P concentrations in the Middle Paraná River: effect of the floodplain lakes?.

MAINE, M.A.; SUÑÉ, N.L.; BONETTO, C.

Carmona, España

Simposio; 4th International Symposium on Phosphate in Sediments; 2003

Water composition and sediment calcium, iron and phosphorus fractions were assessed monthly in the Middle Paraná River and a floodplain lake in permanent contact with it. The same determinations were performed in the Paraná main stem and floodplain lakes in five 5 sections along 400 km of the Middle Paraná stretch in coincidence with a flood and the subsequent low water phase. During the flood soluble reactive phosphorus (SRP) and nitrate decreased downstream along the river. The lakes had higher transparency, conductivity and major ion concentration and lower pH, oxygen, nitrate, SRP and iron than the river. Bottom sediments had an increased Fe/Ca ratio in the floodplain lakes than in the river. River sediments showed higher amounts of the residual P fraction while lakes sediment had an enriched sodium hydroxide-extractable P and an impoverished calcium and residual P fractions. Riverine-contributed SRP and nitrate sustain macrophytic productivity in the lakes. Improved transparency within the lakes indicates suspended matter settlement within them. Decomposition of the litter produced by the macrophytes caused lower oxygen concentrations and pH in the lakes. Denitrification in the water sediment surface is suggested to be an important N pathway. Co-precipitation of SRP with suspended matter and iron is likely contributing to the lower SRP and iron concentrations in lakes than river waters. Transformation of calcium bound P from the incoming river sediments into sodium hydroxide extractable P in the lakes sediment is a major process. The floodplain lakes represent a long-term trap of suspended matter and nutrients.