Pipes or chimneys? For carbon cycling in small boreal lakes, precipitation matters most

Abstract

Are small lakes passive pipes transporting terrigenous organic carbon (dissolved organic carbon [DOC]), or chimneys for CO2 release in the landscape? Using a unique combination of 30‐yr measurements, sediment dating and modeling of a small humic lake and its catchment in southeast Norway, we calculated lateral DOC fluxes and in‐lake retention. Concentrations and fluxes rose significantly, driven by declining sulfur deposition and increased precipitation. In‐lake retention (% of inputs) declined because of higher discharge and lower residence times. DOC removal rates were not sensitive to residence time. Modeled in‐lake DOC removal was driven primarily by microbial metabolism and, secondarily, by flocculation, suggesting that the likely fate of lake‐retained DOC is CO2 evasion to the atmosphere. Precipitation was the overriding landscape control on DOC fluxes and retention. In a wetter climate, small northern lakes will, on balance, function more as pipes than chimneys, with increasing lateral DOC fluxes but little change in CO2 production.