Zooplankton assemblages in the shallow tidal estuary of Mundaka (Bay of Biscay)

Spatial and temporal variability of zooplankton assemblages were studied in the tidal estuary of Mundaka , from May 1981 to July 1982, in relation to factors controlling estuarine hydrodynamics. Salinity range and maintenance of mixed waters were related to the tidal amplitude and rain fall, which drives river discharge. At high tide, the study zone appeared mainly as a polyhaline system, in which Acania bifilosa was the only perennial component of the estuarine-marine assemblage, and true estuarine copepods (genus Eurytemora) were absent. Among neritic holoplankters, Paracalanus parvus and Acartia clausi dominated and no seasonally stable euryhaline and stenohaline assemblages were found. Among meroplankton groups, larvae of Polychaete Spionids was predominant toward the upper estuary, while Bivalve veligers had the lowest penetration in the estuary from the neritic zone. Some nectobenthic organisms associated with muddy benthic habitats, such as Praniza larvae of Paragnatilia formica and ostracod Cytherids, characterized the plankton community of the upper estuary during periods of low abunclanre of estuarine holoplankton. According to the hierarchical clustering of samples from zooplankton data, summer-autumn, winter and spring periods were separated as seasonal units, which also differed with regard to hydrological features. The mixed water persistency and the development of estuarine species were higher in summer, when low freshwater inflows and neap tides coincided. By contrast, high-salinity waters and neritic zooplankton filled the estuary in spring by the effect of spring tides coupled with low freshwater inflows.