Seasonal variation and origin of Particulate Organic Carbon in the lower Garonne River at La Reole (southwestern France)

Using a databank, grouping results on dailymeasurements of Total Suspended Matter (TSM) and Particulate Organic Carbon (POC) on the lower Garonne river at La Reole (upstream limit of the dynamic tide) over the last 20 years, we studied seasonal variations in particulate organic carbon concentrations and fluxes, using TSM concentration classes. The results show seasonal variations in POC concentrations and fluxes, and especially a strong impact of flood events. A qualitative and quantitative hypothesis on the differentiation of POC origin is suggested. On every time scale (daily, seasonally or yearly), this hypothesis allows the estimation of POC inputs from the three different pools: soil, litter and autochthonous production. The Garonne River exports 78 000 tons of POC annually, 54% from soil (0.8 t km^-2 yr^-1), 38% from litter (about 0.55 t km^-1 yr^-1) (km of permanent stream) and 8% from autochthonous production. Most of this material is probably mineralized in the Gironde estuary.