Changes in Water Chemistry Associated with Rainstorm Events Increase Carbon Emissions from the Inflowing River Mouth of a Major Drinking Water Reservoir

Large reservoirs are hotspots for carbon emissions, and the continued input and decomposition of terrestrial dissolved organic matter (DOM) from upstream catchments is an important source of carbon emissions. Rainstorm events can cause a surge in DOM input; however, periodic sampling often fails to fully capture the impact of these discrete rainstorm events on carbon emissions. We conducted a set of frequent observations prior to and following a rainstorm event in a major reservoir Lake Qiandao (China; 580 km²) from June to July 2021 to investigate how rainstorms alter water chemistry and CO₂ and CH₄ emissions. We found that the mean CO₂ efflux (FCO₂) (13.2 ± 9.3 mmol m^-2 d^-1) and CH₄ efflux (FCH₄) (0.12 ± 0.02 mmol m^-2 d^-1) in the postrainstorm campaign were significantly higher than those in the prerainstorm campaign (-3.8 ± 3.0 and +0.06 ± 0.02 mmol m^-2 d^-1, respectively). FCO₂ and FCH₄ increased with increasing nitrogen and phosphorus levels, elevated DOM absorption (a₃₅₀), specific UV absorbance SUVA₂₅₄, and terrestrial humic-like fluorescence. Furthermore, FCO₂ and FCH₄ decreased with increasing chlorophyll-a (Chl-a), dissolved oxygen (DO), and pH. A five-day laboratory anoxic bioincubation experiment further revealed a depletion of terrestrial-DOM concurrent with increased CO₂ and CH₄ production. We conclude that rainstorms boost the emission of CO₂ and CH₄ fueled by the surge and decomposition of fresh terrestrially derived biolabile DOM in this and likely many other reservoir's major inflowing river mouths.