Geographical and seasonal variations in mesozooplankton abundance and biomass in relation to environmental parameters in Lake Shinji-Ohashi River-Lake Nakaumi brackish-water system, Japan

We measured the abundance and biomass of the major taxonomic groups of meso zooplankton at six stations in Lake Shinji-Ohashi River-Lake Nakaumi bracki sh-water system, Japan, monthly for three full years (1995-1997), except fo r one station (for 1 year and 9 months). Over the entire area, copepods ove rwhelmingly dominated the zooplankton community both in terms of abundance (mean: 87.9%) and biomass (83.4%). The remaining taxa were cladocerans (i.e . Diphanosoma brachyurum, Evadone tergestina, Penilia avirostris, Podon leu ckarti and Podon palyphemoides), appendicularians (Oikopleura dioica and Oi kopleura longicauda), chaetognaths (Sagitta crassa) and the larvae of benth os (e.g. polychaetes, bivalves, gastropods and malacostracans), The geograp hical and seasonal variations of the mesozooplankton community were therefo re principally explained by the variations of the copepod community. The ge ographical difference in copepod species composition was associated with sa linity preference or tolerance of respective species. In Lake Shinji, where the salinity was lowest (mean: 4.0), Sinocalanus tenellus was monospecific ally abundant with sporadic occurrence of Pseudodiaptomus inopinus. In Ohas hi River (mean salinity: 9.9), Acartia hudsonica, Acartia sinjiensis, Euryt emora pacifica and Oithona davisae added to the community. At central and s outheast Lake Nakaumi and in Honjo District, where mean salinity ranged fro m 16.4 to 21.7, these four species became more important than S. tenellus a nd P. inopinus. At the entrance of Sakai Strait, where the salinity was hig hest (mean: 24.0), Paracalanus spp. constituted a significant component. Du e to large temperature fluctuation with season, the copepods showed remarka ble seasonal variations in abundance and biomass, with enormous annual peak s in winter-spring. These annual peaks might be attributed to scarce occurr ence of predators. (C) 2000 Elsevier Science B.V. All rights reserved.