Zooplankton abundance in a coastal station off the Ria de Aveiro inlet (north-western Portugal): relations with tidal and day/night cycles

Temporal and vertical variations of zooplankton at a station on the north-western Portuguese shelf, close to the Ria de Aveiro inlet, were studied during two periods of 25 h each in the summer of 1994. Zooplankton sampling was carried out every 2 h, using horizontal plankton tows (mesh size, 500 μm). Sampling was carried out in two depths, 4 and 12 m. The vertical profiles of salinity and temperature were obtained immediately before the plankton sampling. Higher zooplankton concentrations were usually observed during the night and at 4 m depth. Most taxa also occurred inside the Ria de Aveiro, indicating an exchange of planktonic forms between the Ria and the shelf. A classification analysis of the most abundant taxa suggests some groups, which differ according to taxonomic features and their origin. This was attributed to the high variability of the forcing agents that affect zooplankton abundance