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

Abstract

We studied the concentrations and diffusive fluxes of carbon dioxide (CO 2 ) and methane (CH 4 ) in 12 lakes in a size range of 0.004-35 km 2 and mean total organic carbon (TOC) range of 6-25 mg L 21 , located in the southern boreal area of Finland. TOC, mainly originating from the catchment, was the best predictor of concentrations and areal CO 2 effluxes in the whole lake-area range, but among the lakes with an area , 1 km 2 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 CH 4 from the lakes' pelagic zones were related more closely to lake-sizerelated water column stability and turbulent mixing than directly to TOC. The total regional flux estimate from 619 lakes in the region of 1600 km 2 showed that seven lakes with an area of 10-50 km 2 were responsible for 57-69% of the pelagic CO 2 effluxes and about half of the sum of pelagic and littoral CH 4 effluxes from the lakes. The proportion accounted for by the small lakes (area , 1 km 2 ) in the gas effluxes was 1.4-2.1 times higher, 18-26% for CO 2 and 21-26% for CH 4 , than their areal proportion (12.5%) in the landscape, although numerically these small lakes predominate in the whole lake population (96%).Most inland waters worldwide have been shown to be supersaturated with the greenhouse gases (GHG) carbon dioxide (CO 2 ) and methane (CH 4 ), and thus release these gases to the atmosphere (Cole et al. 1994;Bastviken et al. 2004). The GHG supersaturation indicates that freshwater ecosystems play an important role in the global carbon cycle by mineralizing and/or accumulating terrestrial organic carbon (Tranvik et al. 2009). By applying Pareto distribution analysis, Downing et al. (2006) estimated that . 99% of the world's lakes have an area , 1 km 2 and these small lakes form , 43% of the global lake area. Thus, in many recent analyses of lake-rich landscapes, the role of small lakes has received much attention, because the small headwater lakes are more typically characterized by higher concentrations of allochthonous organic carbon than the larger lowland lakes (Xenopoulos et al. 2003;Hanson et al. 2007;Einola et al. 2011). In the boreal region, high fluxes of total organic carbon (TOC), originating mainly from peatland and forested catchments (Kortelainen 1993;Humborg et al. 2010), are among the most important factors contributing to CO 2 supersaturation and oxygen consumption in the lake water columns and sediments (Salonen et al. 1983;Larsen et al. 2011). As a result of anaerobic decomposition of organic matter, high concentrations of CH 4 are typical in eutrophic lakes with an anoxic hypolimnion but also in sheltered forest lakes, which likewise develop marked and prolonged hypolimnetic anoxia. It has been suggested that spring and fall mixing periods are generally the periods of the greatest effluxes of GHG into the atmosphere. However, rainy periods may also be followed by efflux peaks , and thus precipitation influences the inte...

Nitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)

“…Furthermore, seasonal variations of CH 4 and N 2 O fluxes have seldom been described in lakes, and mostly in boreal systems (e.g. Kankaala et al 2013;Miettinen et al 2015). Eddy-covariance allows the direct measurement of CH 4 and N 2 O fluxes to the atmosphere in lakes (e.g.…”

“…Podgrajsek et al 2014;Xiao et al 2014), although fluxes are usually computed from dissolved concentrations in surface waters using estimates of the gas transfer velocity (e.g. Schubert et al 2010;Kankaala et al 2013;Miettinen et al 2015).…”

Greenhouse gas metabolism in Nordic boreal lakes

“…High primary production also increases the hypolimnetic respiration and sediment anoxia that may result in elevated emissions of CO 2 and CH 4 . Alternatively, inputs of inorganic C linked to catchment properties may play a major role in CO 2 release (Kankaala et al 2013;McDonald et al 2013).…”

“…Demarty et al 2011;Kankaala et al 2013;Kortelainen et al 2006b;Lemon and Lemon 1981;Ojala et al 2011;Tremblay et al 2005) (Table 1), while studies addressing all three biogenic GHGs (CO 2 , CH 4 , N 2 O) are very limited (Huttunen et al 2003a;Tremblay et al 2005). …”

A science plan for carbon cycle research in North American coastal waters. Report of the Coastal CARbon Synthesis (CCARS) community workshop, August 19-21, 2014

“…Comparative studies across lakes globally indicate a strong size-dependence in the CO 2 coupling of atmosphere and lakes (Kelly et al, 2001;Kortelainen et al, 2006). In general, larger lakes have surface waters closer to atmospheric equilibrium due to the lesser importance of allochthonous organic matter to integrated carbon budgets (Kankaala et al, 2013).…”