Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO₂ emissions from a major Chinese reservoir

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO₂) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO₂ efflux (FCO₂) of 37.2 ± 29.0 gC m⁻² yr⁻¹, corresponding to 0.02 ± 0.02 TgC yr⁻¹ from this lake. FCO₂ increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ²H-H₂O, while FCO₂ increased with increasing specific UV absorbance (SUVA₂₅₄) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA₂₅₄ and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO₂ (pCO₂) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ¹³C-CO₂ declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO₂ emissions from this lake and potentially other aquatic ecosystems.