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RESUMONutrientes inorgânicos dissolvidos, nitrogênio total (NT) e fósforo total (PT), clorofila-a e salinidade foram analisados no estuário do Rio Cachoeira antes e depois da operação de uma estação de tratamento de esgoto (ETE). As concentrações de nutrientes inorgânicos dissolvidos apresentaram grande variação, com valores desde abaixo do limite de detecção a > 45 µM para silicato, > 30 para o nitrogênio amoniacal e > 28 µM para o nitrito. As concentrações de nitrato foram baixas. A clorofila-a variou desde abaixo do limite de detecção a > 60 µg . L -1. A razão molar de nitrogênio e fósforo inorgânico dissolvido (NID:PID) foi maior que a de Redfield (N:P = 16:1) antes e depois da operação da ETE, decrescendo nos meses seguintes. A razão média SiD:NID foram < 1:1. A SiD:PID foi geralmente < 16:1, indicando que o silicato pode estar limitando a produtividade primária. As concentrações de NT foram altas (56,6 -310 µM). O PT variou de 2,07 a 3,54 µM. As concentrações de nutrientes e clorofila-a encontradas neste estudo são características de ecossistemas eutrofizados. A ETE e outras fontes pontuais contribuem com uma elevada carga de nutrientes para este estuário, levando a eutrofização desse sistema. PALAVRAS-CHAVE:nutrientes, razão molar, clorofila-a, esgotos, eutrofização. ABSTRACT Dissolved nutrients and chlorophyll-a in the estuary of river Cachoeira, northern BrazilDissolved inorganic nutrients, total nitrogen (TN) and phosphorus (TP), chlorophyll and salinity were analyzed in the estuary of River Cachoeira, before and after the operation of a sewage treatment plant (STP). The concentrations of dissolved inorganic nutrients presented great variation, with values from below the detection limit to > 45 µM for silicate, > 30 µM for the ammonium and > 28 µM for nitrite. The concentrations of nitrate were low. The chlorophyll varied from below the detection limit up to 60 µg . L -1. The molar ratio of dissolved inorganic nitrogen to phosphorus (DIN:DIP) was higher than Redfield (N:P = 16:1) before the ETE operation, decreasing for lower ratios in the following months. The mean DSi:DIN was lower than 1:1. DSi:DIP was < 16:1, generally, indicating that silicate may be limiting the primary productivity. The concentrations of TN were high (56.6 -310 µM). TP varied from 2.07 to 3.54. The concentrations of nutrients and chlorophyll found in this study are characteristics of eutrophic ecosystems. The STP and others sources contribute with an elevated load of nutrients for this estuary, and can contribute for the eutrophication of the system.

Sampling was conducted monthly during a transition period between the dry and rainy seasons in order to evaluate the effectiveness of a municipal sewage treatment plant (STP) in eutrophication control. STP effluent and fluvial input data were also estimated. In the dry period, high concentrations of nutrients, chlorophyll a (up to 360 μg L(-1)), and anoxia in bottom waters were observed in the upper portion of the estuary. Nitrate was scarce during the dry months, although high concentrations were observed at the river sources and the upper estuary. The N:P and Si:P molar ratios were usually below 16:1, and the Si:N ratio was higher than 1:1. The fluvial inputs were a greater source of nutrients to the estuary than the STP, but nutrient loading by these effluents were also important in contributing to the eutrophication of the upper estuarine zone, especially in the dry season when symptoms were more intense.

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