Sources and Fate of Organic Matter in Suspended and Bottom Sediments of the Mandovi and Zuari Estuaries, Western India

Organic carbon (OC), total nitrogen (TN) and stable carbon and nitrogen isotopic compositions were measured in suspended particulate matter (SPM) and surface sediment along estuaries of Mandovi and Zuari rivers, two small mountainous river systems in western India during wet and dry seasons, to characterize the sources of organic matter (OM) in these systems. Unlike major rivers, SPM concentrations increase seaward with a general trend of decreasing particulate organic carbon (POC) in these rivers, mostly due to the presence of estuarine turbidity maximum (ETM) located downstream of the estuaries. POC and particulate nitrogen (PN) were higher in the Mandovi than in the Zuari estuary. Except during wet season in the Mandovi, POC/PN and δ¹⁵N were altered by biogeochemical processes in both the estuaries and are not indicators of source organic matter. PN/POC and δ¹³C_org indicated the dominance of terrestrial plant-derived OM and terrestrial soil-derived OM respectively, in the Mandovi and Zuari estuaries during wet season. The δ¹³C_org versus salinity plot indicated increasing proportions of marine OM seaward in both estuaries during dry season. OC and TN in the sediments of both estuaries were much lower than in the overlying suspended matter. The mean δ¹³C_org in the sediment and SPM were similar in both the seasons in Mandovi and only during wet season in Zuari estuary. Uniform mean values of δ¹³C_org in the lower estuary and bay of Zuari indicated efficient mixing of sediments during wet season. Sediments with relatively high δ¹³C_org and low δ¹⁵N in the upper estuary of Zuari were related to anthropogenic contamination by sewage effluents during dry season. It is estimated that each river contributed at least ~20% terrestrial organic carbon (TOC) to the coastal system during wet season and received similar quantity of TOC during dry season. Since there are more than 10,000 small rivers originating from monsoon- dominated and/or mountainous regions globally, it must be appreciated that their total TOC contribution to the coastal system must be substantial.