Circulação hidrodinâmica no Complexo Estuarino de São Marcos (MA – Brasil)

Abstract

This study has mainly evaluated the temporal variation of hydrographic properties, suspended particulate matter (SPM), dissolved oxygen (DO) and circulation during complete tidal cycles (13 hours) along the São Marcos Estuarine Complex (SMEC), in different rainfall regimes. Data were acquired in two moorings in SMEC: in the north (M-I) and south (M-II), during the rainy (February), transition (June) and dry (November) seasons of 2017. The thermohaline structure and DO data were measured with a CTD (EXO2, YSI, Ohio, USA), the estuarine circulation was measured with a 500 kHz ADCP (Sontek, YSI, CA, USA) and the water samples were collected with a Niskin bottle on the surface and about 1 meter above the bottom for the analysis of the SPM. The velocity profiles of the along-channel component (u) presented a bidirectional flow, with a small predominance of ebb (u > 0) between flood currents (u < 0). Generally, the (M-I) presented average values higher than the (M-II), with the means in (M-I) ranging from -0.87 to 0.96 m s-1, while (M-II) presented mean values of -0.60 to 0.71 m s-1. The thermohaline structure showed a vertical quasi-homogeneity behavior and a small difference between the moorings (M-I and M-II) over the three campaigns. Temperatures varied between 27.8 to 30.5 °C, salinities ranged between 16.0 to 35.5 g kg-1 and sigma-t ranged from 7.63 to 22.49 kg m-3. The thermohaline indexes suggest the presence of two water masses, a Coastal Water (CW) during the rainy and dry seasons, whereas during the end of the rainy season was observed the presence of a River Water (RW). High concentrations of SPM (>1000 mg l-1) were observed in the region, with higher mean values than 230 mg l-1 over the 13 hours collected. The highest concentrations of SPM were observed in the dry season and near the bottom. The dissolved oxygen maintained a high level (DO > 190 μmol kg-1), despite the high SPM values found in the region, which are within the minimum limits established by the Brazilian government's regulatory agency (CONAMA). The Richardson layer number indicated a vertical instability condition throughout the SMEC, which corroborates the Potential Energy Anomaly (7.0x10-6 a 1.1x10-5 J m-3 s-1). The estuarine system was classified as type 1a (well-mixed and poorly stratified estuary), except the (M-I) performed in the dry season, which was classified as 2a (partially mixed and with weak vertical stratification). The dominant components governing the salt transport were the river discharge (2,33 to 41,06 kg m-1 s-1), the Stokes drift (-35,95 to -0,40 kg m-1 s-1) and the tidal currents (-6,05 to 2,04 kg m-1 s-1). River discharge was the main salt export mechanism, while Stokes drift was the main salt import mechanism in the region, presenting a contrary direction than expected. The present study represented a great step towards the understanding of the changes that occur in hydrodynamic and materials transport in macrotidal estuaries located in the Amazon Semiarid interface.