O efeito das plumas estuarinas de sistemas tropicais eutróficos sobre a comunidade fitoplanctônica, incluindo espécies potencialmente nocivas, na margem continental sudeste brasileira

Abstract

Differences in the availability of nutrients and light associated with different hydrographic regimes select phytoplanktonic organisms with distinct adaptations. In Brazil, while most of the studies relate the Brazil Current (BC), and its mesoscale activity, to shifts in phytoplankton density, only few actually investigate changes in the abundance and structure of the phytoplanktonic community due to continental influence, especially on the southeastern margin. Above all, it´s essential to know the influence of coastal bays undergoing an advanced process of degradation, such as the Guanabara and Sepetiba bays (RJ) on the adjacent shelf in order to understand the main causes of harmful algae blooms (HABs). With the aim to evaluate the spatial distribution of the phytoplankton community in the portion of the Brazilian continental margin between 22ºS and 26ºS (Santos Basin) were performed 29 oceanographic stations in June 2010 (austral winter). The functional classification based on morphology reflected distinct survival strategies. Coccolithophorids and dinoflagellates, stress-tolerant and colonizing species, dominated oligotrophic waters of the outer continental shelf, characteristics of Tropical Water (TW), where total density was of the order of 104 cells.L-1. Nanoplanktonic flagellates (stress-tolerant), diatoms and filamentous cyanobacteria (ruderals) were the major representatives in the internal and middle shelf, where the action of water masses with higher nutrient concentration, such as the Coastal Water (CW) in the shallower portions and the South Atlantic Central Water (SACW) at subsurface, raised total densities to 104 and 105 cells.L-1. Potentially harmful species were identified throughout the entire area with densities of up to 105 cells.L-1. As the diatoms Ceratoneis closterium, Thalassionema nitzschioides and Thalassiosira cf. delicatula, the dinoflagellates Prorocentrum cf. balticum, P. cf. mexicanum, P. micans and Tripos fusus, besides the phototrophic ciliate Mesodinium rubrum, were the ones that responded better to the presence of the plumes of Guanabara and Sepetiba bays, it is suggested that they must be monitored in order to prevent or control negative effects of any blooms on the coast of the State of Rio de Janeiro. Okadaic acid producing species, such as Dinophysis acuminata, also deserve attention once they are registered at the entrance of the Guanabara bay with densities (103 cells.L-1) considered to be sufficient to trigger harmful effects.