Como nutrientes e sólidos suspensos variam em um estuário hipersalino?

Abstract

The estuary of the Piranji river (CE) is located in a predominantly semiarid region and has a unique characteristic by the inverted salinity pattern during most of the year, being higher in the interior than near the mouth. The nutrients are fundamental for a primary production in the estuarine ecosystem, and their biogeochemical cycles reflect as active variables in the environment, being able to transmit relevant information also related to the current context of climatic changes and anthropization of the environment. Therefore, this work sought to analyze the spatial and seasonal dynamics of the main nutrients and suspended solids in the estuary and to consider natural factors that may cause increase or decrease in the concentrations of the investigated variables. The water samples were collected bi-monthly over the year 2015, during the syzygy ebb tide, in three stations on the estuary: upper (E1), medium (E2) and lower (E3). Temperature, pH, salinity, dissolved oxygen and oxygen saturation were measured in situ in a multiparameter probe and the water transparency was measured with a Secchi disk. The concentrations of the nutrients (N-ammoniacal, nitrite, nitrate, total nitrogen, orthophosphate, total phosphorus and silica) were determined by spectrophotometric methods, while total suspended solids (TSS) determinations were performed by gravimetry. At station E1, were found the highest (62.6 ± 0.17) and lowest (9.4 ± 0.04) salinity values, while in the station closest to the mouth (E3) this variation was lower, 36.9 (± 0) to 39.5 (± 0.7). In general, the highest concentrations of all nutrients were found in the upper station (E1), followed by the medium (E2), indicating a dilution of these concentrations by coastal waters, with higher values to dissolved nutrients during the rainy period. The results of suspended solids during the dry period were higher than the rainy period, and the highest concentrations in all months were related to the E1 station. It was observed an influence of rainfall in both nutrient and suspended solids dynamics, as well as salinity, pH and dissolved oxygen concentrations. In the dry period, despite the low variation of TN results in relation to the previous period, the DIN concentrations fell, while the values of TP and TSS increased, suggesting an environment more favorable to primary productivity. The results of TSS were quite different from other estuaries and, in addition to pointing to low river flow limited by climatic action, indicated changes from anthropic interference, which are also suggested from high concentrations of N-ammoniacal. The salinity pattern was inverted in most of the months, considering the increase of the primary production presumed from the results of nitrogen, phosphorus and suspended solids, as characteristics of the estuary pointed to an adaptation of the environment to the drought conditions and hypersalinity. Keywords: Semiarid estuary. Inverse estuary. Ceará. Water quality. Limnology.