Spatio-temporal variation in δ13CDIC of a tropical eutrophic estuary (Cochin estuary, India) and adjacent Arabian Sea.

Carbon isotopic composition of dissolved inorganic carbon (δ 13 C DIC ) in the Cochin estuary, a tropical eutrophic estuary along the southwest coast of India, and the adjacent coastal Arabian Sea was measured to understand spatio-temporal variability in sources and processes controlling inorganic carbon (C) dynamics in this estuarine-coastal system. δ 13 C DIC in the Cochin estuary showed wide variation during three different seasons (premonsoon: − 12.2 to − 3.26‰; monsoon: − 13.6 to − 5.69‰; and postmonsoon: − 6.34 to + 0.79‰). Detailed mixing curve approximation modeling along with relationships of δ 13 C DIC with dissolved oxygen and nutrients suggest dominant role of freshwater mixing and degassing of CO 2 on DIC dynamics during wet seasons (premonsoon and monsoon). Excess CO 2 brought in by rivers and in situ production due to respiration in the Cochin estuary result into one of the highest pCO 2 observed in estuarine systems, leading to its degassing. During postmonsoon, a relatively dry period with high salinity, calcite precipitation was a major process with calcite saturation index > 1 at few locations. Relatively lower average surface values of δ 13 C DIC in the coastal Arabian Sea (premonsoon: + 0.95‰; monsoon: + 0.88‰; and postmonsoon: + 0.66‰) compared to the predicted open ocean value along with mixing curve modeling suggest dominance of respiration/organic matter (OM) degradation over primary productivity. Estuarine influence on coastal DIC dynamics was observed in nearshore region (~ 10 km), whereas evidence of upwelling was found at farther locations. [ABSTRACT FROM AUTHOR]