Divergent biophysical controls of aquatic CO2 and CH4 in the World’s two largest rivers

Abstract

Carbon emissions to the atmosphere from inland waters are globally significant and mainly occur at tropical latitudes. However, processes controlling the intensity of CO2 and CH4 emissions from tropical inland waters remain poorly understood. Here, we report a data-set of concurrent measurements of the partial pressure of CO2 (pCO2) and dissolved CH4 concentrations in the Amazon (n = 136) and the Congo (n = 280) Rivers. The pCO2 values in the Amazon mainstem were significantly higher than in the Congo, contrasting with CH4 concentrations that were higher in the Congo than in the Amazon. Large-scale patterns in pCO2 across different lowland tropical basins can be apprehended with a relatively simple statistical model related to the extent of wetlands within the basin, showing that, in addition to non-flooded vegetation, wetlands also contribute to CO2 in river channels. On the other hand, dynamics of dissolved CH4 in river channels are less straightforward to predict, and are related to the way hydrology modulates the connectivity between wetlands and river channels.

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

“…The largest fraction of global CO 2 and CH 4 emissions from riverine networks occurs at tropical and sub-tropical latitudes (Bloom et al, 2010;Raymond et al, 2013;Lauerwald et al, 2015;Borges et al, 2015b) that are in general more pristine than their temperate counter-parts. Conversely, the largest fraction of global N 2 O emissions from riverine networks is assumed to occur in human impacted temperate rivers (Seitzinger and Kroeze, 1998;Hu et al, 2016).…”

“…In pristine river networks, CO 2 and CH 4 emissions are driven by instream production related to the degradation of terrestrial organic matter (Cole and Caraco, 2001;Richey et al, 2002), as well as lateral inputs from groundwater and/or wetlands (Abril et al, 2014;Borges et al, 2015aBorges et al, , 2015b. Pristine rivers are usually nitrogen poor and seem to be low sources or even sinks of N 2 O, related to sediment denitrification that removes N 2 O from the water column (Richey et al, 1988;Baulch et al, 2011;Borges et al, 2015a).…”

“…Enhanced nutrient inputs will fuel primary production leading to low CO 2 and high CH 4 concentrations, the latter related to enhanced organic matter delivery to sediments . Other human impacts that affect carbon and nitrogen cycling in river networks that can potentially influence cycling of GHGs are river bank stabilization and floodplain drainage that disrupt the river-wetland connectivity that is important for CO 2 and CH 4 dynamics in rivers (Abril et al, 2014;Teodoru et al, 2015;Borges et al, 2015aBorges et al, , 2015bSieczko et al, 2016). The introduction of invasive animal species such as the zebra mussel (Dreissena polymorpha) in US rivers and lakes (Caraco et al, 1997;Evans et al, 2011) (Hussner, 2012), some with high production and biomass (Hussner, 2009); invasive floating macrophytes such as the water hyacinth (Eichhornia crassipes) have been documented to increase CO 2 and CH 4 levels in tropical rivers (Koné et al, 2009(Koné et al, , 2010, but this remains undocumented in temperate rivers.…”

Biogeochemical contrast between different latitudes and the effect of human activity on spatio-temporal carbon cycle change in Asian river systems

“…These trends imply the predominance of the seasonality in in-stream respiration of organic carbon fixed originally on land and along river margins and mobilized into flowing waters, as suggested 10 by the previous observed data (Richey et al, 2002;Rudorff et al, 2011). (Schlunz and Schneider, 2000;Coynel et al, 2005;Aufdenkampe et al, 2011;Dai et al, 2012;Raymond et al, 2013;Hartman 15 et al, 2014;Borges et al, 2015) in order to estimate CO2 evasion to the atmosphere, sediment storage, and TOC, DOC, POC, DIC-flux to the ocean in previous data. Because the new model can simulate both horizontal transport to the ocean and vertical fluxes and includes aquatic metabolism and terrestrially derived carbon together in major rivers, the result implied that there is active interplay between inorganic and organic carbon (DOC, POC, DIC, pCO2, etc.)…”

“…While there was some scattering of the 20 simulated result in comparison with the range of compiled data, such as overestimation of TOC and DOC greatly affected by the values in South America, it is interesting that there was great variability of the seasonal carbon budget among each continent, being greatly affected by various hydro-meteorological conditions, land use, topography, and latitudinal difference, etc. ; more organic carbon is exported where wetlands are located in humid tropical and non-carbonate boreal regions (for example, TOC flux = 299.78 TgC/yr in South America, and = 134.74 TgC/yr in Asia), and more inorganic carbon is exported in areas of high 25 carbonate weathering, soil respiration, and groundwater flow in temperate and boreal regions of carbonate terrain, particularly Africa (DIC flux = 171.85 TgC/yr) (Wang et al, 2013;Borges et al, 2015).…”