Evidence for a delayed response of riverine phosphorus exports from increasing agricultural catchment pressures in the Lough Neagh catchment

Total phosphorus (TP) exports from the rivers draining into Lough Neagh increased between 1974 and 2000 despite lower TP inputs to the rivers from point sources. Over this period annual diffuse exports of dissolved reactive P (DRP) increased by 238%, from 17 to 57 kg P km22 with an annual rate increase of 1.6 6 0.3 kg P km22 yr21. Dissolved unreactive P exports increased by 0.4 6 0.2 kg P km22 yr21, but particulate P exports did not increase. Annual exports of these three P fractions were positively correlated with annual runoff. The catchment of Lough Neagh has experienced an intensification of agriculture, with phosphorus inputs of manures and fertilizers to agricultural land increasing from 1,176 kg P km22 yr21 in 1925 to 3,823 kg P km22 yr21 in 2000. However 85% of this increase predated 1975. Increasing DRP exports after 1974 were better correlated with increasing soil P, and 70% of the increase in P accumulation by soils since 1925 occurred after 1974. Land use remained stable after 1974 with a historically low component of arable (,5%) and a dominance of grassland (.63%). The principal decade of intensification was in the 1940s, but this coincided with a greater arable component of land use. This period predated the increase in diffuse DRP exports. Intensively managed grasslands appear to be vulnerable to high DRP exports, reflecting surface application of manures, accumulation of soil P at the surface, and the creation of bypass flow pathways that facilitate the loss of P from soil to water. Lough Neagh, located in northeast Ireland, experienced algal blooms in the late 1960s. Regular monitoring of the lake and inflowing rivers began in 1970, and these data were used in the OECD study on eutrophication (OECD 1982). Waste water treatment works were found to be the largest source of phosphorus (P) in the 1970s, but the emphasis was on dissolved reactive P (DRP) because of its high bioavailability (Table 1). Based on rates of diatom frustule accumulation in the lake sediments, Battarbee (1977) also interpreted the enrichment history of the lake within a framework of increasing inputs of P from urban areas. Since 1981, point source discharges of P have been declining, but, despite this, total P (TP) concentrations in Lough Neagh increased from 110 mg P L21 in 1980 to 145 mg P L21 in 2000 (Foy et al. 2003). In reviewing the effects of point source discharges, Foy et al. (2003) concluded they provided a good explana1 Also a member of staff of The Queen’s University of Belfast. 2 Corresponding author ([email protected]).