Seasonal succession of zooplankton in the north basin of Lake Biwa

To examine the seasonal succession of the entire zooplankton community in Lake Biwa, zooplankton biomass (on an areal basis) and its distribution patterns among crustaceans, rotifers and ciliates were studied in the north basin from April 1997 to June 1998. Seasonal changes in phytoplankton and population dynamics of Daphnia galeata were also examined to assess food condition and predation pressure by fish. From March to November, crustaceans dominated zooplankton biomass, but rotifers and ciliates were dominant from December to February. Among crustaceans, Eodiaptomus japonicus was the most abundant species, followed by D. galeata. Zooplankton biomass increased from January to a peak in early April, just before the spring bloom of phytoplankton, then decreased in mid-April when mortality rate of D. galeata increased. From mid-June, zooplankton increased and maintained a high level until the beginning of November. During this period, both birth and mortality rates of D. galeata were relatively high and a number of rotifer and crustacean species were observed. However, their abundances were very limited except for E. japonicus which likely preys on ciliates and rotifers. In Lake Biwa, food sources other than phytoplankton, such as resuspended organic matter from the sediments, seems to play a crucial role in zooplankton succession from winter to early spring, while zooplankton community seems to be regulated mainly by fish predation from summer to fall.