Observations on the spatial and temporal variation in the phosphorus status of lakes in the British Isles

Abstract. An examination of total phosphorus (TP) concentrations from 902 lakes in England, Scotland and Northern Ireland (NI), suggest that only Scottish lakes have a high percentage (73%) of oligotrophic waters (TP<10 μg PI^-1). The TP status of upland lakes in NI was greater than Scottish lakes particularly if lake catchments were afforested. Although lowland lakes in NI drain a predominately non-urbanized landscape, 38% of lakes below 100 m had TP concentrations > 100 μg PI^-1 and only 29% <35 μg PI^-1. English lakes tended to have higher TP concentrations (70% > 101 μg PI^-1) which may reflect P inputs from sewage treatment works (STWs) although lakes draining agricultural catchments frequently produced high TP concentrations. Between 1985 and 1995, annual point source TP inputs to Loch Leven, Scotland, declined by 8 tonnes P or 40% of the 1985 TP loadings to the Loch. As point source inputs were proportionally richer in dissolved molybdate reactive phosphorus (MRP) than diffuse inputs, the MRP loading was reduced by 46%. From 1974 to 1995, TP concentrations in Lough Neagh (NI) increased despite reduced TP inputs from STWs. Partitioning of annual TP loadings from two major inflowing rivers to Lough Neagh, showed river MRP loadings from non-point sources had been increasing at annual rates of 1.9 and 2.3 kg P km^-2. The remaining non-MRP river loadings had not been influenced by lower TP loads from STWs and showed no tendency to increase with time. Insufficient data is available from other lake systems in the British Isles to judge whether the increase in non-point source MRP loadings observed in the Lough Neagh catchment has been repeated elsewhere.