Surface pCO and air-water CO fluxes dominated by submerged aquatic vegetation: Implications for carbon flux in shallow lakes.

Inland lakes are crucial for processing, storing, and releasing carbon dioxide (CO), and they play a significant role in the global carbon cycle and climate change. Studies have shown that inland lakes are mostly supersaturated in CO, making them significant sources to the atmosphere. However, estimating CO fluxes from inland lakes is still challenging due to large variations in surface water CO partial pressure (pCO). Submerged aquatic vegetation (SAV) is widely found in aquatic ecosystems, especially in shallow lakes. However, their role in lake-wide carbonate chemistry has not been thoroughly investigated. Accurately measuring air-water CO exchange and understanding the environmental factors that control these fluxes in vegetated ecosystems are essential for reducing uncertainties in global CO emission estimates. In this study, high-resolution (3-h interval) field measurements were made along the nearshore of eastern Lake Taihu during the SAV growing seasons to examine their effects on surface water pCO and air-water exchange. Our results showed evident daily variations in water chemistry and air-water fluxes. Daytime air-water CO exchange switched from sinks in summer to sources in autumn. The vegetation sites were observed to be strong CO sources consistently at night. The density of aquatic vegetation was found to be positively correlated with the daily range of pCO, highlighting their role in regulating surface water carbonate chemistry. Negative correlations were found between water depth and surface pCO. These results highlight the importance of aquatic vegetation and daily variations in reducing uncertainties in carbon budgets of shallow aquatic systems.