Regional contribution of CO₂ and CH₄ fluxes from the fluvial network in a lowland boreal landscape of Québec

Abstract

Abstract Boreal rivers and streams are known as hot spots of CO 2 emissions, yet their contribution to CH 4 emissions has traditionally been assumed to be negligible, due to the spatially fragmented data and lack of regional studies addressing both gases simultaneously. Here we explore the regional patterns in river CO 2 and CH 4 concentrations ( p CO 2 and p CH 4 ), gas exchange coefficient ( k ), and the resulting emissions in a lowland boreal region of Northern Québec. Rivers and streams were systematically supersaturated in both gases, with both p CO 2 and p CH 4 declining along the river continuum. The k was on average low and increased with stream order, consistent with the hydrology of this flat landscape. The smallest streams (order 1), which represent < 20% of the total river surface, contributed over 35% of the total fluvial greenhouse gas (GHG) emissions. The end of winter and the spring thaw periods, which are rarely included in annual emission budgets, contributed on average 21% of the annual GHG emissions. As a whole, the fluvial network acted as significant source of both CO 2 and CH 4 , releasing on average 1.5 tons of C (CO 2 eq) yr −1 km −2 of landscape, of which CH 4 emissions contributed approximately 34%. We estimate that fluvial CH 4 emissions represent 41% of the regional aquatic (lakes, reservoirs, and rivers) CH 4 emissions, despite the relatively small riverine surface (4.3% of the total aquatic surface). We conclude that these fluvial networks in boreal lowlands play a disproportionately large role as hot spots for CO 2 and more unexpectedly for CH 4 emissions.