Abstract
Inflows of phosphorus (P) to freshwaters from the wastewater and agriculture sectors of our food systems continue to reduce aquatic biodiversity and threaten human health. A range of analytical tools were applied to investigate the contributions of these two sectors to riverine P pollution in the catchments of the Somerset Levels and Moors (SLM) region of England. A substance flow analysis of current sector P use in 2021 identified a low food system P use efficiency (48–75%) and variable P input pressures of unused P of 2.1–8.5 kg ha −1 year −1 driven by livestock feed imports and a high population density. Despite recent reductions in wastewater P discharges, concentration (C) and flow (Q) analysis found both point source and diffuse source signals in the river P record. River soluble reactive P (SRP) and total P (TP) flux in a range of SLM sub-catchments ranged from 0.5–1.5 and 0.75–2.4 kg ha −1 year −1 , respectively. The flux of SRP and TP associated with diffuse sources (calculated by CQ analysis) averaged 0.3–0.5 and 0.5–0.95 kg ha −1 year −1 , respectively, and was significantly positively correlated ( r 2 0.6, p < 0.01) to the agricultural P surplus in these sub-catchments. The large intercept (0.28 kg SRP ha −1 and 0.56 kg TP ha −1 ) of this relationship suggested that the historical legacy P store was the main pollution threat from agriculture. An inventory of agricultural P inputs and P outputs over the last 150 years together with a catchment soil analysis programme confirmed a legacy of soil P accumulation amounting to an average 2 t ha −1 . An analysis of the potential impact of reductions in the sector P input pressure on river P concentrations suggested that on-going and planned increases in wastewater P removal efficiency must be supplemented with a major system change towards drawdown of legacy soil P reserves to lower the P pollution threat in this nationally important region.