for middle size fractions (2-8 and 8-20 μm) than the smallest and largest size fractions, indicating that the zooplankton communities selectivity remove section 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 fraction was main food of zooplankton. When abundance of sestor, <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.