Sediment cores were taken from Friary Lough, a small eutrophic lake in Co. Tyrone. Whole-lake-basin phosphorus accumulation rates were measured by chemical analysis and sediment dating and historical phosphorus outflow fluxes were estimated using a diatom inferred - total phosphorus (DI-TP) reconstruction. The summation of both sedimentary and outflow losses, in a reformulation of the Vollenweider (1975) steady-state lake phosphorus model, yielded a phosphorus loading history from 1906 to 1995. Results show P loading to have been unchanging from 1906 to c.1945 at 0.55-0.72kg ha-1 yr-1. Since c.1946, P loads have steadily increased up to 1.10-1.35kg ha-1 yr-1, which, when adjusted to take account of variable runoff, is an increase of 2.2-2.8 micro-mg TP L-1 yr-1 in influent waters over the 50 year period. Changes in P loading are broadly coincident with a change in landuse to exclusively grassland agriculture.

Results were augmented with sedimentary iron, manganese, calcium and sodium analyses and show that internal P loading, while operating in the hypolimnion, is not strong enough to excessively fertilise the epilimnion and incur losses to the outflow. They also show that whole-basin sediment yields have been largely unchanging since 1906 at 10t km-2 yr-1 despite a change from mixed to grassland agriculture. The exception is deltaic sedimentation which appears to have increased over the dated period and especially since c.1946.

Total P loads from drumlin soils appear to be large and associated with a high particulate (PP) fraction. Losses to the sediments are greater than outflow losses and an analysis of lake P models with the reconstructed data is presented.

The main lake inflow sub-catchment (61ha) was gauged and sampled for phosphorus fractions over a 12 month period and also intensively during eight storm events. Data were used to construct a contemporary P influent budget which, for TP, was in agreement with the most recent lake P load reconstruction. Particulate P was found to be the dominant fraction with large storms yielding PP loads which were 60-70% of the total P storm load. Storms were separated into fast (overland and preferential) flowpaths and soil-groundwater matrix flow using a silica-based tracing model. The particulate P fraction showed the greatest dependence on the volume of fast flowpaths in a storm event but was not mirrored by the soluble reactive phosphorus fraction. This may be due to SRP leaching into matrix water or P transformation during transport. Results were augmented with soil textural and nutrient analyses which show heavily gleyed drumlin caps and freer draining side slopes, and an area-weighted Olsen P concentration of 36mg kg-1. The implications for modelling and managing P loss are discussed with regard to hydrological paradigms.