Modeling inorganic carbon dynamics in the Seine River continuum in France

Abstract

Abstract. Inland waters are an active component of the carbon cycle where transformations and transports are associated with carbon dioxide (CO2) outgassing. This study estimated CO2 emissions from the human-impacted Seine River (France) and provided a detailed budget of aquatic carbon transfers for organic and inorganic forms, including the in-stream metabolism along the whole Seine River network. The existing process-based biogeochemical pyNuts-Riverstrahler model was supplemented with a newly developed inorganic carbon module and simulations were performed for the recent time period 2010–2013. New input constraints for the modeling of riverine inorganic carbon were documented by field measurements and complemented by analysis of existing databases. The resulting dissolved inorganic carbon (DIC) concentrations in the Seine aquifers ranged from 25 to 92 mg C L−1, while in wastewater treatment plant (WWTP) effluents our DIC measurements averaged 70 mg C L−1. Along the main stem of the Seine River, simulations of DIC, total alkalinity, pH and CO2 concentrations were of the same order of magnitude as the observations, but seasonal variability was not always well reproduced. Our simulations demonstrated the CO2 supersaturation with respect to atmospheric concentrations over the entire Seine River network. The most significant outgassing was in lower-order streams while peaks were simulated downstream of the major WWTP effluent. For the period studied (2010–2013), the annual average of simulated CO2 emissions from the Seine drainage network were estimated at 364±99 Gg C yr−1. Results from metabolism analysis in the Seine hydrographic network highlighted the importance of benthic activities in headwaters while planktonic activities occurred mainly downstream in larger rivers. The net ecosystem productivity remained negative throughout the 4 simulated years and over the entire drainage network, highlighting the heterotrophy of the basin. Highlights CO2 emission from the Seine River was estimated at 364±99 Gg C yr−1 with the Riverstrahler model. CO2 riverine concentrations are modulated by groundwater discharge and instream metabolism. CO2 emissions account for 31 % of inorganic carbon exports, the rest being exported as DIC.