The effects of the anthropogenic process on the trophic status of the Caete estuary (located 150 km southwest of the Amazon delta) were undertaken under different climate conditions. To do this, oceanographic data were carried out, covering the dry season of 2010 (August-December: rainfall 363 mm), and the wet (January July, rainfall 2483 mm) and dry seasons of 2011 (August-October, rainfall 135 mm). During the dry season (2010 and 2011) the mean discharge of the Caete River was less than 20.0 m(3) s(-1), while, during the wet season, the mean discharge increased to 73.6 m(3) s(-1). Tidal ranges oscillated between 2.2 and 3.8 in, and current speeds varied from 0.6 and 0.7 m s(-1) in both seasons (wet and dry). Salinity (under 3.0) and pH (under 6.0) were typical of fluvial systems. Turbidity (up to 150 NTU), dissolved oxygen (6.7 mg L-1) and dissolved nutrient (nitrite, nitrate, phosphate and silicate, but not ammonium) concentrations reached their maximum levels during the dry season, when the runoff decreased. The high chlorophyll a concentrations (values of up to 10.0 mg m(-3)) were typical of environments with high levels of primary productivity. According to the trophic index (TRIX), the waters of the Caete estuary were characterized by moderate levels of eutrophication and good water quality during the wet season, shifting to high levels of eutrophication and bad water quality during the dry season. A comparative analysis indicates that eutrophication is less intense during neap tides in comparison with spring tides.

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