SEASONAL COMPOSITION OF MEROPLANKTON IN THE DUNKELLIN ESTUARY, GALWAYBAY

Dunkellin estuary, Galway Bay, is a shallow embayment, where the domin ant process is tidal mixing. The abundance and distribution of the mer oplankton were studied monthly between December 1984 and June 1986 usi ng Clarke-Bumpus plankton samplers fitted with 0.16mm aperture nylon-m esh nets. During the sampling period, salinity ranged between S4.5 and S34.5, water temperature between 4 degrees C and 18 degrees C, and ch lorophyll a between less than 1mg m(-3) and 29mg m(-3). Meroplankton, all of coastal/marine origin, accounted for 58% of zooplankton numbers recorded during the sampling period. Meroplankton abundance showed a marked seasonality, with a pronounced early spring peak from February to April. Early spawners included larvae of barnacles, polychaetes, ga stropods, eggs and larvae of fish and ephyrae of Aurelia aurita. Cypho nautes larvae and the hydromedusae Rathkea octopuncata and Phialella q uadrata appeared later in April. Summer spawners included larvae of ga stropods, echinoderms, ascidians, decapods and other species of hydrom edusae. Abundance and diversity of meroplankton was lowest during the winter months. Barnacle nauplii were the most abundant group, accounti ng for 70% of meroplankton numbers. Polychaete larvae were next in imp ortance in the inner estuary, while gastropods were more abundant at t he seaward end. The relationship between meroplankton abundance and en vironmental variables is discussed.