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...

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Based on theories of mire development and responses to a changing climate, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the current net C-exchange in mires requires measurements of all relevant fluxes. Estimates of annual total carbon budgets in mires are still very limited. Here, we present a full carbon budget over 2 years for a boreal minerogenic oligotrophic mire in northern Sweden (64111 0 N, 19133 0 E). Data on the following fluxes were collected: land-atmosphere CO 2 exchange (continuous Eddy covariance measurements) and CH 4 exchange (static chambers during the snow free period); TOC (total organic carbon) in precipitation; loss of TOC, dissolved inorganic carbon (DIC) and CH 4 through stream water runoff (continuous discharge measurements and regular C-concentration measurements). The mire constituted a net sink of 27 AE 3.4 ( AE SD) g C m À2 yr À1 during 2004 and 20 AE 3.4 g C m À2 yr À1 during 2005. This could be partitioned into an annual surfaceatmosphere CO 2 net uptake of 55 AE 1.9 g C m À2 yr À1 during 2004 and 48 AE 1.6 g C m À2 yr À1 during 2005. The annual NEE was further separated into a net uptake season, with an uptake of 92 g C m À2 yr À1 during 2004 and 86 g C m À2 yr À1 during 2005, and a net loss season with a loss of 37 g C m À2 yr À1 during 2004 and 38 g C m À2 yr À1 during 2005. Of the annual net CO 2 -C uptake, 37% and 31% was lost through runoff (with runoff TOC4DIC ) CH 4 ) and 16% and 29% through methane emission during 2004 and 2005, respectively. This mire is still a significant C-sink, with carbon accumulation rates comparable to the long-term Holocene C-accumulation, and higher than the C-accumulation during the late Holocene in the region.

[1] The Krycklan Catchment Study (KCS) provides a unique field infrastructure for hillslope to landscape-scale research on short-and long-term ecosystem dynamics in boreal landscapes. The site is designed for process-based research assessing the role of external drivers including forest management, climate change, and long-range pollutant transport on forests, mires, soils, streams, lakes, and groundwater. The overarching objectives of KCS are to (1) provide a state-of-the-art infrastructure for experimental and hypothesis-driven research, (2) maintain a collection of high-quality, long-term climatic, biogeochemical, hydrological, and environmental data, and (3) support the development of models and guidelines for research, policy, and management.

Abstract:A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration-discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM.

Despite an increasing number of empirical investigations of catchment transit times (TTs), virtually all are based on individual catchments and there are few attempts to synthesize understanding across different geographical regions. Uniquely, this paper examines data from 55 catchments in five geomorphic provinces in northern temperate regions (Scotland, United States of America and Sweden). The objective is to understand how the role of catchment topography as a control on the TTs differs in contrasting geographical settings. Catchment inverse transit time proxies (ITTPs) were inferred by a simple metric of isotopic tracer damping, using the ratio of standard deviation of d 18 O in streamwater to the standard deviation of d 18 O in precipitation. Quantitative landscape analysis was undertaken to characterize the catchments according to hydrologically relevant topographic indices that could be readily determined from a digital terrain model (DTM). The nature of topographic controls on transit times varied markedly in different geomorphic regions. In steeper montane regions, there are stronger gravitational influences on hydraulic gradients and TTs tend to be lower in the steepest catchments. In provinces where terrain is more subdued, direct topographic control weakened; in particular, where flatter areas with less permeable soils give rise to overland flow and lower TTs. The steeper slopes within this flatter terrain appear to have a greater coverage of freely draining soils, which increase sub-surface flow, therefore increasing TTs. Quantitative landscape analysis proved a useful tool for intercatchment comparison. However, the critical influence of sub-surface permeability and connectivity may limit the transferability of predictive tools of hydrological function based on topographic parameters alone.

Total organic carbon (TOC) concentrations from seven boreal catchments in northern Sweden were monitored between June 1996 and May 1998 to examine spatial and temporal variations in streamwater TOC export and its relationship with catchment characteristics. The annual average export of TOC ranged between 36 and 76 kg ha -1 yr -1 and correlated positively with the areal extent of wetlands (r 2 = 0.72; p = 0.03). The daily output of TOC was 5-11 times higher during the spring than during any other season. In total, the four week long spring period contributed between 50% and 68% of the annual TOC export from the seven catchments. The relative im- Aquatic Sciencesportance of the spring snow melt period for the annual TOC export, however, correlated negatively with the percentage of wetlands (r 2 = 0.83; p < 0.01). We suggest that the smaller relative importance of the spring runoff period for the annual TOC export from wetland dominated catchments is a result of the hydrological flow paths associated with the snow melt period. While a large fraction of the spring runoff from forested areas reaches the stream via subsurface flow paths across riparian soils rich in TOC, the flow paths through wetland dominated systems include a much larger component of low-TOC snow melt water via surface flow over ice and frozen peat.