Rainstorms drive the carbon dioxide emissions during the algae-growing season in a large eutrophic lake

Lakes are sources of atmospheric carbon dioxide (CO₂), contributing to global climate change. Temporal variations in lake CO₂ emissions are pronounced, with algal growth and precipitation identified as important drivers. Eutrophic lakes often act as atmospheric CO₂ sinks during the growing season. However, these lakes can also emit CO₂ during the same period, a paradox that we hypothesize is driven by precipitation. This study tests this hypothesis and examines how rainstorms influence lake CO₂ emissions. To investigate, seven buoys were deployed in eutrophic Lake Taihu and a major inflow river to monitor water quality at 4-h intervals throughout 2021. CO₂ flux (FCO₂) was calculated using integrated methods, including gas diffusion models, the CO2calc program, and machine learning algorithms, based on water quality and meteorological data. Results revealed that only rainstorms (daily rainfall >50 mm) significantly increased FCO₂. Although only three rainstorm events occurred, they accounted for 12.15% of the annual CO₂ emissions. During the growing season, the lake was a net CO₂ source, but without rainstorm-induced emissions, it would have functioned as a CO₂ sink, highlighting the crucial role of rainstorms in shifting lake dynamics. Piecewise structural equation modeling indicated that both abiotic factors (e.g., gas transfer velocity) and biotic factors (e.g., aquatic metabolism) influenced by rainstorms contributed to the elevated FCO₂. These findings suggest that future reductions in lake FCO₂ due to eutrophication, combined with more frequent rainstorms under climate change, could amplify the impact of extreme precipitation on CO₂ emissions.