DISTRIBUTION AND COMPOSITION OF SMALL-SIZE ZOOPLANKTON FRACTION IN A TEMPERATE SHALLOW ESTUARY (WESTERN PORTUGAL)

The variation of small zooplankton abundance and distribution patterns was studied in a shallow temperate southern European estuary (Mondego estuary, Portugal). Monthly plankton sampling was carried out from February 2005 to July 2006 at five stations, in both arms of the estuary - north and south, with subsurface 63 mu m mesh tows. Concomitant hydrological parameters were measured. 84 zooplankton taxa were identified in the northern and southern arms stations. Copepods were the most abundant group (95.6% of the total abundances). Nauplii were the most abundant copepod development stage (80.3% of the total), followed by copepodites (14.4%). Marine zooplankters were reported at the mouth of the estuary and downstream stations while freshwater and estuarine species dominated in the inner part of the estuary. Statistical analysis (RDA) correlated marine species with high salinity and temperature while freshwater and estuarine species were correlated with low salinity and temperature and high nutrient concentrations. The relationship between species distribution and environmental parameters suggests adaptive strategies of the first stages of copepods to hydrological variations which explain the adults' abundances.