Fluvial Connectivity Impacts Carbon Biogeochemistry in a Tropical Mangrove Delta, Sundarban, India.

Coastal estuaries are hotspots of biogeochemical cycling, biodiversity, and sediment cycling, yet the drivers of carbon cycle processes remain poorly constrained. This study elucidates how hydrological connectivity influences carbon biogeochemistry in the Indian Sundarban over two monsoonal cycles spanning pre-monsoon, monsoon, and post-monsoon seasons. A spatially extensive sampling strategy compared channels connected to perennial freshwater flow with channels isolated from feeding rivers. Linear mixed-effects modelling showed dissolved organic carbon (DOC) and particulate organic carbon (POC) varied significantly with both season and connectivity. DOC peaked pre-monsoon and POC during the monsoon, with higher concentrations in connected sites. Dissolved inorganic carbon (DIC) declined during the monsoon but showed no connectivity effect. Elevated DOC relative to conservative mixing was attributed to freshwater runoff or groundwater input. Isotope data indicated POC respiration dominated during pre- and post-monsoon, while DOC flocculation-controlled monsoon POC dynamics, particularly in connected sites. Carbonate dissolution regulated pre-monsoon DIC in general, while organic matter degradation dominated in the monsoon and post-monsoon periods. CO₂ efflux, measured across all sites (1.7–297.6 mmol C m⁻² d⁻¹), was consistently a source to the atmosphere and 2–4 times higher in connected channels, with higher turbulence driving maximum fluxes in upper reaches. Our findings demonstrate that hydrological connectivity fundamentally structures estuarine carbon cycling, lowering organic carbon concentrations and enhancing CO₂ fluxes. Thus, shifts in global coastal delta sediment dynamics and subsequent riverine impacts, may significantly change global deltaic carbon cycle processes.