Zooplankton Assemblages from a Tidal Channel in the Bahía Blanca Estuary, Argentina*

In this study we investigate for the first time the zooplankton assemblages in a relatively small tidal channel called Bahía del Medio, situated in the middle-outer area of the Bahía Blanca Estuary, South of the Main channel. We focused the study on microand mesozooplankton assemblages with emphasis on tintinnids, copepods and meroplankton along the annual cycle 1997-1998. Fifteen species of tintinnids belonging to 6 genera were observed in the study area, the mean total tintinnid abundance fluctuating between 3.4 x 10 ind. m and 5.1 x 10 ind m. The mesozooplankton comprised 11 taxa, total abundance fluctuating between 449 ind m in March 1997 and only 1 ind m in October 1997. The channel proved to be spatially homogeneous in its physical and biochemical features. Microand mesozooplankton abundance displayed a high degree of spatial homogeneity, though the seasonal variation in both environment and zooplankton was significant. Comparison with the zooplankton observed at a station in the Main channel of the estuary showed differences in number of taxa represented, a general lower abundance of both assemblages and the presence of some rare species, a fact which is discussed.