Gas exchange of CO2 in a turbulent and eutrophic river

Titre du congrès :Open Science Conference on the GHG Cycle in the Northern Hemisphere Ville du congrès :Crète Date du congrès :14/11/2006

Rivers are well documented in terms of lateral carbon fluxes but poorly in terms of vertical carbon fluxes (CO2 water-air exchanges). Although the general average CO2 supersaturation of river waters is well known, a rigorous up-scaling of CO2 emissions by world rivers is hampered by large spatial and temporal variations in water pCO2 due to export from watershed and in-stream processes and by a lack of an adequate parameterization of the gas transfer velocity (K600). Here, we report direct field measurements of water and air pCO2 and water-air CO2 fluxes during four seasons in an eutrophic and turbulent river. CO2 fluxes and gas transfer velocities were estimated with three different techniques: the floating chamber, the eddy covariance and the "delta"method, based on diurnal pCO2 records with an equilibrator. The eutrophic Loire River behaved as a source of atmospheric CO2 during fall (Nov 2005: F(CO2) = +30mmol.m-2.d-1) and winter (Feb. 2003: F(CO2) = +80mmol.m-2.d-1) and as a sink of CO2 in spring (May 2005 F(CO2) = -30mmol.m-2.d-1) and summer (Sept 2004: F(CO2) = -60mmol.m-2.d-1). Strong diurnal variations in water pCO2, due to photosynthesis and respiration were observed, except in winter. The three methods used gave consistent CO2 fluxes and gas transfer velocities, the later being very high, in the range of 20-50 cm.h-1. The spring and summer CO2 uptake generates a lateral transport of algal carbon to the estuary. Previous works show however that this material is further totally mineralized, leading to a very high summer CO2 degassing in the estuary (F(CO2) = 280mmol.m-2.d-1). Throughout the year, the river-estuary continuum is a net CO2 source, with a magnitude similar to the organic carbon net export to the Atlantic Ocean.