Caracterização oceanográfica de um canal de maré e da zona costeira adjacente: canal de Barra de Guaratiba, Baía de Sepetiba, RJ

Abstract

On the eastern boundary of Sepetiba bay lies a tidal channel system that debouches into the ocean through a 4m-deep channel located in Barra de Guaratiba, between Marambaia barrier island and the continent. With the main goal of contributing new information about the oceanographic characteristics of this poorly studied area, sampling campaigns consisting of anchor stations and transects were carried out between October/2013 and February/2014. Two anchor stations placed at the extremities of the tidal channel system, at its inner limit and at the Barra de Guaratiba mouth, were simultaneously occupied for 50 and 25 hours, respectively, in October/2013 and February/2014. Measurements were conducted under springtide conditions and included water level, currents, water properties (temperature, conductivity, transmissivity, fluorescence and dissolved oxygen), suspended particulate concentration (SPM) and surface sediment. Additionally, three transects roughly extending 800m off the Barra de Guaratiba tidal channel mouth were sampled in December/2013. The results have shown that the Barra de Guaratiba tidal channel is highly dynamic with current speeds reaching 1.8 m s-1 in October/2013, when the passage of a cold front induced strong SW winds and increased rainfall. During this same campaign a series of vortices were seen entering the channel, carrying high (9 g L-¹) concentrations of suspended sand. In February/2014, under fairweather conditions, currents reached 1.4 m s-1. Both, in October/2013 and February/2014, the strongest currents were measured during the time of current direction inversion, roughly 1:45 hr before each high water, when water flows towards the ocean while water level continuously rises. This counterintuitive behavior is possibly related to pressure gradients developed inside Sepetiba bay due to the large volume of water that enters the bay through its main connection with the sea at its western boundary, forcing the water flow through the anastomosed tidal channels that discharge through Barra de Guaratiba. Temporal variation of SPM concentration followed the current speed pattern, reaching the highest values during ebb tides, when flows are more intense, suggesting local ressuspension and advection of turbid waters from the bay. Barra de Guaratiba tidal channel also influences the nearby coastal ocean as attested by the variability of the water properties measured in December/2013, particularly along the transect that follows its main orientation. Moreover, surface sediment grain size distribution also indicates its influence through the increased occurrence of medium sands, similar to those found inside the channel, only approximately 500m offshore the channel s mouth. Winds play an important role in the channel s dynamics, intensifying the ebb/flood currents when aligned with their orientation. South Atlantic Central Water (SACW) was found in the sampling campaigns inside Guaratiba channel (February/2014) and in the coastal area (December/2013), when it was detected close to the bottom of the outermost stations