Sources of and processes controlling CO2 emissions change with the size of streams and rivers

Abstract

Carbon dioxide (CO 2 ) evasion from streams and rivers to the atmosphere represents a substantial flux in the global carbon cycle 1-3 . The proportions of CO 2 emitted from streams and rivers that come from terrestrially derived CO 2 or from CO 2 produced within freshwater ecosystems through aquatic metabolism are not well quantified. Here we estimated CO 2 emissions from running waters in the contiguous United States, based on freshwater chemical and physical characteristics and modelled gas transfer velocities at 1463 United States Geological Survey monitoring sites. We then assessed CO 2 production from aquatic metabolism, compiled from previously published measurements of net ecosystem production from 187 streams and rivers across the contiguous United States. We find that CO 2 produced by aquatic metabolism contributes about 28% of CO 2 evasion from streams and rivers with flows between 0.0001 and 19,000 m 3 s −1 . We mathematically modelled CO 2 flux from groundwater into running waters along a stream-river continuum to evaluate the relationship between stream size and CO 2 source. Terrestrially derived CO 2 dominates emissions from small streams, and the percentage of CO 2 emissions from aquatic metabolism increases with stream size. We suggest that the relative role of rivers as conduits for terrestrial CO 2 e ux and as reactors mineralizing terrestrial organic carbon is a function of their size and connectivity with landscapes.Inland waters play a central role in the global carbon (C) cycle by transforming, outgassing and storing more than half of the C they receive from terrestrial ecosystems before delivery to oceans 1-3 . Terrestrial C inputs to freshwaters are often of similar magnitude to terrestrial net ecosystem production (NEP; refs 1,2,4). Consequently, ignoring inland waters in landscape C budgets may overestimate terrestrial CO 2 uptake and storage 1,5 . In fact, not accounting for terrestrial C exports to and emissions from freshwaters could bias terrestrial NEP and net ecosystem exchange measurements by 4-60% (refs 6-8). Despite small areal coverage, running waters are hotspots for CO 2 emissions 3,9 , with high rates of outgassing relative to lake and terrestrial ecosystems on an areal basis 3,10,11 . Given their significant role in landscape C transformations, transport and emissions, there is a fundamental need to understand rates and drivers of C cycling in running waters.A mechanistic understanding of the processes regulating CO 2 emissions from streams and rivers is necessary for sound predictions of the present and future role of freshwaters in global C cycling and the climate system. Inland waters are often supersaturated with CO 2 due to inputs of terrestrially derived CO 2 and in situ aquatic mineralization of terrestrial OC (refs 12-15) (hereafter, 'internal production') as well as abiotic CO 2 production (Supplementary Section 1). CO 2 concentrations and emissions from running waters will thus vary with changes in land cover, climate, terrestrial ecosystem processes, land-water c...

Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)

“…9). Such a pattern has been previously reported for CO 2 in US rivers (Butman and Raymond, 2011), and might be interpreted as resulting from decreasing input of CO 2 rich groundwater with increasing Strahler order (Hotchkiss et al, 2015). However, this is inconsistent with the decreasing pattern of CH 4 with Strahler order since groundwater in the Meuse basin has low CH 4 concentrations (Fig.…”

“…Groundwater can be a source of GHGs to riverine systems in particular in lower order streams and headwaters (Johnson et al, 2008;Hotchkiss et al, 2015). The comparison of CO 2 and CH 4 in groundwater and riverine waters of the Meuse basin (Fig.…”

Regional-scale lateral carbon transport and CO<sub>2</sub> evasion in temperate stream catchments

“…In addition to soil respiration, mineral weathering also contributes to DIC in stream water. The relative importance of soil respiration and weathering varies depending on geology and the presence of wetlands in the area (Hotchkiss et al, 2015;Lauerwald et al, 2013;Jones et al, 2003). In the present study, 16 % of the catchment areas contained carbonate bedrock.…”

Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater

“…Although the river continuum concept has been useful in explaining gradual longitudinal variations in CO 2 from headwater streams to lowland rivers corresponding to the changing balance between autotrophy and heterotrophy (Borges and Abril, 2012;Koehler et al, 2012;Catalán et al, 2016;Hotchkiss et al, 2015), it has been increasingly recognized that rivers are often divided into discrete 450 segments such as reaches impounded by dams (Ward and Stanford, 1983;Poole, 2002) and eutrophic reaches receiving wastewater (Garnier and Billen, 2007;Yoon et al, 2017;Park et al, 2018). Previous studies of DOM biodegradation have often assumed a selective degradation of labile components of riverine organic matter along the continuum with minimal to low levels of anthropogenic perturbations (Koehler et al, 2012;Weyhenmeyer et al, 2012;Catalán et al, 2016).…”