Spatio-temporal variability in the sources of particulate organic carbon and nitrogen in a tropical Godavari estuary

In order to evaluate the impact of seasonal river discharge on the spatio-temporal variability in the major sources of particulate organic matter (POM) in the tropical Godavari estuary, India, a comprehensive study was conducted for one year from head to mouth of the estuary. Higher concentrations of both particulate organic carbon (POC) and nitrogen (PN) were observed during the peak discharge period followed by moderate and lean discharge periods associated with magnitude of discharge. POC:Ch1-a ratio and isotopic composition of particulate organic carbon (delta C-13) and nitrogen (delta N-15) suggest that predominant contribution of OM is from in-situ production during the moderate discharge period associating with higher phytoplankton biomass. Contribution from in-situ sources dominates over terrestrial sources during the lean discharge period also. However, both terrestrial and in-situ sources contributed during the peak discharge period. Discharge mobilised significant amounts of terrestrial OM such as the debris of land plants, agricultural crops, forests and soils in the catchment during peak and moderate discharge periods. Strong stratification during the moderate discharge period enhanced phytoplankton production, mainly by freshwater algae. Dual isotope mixing model analysis revealed that freshwater phytoplankton contributed significantly to the POM pool (20-90%) during the moderate discharge period whereas contribution from terrestrial C-3 plants (20-40%) dominated during the peak discharge period. Our study demonstrates that river discharge modifies content and source of OM to the estuary that may have significant impact of heterotrophic activity.