Isotopic composition as a tool for assessment of origin and dynamic of organic matter in tropical freshwater.

The isotopic composition (13C and 15N) and C:N ratio of sediment and particulate were investigated to assess the origin and dynamic of the organic matter in two river basins located in the Parana State, Brazil. The main sources of organic matter, plants, litter and soil, were analyzed in the study. This study was carried out during the dry and wet seasons. Although organic matter contribution for the particulate material has been mixed, the main contribution in the two seasons for both rivers was autochthonous characterized by delta13C and C:N ratio values ranging from -31.0 to -21.8 per thousand and from 5.1 to 11.4, respectively. The soil with delta13C values ranging from -25.9 to -20.6 per thousand, and C:N ratio from 10.2 to 15.8, was the main carbon source for the river sediment for the two tributaries in the wet and dry seasons. The sediment delta15N values for both the rivers in the wet season showed an 15N enrichment trend in most of studied sites when compared to the dry season values, suggesting major input of nutrients as dissolved organic nitrogen and sewage nitrate. In the dry season, 14N enriched compounds, could have been used by phytoplankton, causing 15N enrichment. The isotope and C:N ratio data clearly showed that the agricultural activity has been influencing the origin and dynamic of organic matter in the two rivers, with potential biochemical consequences for the lower basin of the major Tibagi river.