Distributions and Sources of Bulk Organic Matter in Sediments of Five Estuaries in Todos os Santos Bay, Brazil.

Total organic carbon (TOC), total nitrogen (TN) and their δ¹³C and δ¹⁵N values were determined in cores and surficial sediments to evaluate organic matter contents and sources. Samples were collected along the salinity gradient of five estuaries (Paraguaçu, Subaé, Jaguaribe, São Paulo and Mataripe) of the Todos os Santos Bay (BTS), the second largest bay in Brazil. TOC values ranged from 0.7 to 2.7% and TOC/TN ranged between 1.8 and 8.3. Overall, a slight seaward decrease in %TOC as well as C/N is observed. Values of TOC/TN lower than the Redfield ratios were found in most samples from São Paulo and Mataripe estuaries. This is possibly the result of the anthropogenic impacts caused by oil related activities developed in the region. However, natural causes, such as preferential sorption of inorganic N or bacterially derived organic matters in the sediment, should not be dismissed. Indeed, the influence of inorganic N was also indicated by the low correlation found between TOC and TN values in some estuaries and the positive intercept observed in others, the suggesting that they come from different sources. In general, δ¹³C values ranged from -26.9 to -20.3‰ and enrichment was observed seaward. Consequently, δ¹³C has a negatively correlation with the TOC /TN values, except for Subaé River. In all estuaries the δ¹³Cvalues suggest a mixed marine/terrestrial source of the organic matter with an increase of marine influence towards the river mouth.