Qualidade da água no estuário do Rio Pacoti sob influência da carcinicultura

Abstract

The northeastern region of Brazil has a prominent role in the production of the marine shrimp Litopenaeus vannamei, with Ceará being one of the biggest Brazilian producers. In order to carry out this production, the estuaries are important supplying bodies and receptors of water from shrimp farms. The effluents from shrimp farming have high concentrations of nutrients and organic matter, and, in order to mitigate their negative impacts on the environment, settling basins are built to reduce nutrients, suspended material and organic matter from the effluent. Therefore, the objectives of this work were to observe the possible effects of the discharge of effluent from shrimp ponds on the water quality of the Pacoti estuary (CE), to analyze the seasonal variation in the estuarine water quality, analyze changes in the physicochemical parameters of the water after the grow-out process and to evaluate the effectiveness of the local settling basin. Water samples were collected monthly, from May to October 2017, in five points: in the area of the estuary where the water is collected to supply tanks, at the points where the crude and final effluent are released, at the point of the estuary located in front of the effluent discharge and at the point of the estuary downstream of the discharge. In order to evaluate the water quality, the following parameters were analysed: temperature, salinity, pH, turbidity, dissolved oxygen, total suspended solids, orthophosphate, total phosphorus, ammoniacal nitrogen, nitrite, nitrate and chemical oxygen demand. The values obtained for most of the parameters (except for dissolved oxygen, total phosphorus and ammoniacal nitrogen) indicated good water quality conditions in the estuary. There were no major seasonal changes in the parameters of water quality, except for dissolved oxygen, which was lower in the period of higher rainfall, and orthophosphate, which was higher during this period. It was observed that the concentration of suspended matter, nutrients and organic matter in the effluent was higher than that of the influent water and the concentrations depend on the duration of the cycle and the frequency of water changes. There were no changes in the physicochemical parameters of the estuary water with the release of effluent. There was a removal of suspended matter, nutrients and organic matter in the settling basin, but its efficacy depends on the state of conservation of the basin and the retention time of the effluent. With the study, it was noticed that effects on water quality resulting from the release of shrimp effluent can not always be directly perceived, considering the distance between the point of effluent discharge and the water body.