Variação de curta escala temporal do fitoplâncton no setor leste da Baía de Sepetiba e Canal de Guaratiba (RJ)

Abstract

Estuarine systems are characterized by transient hydrographical and biological gradients, influenced by both short-term and mesoscale phenomena. Tides generally induce short-term variations, causing both vertical and horizontal water column mixing. Mesoscale processes, such as the intrusion of shelf water masses due to coastal upwelling, may promote phytoplankton development in coastal systems. The study sites, the Eastern Sector of Sepetiba Bay and Barra de Guaratiba Channel, stand for growing eutrophication and lack of studies on phytoplankton community. This study aims to investigate phytoplankton structure and dynamics focusing potentially harmful species in the Eastern Sector of Sepetiba Bay and Barra de Guaratiba channel. The methodological approach assesses phytoplankton community variation considering oceanographic variables in two complete tidal cycles. Two campaigns (summer and winter) were performed, considering mesoscale processes, such as the presence of the South Atlantic Central Water (SACW). Sampling was carried out in two moorings simultaneously, one in the Eastern Sector of Sepetiba Bay (BS) and another in Barra de Guaratiba Channel (BGC). The occurrence of SACW was already reported in literature from the main entrance of Sepetiba Bay to Guaíba Island, however its presence in CBG was recently reported in the studies coming from the research projects in which this dissertation is included. The SACW lead to changes in the phytoplankton community structure, increasing diatom contribution in the CBG associated with the intrusion of this water mass, while dinoflagellates and flagellates were associated with the Coastal Water (CW). Considering the Intermediate Disturbance Hypothesis (IDH), episodes of higher disturbances occurred in CBG with SACW intrusion and tidal currents reversals during summer campaign; and with rainfall events and higher current speeds in the winter sampling. Intermediate disturbance situations with greater species richness and diversity occurred in both BS and BGC, associated with temperature variations and suspended particulate matter (SPM) supply, influenced by tidal channels. The potentially harmful species Prorocentrum lima and diatoms of the complex Pseudonitzschia delicatissima were observed in low densities in CBG, during tidal currents inversions, while the dinoflagellate D. acuminata occurred in BS during both campaigns. The occurrence of potentially harmful algae in the study area, related to CW influenced by the tidal channels, is note worth and deserves a monitoring program for this area of fishing, mariculture and recreation activities.