Sources and fate of organic matter in a hypersaline lagoon: A study based on stable isotopes from the Pulicat lagoon, India

Stable carbon (delta C-13) and nitrogen (delta N-15) isotopes of organic matter (OM) in bed sediments and suspended solids are thoroughly investigated in the Pulicat lagoon, India, in pre-South West (SW) monsoon (June 2018) and post-North East (NE) monsoon (March 2019) to understand the response of OM in salt stress conditions. A near absence of an external supply of water and intense evaporation, as suggested by higher hydrogen and oxygen isotope values (delta D and delta O-18) of the lagoon water, led to hypersaline conditions in the lagoon. Despite a long period of osmotic stress, a high OM concentration in suspended solids in post-NE monsoon suggests that autochthonous production is unaffected by salt stress conditions. Locally at different sites, the difference in delta C-13 (-4.9% to +1.4%) and delta N-15 (-4.1% to +1.6%) values of OM between suspended solids and bed sediments are higher in pre-SW monsoon compared to post-NE monsoon. The negative isotopic difference is caused by benthic respiration of OM and cation exchange with clay bound ammonium in bed sediments, whereas the positive difference is the result of cellulose decomposition in areas dominated by seagrasses. However, in post-NE monsoon, wind-induced re-suspension of bed sediments reduce the differences in delta C-13(-2.3% to -0.1%) and delta N-15 (-2.1% to +3%) values. The source apportionments of delta N-15 values suggest inputs from sewage and fertilizers. Additionally, seagrass-detritus dislodged by fishing activities favors primary production. Overall, we suggest that the impact of the hypersaline conditions on in-situ productivity can be suppressed if wind activity and nutrient re-cycling are dominant. The present study is unique as it addresses the processes that operate in a hypersaline lagoon during the short-term failure of monsoon. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study revealed that despite the high salinity conditions in the lagoon, autochthonous production remains stable, with nitrogen isotope values mainly coming from sewage and fertilizer inputs. Additionally, the impact of wind activity and nutrient re-cycling on productivity cannot be underestimated.