Variabilidade temporal da qualidade da água e importância da descarga do canal da Barra de Guaratiba : Baía de Sepetiba, RJ

Abstract

Located on the eastern of the Sepetiba bay, there is a system of tidal channels that debouches into the region of Barra de Guaratiba, between Marambaia barrier island and the continent. In order to evaluate the temporal variability of water quality and importance of the discharge channel of the Barra de Guaratiba as exporter and/or importer of nutrients to the adjacent coastal zone, were realized two fundeios of 25 hours each, both in springtide, in October 2013 (spring) and February 2014 (summer). Measurements were conducted under springtide conditions and were realized on at the mouth of the Bacalhau (local depth of ~ 4 m) where the following variables were analyzed: water temperature, salinity, particulate matter in suspension, inorganic dissolved nutrients (NH4+, Si, PO43-, NO2- e NO3-), organic carbon dissolved and intensity and direction of tidal currents. The results show that the channel of Barra de Guaratiba is a very dynamic environment, with intense currents particularly when there has been a passage of a cold front, strong influence of winds and high rainfall during the october 2013 campaign. In february 2014 campaign, in normal weather conditions, the intensity of currents reached 1,4 m s-1 on PM2. The moments of greater intensity of currents correspond to investments of your sense, observed about of 1:45 before each high water. In the 2013 campaign, temperatures were low and salinity presented the minimum of 0,33. The campaign showed export of MPS (median of 69, 00 mg L-1 at ebb tide), NH4+, NO2- and Si. The medians on ebb tide to the NH4+ (27,59 μmol L-1) and Si (29,06 μmol L-1) were, respectively, five and two times greater than in the flood tide. It has not possible to define the behavior of the PO43-, NO3- and COD in the estuary. In the 2014 campaign, the temperature medians were 15,73° C (ACAS) and 23,98° C (AC) and salinity always exceeding 34,7. The MPS had a behavior exporter with the median in the ebb tide of 65,78 mg L-1. The channel showed importer of NO3- (maximum of 83,94 μmol L-1 - ACAS) and exporter for Si (median of 12,82 μmol L-1), NH4+ (maximum of 5,09 μmol L-1), PO43- (presence of eutrophic conditions in the environment and pollution) and COD, all associated with the AC after the high water. So, in both campaigns, the channel exported most of the components analyzed, showing your importance in fertilization of the coastal environment adjacent to it.