Managing phosphorus input pressures for improving water quality at the catchment scale.

Phosphorus (P) pollution of freshwater is an endemic threat to water quality and aquatic biodiversity. To better define the contributions of the two main food system sectors (agriculture and wastewater) responsible for freshwater P pollution, we investigated how the magnitude and distribution of sector P input pressures calculated using Substance Flow Analysis (SFA) linked to the P pollution threat across four distinct physiographic regions of the River Stour catchment (1260 km) in Dorset, England. Agricultural P input pressures (-1 to 7 kg ha yr) were dependent on the amount of livestock feed imports and resulting manure loadings to land, whilst food imports and population densities were the main driver of the human net P inputs of up to 13 kg ha yr. Total P input pressures (i.e. Net Anthropogenic P Inputs (NAPI)) were positively correlated (r 0.8-0.9) to riverine P flux of up to 6 kg ha yr across the catchment. Using measured river P concentration (C) and flow discharge (Q) analysis to distinguish monitoring stations capturing mainly diffuse P sources (termed diffuse stations), estimated riverine P fluxes attributable to agriculture varied up to 0.92 kg ha yr depending on the surplus P inputs applied to land. A combination of enhanced wastewater P removal and reduced surplus agricultural P inputs was required to improve water quality. For example, the P pressure-river P flux relationship at diffuse stations suggested that in the catchment area dominated by livestock production, removing the agricultural P surplus of 7 kg ha yr would reduce annual average river SRP concentrations in this area by a third to 0.23 mg L, but still well above the target concentration for eutrophication control (0.08 mg L). Our approach of linking SFA outputs to measured river P data provides a potential complimentary and internationally relevant methodology to evidence effective sector mitigation targets and policies in catchments, and its further testing in other catchments is recommended.