Total Aquatic Carbon Emissions Across the Boreal Biome of Quebec Driven by Watershed Slope

Inland waters emit large amounts of CO2 and CH4 to the atmosphere, partially offsetting the sequestration of carbon in terrestrial ecosystems. However, the incorporation of inland waters into landscape carbon budgets remains challenging, hampered by a lack of studies that consider both carbon gases and the variety of aquatic systems (streams, rivers, and lakes). Here we develop a whole-network assessment of total aquatic carbon emissions for a set of large watersheds in boreal Quebec, Canada. Expressed per unit watershed area, our estimates of total (CO2 + CH4) aquatic carbon emissions range between 11 and 38 g C m(-2) yr(-1) and cannot be predicted from the size of the watershed or the total surface area of aquatic systems. Rather, we show that total aquatic emissions vary across the boreal landscape of Quebec as a function of the average slope of the watershed, which indirectly accounts for the configuration of aquatic networks, the physical forcing that influences gas exchange in fluvial systems, and the potential amount of soil carbon reaching aquatic systems. Total aquatic carbon emissions in boreal Quebec are of the same range and magnitude of variation than other components of the boreal carbon budget and could offset terrestrial net ecosystem productivity by as much as 38%. Plain Language Summary Inland waters are natural emitters of greenhouse gases to the atmosphere, partially offsetting the CO2 uptake by agricultural, forest, and wetland ecosystems. Nevertheless, the role of inland waters is rarely incorporated into regional greenhouse gas inventories. One of the main reasons is that there are very few estimates that integrate CO2 emissions from streams, rivers, and lakes at the large scale, and even fewer that in addition consider methane. Here we present the first assessment of total aquatic carbon emissions at the regional scale, which we developed for the boreal biome of Quebec, Canada, and explore how these aquatic emissions vary across this northern landscape. We show that total aquatic carbon emissions covary nonlinearly with the average slope of the watershed, such that maximal emissions occur both in the flattest and steepest landscapes. We further demonstrate that aquatic emissions could offset terrestrial carbon sinks by as much as 38%, and provide simple tools to incorporate inland waters into regional greenhouse gas inventories.