In-lake carbon dioxide concentration patterns in four distinct phases in relation to ice cover dynamics

Abstract:

Global carbon dioxide (CO₂) emission estimates from inland waters include emissions at ice melt that are based on simple assumptions rather than evidence. To account for CO₂ accumulation below ice and potential emissions into the atmosphere at ice melt we combined continuous CO₂ concentrations with spatial CO₂ sampling in an ice-covered small boreal lake. From early ice cover to ice melt, our continuous surface water CO₂ concentration measurements at 2 m depth showed a temporal development in four distinct phases: In early winter, CO₂ accumulated continuously below ice, most likely due to biological in-lake and catchment inputs. Thereafter, in late winter, CO₂ concentrations remained rather constant below ice, as catchment inputs were minimized and vertical mixing of hypolimnetic water was cut off. As ice melt began, surface water CO₂ concentrations were rapidly changing, showing two distinct peaks, the first one reflecting horizontal mixing of CO₂ from surface and catchment waters, the second one reflecting deep water mixing. We detected that 83% of the CO₂ accumulated in the water during ice cover left the lake at ice melt which corresponded to one third of the total CO₂ storage. Our results imply that CO₂ emissions at ice melt must be accurately integrated into annual CO₂ emission estimates from inland waters. If up-scaling approaches assume that CO₂ accumulates linearly under ice and at ice melt all CO₂ accumulated during ice cover period leaves the lake again, present estimates may overestimate CO₂ emissions from small ice covered lakes. Likewise, neglecting CO₂ spring outbursts will result in an underestimation of CO₂ emissions from small ice covered lakes.