Processos hidrodinâmicos e implicações na carga de nutrientes no estuário do Rio São Francisco decorrente das reduções de vazões

Abstract

The São Francisco River estuary located on the border between the states of Alagoas and Sergipe (10° 26'S and 036° 25'W) suffers from the regularization of flow in the various hydrographic sectors of the river. Such regularization directly influences the transportation of nutrients and sediments to the adjacent coastal region, and in turn, alter the chemical and biological conditions within the estuarine sector. The present work aimed to understand how the successive flow reductions influence the nutrient load within this ecosystem. Physical-chemical parameters collections were performed through the YSI 6600 multi-parameter probe and current velocities with Sontek 1.5 MHz ADCP, in addition to water sampling to determine the concentration and dissolved inorganic nutrients loads, chlorophyll-a, and total suspended sediments. ERA-Interim reanalysis data were used to represent the wind conditions at 1000 hPa during the study period. The vertical velocity profiles of the current presented higher values on the surface and decreased towards the bottom, and overall it was observed higher velocity in July. The average temperature was 28.8°C in February and 26°C in July. For salinity an average of 18.17 and 18.01 UPS was observed in February and July respectively, being directly related to the influence of the tide and the small variability of the fluvial flows. The wind showed magnitude varying from 2 to 10 m.s-1. The concentrations of dissolved inorganic nutrients, TSS, turbidity, pH and chlorophyll-a were higher during the rainy season. Despite the active control of outflows, the erosion of the margins, physical and geochemical processes along with local precipitation would be increasing nutrients in the estuary. The exception was the silicon that presented higher concentrations in February, due to its abundance in the soils. Higher nutrient loads were observed for TSS (greater than 1,2x104 t/month) followed by silica (greater than 1,1x104 t/month), and reduced for PO4-3 (maximum of 1,1x10 t/month), HPO4-2 (maximum of 4,5x10 t/mês) and NID (maximum of 2,6x102 t/month in July). The regularization of the flow changed the nutrient load and suspended sediments, bringing impacts to the aquatic biota and to the estuary itself, reaching oligotrophic condition, in addition to greater saline intrusion. The nutrient increment was mainly related to local precipitation and the estuarine geochemistry.