Patterns inpCO₂in boreal streams and rivers of northern Quebec, Canada

Abstract

[1] Here we examine the patterns in carbon dioxide partial pressure (pCO 2 ) measured in a number of small boreal streams (<5 km in length) in the northwestern boreal region of Québec during the ice-free season and compare these to the patterns found in a major river (Eastmain River) and in a tributary in the same region. All systems were consistently supersaturated in CO 2 (range 450 to 5000 matm) streams having both higher (mean 1850 matm) and more variable pCO 2 than that of rivers (range 550 to 800 matm). Stream pCO 2 was positively related to DOC concentration and stream segment length, both suggesting a direct influence of the surrounding landscape. Calculated stream water-air CO 2 fluxes ranged from 700 to over 3000 mg C m À2 d À1 , up to 2 orders of magnitude higher than those measured in large rivers and lakes of the same region. Small streams, despite their extremely reduced areal coverage (1% of the aquatic surface), accounted for 25% of the total aquatic C emissions, and the resulting areal stream fluxes were comparable to those measured in different soils or wetlands in the region.

Daily and seasonal variability of CO2 saturation and evasion in a free flowing and in a dammed river reach

“…Such evidence is confirmed by summer mass balances of inorganic carbon, revealing a net decrease of loads from S1 to S2 (see later). CO 2 evasion rates calculated for the northern reach of the Mincio River were close to those reported for streams (Teodoru et al, 2009) or unregulated rivers (Koné et al 2009) (Supplementary Tab. 1).…”

“…In large rivers variations of CO 2 levels may occur, but over a seasonal time scale or over large spatial scales, of the order of hundreds km (Raymond et al, 1997). On the contrary, processes within the water mass or occurring in the hyporheic zone have the potential to alter CO 2 levels in small rivers, at much shorter or smaller temporal and spatial scales (Neal et al, 2002;Teodoru et al, 2009). This is likely due to frequent (i.e., daily) variations of the equilibrium between heterotrophic and autotrophic processes within the river, in particular in those segments that are macrophyte or phytoplankton dominated as the two reaches considered in this study (Neal et al, 2002).…”

“…Excess respiration of organic carbon from terrestrial inputs maintains elevated CO 2 supersaturation values, with moderate seasonal variations and very limited control by primary producers (Duarte and Prairie, 2005;Butman and Raymond, 2011). Intermediate and large rivers display limited spatial and temporal variations of CO 2 saturation values when compared to streams and small rivers likely due to higher buffer capacity, different hydrology and more constant organic input from watersheds Waldron et al, 2007, Teodoru et al, 2009.…”

Groundwater data improve modelling of headwater stream CO&lt;sub&gt;2&lt;/sub&gt; outgassing with a stable DIC isotope approach

“…The main reasons are uncertainties in groundwater input as well as poorly defined surface areas and gas trans-fer velocities (Marx et al, 2017a;Schelker et al, 2016). In addition, pCO 2 and subsequent CO 2 outgassing fluxes typically decline rapidly from stream source areas to river sections further downstream (van Geldern et al, 2015;Stets et al, 2017). Poor definition of these gradients adds another uncertainty to the global carbon budget.…”

“…Most often fluxes are calculated from pCO 2 and gas transfer velocities (Teodoru et al, 2009;Raymond et al, 2012;Lauerwald et al, 2015;van Geldern et al, 2015). However, because of large variabilities of gas transfer velocities on different spatial and temporal scales, this type of determination remains controversial, especially for small-scale applications (Marx et al, 2017a;Regnier et al, 2013;Schelker et al, 2016).…”