C:N:P ratios and stable carbon isotope compositions as indicators of organic matter sources in a riverine wetland system (Moji-guaçu River, São Paulo-Brazil)

Abstract

Particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP) concentrations, and stable carbon isotope compositions (δ13C) are reported for Moji-Guacu River (suspended matter) and Infernao Lake (suspended matter and bottom sediment) samples collected during the entire 1990/91 hydrologic year. POC and PN concentrations for both systems and riverine PP concentrations peaked during the wet season, but only riverine PP concentrations peaked together with the river discharge, thus characterizing the regional watershed (main river) as the principal source of this phosphorus. Lacustrine POC, PN, and PP displayed maximum concentrations before discharge peak, suggesting that local sources (lake basin, wetlands, and internal production) are more important in the lake case. A less pronounced increase in riverine PN than in POC or PP increments also was documented, possibly due to denitrification occurring in the river-wetland system. The principal source of organic matter in the Moji-Guacu River is watershed inputs. The origin of suspended matter in the oxbow lake is mainly phytoplankton, while bottom sediments are composed by a more refractory fraction of organic matter derived from local runoff, lake macrophytes, and riverine POM, with some contribution of lake POM subjected to alteration by early diagenetic processes.