Nutrient dynamics in the Paraná River Delta: relationship to the hydrologic regime and the floodplain wetlands

Abstract

South America contains the most extensive freshwater wetlands in the world, associated with the floodplains of the Amazon, Orinoco and Paraná rivers. The Paraná River delta is a large wetland system strongly influenced by the flood/drought cycles of the river. Water quality remains largely unreported at the regional scale. The present study aims to evaluate the effect of increasing anthropogenic disturbances on nutrient concentrations in the Paraná deltaic system. Within this framework, the river-floodplain interaction is examined. Fourteen sites located along 300 km were sampled over the course of three successive years. The results showed a large spatial homogeneity: most of the studied parameters showed a higher temporal than spatial variability. The temporal variability was associated with flood/drought cycles of the river, while the differences among the sampling sites were related to nearby anthropic activities. Significantly lower nitrate, soluble reactive phosphorus, and suspended matter concentrations occurred in coincidence with high hydrological connectivity, particularly during the El Niño-Southern Oscillation (ENSO) flooding, compared to the medium hydrological connectivity conditions, suggesting the effect of river-floodplain interactions. In contrast, ammonium concentrations were significantly higher after the floods. Also, nitrate concentrations were significantly higher in the upper deltaic stretch than in the middle or lower stretches, despite the loads contributed by several affluents with higher concentrations than those in the Paraná River. The present results demonstrate the extent to which the Paraná River delta removes suspended matter, nitrate, and P, thus providing an enormously valuable ecosystem service by maintaining acceptable water quality in the river despite increasing contaminant loads. This study supplies a detailed baseline of water quality and knowledge about ecosystem functions in the deltaic wetlands, useful in the design of conservation policies for safeguarding the largest floodplain in Argentina.