Composition, Distribution and Seasonal Abundance of Zooplankton in a Shallow, Seasonally Closed Estuary in Temperate Australia

Nets of 53- and 500-μm mesh sizes were used to collect zooplankton and fish larvae, respectively, from the lower, middle and upper regions of the large basin and saline reaches of a major tributary river of the seasonally closed Wilson Inlet in each month between July 1988 and June 1989. Copepods, of which approximately two-thirds were naupliar stages, contributed 52% to the total number of zooplankton. The dominant copepods wereOithona simplexandAcartia simplex. Copepod nauplii was the most abundant single group, reaching a density of 680768m−3in one region in January and having a mean monthly density for the whole system of 147166m−3. Other numerically important taxa were the tintinnid ciliatesTintinnopsissp. a,Helicostomellasp. andFavellasp., the rotiferSynchaetacf.baltica, and the meroplanktonic stages of molluscs and polychaetes. The densities of the 10 most abundant taxa did not exhibit sharply defined seasonal peaks, except forS.cf.baltica, which peaked in summer. The densities of the other abundant taxa remained high for between 5 and 10 months between late winter and the early winter of the following year. The mean monthly density of zooplankton was very high (447238m−3), presumably reflecting a combination of the use of a fine-mesh net, the high nutrient loading of this system and the relative stability of the water mass within the estuary basin that results from a restricted exchange of water with the sea. Although the density of the total zooplankton community reached a maximum in mid-summer, densities were also high in all but the mid-winter to early spring months, when salinities and temperatures were at their lowest. The zooplankton community in Wilson Inlet changed progressively throughout the year as temperature and salinity also changed. The zooplankton assemblages in the lower, middle and upper basin at any one time were similar, as were the salinities, but they differed from those 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 the zooplankton were euryhaline marine rather than estuarine. The seasonal densities of zooplankton and fish larvae were correlated in both the middle basin and Denmark River. Since even copepod nauplii, the main prey of fish larvae, were typically at least 20000 times more abundant than fish larvae, it is unlikely that the zooplankton food supply of fish larvae in Wilson Inlet was limiting.