Effects of the hydrosedimentological regime on nitrogen transport and speciation in a large subtropical floodplain river

Abstract

Hydrosedimentological conditions of floodplain rivers can affect nutrient delivery processes. This study aimed to evaluate the effects of sedimentological and hydrological regimes on N speciation and transport in the floodplain-river system of the Middle Paraná. Relations of sedimentological and hydrological regimes, assessed through turbidity and hydrometric level, with N speciation and concentration were analyzed. Simple linear regressions were performed to assess whether N load changed within the main channel in response to hydrological and sedimentological regimes. From the main channel to the floodplain, dissolved inorganic N decreased. However, the most isolated lake had the highest N concentration, almost totally in organic form. Turbidity was negatively associated to concentrations of nitrate-N (N-NO3-), nitrite-N (N-NO2-) and dissolved organic N (DON), and relative contributions of these N forms to total N (TN). Contrarily, it was positively correlated to particulate N (PN) and ammonium-N (N-NH4+). Hydrometric level was positively associated to DON concentration and its relative contribution at the main channel; but negatively associated to DON, PN and TN concentrations in the most isolated lake. Simple linear regressions showed that the sedimentological regime explained significantly all N forms, but not TN load. The flood increased TN and mainly DON load. The results show that the hydrosedimentological regime largely affects N transport and speciation. The sediment peak incorporates PN to the system and affects dissolved N speciation probably through effects of suspended particles on redox reactions. The increase of N inputs to the fluvial system during the flood could be caused by DON exportation from the floodplain.