Relationship between sediment chemistry and phosphorus concentrations at baseflow in rivers of the New Zealand National River Water Quality Network

Stream sediments can act as a source or a sink of dissolved (filtered) phosphorus (P) via abiotic and biotic processes. The cumulative action and magnitude of abiotic processes has been quantified by the equilibrium P concentration at zero net sorption or desorption (EPC0). The EPC0 was determined in 76 large rivers of contrasting climate, topography, and geology across New Zealand. Measurements of EPC0 (0.004–0.065 mg L-1) indicated sediments were acting as a source of filtered reactive P (FRP) to the water column. The EPC0 was related to the proportion of intensive agriculture in the catchment, the concentration of readily available P in the sediment, sediment size, and catchment slope and elevation. Determination of EPC0 will yield a relative assessment of the sediment’s ability to supply P to the water column especially at baseflow. Furthermore, the EPC0 may be less prone to short-term variation (e.g., diurnal patterns) compared with grab samples. This information will help target efforts to mitigate FRP concentrations in rivers by managing sediment inputs. Additional work is required to determine, for instance, how long an EPC0 measurement remains valid before new sediment is deposited or existing sediment is scoured.