Tidal-Induced Changes in the Zooplankton Community of an Amazon Estuary

Because variations in the zooplankton community are common in macrotidal estuaries, the composition and density of these organisms in the estuary of the Paracauari River, an equatorial macrotidal system of the Amazon coastal zone, was studied during spring and neap tides to 'evaluate fluctuations related to variations in the physical and chemical characteristics of the water. Significant differences between tidal periods were recorded for salinity, pH, and dissolved oxygen. Phosphate concentrations were significantly higher in the spring tide. In all, 34 taxa were identified, with the Cirripedia dominating in both tides. Total density was highest in the spring tide (1871.6 +/- 1248.4 individuals m(-3)), whereas chlorophyll a was lowest (10.46 +/- 3.48 mg m(-3)). Diversity was also higher in the spring tide. Multivariate analyses indicated the formation of two distinct groups representing the spring and neap tides. The zooplankton comprised mainly estuarine and coastal species. Oithona oswaldocruzi, Paracalanus quasimodo, and Acartia tonsa were the most common, and their density was related directly to tidal oscillations.