CARACTERIZAÇÃO ELEMENTAR, ISOTÓPICA E BIOGEOQUÍMICA DA MATÉRIA ORGÂNICA PARTICULADA E DISSOLVIDA NO COMPLEXO ESTUARINO DA BAÍA DE SÃO MARCOS, SÃO LUÍS - MA.

Abstract

The estuaries are transitional coastal environments, which act as a natural filter and site for the exchange of water and matter. According to their characteristics, estuaries can import, export or retain organic matter (OM). The OM is a biochemical complex formed by several organic compounds, and can be in the environment both in the form of dissolved organic carbon (DOC) and in particulate organic carbon (POC). Determining the origin of the organic carbon can help in the dynamics of the OM, to trace its origin more precisely, the isotope composition is reconciled with the use of the stable isotopes of carbon (δ13C) and nitrogen (δ15N). In addition, elemental composition, through the atomic ratio C:N. The São Marcos Bay is an estuarine complex - a very well mixed estuary - of great economic and environmental importance for the state of Maranhao. Preserved mangroves, tidal plain and sandy beaches, form it. The objective of this study was to understand the composition of the organic matter in the water column of this environment under constant influence of the macro tide of the semidiurnal type. Three samples were taken in August / 2016 (period with low rainfall and fluvial influence), all in quadrature tide. In total, 3 transects totaling 14 sampling points were used, water sampling was performed in two layers, subsurface (0.5m below the surface) and bottom (0.5m before touching the bottom), for the following determinations: suspension, dissolved organic carbon, particulate organic carbon, stable isotopes of carbon (δ13C) and nitrogen (δ15N), atomic ratio (C/N). Concomitantly, in situ were measured: electrical conductivity, pH, dissolved oxygen (DO) and temperature (°C). At the end of each day of collection, the material was be transported to the Laboratory of Biogeociclos of the Chemical Constituents of Water (LABCICLOS), at UFMA, where the samples were filtered and later analyzed at the Environmental Sciences Laboratory (LCA), at the UENF. The results of temperature and salinity did not present a significant vertical stratification, but horizontal stratification was be observed. The MPS values were higher from the middle to the innermost part of the bay. The organic carbon had higher concentrations in the dissolved form than the particulate, varying between the depths and the sampling points. The concentrations of total nitrogen (NT) were low, be related to the low influence of nitrogen compounds. The isotopic and C/N ratio indicated that mangrove was the most abundant source of DOC in this estuary. This study ended up emphasizing the complexes patterns of distribution and the multiple sources of MO, mainly in this coastal environment.