Lake‐size dependent physical forcing drives carbon dioxide and methane effluxes from lakes in a boreal landscape

We studied the concentrations and diffusive fluxes of carbon dioxide (CO2) and methane (CH4) in 12 lakes in a size range of 0.004–35 km2 and mean total organic carbon (TOC) range of 6–25 mg L−1, located in the southern boreal area of Finland. TOC, mainly originating from the catchment, was the best predictor of concentrations and areal CO2 effluxes in the whole lake-area range, but among the lakes with an area < 1 km2 having an anoxic hypolimnion during summer, the areal effluxes were also related to lake size and an index of turbulence. The concentration and areal effluxes of CH4 from the lakes' pelagic zones were related more closely to lake-size-related water column stability and turbulent mixing than directly to TOC. The total regional flux estimate from 619 lakes in the region of 1600 km2 showed that seven lakes with an area of 10–50 km2 were responsible for 57–69% of the pelagic CO2 effluxes and about half of the sum of pelagic and littoral CH4 effluxes from the lakes. The proportion accounted for by the small lakes (area < 1 km2) in the gas effluxes was 1.4–2.1 times higher, 18–26% for CO2 and 21–26% for CH4, than their areal proportion (12.5%) in the landscape, although numerically these small lakes predominate in the whole lake population (96%).

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