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

Abstract

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 δ13CDIC with significant variability (– 4.28 to – 2.21‰). 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 δ13CDIC - 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 ~214 μatm higher pCO2 and 1.13 times higher FCO2 during low tide than high tide. On diurnal basis, the estuary released ~1348 mg CO2 per m2 of surface area to the regional atmosphere.