The evaluation of water quality works as an indicator, relating the quality standards with the levels of environmental degradation, being of great importance for the preservation of biodiversity. The present work aims to diagnose the processes of alteration in the quality of the surface water of the Marapanim River estuary, NE-PA. Water samples were collected in the dry (Nov-Dec/2002) and rainy (March/2003) periods, according to ebb and flood tides, in six points along the estuary, and submitted to physical (temperature, pH, salinity, electrical conductivity), chemical (dissolved oxygen-OD, total alkalinity, chloride, sulfate, Ca, Mg, Na, K) and nutrient (silicate and phosphate) analyses. Salinity showed higher values in the dry period, decreasing in the rainy period, decreasing from the mouth to the headwaters. The pH presented high values in the dry period, reducing in the rainy period, indicating more acidic conditions due to the higher volume of water in the river and the large amount of organic matter produced in this period. The values of conductivity and total alkalinity were directly proportional to the parameters mentioned above. The concentration of DO revealed lower values during the dry period, caused by the oxidation of organic material carried to the river. Higher concentrations of chloride, sulfate, Ca, Mg, Na and K ions were also recorded in the dry period, probably due to the reduced input of fresh water. Low concentrations of the nutrients silicate and phosphate were detected, with no concentrations of silicate at the upstream point and phosphate downstream of the estuary during the ebb tide. Therefore, the influence of tidal regimes and seasonality, in the analytical results, are quite significant, because they are responsible for the balance of physicochemical and biological processes in the estuary.
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