SUMMER ZONATION AND DEVELOPMENT OF ZOOPLANKTON POPULATIONS WITHIN A SHALLOW MESOTIDAL SYSTEM - THE ESTUARY OF MUNDAKA

Two sampling series carried out in the estuary of Mundaka in July and August 1990 allowed us to obtain detailed information on the summer zo nation and development of zooplankton populations under particular env ironmental conditions. i.e. very low river discharge throughout the st udy period. Copepods (largely dominated by Acartia bifilosa) and gastr opod larvae were not only the dominant taxa, but also the main cause o f the spatial and temporal patterns observed in the micro and mesozoop lankton assemblages. Both showed maxima around 30 parts per thousand, salinity, but higher densities of microzooplankton were usually found at higher salinities than those corresponding to mesozooplankton, due mainly to the seaward skewed distribution of copepod nauplii. Large ci liates and rotifers distributed upstream than former taxa, dominating in salinities below 25 parts per thousand, but their contribution to t otal zooplankton was relatively low. Temporal variations in the copepo d abundance reflected the development of successive cohorts of A. bifi losa, with an estimated generation time of about 25 days. The observed synchrony between the abundance of gastropod larvae and tidal amplitu de supports the hypothesis that larval release took place around the s pring tides, following a semilunar rhythmicity. As a result of these p eriodical fluctuations in the abundance of gastropod larvae, the compo sitional structure of the zooplankton community showed noticeable shor t-time changes in synchrony with the neap-spring tidal cycle.