COMPOSITION, DISTRIBUTION AND SEASONAL ABUNDANCE OF ZOOPLANKTON IN A SHALLOW, SEASONALLY CLOSED ESTUARY IN TEMPERATE AUSTRALIA

Nets of 53- and 500-mu m mesh sizes were used to collect zooplankton a nd fish larvae, respectively, from the lower, middle and upper regions of the large basin and saline reaches of a major tributary river of t he seasonally closed Wilson Inlet in each month between July 1988 and June 1989. Copepods, of which approximately two-thirds were naupliar s tages, contributed 52% to the total number of zooplankton. The dominan t copepods were Oithona simplex: and Acartia simplex. Copepod nauplii was the most abundant single group, reaching a density of 680 768 m(-3 ) in one region in January and having a mean monthly density for the w hole system of 147 166 m(-3). Other numerically important taxa were th e tintinnid ciliates Tintinnopsis sp. a, Helicostomella sp. and Favell a sp., the rotifer Synchaeta cf. baltica, and the meroplanktonic stage s of molluscs and polychaetes. The densities of the 10 most abundant t axa did not exhibit sharply defined seasonal peaks, except for S. cf. baltica, which peaked in summer. The densities of the other abundant t axa remained high for between 5 and 10 months between late winter and the early winter of the following year. The mean monthly density of zo oplankton was very high (447 238 m(-3)), presumably reflecting a combi nation of the use of a fine-mesh net, the high nutrient loading of thi s system and the relative stability of the water mass within the estua ry basin that results from a restricted exchange of water with the sea . Although the density of the total zooplankton community reached a ma ximum in mid-summer, densities were also high in all but the mid-winte r to early spring months, when salinities and temperatures were at the ir lowest. The zooplankton community in Wilson Inlet changed progressi vely throughout the year as temperature and salinity also changed. The zooplankton assemblages in the lower, middle and upper basin at any o ne time were similar, as were the salinities, but they differed from t hose in the river, where salinities were usually lower. Although there was only a limited exchange of water between the basin and the ocean, and salinities in the basin never exceeded 29, the main species in th e zooplankton were euryhaline marine rather than estuarine. The season al densities of zooplankton and fish larvae were correlated in both th e middle basin and Denmark River. Since even copepod nauplii, the main prey of fish larvae, were typically at least 20 000 times more abunda nt than fish larvae, it is unlikely that the zooplankton food supply o f fish larvae in Wilson Inlet was limiting. (C) 1995 Academic Press Li mited