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

Using a databank, grouping results on daily measurements of Total Suspended Matter (TSM) and Particulate Organic Carbon (POC) on the lower Garonne riv er at La Reole (upstream limit of the dynamic tide) over the last 20 years, we studied seasonal variations in particulate organic carbon concentration s and fluxes, using TSM concentration classes. The results show seasonal va riations in POC concentrations and fluxes, and especially a strong impact o f flood events. A qualitative and quantitative hypothesis on the differenti ation of POC origin is suggested. On every time scale (daily, seasonally or yearly), this hypothesis allows the estimation of POC inputs from the thre e different pools: soil, litter and autochthonous production. The Garonne R iver 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) a nd 8% from autochthonous production. Most of this material is probably mine ralized in the Gironde estuary.