Tidal scale dissolved inorganic and particulate organic carbon dynamics in a tropical estuary

Dissolved inorganic carbon (DIC) and particulate organic carbon (POC) dynamics in the world's estuaries have been studied extensively at monthly, seasonal, and annual time scales with particular focus on their concentrations and export fluxes to the coastal oceans. However, given the dynamic nature of the estuaries, the effect of tidal and diel cycles on the processes modulating DIC and POC dynamics remains obscure. To decipher the biogeochemical processes at tidal scale, DIC and POC concentrations and their carbon and nitrogen isotopic compositions were measured across the salinity gradient at every high and low tide for nine consecutive days (14–23 October 2019) in the Mahanadi estuary, a tropical estuary at the east coast of India. Showing contrasting differences across salinity gradient in DIC, POC and their isotopic compositions, DIC and POC were significantly different during high and low tide in the mixing zone only during spring duration. This showed the effect of spring-neap tidal cycle owing to water level fluctuations and mixing intensity in the estuarine mixing zone. Linear least-squares regression models indicated carbonate and/or silicate weathering by biogenic CO₂ to be the probable DIC source in the freshwater region of the estuary. Deviations of observed DIC concentrations and δ¹³C_DIC from the conservative mixing values suggested pronounced alteration of DIC source signature in the mixing zone. A process-based model approach aimed at delineating possible biogeochemical processes affecting DIC dynamics indicated calcite dissolution during low tide and calcite precipitation during high tide to be dominant processes in the mixing zone. Additionally, signatures of more than one simultaneous biogeochemical process modulating the DIC dynamics were also observed. POC pool in the mixing zone was largely influenced by its removal through rapid remineralization during both high and low tides. Graphical plots also showed that POC in the mixing zone and at the saline location was significantly affected by processes such as degradation, whereas it was only slightly affected in the freshwater region of the estuary. δ¹³C_POC, along with the C/N ratio of POM, indicated that C3 plants and/or their derived soil were the major source of POM in the freshwater, whereas the higher contribution of riverine POM and marine phytoplankton was observed in the mixing zone and saline location, respectively.