Tracing terrestrial versus marine sources of dissolved organic carbon in the largest monsoonal Godavari estuary in India using stable carbon isotopes

Abstract

To examine the sources of dissolved organic carbon (DOC) and its modifications within the estuary, time-series sampling was conducted at 10 stations covering the entire salinity range in the Godavari estuary.The salinity of the upper estuary was close to zero and increased to 32 at the lower estuary during the dry period whereas the entire estuary was filled with fresh water during the wet period.The estuary received a high concentration of suspended particulate matter (SPM) during the wet compared to the dry period.The concentration of DOC and DON was 1.5-3 times higher during the wet than dry period with DOC: DON ratios of 5-20.The depleted isotopic composition of DOC (?13CDOC) was observed during the dry period in the upper estuary and it was enriched towards the lower estuary suggesting that terrestrial organic matter (mainly C3 plant) is the source in the upper and marine sources in the lower estuary.Highly enriched ?13CDOC values were noticed during July-August, which was associated with peak discharge, suggesting that soil organic matter may be a potential source.Enhanced phytoplankton biomass was observed during October-November associated with depleted ?13CDOC from that peak discharge period suggesting DOC released by phytoplankton bloom by phytoplankton bloom (Merismopedia sp.) by freshwater algae may be contributed significantly to the DOC pool.Based on the deviation from the simple mixing line between the upper and lower estuary, the removal of DOC seems to be occurring through microbial decomposition and photo-oxidation during the dry period.The modification of DOC within the estuary may be minimal due to the low residence of water (<1 day) during the wet period.As a result, a significant amount of terrestrial carbon is transported to the coastal ocean during the wet period and its impact is reported to be seen up to 15-25 km from the coast in the coastal Bay of Bengal.