Zooplankton Distribution in the Guaraú River Estuary (South-eastern Brazil)

The structure and seasonal variability of zooplanktonic associations were analysed over a 1-year period in the Guaraú River Estuary (south-east Brazil). The spatial segregation of the zooplankton was mainly conditioned by the longitudinal salinity gradient, as revealed by multivariate analysis. The upper estuary was dominated by a large indigenous population of the estuarine copepod Pseudodiaptomus richardi. The estuarine marine copepods Acartia lilljeborgi and Oithona hebes, together with meroplanktonic organisms, co-occurred with P. richardi in the middle estuary. The maintenance of high standing-stocks of marine euryhaline species such as Paracalanus crassirostris and Pseudodiaptomus acutus in the outer estuary, largely depended on recruitment from the coastal water. Temperature fluctuations influenced the seasonal occurrence of the zooplankton, with higher densities occurring in the warmer months. Increased river inflow caused by summer rainfalls had an evident but comparatively smaller effect on the zooplankton abundance. Predation on rotifers and grazing on nanoplankton are probably important factors regulating the local zooplankton dynamics.