Sources and transformations of organic matter in sediments of Asia's largest brackish water lagoon (Chilika, India) and nearby mangrove ecosystem

The present study aimed to understand sources and transformations of organic matter along with cycling of nitrogen and carbon in sediments of two geographically close but ecologically distinct wetlands located on the east coast of India viz. Chilika lagoon (Asia's largest brackish water lagoon) and Bhitarkanika mangrove. The study also investigates potential nitrogen loss pathways in the bottom sediments and explores stables isotopes as a proxy for the source identification of sediment organic matter in shallow aquatic ecosystems. For this purpose, the isotopic compositions of organic carbon and nitrogen (δ¹³C_org and δ¹⁵N) and its contents (% C_org and % N) were measured at different depths in sediment cores collected from the Chilika lagoon (eight cores) and Bhitarkanika mangrove forest (three cores). Overall, the mean δ¹³C_org and % C_org in the lagoon were − 21.10 ± 0.79‰ and 0.84 ± 0.47%, respectively; whereas the same for mangrove cores were − 24.56 ± 0.80‰ and 1.04 ± 0.26%, respectively. Similarly, average δ¹⁵N and % N in the lagoon cores were 4.15 ± 0.63‰ and 0.11 ± 0.05%, respectively; for mangrove cores, the values were 4.28 ± 0.50‰ and 0.07 ± 0.01%, respectively. Isotopic composition and elemental ratios indicated organic matter in the sediments of Bhitarkanika mangrove to be a mixture of terrigenous and marine origin with relative dominance of terrestrial influence. A significant increase in δ¹³C_org of sediment organic matter compared to suspended particulate organic matter in the Chilika indicated transformation of organic matter in the water or sediment column through mineralization and diagenetic alterations. The δ¹⁵N of sediment or particulate organic matter did not show clear evidence of nitrogen loss in the recent past in these two ecosystems through processes such as denitrification. The absence of a relationship between δ¹³C_org of particulate and sediment organic matter in the Chilika indicated lack of efficient exchange between suspended and sediment organic matter.