Natural and human controls of water quality of an Amazon estuary (Caeté-PA, Brazil)

•Nutrients and thermotolerant coliforms input is intensified by the absence of a public sanitation system.•Mangrove outwelling increases the nutrient input to the estuary.•During the dry season estuary is more susceptible to retaining nutrients.•Pronounced eutrophic has been observed in the upper estuary near the pollution sources.•Nutrient concentrations in the upper sector are similar to highly populated areas.

Estuarine waters are usually eutrophic due to nutrients input derived from natural and anthropogenic sources. In the Caeté Estuary, nutrients and thermotolerant coliforms input is intensified by the absence of a public sanitation system and by mangrove outwelling. This input is more accentuated in the upper sector of the estuary where 90% of the local population is concentrated and a high incidence of commercial activities (public markets, ice factories and boat repairing among others) is located. As a consequence, eutrophic waters with high concentrations of thermotolerant coliforms (up to 1100 MPN/100 ml) were observed during the dry season when dilution and/or advection are less effective. Eutrophication, though less pronounced, also occurred in the other sectors (middle and lower), but these results seem to indicate that this could be a natural condition of an Amazon region which is naturally enriched with a high amount of nutrients. On the other hand, nutrient concentration values in the upper sector are similar to those found in other highly populated and industrialized Brazilian estuaries. Taking into account that the population increases by about 20% per decade in the region, this growth may lead to a significant increase in human pressure and impacts on the area, mainly during periods when the estuary is more susceptible to retaining nutrients (e.g., during the dry season). In order to improve water quality, the DPSIR framework was used as a guide for proposing potential measures to be implemented in the Caeté Estuary based on (i) urban planning to control sewage discharges, (ii) construction of water treatment infrastructures to reduce untreated effluents inputs and (iii) water use regulations as a function of the current water quality.