Contributions of diffuse and point sources to the phosphorus loads in the River Main over a 22-year period

Time series from 1974 to 1995 of the soluble reactive phosphorus (SRP), soluble organic phosphorus (SOP) and particulate phosphorus (PP) annual loads from the River Main, Northern Ireland (NI), were compared with changes in sewage derived phosphorus (P) loadings and the manure P load produced by farm animals in the catchment. A P reduction programme at the largest sewage treatment works in the catchment from 1980 had only a limited impact on river SRP loads from 1981 to 1984. River loads then increased, so that by 1995 they were 15 tonnes P higher than in 1974 despite a net 18 tonnes P reduction in the sewage derived P load over that period. When allowance was made for SRP inputs from towns and rural septic tanks, the remaining river SRP loads showed a pronounced upward trend from 1974 to 1995 with the rate of increase accelerating after 1986. This change point coincided with a change point in the manure P production time series which also showed an upward trend after 1986. Prior to that 1996 manure P production had been relatively constant form 1974. The increase in diffuse SRP loads from 1974 to 1986 was attributed to increasing soil P. A P budget for NI agriculture from 1974 to 1995 showed that the cumulative surplus was 28 tonnes P km–2, while over the same period background or diffuse river loads increased by 0.058 tonnes P km–2 from 0.024 tonnes P km–2 in 1974 to 0.082 tonnes P km–2 in 1995. Annual flow was positively correlated with river loads of SRP, SOP and PP. Changes in sewage derived P were significantly correlated with SRP but not with either SOP or PP loads. Neither SOP nor PP loads showed any trend with time.