Sources, distribution and preservation of organic matter in a tropical estuary (Godavari, India)

Abstract

Major sources and distribution of Organic Matter (OM) were examined in a tropical monsoonal estuary, the Godavari, using content and isotopic signatures in suspended particulate matter and surface sediments during no-discharge period. The lower (<10 km from the mouth) and upper (>10 km) estuaries have shown remarkably different features with reference to the sources, distribution and preservation of OM. The upper estuary is characterized by relatively high content of Particulate Organic Carbon (POC) and chlorophyll-a (Chl-a) and low suspended matter and sediment OC. In contrast, the lower estuary is characterized by relatively low content of particulate OC and Chl-a and high suspended matter and sediment OC. Our data and model results showed that suspended particulate OM is derived predominantly from freshwater phytoplankton in the upper (∼85%) and lower (∼60%) estuaries, with a minor contribution from estuarine phytoplankton (∼25%) and terrestrial C₃ plants (∼10%) in the lower estuary, during the study period. On the other hand, sediment OM in the upper and lower estuaries derived from both autochthonous and allochthonous sources with nearly equal contributions. Although relatively high in situ production was observed in the upper than the lower estuary, relatively low sediment OC was found in the former than the latter region, suggesting that strong influence of preservation conditions rather than surface biological production on OC accumulation in sediments. This is attributed to intense transformation of OM by heterotrophs in the upper than the lower estuary because of prevailing conducive conditions for degradation of OM in the upper estuary during the study period.