The farmgate phosphorus balance as a measure to achieve river and lake water quality targets.

This short communication proposes a pathway for achieving river and lake water quality phosphorus (P) targets using the agricultural farmgate P balance (FPB). The context is the internationally important Lough Neagh and general river network in Northern Ireland (NI). A meta-analysis shows a direct and strong linear relationship between the FPB and, with a one-year lag, the mean soluble reactive phosphorus (SRP) concentration for ninety-three river sites over eighteen years (R² = 0.73, p < 0.01) during quiescent conditions. The model suggests that a reduction of the national FPB to 5.5 kg ha^-1 yr^-1 will improve up to 100% of river SRP mean annual concentrations to at least the moderate/good boundary target for SRP status, and 25% to the good/high status. In Lough Neagh, the moderate/good boundary is an in-lake mean annual total P (TP) concentration target of 0.044 mg L^-1. The annual TP load normalised to a flow weighted mean concentration (FWMC) required to achieve this target in the eight major Lough Neagh rivers is 0.109 mg L^-1. Applying a five-year time lag to the TP FWMC data when compared to the FPB also indicates 5.5 kg ha^-1 yr^-1 as a way to reach this in-lake target (R² = 0.72, p < 0.01). Allowing for non-FPB sources being part of P mitigation strategies would either speed up the process of P reductions in rivers and to lakes, or relieve the burden to the agricultural sector if a FPB was increased proportionally to an optimum target.