Reduction in Riverine Freshwater Supply Changes Inorganic and Organic Carbon Dynamics and Air-Water CO2 Fluxes in a Tropical Mangrove Dominated Estuary

Reduction in riverine freshwater supply due to climate change as well as anthropogenic activities are documented throughout the globe. How river discontinuity in upstream reaches and the subsequent reduction in freshwater influx alter inorganic and organic carbon dynamics in downstream estuaries adjacent to mangroves has been rarely reported. We investigated the dynamics of the inorganic carbon system and organic matter (OM) in two Indian estuaries near mangroves; riverine freshwater supply to the Matla Estuary was reduced and that to the Dhamra Estuary was uninterrupted. Seasonal sampling was conducted over an annual cycle. We used elemental and stable isotope signatures to delineate the sources of OM and dissolved inorganic carbon (DIC). We found that compared to the Dhamra, the reduced riverine freshwater supply to the Matla increased the marine influence in the estuary on the OM degradation pathways and decreased CO2 emissions to the atmosphere. In the Dhamra, higher seasonal variability in biogeochemical pathways, facilitated high internal carbonate buffering capacity; in contrast in the Matla, the greater marine influence increased the carbonate buffering capacity, resulting in retention of higher DIC concentrations and low CO2 emissions. Dissolved and particulate organic carbon concentrations were higher in the Dhamra than the Matla, indicating higher riverine supply of these. The present study can contribute an overlooked effect of long-term changes in riverine freshwater supply on the carbon dynamics of mangrove-dominated estuaries, which might help to improve the understanding of coastal carbon budgets in a changing world.