Carbon dioxide, methane and nitrous oxide emissions from the human-impacted Seine watershed in France

Abstract

Greenhouse gas (GHG) emissions from rivers and lakes have been shown to contribute significantly to global carbon and nitrogen cycling. In temperate and human-impacted regions, simultaneous carbon dioxide, methane and nitrous oxide emissions from aquatic systems are poorly documented. We estimated carbon dioxide (CO) concentrations in the Seine hydrosystem (71,730 km, France) using direct measurements, and calculations of CO partial pressures from 14 field campaigns conducted between 2010 and 2017, and compared them to methane (CH) and nitrous oxide (NO) concentrations. In the main stem of the Seine River, CO showed the same spatial gradient as NO and CH with peaks in concentration downstream from the arrival of effluents from wastewater treatment plants enriched in organic matter, thus favoring mineralization. It is likely that high CO concentrations upstream were due to organic carbon inputs from soils and enriched CO groundwater discharges, whereas high NO and CH upstream values were likely due to denitrification in riparian wet areas and anoxic decomposition of organic matter-rich wetlands, respectively. In addition, seasonal variations in all three GHGs were observed with higher concentrations in summer when higher temperatures promote mineralization and low water reduces the dilution of organic matter mainly originating from WWTP effluents. GHG emissions were calculated and compared with agricultural and nonagricultural (urban, transport) fluxes in the basin. In the Seine River network, CO emissions dominated riverine GHG emissions, reaching 95.3%, while NO and CH emissions accounted for 4.4% and 0.3%, respectively. These indirect emissions from the hydrosystem were estimated to account for 3.7% of the total GHG emissions from the basin that amounted to 61,284 Gg COeq yr. Comparatively, direct agricultural and nonagricultural GHG emissions were estimated at 23.3% and 73.0%., respectively.

Anthropogenic inputs from a coastal megacity are linked to greenhouse gas concentrations in the surrounding estuary

“…Elevated surface concentrations of CH 4 (3.1×) and CO 2 (1.6×) in urban tributaries, compared with adjacent midchannel sites, supported H2. The higher surface values of CH 4 and CO 2 quantified in urban tributaries and tidal straits, each containing up to 50 sewage delivery sites (NYCDEP ), are likely the result of direct inputs of GHGs transported along with sewage discharges, which have been quantified directly in streams and inland waters (Alshboul et al ; Martinez‐Cruz et al ; Smith et al ), but not in estuaries until recently (Garnier et al ; Marescaux et al ).…”

“…In addition, we did not measure N 2 O, a GHG with 298× warming potential compared with CO 2 over a 100‐yr period (Ciais et al ). Quantifying N 2 O is of major import for estuaries as denitrification has been demonstrated to be an important GHG source (Marescaux et al ) in urban estuaries and surrounding wetlands due to high anthropogenic inputs (Schlesinger ; Doney ).…”

“…CH 4 concentrations typically decrease with increasing estuarine salinity because sulfate carried with seawater increases competition between sulfate reducers and methanogens, limiting CH 4 production in more saline waters (Bartlett et al ; Capone and Kiene ). However, rivers and estuaries with large urban and/or agricultural influences including the Scheldt, Pearl, Seine, and Hudson River estuaries/watersheds demonstrated supersaturated CO 2 and CH 4 concentrations in downstream regions (Kempe ; Raymond et al ; Frankignoulle et al ; Abril et al ; Middelburg et al ; Chen et al ; Regnier et al ; Marescaux et al ), suggesting that anthropogenic inputs can alter upstream/downstream trends. Nevertheless, data explicitly linking anthropogenic inputs to enhanced estuarine GHG efflux are lacking (Burgos et al ).…”

Impact of Land Cover Types on Riverine CO2 Outgassing in the Yellow River Source Region

“…By comparison, the grassland rivers have been substantially affected by human activities, especially grazing. Consequently, besides the physical erosion, the pollutants (e.g., Yak excreta) produced by human activities are also important sources of degradable DOC [56], which may have been decomposed to enhance riverine CO 2 evasion [57]. Moreover, the DIC is also an important source of riverine CO 2 in grassland areas.…”

Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852–2014): From traditional agriculture to conventional intensive systems

“…Within the country, the Seine watershed is one emblematic example of the former specialised cropping systems, while Bretagne is a region of typical intensive livestock farming systems highly disconnected from croplands. Marescaux et al (2018) established the GHG budget for the Seine Basin, including the hydrosystem network, the agricultural and non-agricultural sectors. Of the approximately 61,000 ktons CO 2 Eq yr −1 emitted from the whole Seine Basin, non-agriculture GHGs were shown to dominate the total emissions (73%), while the agricultural sector amounted to 23% and emissions by rivers reached 4%.…”