Fluvial connectivity drives carbon cycle dynamics in a tropical mega‐delta

Abstract

Coastal estuaries are hotspots of biogeochemical cycling, biodiversity, and sediment processing, yet the drivers of carbon cycle processes remain poorly constrained. Here, we elucidate the influence of hydrological connectivity on carbon biogeochemistry in the Indian Sundarban over successive monsoon seasons by comparing hydrologically connected channels with perennial freshwater flow to channels isolated from feeding rivers. Results demonstrate dissolved organic carbon (DOC) and particulate organic carbon (POC) varied significantly with both season and connectivity. Dissolved organic carbon peaked pre‐monsoon and POC during the monsoon, with higher concentrations in hydrologically connected sites. Dissolved inorganic carbon (DIC) declined during the monsoon season but showed no connectivity effect. Elevated DOC relative to conservative mixing was attributed to freshwater runoff or groundwater input. Isotope data (δ 13 C) indicated POC respiration dominated during pre‐ and post‐monsoon, while DOC flocculation during the monsoon controlled POC dynamics, particularly in connected sites. Carbonate dissolution primarily regulated pre‐monsoon DIC, while organic matter degradation dominated in the monsoon and post‐monsoon periods. CO 2 efflux, measured across all sites, was consistently a source to the atmosphere and two to four times higher in connected channels, with higher turbulence driving maximum fluxes. Our findings demonstrate that hydrological connectivity fundamentally structures estuarine carbon cycling, lowering organic carbon concentrations and enhancing CO 2 fluxes. Shifts in global coastal delta sediment dynamics, in association with anthropogenic river management, therefore, have the potential to significantly alter delta carbon dynamics on a global scale.

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