Evolution of CO₂ flux over 60 years: Identifying source and sink changes caused by eutrophication of Hulun Lake

Understanding the carbon cycling process and assessing the carbon sequestration potential in freshwater lakes relies heavily on their source-sink relationship. However, human activity and climate change have obscured the clarity of this relationship and its driving mechanisms, particularly in northern grassland lakes. This study focused on Hulun Lake, the largest grassland lake in northern China, to quantitatively analyze the carbon dioxide exchange flux (FCO2) at the water-air interface from 1963 to 2023. The analysis revealed significant seasonal, interannual, and decadal variations in the FCO2. Over the past 60 years, FCO2 varying significant in seasons and years has notably decreased, averaging 0.324 ± 0.106 gC·m^-2·d^-1. Notably, there was a qualitative change in FCO2 from "sink" (0.161 ± 0.109 gC·m^-2·d^-1) to "source" (-0.130 ± 0.087 gC·m^-2·d^-1)between 2019 and 2020. From 1963 to 2019, the lake acted as a CO₂ source, releasing an average flux of 0.438 ± 0.111 gC·m^-2·d^-1. During this period, FCO2 was the highest in spring, followed by summer, and the lowest in autumn and winter when the lake was covered by ice. In 2020, the lake transitioned into a CO₂ sink with an average FCO2 of -0.248 ± 0.042 gCm^-2·d^-1 from 2020 to 2023. During this period, FCO2 peaked in autumn, followed by summer and spring, and was lowest in winter when the lake was ice covered. A structural model equation (SEM) was employed to analyze the effects of various factors, including physical, chemical, and biological aspects, on FCO2 and the source-sink pattern of Hulun Lake. This study suggested that lake eutrophication, compounded by global warming, may be the primary driving force behind these changes. Rising temperatures and eutrophication enhanced the primary productivity of the lake. The amount of CO₂ fixed through photosynthesis surpassed that emitted by respiration. Consequently, the eutrophication may alter the CO₂ exchange pattern in Hulun Lake, shifting it from a "source" to a "sink".