Synergy between nutrients and warming enhances methane ebullition from experimental lakes

Lakes and ponds are important natural sources of the potent greenhouse gas methane (CH 4 ), with small shallow waters identified as particular hotspots 1 , 2 . Ebullition (bubbles) of CH 4 makes up a large proportion of total CH 4 flux 3 , 4 . However, difficulty measuring such episodic events 5 makes prediction of how ebullition responds to nutrient enrichment and rising temperatures challenging. Here, the world’s longest running, mesocosm-based, shallow lake climate change experiment was used to investigate how the combination of warming and eutrophication (that is, nutrient enrichment) affects CH 4 ebullition. Eutrophication without heating increased the relative contribution of ebullition from 51% to 75%. More strikingly the combination of nutrient enrichment and experimental warming treatments of +2–3 °C and +4–5 °C had a synergistic effect, increasing mean annual ebullition by at least 1900 mg CH 4 -C m −2 yr −1 . In contrast, diffusive flux showed no response to eutrophication and only a small increase at higher temperatures (average 63 mg CH 4 –C m −2 yr −1 ). As shallow lakes are the most common lake type globally, abundant in highly climate sensitive regions 6 and most vulnerable to eutrophication, these results suggest their current and future contributions to atmospheric CH 4 concentrations may be significantly underestimated. The combination of nutrient enrichment and warming has a synergistic effect on rates of methane ebullition from experimental lakes. This suggests methane emissions from shallow lakes may be significantly underestimated.