Frequent algal blooms dramatically increase methane while decrease carbon dioxide in a shallow lake bay

Freshwater ecosystems play a key role in global greenhouse gas estimations and carbon budgets, and algal blooms are widespread owing to intensified anthropological activities. However, little is known about greenhouse gas dynamics in freshwater experiencing frequent algal blooms. Therefore, to explore the spatial and temporal variations in methane (CH4) and carbon dioxide (CO2), seasonal field investigations were performed in the Northwest Bay of Lake Chaohu (China), where there are frequent algal blooms. From the highest site in the nearshore to the pelagic zones, the CH4 concentration in water decreased by at least 80%, and this dynamic was most obvious in warm seasons when algal blooms occurred. CH4 was 2-3 orders of magnitude higher than the saturated concentration, with the highest in spring, which makes this bay a constant source of CH4. However, unlike CH4, CO2 did not change substantially, and river mouths acted as hotspots for CO2 in most situations. The highest CO2 concentration appeared in winter and was saturated, whereas at other times, CO2 was unsaturated and acted as a sink. The intensive photosynthesis of rich algae decreased the CO2 in the water and increased dissolved oxygen and pH. The increase in CH4 in the bay was attributed to the mineralization of autochthonous organic carbon. These findings suggest that frequent algal blooms will greatly absorb more CO2 from atmosphere and increasingly release CH4, therefore, the contribution of the bay to the lake's CH4 emissions and carbon budget will be major even though it is small. The results of this study will be the same to other shallow lakes with frequent algal bloom, making lakes a more important part of the carbon budget and greenhouse gases emission.