Transports fluviaux de carbone organique par la Garonne à l'entrée de l'estuaire de la Gironde. / Fluvial transport of organic carbon by the Garonne river into the Gironde estuary

Abstract

In order to draw a reliable budget of the organic matter fluxes arriving in the Gironde estuary, weekly measurements of organic carbon content in particulate and dissolved phases over three successive annual hydrological cycles were carried out in the Garonne river water, at the boundary of the estuarine system. As the sampling period presented an important hydrological deficit (about 40%). the estimated carbon budgets are much lower than those previously proposed from data of poorer quality. Nevertheless, this study allowed us to determine precisely the relationships between water discharge, suspended matter concentration and organic carbon content, to better define the origins of organic matter (its phytoplanctonic and pedological components) and to highlight effects of dryness (dissolved organic carbon contents ; impact of a flood following a long period of low discharge). Finally, by applying the different models developed for the study period cycles to an average year (mean annual river discharge of 650 m³.s⁻¹). it is proposed mean carbon fluxes entering the estuary. It appears that, by comparing these fluxes to those exported into the ocean at the mouth of the Gironde estuary. about 55% of the particulate organic carbon input disappear in the estuary after mineralization. This biogeochemical process is confirmed by different studies on the high CO₂ emission into these continent-ocean interfaces.