Spatial evaluation of fluvial influence inferred from foraminifera distribution and organic geochemistry Bianca A. D. M. Parizotto; Carla Bonetti; Jarbas Bonetti & Ruy A. Wolff Coastal Oceanographic Laboratory – Federal University of Santa Catarina, Brazil email@example.com Baía Norte (27°23’ – 27°35’S; 48°33’– 48°30’W) is a body of shallow marine water located in the southern region of Brazil (Santa Catarina State). Its surface area is 146 km2, and its depth is usually lower than 6 meters. It has a connection to the Atlantic Ocean by means of a channel of 12 m maximum depth. Estuaries, mangroves, small sand beaches, river deltas, tidal flats and rocky shores are the main habitats bordering this embayment. Urban developments are also present and are concentrated mainly in and around the cities of Florianópolis, São José and Biguaçu. Among the economic activities developed in this area, tourism, traditional fishing, and oyster and mussel production play an important role. However, the sustainability of these activities depends on local water quality, which should be a constant concern, since sewage treatment is insufficient, and untreated wastes are discharged directly into the creeks and rivers that reach the bay. Fluvial inputs are probably the most important sources of terrigenous constituents and pollutants to Baía Norte sediments. The major freshwater contributors in the area are the Biguaçu, Ratones and Itacorubi rivers. The first one is located on the mainland side of the bay and has the biggest local catchment (383 km2), while Ratones (91 km2) and Itacorubi (28 km2), both located at an island margin, are less important. In this study, irregularities in Recent foraminifera distribution and sedimentary organic matter composition were used as indicators of the sites that are most susceptive site to the influence of fluvial inputs. Thirty eight surface sediment samples were obtained in a 2 km grid resolution and analyzed in relation to their biological and geochemical characteristics (total organic matter as well as carbon, nitrogen, sulfur, biodetritic carbonate, and grain size distribution). The sediments of the area can be described as predominantly muddy, with carbonate contents between 0.3 and 3.8% and total organic matter between 2.5 and 8.3%. High values of C:N ratio (between 9.3 and 19.4) suggest a significant continental contribution at places. Although sediments are relatively rich in organic compounds, the values of C:S ratio (above 5) suggest organic matter degradation under well oxygenated conditions at the bottom. In general, the values of benthic foraminiferal density and richness are correlated to the depth and negatively to the total organic matter content and increase of clay in the sediments. The suborder Miliolina, as well as the family Nonionidae (mainly Pseudononion atlanticum), present a significant negative correlation with organic matter and clay percentage, increasing towards the marine areas less influenced by inner waters. Samples with low density of foraminifera tests, which also show a higher proportion of living forms in the total population and the dominance of the genus Ammonia, could be related to local influence of fluvial input into the embayment. Although the families Bolivinidae and Buliminellidae (mainly Buliminella elegantissima, Brizalina striatula and Bolivina sp.) are abundant in all the study area, their occurrences are more important at depths greater than 4 meters, suggesting that these taxa are less tolerant than Ammonia to environmental instability induced by mixing processes between fluvial and marine fluxes. The author thanks the CNPq (CT-Hidro Program, process number 142434/2004-3) for financial support.
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