RIVER-FLOODPLAIN INTERACTIONS - NUTRIENT CONCENTRATIONS IN THE LOWER PARANA RIVER

Changes in water chemistry along the lower 500 km stretch of the Paran a River were assessed. The water composition of the river and of a rep resentative floodplain marsh were compared. Short-term changes in nutr ient concentrations were monitored in marsh enclosures filled with riv er water in order to simulate the nutrient dynamics under floodplain i nundation. Experiments were performed by resuspending river suspended matter (SM) in synthetic river water acidified to floodplain pH values and short term changes in soluble reactive phosphorus (SRP) and calci um recorded. Marsh water exhibited strong depletions of oxygen and nit rate. Dissolved free CO2 was an order of magnitude higher and pH lower than in river waters. In the marsh, the net heterotrophic metabolism below the water surface seems to be the most plausible explanation for such features. The lack of a significant downstream increase in nitra te concentration in the river, in spite of large cultural inputs, was consistent with the low nitrate concentrations observed in marsh water s and the fast nitrate disappearance in enclosure experiments, suggest ing large losses by denitrification. A decrease in SM along the Lower Parana River would indicate high sediment retention within the floodpl ain. Calcium, bicarbonate, and soluble reactive phosphorus (SRP) conce ntrations were higher in the marsh than in the river. High marsh SRP c oncentrations are likely to originate from the weathering of river SM upon sedimentation, in response to the reducing and acidic marsh envir onment. The observed release of calcium and SRP upon acidification of SM is consistent with the higher contents observed in the marsh. Downs tream increases in calcium, bicarbonate, and SRP along the river cours e suggest the effect of a large water exchange between the river and i ts floodplain. The downstream increase in the P content of SM was corr elated to SRP concentration. Mean inorganic nitrogen/SRP ratio, by wei ght, decreased from 8 in the upmost sampling site, to 3.6-3.8 close to the river mouth, being 0.6-0.8 in the floodplain marsh. The TOC and P OC correlations to hydrometric level and the fact that both parameters attained their maxima at the receding stage after the flood peak, sug gest the floodplain origin of the river organic loading. Conductivity, sodium, potassium, chloride, and sulphate increased from Santa Fe to Rosario, without further changes downstream, suggesting an important c ontribution from the Salado River, a tributary of high salinity that m erges with the Parana River downstream from Santa Fe.