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

Hydrosedimentological conditions of floodplain rivers can affect nutrient delivery processes. This study evaluated the effects of sedimentological and hydrological regimes on nitrogen (N) speciation and transport in the floodplain-river system of the Middle Parana. 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 with concentrations of nitrate-N (NO3-N), nitrite-N (NO2-N), and dissolved organic N (DON), and relative contributions of these N forms to total N (TN) but was positively correlated to particulate N (PN) and ammonium-N (NH4-N). The hydrometric level was positively associated with DON concentration and its relative contribution at the main channel but negatively associated with DON, PN, and TN concentrations in the most isolated lake. Simple linear regressions showed that the sedimentological regime significantly explained all N forms but not TN load. Flooding 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.