Avaliação espaço-temporal da hipoxia no estuário do Rio Itajaí-Açu entre os anos de 2003 e 2012

Abstract

The Itajaí-Açu river estuary, on the central northern coast of Santa Catarina, drains into an area of 15,500 km² that has some important cities like Blumenau, Brusque, Navegantes and Itajaí. In the final portion of the estuary, critical concentrations of dissolved oxygen and the formation of signs of hypoxia have been reported in previous studies, particularly in low fluvial discharges. The aim of this study was to evaluate the main factors that influence water quality and the occurrence of states of hypoxia in the estuary. Data were compiled from nearly nine years of monitoring of the lower estuary. This enabled recognition of the different forms that lead to states of low dissolved oxygen concentrations. The first is trigged by events that cause disturbances in the system, such as the occurrence of works in the port region or dredging works to increase the river depth; and the traditional form, in which the minimum concentration develops at the outer edge of the saline intrusion, between discharge values of 60 to 200 m³.s-1 due to the greater stratification. The lowest recorded concentrations of dissolved oxygen were 2.0 mg.L-1, 1.59 mg.L-1 and 1.45 mg.L-1. The direct consequence of the traditional form is deterioration in water quality of the estuary, since the lower fluvial discharge does not dilute the pollutants, making the discharges proportionally greater, increasing the concentrations of ammoniacal nitrogen and reaching a maximum of 84% in DIN. The benthic macrofauna was not directly influenced by variations in dissolved oxygen. The conceptual model indicated that fluvial discharge directly contributes to the saline intrusion and variations in DO. It is concluded that there is a greater likelihood of hypoxia when the fluvial discharge values are below 180 m³.s-1. Fluvial discharge also accounted for most of the changes in conditions of the system