Biogeochemical response of a secondary-salinised floodplain wetland to experimental freshening

ABSTRACT An experimental flooding with freshwater was used to evaluate the biogeochemical impacts of freshening in a semi-arid floodplain wetland that had undergone severe secondary salinisation (Loveday Disposal Basin, Australia). Filling the wetland with freshwater lowered electrical conductivity (EC) from ~60 to 13 dS m–1. This freshening was accompanied by substantial declines in total P (TP), filterable reactive P (FRP), dissolved organic C (DOC), total organic N (TON), dissolved organic N (DON), NH4+ and molybdenum-reactive Si (MoR Si) concentrations. Owing to the semiarid climate, the water level receded and EC increased to pre-flooding conditions within seven months. During this drying phase, some chemical species maintained lower concentrations then pre-flooding (TP and FRP), others returned to within their original range (DOC, TON, DON), and others went above their original range (NH4+ and MoR Si), the latter reflecting some bank discharge of groundwater. Unexpectedly low turbidity and P concentrations during the drying phase were promoted by the development of an extensive filamentous algal mat following flooding, which limited sediment resuspension as the water level receded. The transient response to freshening highlights that salinity is a key driver of biogeochemical cycles in semi-arid wetlands.