Carbon dioxide emissions from temperate reservoirs and pit lakes of different trophic states

The abundance of constructed inland waters, such as artificial ponds and reservoirs, has increased over the last decades. Compared to natural lakes, reservoirs and retention basins can exhibit a higher accumulation of organic carbon and a higher emission of greenhouse gases. The magnitude and dynamics of related emission fluxes are important to understand the role of inland waters in the global carbon cycle. However, origin and use of artificial waterbodies are diverse, and it is unclear whether current gas flux estimates adequately represent all important types and conditions of artificial inland waters. This study compares drivers of CO₂ and CH₄ emissions from 2 types of temperate artificial waters widespread in central Europe: reservoirs and artificial lakes in former open-pit lignite mines (pit lakes). We measured CO₂ exchange at the water surface, calculated surface water CO₂ from long-term dissolved inorganic carbon and pH data, and measured CO₂ and CH₄ concentrations throughout the water column. We found that an older, eutrophic pit lake showed seasonal variations in CO₂ fluxes including uptake during summer and release during turnover, similar to 3 studied freshwater reservoirs. By contrast, reacidification and repeated addition of carbonate (liming) led to sustained CO₂ supersaturation in a young, oligotrophic pit lake throughout the year, potentially making it a steady source of CO₂ to the atmosphere. This finding implies that on an annual level, pH regulation and lake maturity largely determine whether pit lakes act as sources of CO₂ (young, slightly acidic) or sinks of CO₂ (older, neutral pH).