Diurnal carbon dynamics in a mangrove-dominated tropical estuary (Sundarbans, India)

Based on a 24h of time-series study, we report the effects of a tidal cycle on carbon biogeochemistry of a mangrove dominated tropical estuary (the Sundarbans) located in the eastern part of India. Salinity, dissolved oxygen, and pH showed clear tidal variability with relatively higher values during high tide than low tide. Dissolved inorganic carbon (DIC) concentrations varied over a narrow range (1.92-2.19 mM) with relatively higher values during low tide; reverse trend, however, was noticed for delta C-13(DIC) with significant variability (- 4.28 to - 2.21 parts per thousand). During low tide, along with estuarine mixing, preliminary evidences for influences of biogeochemical (such as organic carbon mineralization, sulfate reduction, and denitrification) and hydrological processes (porewater exchange) were found on DIC dynamics. The delta C-13(DIC) - DIC relationship suggested respiration of marine plankton to be one of the possible sources for DIC. Dissolved organic carbon showed tidal influence during high tide with a signal of porewater mediated addition during low tide. Both particulate organic carbon and particulate nitrogen concentrations reached the maximum during low tide with stable isotopic compositions showing predominantly marine signature along with the possibility of biogeochemical modifications within the estuary. Marine water contribution together with organic carbon mineralization and possible porewater influx resulted in similar to 214 mu atm higher pCO(2) and 1.13 times higher FCO2 during low tide than high tide. On diurnal basis, the estuary released similar to 1348 mg CO2 per m(2) of surface area to the regional atmosphere.