The Oona Water (102 km2) is a tributary of the Blackwater River (1,480 km2), an Irish cross border catchment and the largest of the six influent rivers to Lough Neagh. An intensive investigation into the magnitude of phosphorus and sediment transfers from field (0.15 km2), farm (0.62 km2) and landscape (84.50 km2) scale sub-catchments showed that total phosphorus transfers were 1.73, 1.82 and 2.50 kg/ha, respectively, during the 2001-2002 hydrological year. Two important features of these data were noted. Firstly, higher transfers from the landscape scale sub-catchment were related to phosphorus inputs between storm events. These were mainly in the soluble form and maintained the river in a hypertrophic state during low flow despite there being no major point source discharges in the catchment. A mass P balance estimate of all domestic wastewater effluents indicated that this is a minor source but may have major impacts at extreme low flows. Secondly, despite the Oona Water being a grassland catchment the main phosphorus fraction recorded was in the particulate form (>50%) and strongly correlated with suspended sediments (SSs), manganese and iron during both storm and non-storm periods. Previous Irish studies have indicated that the main edge-of-field phosphorus transfers from grassland soils are in the soluble form. While erosive overland flow cannot be ruled out from soils of low permeability in the Oona Water, it is also likely that soluble P is entrained to equilibrium by manganese and iron rich SSs from multiple sources that will include stream bank and bed sediments. © 2006 Springer Science+Business Media B.V.