Increase in productivity enhances CH4 but limits N2O production in a shallow tropical lake experiencing seasonal volume reduction and salinization.

Lakes are hotspots for carbon and nitrogen cycling and a major contributor to the natural fluxes of greenhouse gases [GHGs: methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O)] to the atmosphere. Lake volume reduction and salinization are major threats to lakes across the globe, and their effect on lake biogeochemistry warrants detailed investigation. Given the majority of the studies addressing this crisis are from the higher latitudes, the response of tropical lakes, and shallow lakes in particular, to changing magnitude of environmental stressors is a major research gap. The present study investigates the effect of lake volume reduction and increasing salinity on the productivity of a shallow tropical lake and ultimately on the GHGs dynamics. Stable oxygen isotopic composition of lake water indicated that evaporative water loss was the major cause for lake volume reduction, which eventually led to increasing ion concentration and salinity. The dissolved inorganic carbon and nitrogen pools also increased as the lake volume reduced, leading to increased primary productivity when the lake was shallow and salty. Consequently, the availability of organic matter enhanced CH4 and CO2 but limited N2O production. However, the strong positive fluxes of CH4 and CO2 masked the weak N2O sink during the productive period and resulted in an overall net source of GHGs (CO2 equivalent). This study bridges lake hydrology with its biogeochemistry and highlights the significance of shallow tropical lakes as conduits of GHGs in a future warming scenario.