The CO 2 -equivalent balance of freshwater ecosystems is non-linearly related to productivity.

Eutrophication of fresh waters results in increased CO(2)uptake by primary production, but at the same time increased emissions of CH(4)to the atmosphere. Given the contrasting effects of CO(2)uptake and CH(4)release, the net effect of eutrophication on the CO2-equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO(2)and CH(4)diffusion, CH(4)ebullition) and CH(4)oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39 mu g/L until hypereutrophic 939 mu g/L) and planktivorous fish in half of them. We found that the CO2-equivalent balance had a U-shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO2-equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19 mmol O-2 m(-3) day(-1). Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO2-equivalents due to enhanced CO(2)uptake, but continued eutrophication enhances CH(4)emission and transforms freshwater ecosystems to net sources of CO2-equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO2-equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO2-equivalent balance of fresh waters.