River-floodplain interactions: nutrient concentrations in the Lower Parana River

Changes in water chemistry along the lower 500 km stretch of the Parana River were assessed. The water composition of the river and of a representative floodplain marsh were compared. Short-term changes in nutrient concentrations were monitored in marsh enclosures filled with river water in order to simulate the nutrient dynamics under floodplain inundation. 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 calcium recorded. Marsh water exhibited strong depletions of oxygen and nitrate. 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 nitrate concentration in the river, in spite of large cultural inputs, was consistent with the low nitrate concentrations observed in marsh waters and the fast nitrate disappearance in enclosure experiments, suggesting large losses by denitrification. A decrease in SM along the Lower Parana River would indicate high sediment retention within the floodplain. Calcium, bicarbonate, and soluble reactive phosphorus (SRP) concentrations were higher in the marsh than in the river. High marsh SRP concentrations are likely to originate from the weathering of river SM upon sedimentation, in response to the reducing and acidic marsh environment. The observed release of calcium and SRP upon acidification of SM is consistent with the higher contents observed in the marsh. Downstream increases in calcium, bicarbonate, and SRP along the river course suggest the effect of a large water exchange between the river and its floodplain. The downstream increase in the P content of SM was correlated to SRP concentration. Mean inorganic nitrogen/SRP ratio, by weight, 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 POC correlations to hydrometric level and the fact that both parameters attained their maxima at the receding stage after the flood peak, suggest 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 contribution from the Salado River, a tributary of high salinity that merges with the Parana River downstream from Santa Fe.