Seasonal and spatial variability of the partial pressure of carbon dioxide in the human-impacted Seine River in France

Abstract

Carbon evasion from rivers is an important component of the global carbon cycle. The intensification of anthropogenic pressures on hydrosystems requires studies of human-impacted rivers to identify and quantify the main drivers of carbon evasion. In 2016 and 2017, four field campaigns were conducted in the Seine River network characterized by an intensively cropped and highly populated basin. We measured partial pressures of carbon dioxide (pCO2) in streams or rivers draining land under different uses at different seasons. We also computed pCO2 from an existing data set (pH, water temperature and total alkalinity) going back until 1970. Here we report factors controlling pCO2 that operate at different time and space scales. In our study, the Seine River was shown to be supersaturated in CO2 with respect to the atmospheric equilibrium, as well as a source of CO2. Our results suggest an increase in pCO2 from winter to summer in small streams draining forests (from 1670 to 2480 ppm), croplands (from 1010 to 1550 ppm), and at the outlet of the basin (from 2490 to 3630 ppm). The main driver of pCO2 was shown to be dissolved organic carbon (DOC) concentrations (R2 = 0.56, n = 119, p < 0.05) that are modulated by hydro-climatic conditions and groundwater discharges. DOC sources were linked to land use and soil, mainly leaching into small upstream streams, but also to organic pollution, mainly found downstream in larger rivers. Our long-term analysis of the main stream suggests that pCO2 closely mirrors the pattern of urban water pollution over time. These results suggest that factors controlling pCO2 operate differently upstream and downstream depending on the physical characteristics of the river basin and on the intensity and location of the main anthropogenic pressures. The influence of these controlling factors may also differ over time, according to the seasons, and mirror long term changes in these anthropogenic pressures.

Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach

“…Our simulations describe a heterotrophic system characterized by a moderate biogeochemical processing and relatively little production in the estuarine section as suggested by previous studies (Garnier et al, 2001). One of the main controls of the biogeochemical dynamics of the Seine river network is the significant anthropic pressure which translates into large nutrients and organic matter point sources, in particular in Achères downstream the WWTP SAV and Rouen as was evidenced by Servais et al (1991), Romero et al (2016), and Marescaux et al (2018). Although the timing of the simulated phytoplankton bloom is late by a few weeks, the amplitude of the nutrient consumption is similar to observation and the decrease in dissolved silica confirms the domination of diatoms within phytoplankton biomass (Morelle et al, 2018;Garnier unpublished).…”

“…•s −1 over the 2010-2013 period (Marescaux et al, 2018). The annually average river discharge of 480 m 3…”

“…Integrated Budget of the Seine Estuarine/River Network Every year, 1,204 Gg•C are injected into the Seine river network, including 101 Gg•C•yr −1 under the form of organic carbon as diffuse and point sources (Marescaux et al, 2018) providing a ratio of OC/IC of around 0.1. This organic to inorganic carbon ratio drops below 4% at the estuarine mouth where 716 and 29 Gg•C•yr −1 of DIC and TOC are exported to the sea, respectively.…”

“…Fully transient simulations over the entire year 2010 are performed to quantify the annually integrated CO 2 exchange at the airwater interface and better understand the evolution over the year and along the estuarine-river continuum of the CO 2 exchange with the atmosphere and organic carbon degradation as well as the longitudinal variations of inorganic to organic carbon ratio in response to anthropic pressures. 2010 was selected as a year representative of the average hydrology of the Seine river network both in terms of annually averaged water discharge and seasonal distribution of the flow (Marescaux et al, 2018). While Riverstrahler, and its simplistic derived application to the estuary, has been thoroughly validated for the Seine river over the years (Garnier et al, 2008(Garnier et al, , 2010, this integrated study is the first application of C-GEM to the Seine estuary.…”

Modeling inorganic carbon dynamics in the Seine River continuum in France

“…Direct measurements of pCO 2 along the drainage network are still too scarce to accurately support temporal and spatial analyses of CO 2 variability. While calculations from pH, temperature and alkalinity may help reconstruct spatiotemporal patterns of CO 2 dynamics (Marescaux et al, 2018b), only modeling tools can predict the fate of carbon in whole aquatic systems. Indeed, modeling approaches have made it possible to simulate and quantify carbon https://doi.org/10.5194/hess-2019-601 Preprint.…”