Riverine fluxes of dissolved inorganic nitrogen may be underestimated, especially in the gated estuaries: Influence of suspended sediments

Abstract

The flux of dissolved inorganic nitrogen (DIN), predominantly nitrate (NO3−) and ammonium (NH4+), from land to coastal waters via rivers is commonly estimated by a simple calculation multiplying water flux by nitrogen concentration. Often, the subsequent interactions involving NO3− and NH4+ adsorption or desorption on suspended sediments are overlooked. To better understand the impact of these interactions on the overall NO3− and NH4+ sorption or desorption and, consequently, the mobility and transport to the coastal zone, a series of NO3− and NH4+ adsorption and desorption experiments were conducted. These experiments involved varying suspended sediment concentrations (1 to 40 g·L− 1), particle sizes, and salinities (0 to 30) to assess their potential effects. Furthermore, we investigated how different sea-salt ions influenced these processes by comparing the effect of NaCl solution and real seawater solutions. In the estuarine environment, it was observed that the desorption of NO3− and NH4+ from suspended sediments is more pronounced than adsorption. Notably, when suspended particle concentrations were low and salinity levels were high, the release of NO3− and NH4+ from sediments was significantly intensified. Moreover, under polyhaline conditions, there was a further increase in the desorption of NO3− and NH4+. This enhanced release from suspended sediments facilitates the transport of DIN to nearshore waters, especially significant in gated estuaries with sediment resuspension. Ignoring these dynamics could lead to underestimations of DIN transport in river-estuary systems.