[1] The Congo (Zaire) River, the world's second largest river in terms both of water discharges and of drainage area after the Amazon River, has remained to date in a near-pristine state. For a period between 2 and 6 years, the mainstream near the river mouth (Brazzaville/Kinshasa station) and some of the major and minor tributaries (the Oubangui, Mpoko, and Ngoko-Sangha) were monitored every month for total suspended sediment (TSS), particulate organic carbon (POC), and dissolved organic carbon (DOC). In this large but relatively flat equatorial basin, TSS levels are very low and organic carbon is essentially exported as DOC: from 74% of TOC for the tributaries flowing in savannah regions and 86% for those flowing in the rain forest. The seasonal patterns of TSS, POC, and DOC show clockwise hysteresis in relation to river discharges, with maximum levels recorded 2 to 4 months before peak flows. At the Kinshasa/Brazzaville station, the DOC distribution is largely influenced by the input from the tributaries draining the large marshy forest area located in the center of the basin. There is a marked difference between specific fluxes, threefold higher in the forest basins than in the savannah basins. The computation of inputs to the Atlantic Ocean demonstrates that the Congo is responsible for 14.4 Â 10 6 t/yr of TOC of which 12.4 Â 10 6 t/yr is DOC and 2 Â 10 6 t/yr is POC. The three biggest tropical rivers (the Amazon, the Congo, and the Orinoco), with only 10% of the exoreic world area drained to world oceans, contribute $4% of its TSS inputs but 15-18% of its organic carbon inputs. These proportions may double when considering only world rivers discharging into the open ocean.Citation: Coynel, A., P. Seyler, H. Etcheber, M. Meybeck, and D. Orange (2005), Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River, Global Biogeochem. Cycles, 19, GB4019,

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Abstract:Water draining from a large agricultural catchment of 1 110 km 2 in southwest France was sampled over an 18-month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1Ð8 t km 2 y 1 and 0Ð7 t km 2 y 1 , respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC-discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment.

The Nivelle River, a typical Pyrenean mountainous watershed reaching the Bay of Biscay (Atlantic Ocean), was sampled with high resolution during 1996. The particulate organic carbon (POC) contents during successive floods shows that there is a graduated impoverishment of the organic fraction of suspended particulate matter (SPM) from the first flood to the next ones, reaching a threshold value (3%) attributed to allochtonous fraction (soil). On the basis of the high frequency data of water discharge and POC concentration, an annual POC flux was established: 845 tons, corresponding to a specific POC flux of 5.3 tC km À2 yr À1 . This value was obtained during a dry period and must be considered as a minimum value for longer time scale. The POC originated mostly from soil (55%) and riparian/litter ($40%) with a very minor (<5%) contribution of autochthonous POC. Thirty-two percent of the annual POC flux was carried in 1% of time and 66% in 10% of time. The specific POC yield, 5.3 tC km À2 yr À1 , if extended to the whole mountainous area of the southern coast of the Bay of Biscay (19,000 km 2 ), leads to an estimated POC flux around 100,000 t yr À1 . Although small Cantabrian mountainous rivers contributed to only 28% of the freshwater discharge in the Bay of Biscay, their POC load was estimated to account for 70% of the total POC inputs in the Bay.