ORGANIC CARBON TRANSPORT IN A LARGE TROPICAL RIVER, THE GODAVARI (INDIA): LABILE VERSUS REFRACTORY FRACTIONS AND IMPLICATION OF CARBON INPUTS TO THE BAY OF BENGAL

This study gives an insight into the source of organic carbon and nitrogen in the Godavari river and its tributaries, the yield of organic carbon from the catchment, seasonal variability in their concentration and the ultimate flux of organic and inorganic carbon into the Bay of Bengal.

Particulate organic carbon / particulate organic nitrogen (POC/PON or C/N) ratios revealed that the dominant source of organic matter in the peak season is from the soil (C/N=8-14), while in the rest of the seasons, the river-derived (in-situ) phytoplankton is the major source (C/N=1-8).

Amount of organic materials carried from the lower catchment and flood plains to the oceans during the peak season are 3 to 91 times higher than in the moderate and lean seasons. Large-scale erosion and deforestation in the catchment has led to higher net yield of organic carbon in the Godavari catchment when compared to other major world rivers.

The total flux of POC, and dissolved inorganic carbon (DIC) from the Godavari river to the Bay of Bengal is estimated as 756 x 10⁹ g yr^-1 and 2520 x 10⁹ g yr^-1 respectively. About 22% of POC is lost in the main channel because of oxidation of labile organic matter, entrapment of organic material behind dams/ sedimentation along flood plains and river channel; the DIC fluxes are conservative through out the river channel.

Finally, the C/N ratios (~12) of the ultimate fluxes of particulate organic carbon suggest the dominance of refractory / stable soil organic matter that could eventually get buried in the coastal sediments on a geological time scale.