Geographical and seasonal variations in taxonomic composition, abundance and biomass of microzooplankton across a brackish-water lagoonal system of Japan

The taxonomic composition, abundance and biomass of microzooplankton were studied at eight stations in Lake Shinji-Ohashi River-Lake Nakaumi brackish-water system, Japan, monthly from April 1998 to March 1999. Over the entire area, naked ciliates numerically dominated the microzooplankton community (annual mean 39.6%) followed, in order, by tintinnids (30.3%), copepod nauplii (24.6%) and rotifers (5.5%). The abundance of each taxonomic group of microzooplankton varied geographically due to large salinity variations (range 1.5-33.3 p.s.u.). It was notable that naked ciliates occurred overwhelmingly in Lake Shinji (54.9% of total microzooplankton) and rotifers were relatively numerous in Lake Shinji (8.8%) and Ohashi River (11.1%), where the salinity was lower (annual mean 4.1 and 13.6 p.s.u., respectively) than in Lake Nakaumi and Sakai Strait (26.3 and 29.8 p.s.u., respectively). Owing to large seasonal temperature variation (range 5.4-29.8degreesC), the abundance of microzooplankton showed marked seasonal variations, being higher in spring and summer than in the remaining seasons. A total of 49 species of tintinnids were identified, and 15 of these species reached concentrations >500 individuals l(-1). The occurrence of most tintinnid species was confined to certain months or locations, closely associated with species-specific temperature and salinity preference and/or tolerance. In this eutrophic system food supply for microzooplankton might be sufficient due to extremes high chlorophyll a concentration (annual mean 8.8 mug l(-1) in Sakai Strait to 22.6 mug l(-1) in Lake Shinji). However, microzooplankton biomass remained moderate (range 0.19-18.7 mug C l(-1)) due probably to heavy predation by mesozooplankton, which inhabit this brackish-water system at extremely high biomass.