Dominant Production of Dissolved Inorganic Carbon by Organic Matter Degradation in a Coastal Lagoon: Evidence from Carbon Isotopes

Coastal oceanic settings are biogeochemically active zones and play a dominant role in the global carbon cycle. In this contribution, we have investigated the spatial distribution of dissolved inorganic carbon (DIC) and δ¹³C_DIC along the salinity gradient of a large tropical coastal lagoon (Chilika, India) and major source waters (river, groundwater) to the lagoon for three different (pre-monsoon, monsoon, and post-monsoon) seasons. These data were used to constrain internal cycling and DIC fluxes to the Bay of Bengal. The average [DIC] and δ¹³C_DIC values of the Chilika, although they exhibit significant variation within a season, are found comparable for the pre-monsoon (1.8 ± 0.6 mM; −5 ± 3 ‰), monsoon (1.7 ± 0.4 mM; −4 ± 2 ‰), and post-monsoon (1.9 ± 0.3 mM; −4 ± 3‰) samples. Co-variation between DIC (and δ¹³C_DIC) and salinity during all three seasons deviates from the theoretical mixing line (TML) between river and seawater, indicating nonconservative behavior of DIC in the lagoon. The magnitude of the DIC and δ¹³C_DIC deviations from their corresponding TML points to dominancy of organic matter degradation in causing this nonconservative trend. Additionally, the pre-monsoon samples also show a minor effect of calcite precipitation on the [DIC]. The LOICZ model estimates that the DIC flux from this lagoon to the ocean is higher than that reported for several peninsular Indian rivers, despite their high (378 km³/yr) freshwater influxes to the Bay of Bengal. This disproportionally higher DIC flux is linked to remineralization of organic matter, underscoring its importance in regulating the inorganic carbon cycle of this highly productive coastal system.