Ecosystem maturity modulates greenhouse gases fluxes from artificial lakes

Lentic ecosystems play a major role in the global carbon cycling but the understanding of the environmental determinants of lake metabolism is still limited, notably in small artificial lakes. Here the effects of environmental conditions on lake metabolism and CO2 and CH4 emissions were quantified in 11 small artificial gravel pit lakes covering a gradient of ecosystem maturity, ranging from young oligotrophic to older, hypereutrophic lakes. The diffusive fluxes of CO2 and CH4 ranged from −30.10 to 37.78 mmol m−2 d−1 and from 3.05 to 25.45 mmol m−2 d−1 across gravel pit lakes, respectively. Nutrients and chlorophyll a concentrations were negatively correlated with CO2 concentrations and emissions but positively correlated with CH4 concentrations and emissions from lakes. These findings indicate that, as they mature, gravel pit lakes switch from heterotrophic to autotrophic-based metabolism and hence turn into CO2-sinks. In contrast, the emission of CH4 increased along the maturity gradient. As a result, eutrophication occurring during ecosystem maturity increased net emissions in terms of climate impact (CO2 equivalent) due to the higher contribution of CH4 emissions. Overall, mean CO2equivalent emission was 7.9 g m−2 d−1, a value 3.7 and 4.7 times higher than values previously reported in temperate lakes and reservoirs, respectively. While previous studies reported that lakes represent emitters of C to the atmosphere, this study highlights that eutrophication may reverse lake contribution to global C budgets. However, this finding is to be balanced with the fact that eutrophication also increased CH4 emissions and hence, enhanced the potential impact of these ecosystems on climate. Implementing mitigation strategies for maintaining intermediate levels of maturity is therefore needed to limit the impacts of small artificial waterbodies on climate. This could be facilitated by their small size and should be planned at the earliest stages of artificial lake construction.