Résumé
The floodplains of the Amazon basin influence the hydrology and fluxes of suspended solids and solutes of the rivers throughout the basin. Our study focused on the floodplain of Lago Grande de Curuaï¿1/2 (ï¿1/2bidos, Brazil), a 4000 km2 segment including its local upland catchment representative of the lower Amazon floodplain. The floodplain contains large lakes temporally or permanently inter-connected linked to the Amazon River through a system of channels. In a first step, based on in situ water level and discharge data and satellite data acquired from 1997 through 2003, we calculated the exchanges between the floodplain and the river and described the temporal dynamics of lake water derived from river flooding, rainfall, runoff, and exchange with groundwater over six years. The Amazon River dominated the inputs of water to the lake year- round, accounting about 53% of the annual total; rainfall and runoff accounted for about 10% and 8%, respectively while seepage from groundwater system and local upland catchment accounted for 22%. The hydrologic residence time of the lake is about 2.6 months, and the floodplain always was a net contribution of water to the river. The exported volume varied between 2.1 and 4.4 km3 depending on water year and represented about one third of the maximal storage reached each year. In a second step, using chlorine as a conservative tracer of water mixture and water level temporal series derived from T/P and ENVISAT altimetry radar, we computed exchanges between the major lakes constituting the floodplain and the water residence time. Depending on their degree of connection with the Amazon River, the relative importance of each water source and the water residence time greatly differ among lakes. In a third step, the comparison of the observed concentrations with the conservative model outputs enabled us to interpret spatial heterogeneities of some chemical elements (major elements principally) and to identify some of the main biochemical processes taking place in the floodplain. In particular, the comparison of the model results with the observed Ca, K, Sr and Ba concentrations could be induced by incorporation in the vegetation pool. Finally exchanged fluxes with the Amazon River as well as residence time of some essential elements were computed.