Freshwater influence and water masses interaction on the foraminifera along the SW Atlantic continental shelf

The focus of this study is the influence of freshwater runoff from Plata River and Patos Lagoon on the distribution of benthic foraminifera along the SW Atlantic Continental Shelf. Surface sediments were collected with a bottom snapper along the 50, 100 and 150 m isobaths from 27 degree to 38 degree S. Analyses of 26 samples from winter and 21 from summer demonstrate that despite the seasonal migration of the Subtropical Shelf Front, a strong influence of SSF on foraminiferal associations persists between 33 degree and 34.5 degree S. Salinity, temperature, and dissolved oxygen show significant seasonal variations, but the geographic distribution of foraminiferal species is related strongly to freshwater runoff, especially that from the Plata River. The species diversity is lower at stations closer to the Plata mouth and higher at those from the Patos lagoon and farther north. Regional relative abundances of dominant species follow clear latitudinal gradients, and three main foraminiferal associations indicate three different water masses. Association A, with >30% Buccella peruviana and Bulimina marginata, indicates the presence of cold Sub Antarctic Shelf Water (SASW) in winter, from 33.5 to 38.3 degree S, but mainly in the southern sector. In summer, however, at stations deeper than 100 m, the abundance of this association decreases to 55 m depths in the northern part of the area (27-33 degree S) in both seasons, and is linked with Brazil Current waters. In four areas of the shelf, continental freshwater influence is detected by the presence of Association C (Ammonia beccarii, Bolivina striatula, and Buliminella elegantissima). The strongest signal, between 34 and 35.5 degree S, is from the Plata River in winter. The second freshwater signal, from the Patos Lagoon, is observed between 32.9 and 33.2 degree S in winter but farther south in summer (33.5-34.5 degree S). The third freshwater signal is observed between 29 degree and 29.8 degree S, and marks the influence of the Laguna estuarine system. The northern most fresh water influence is from the Itajai-Acu River, recorded between 27.3 and 27.5 degree S in winter, but extending southward (to 28.8 degree S) in summer.