Methane and Nitrous Oxide Emissions From an Anthropogenically Transformed Lake (Lake Licheńskie, Poland).

For over 50 years Lake Licheńskie (LLi), central Poland, has been involved in the cooling system of two power plants (PP). Owing to the discharge of cooling waters the lake was prone to considerable environmental changes which involved its thermal structure, mixing regime and water parameters. In this study we investigated how the man‐made transformations affected greenhouse gas emissions (GHG; CH4 and N2O) from the lake. The GHG emissions in Lake Licheńskie were monitored at the deepest site from December 2014 to November 2015 and between March 2022 and February 2023. The values obtained were compared to reference lakes encompassing 10 natural and undisturbed inland and coastal lakes in Poland. Our results revealed that LLi was a net source of CH4 and N2O to the atmosphere but the fluxes were low. The mean annual diffusive fluxes were 0.21–0.38 mmol·m−2 d−1 for CH4 and 4.90–7.40 μmol·m−2 d−1 for N2O. The CH4 emissions were significantly lower than in most of reference lakes, while the N2O emissions were comparable. Therefore, the human intervention resulted in reduction of CH4 release from LLi but it had minor effect on the N2O. The most likely reason for the low direct fluxes of GHGs from the surface waters was the high flushing rate of the lake and export of dissolved gases to adjacent lakes and canals. Hence, the overall emission from the connected lake and canal system was not mitigated by the man‐made changes to the lake system. Plain Language Summary: CH4, and N2O are potent greenhouse gases (GHGs). Their emission from lakes is often stimulated by human intervention in the ecosystem. Lake Licheńskie, central Poland, represents a highly disturbed lacustrine system, which, together with a few other lakes, is a part of a cooling system of two electric power plants. Consequently, since 1960s the lake has received considerable discharge of cooling waters which changed lake water chemistry and hydrodynamics. In this study we tried to find out how these multifaceted changes affected greenhouse gas production and emission in the lake. For this purpose we collected water chemistry data and measured diffusive emissions of the GHGs from Lake Licheńskie and a few natural undisturbed lakes in Poland. The latter lakes served as reference lakes. Our data revealed that Lake Licheńskie emitted comparable amounts of N2O albeit less CH4 than natural lakes. This lower emission of CH4 in Lake Licheńskie was a result of the very high flushing rates of the lake by circulating cooling waters. The GHG emissions were most likely released further downwards in the water receiving canals. Key Points: Lake Licheńskie (Poland) has been highly transformed by long‐term discharge of the effluents from of a power plantLake Licheńskie shows lower diffusive emissions of CH4 than natural lakesGreenhouse gas emissions from Lake Lichenskie are more strongly related to a very high flushing rate than to thermal pollution [ABSTRACT FROM AUTHOR]