Caracterização das condições oceanográficas em estuários de macromaré: complexo estuarino Arraial-São José (MA)

Abstract

This study evaluated how the minimization of river flows influences the hydrodynamic circulation and transport of materials and constituents in the Arraial-Sao Jose Estuarine Complex (ASJEC), seasonally. Twelve field campaigns were initially carried out in the estuaries of Perizes and Sampaio rivers, during the rainy season, transition season (rainy/dry), and dry season in the years of 2014 and 2015, under neap and spring tide conditions. After that, data were acquired along the ASJEC, during the rainy and dry seasons of 2017, under spring tide conditions, where two Eulerian profiles were made in complete cycles of tide (13 hours). The results for Perizes and Sampaio rivers estuaries suggest that through the thermal and saline horizontal gradients, the year of 2014 presented warmer and less saline waters, whereas for the year of 2015, we observed cooler and saline waters. This fact, when linked to volume transport and Suspended Particulate Matter (SPM) flows, suggests a minimization of river flows for the year of 2015, making it easy the propagation of the tidal wave toward the inner estuary. The results of the percentages of fresh water were more significant during the rainy season, except for the year of 2015, which residence times were less than one hour, presenting dry period. High concentrations of SPM associated to lower volume transports suggest a hydraulic barrier due to the change/inversion of the baroclinic pressure gradient force, resulting in net and material retention, mainly in neap tide. Through the thermal and saline structures observed in ASJEC, high temperatures and typically estuarine salinities in the rainy season, and cooler and saline water in the dry season, we suggest a Mixing Zone/Coastal Zone (MZ/CZ) interface near the anchor station  (located at the coastal zone) and a Tidal River Zone /Mixing Zone (TRZ/MZ) interface near the anchor station  (located in an inner region, within the complex). These thermohaline structures were almost vertically homogeneous, indicating conditions of vertical instability. The currents intensities were more intense in flood tide, with residual velocities close to zero, showing the importance of the marine component in the region, in which estuarine system in the rainy season was classified as type 2a at the MZ/CZ interface and 1a at the TRZ/MZ interface, whereas in the dry season, it was classified as type 1a in both interfaces. The salt transport was dominated by the fluvial discharge in most cases, which total salt transport was toward the inner estuary, suggesting a salt accumulation in the ASJCE, most likely associated with the entrance of marine waters. Coastal water intrusion intensifies the baroclinic component of the pressure gradient force, generating the hydraulic barrier, and may indicate that MPS flows from the continental drainage to the estuary and consequently to the adjacent platform is interrupted and the residence time increased. The continuation of this study may generate, in a medium term, a set of environmental information that will help in the decision-making of water resources managers taking into account the changes in land use in the drainage basin or a reality of climate change (sea level rise), mitigating its potential impacts.