Sources of organic matter in Chilika lagoon, India inferred from stable C and N isotopic compositions of particulates and sediments

Abstract

Abstract Stable isotopic compositions (δ13C and δ15N) and C/N ratios of suspended particulate organic matter (POM) and surface sediment organic matter (SOM) in the Asia's largest lagoon (Chilika, India) were analyzed to identify spatial and seasonal variabilities in sources of organic matter. The variability of POM composition (δ13C: −23.5‰ to −27.9‰, δ15N: 2.1‰ to 7.5‰, C/N: 9.5 ± 0.9) collected during monsoon, a period of highest river discharge, suggests dominant input of terrestrial organic matter, whereas wintertime POM (δ13C: −22.3‰ to −27.7‰, δ15N: −0.2‰ to 4.8‰, C/N: 9.1 ± 1.0) exhibits a mixed source of agricultural runoff, and lagoon phytoplankton and cyanobacteria. The composition of POM collected during summer/dry season (δ13C: −21.2‰ to −26.2‰, δ15N: 2.0‰ to 6.0‰, C/N: 8.9 ± 1.1) indicates enhanced lagoon phytoplankton and bacterial productivity. Spatial variability of POM isotopic composition clearly shows more contribution of terrestrial sources in the northern sector- influenced by perennial Mahanadi River distributaries- compared to the central and southern sectors. The isotopic compositions (δ13C: −20.9‰ to −22.9‰, δ15N: 1.9‰ to 6.6‰) and C/N ratios (10.1 ± 1.3) of SOM indicate major contribution from terrestrial and macrophyte sources and minor contribution from phytoplankton and/or cyanobacteria. The northern sector is also heavily influenced by urban/industrial wastewater input, whereas the outer channel with higher salinity remains dominated by marine organic matter. Western part of the central sector is significantly influenced by untreated domestic sewage discharged from nearby townships and villages, which endangers the lagoon ecosystem.