The CO₂‐equivalent balance of freshwater ecosystems is non‐linearly related to productivity

Abstract

Abstract 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 CO 2 ‐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 µg/L until hypereutrophic 939 µg/L) and planktivorous fish in half of them. We found that the CO 2 ‐equivalent balance had a U‐shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO 2 ‐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 CO 2 ‐equivalents due to enhanced CO 2 uptake, but continued eutrophication enhances CH 4 emission and transforms freshwater ecosystems to net sources of CO 2 ‐equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO 2 ‐equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO 2 ‐equivalent balance of fresh waters.