A phosphorus budget for Northern Ireland: inputs to inland and coastal waters

The European Union (EU) Water Framework Directive (200/60/EC) requires Member States to restore all surface waters to ‘good ecological status’ by 2015. There is growing recognition that phosphorus (P) is the key nutrient in controlling the degree of eutrophication of inland waters. Recent increases in the diffuse P enrichment of surface waters from Northern Ireland agriculture, operating at a P surplus of 14.8 kg ha−1 year−1 have nullified the impacts of programmes to reduce P from point sources. The aim of the present study was to identify an overall P budget for the inland and coastal waters of Northern Ireland and to apportion the total to diffuse and point sources. To calculate the diffuse agricultural contribution, export coefficients were used. These had been derived for total P previously by multiple regression analysis for a number of CORINE land cover classes. A GIS was employed to estimate total P losses for these land use types for the whole land cover of Northern Ireland. By this method it was determined that 1130 tonnes of total P are exported from agricultural land to inland and coastal waters. In addition there are other diffuse contributions from urban streets and surfaces, moorland, forests and peat bogs, which total 165 tonnes of total P. The human contribution to the overall P budget was divided into mains-sewered households (945 tonnes), households connected to septic tanks (118 tonnes) and industrial discharges (40 tonnes). Overall agriculture contributes some 48% of all P exports to inland and coastal waters. As a disproportionate amount of the effluent from waste water treatment works discharges directly to sea or estuary, the agricultural contribution to inland waters is increased to 58%.