OBSERVATIONS ON THE SPATIAL AND TEMPORAL VARIATION IN THE PHOSPHORUS STATUS OF LAKES IN THE BRITISH-ISLES

An examination of total phosphorus (TP) concentrations from 902 lakes in England, Scotland and Northern Ireland (NI), suggest that only Scot tish lakes have a high percentage (73%) of oligotrophic craters (TP<10 mu g Pl(-1)). The TP status of upland lakes in NI was greater than Sc ottish lakes particularly if lake catchments mere afforested. Although lowland lakes in NI drain a predominately non-urbanized landscape, 38 % of lakes below 100 m had TP concentrations > 100 mu g Pl(-1) and onl y 29% < 35 mu g Pl(-1). English lakes tended to have higher TP concent rations (70% > 100 mu g Pl(-1)) which may reflect P inputs from sewage treatment works (STWs) although lakes draining agricultural catchment s frequently produced high TP concentrations. Between 1985 and 1995, a nnual point source TP inputs to Loch Leven, Scotland, declined by 8 to nnes P or 40% of the 1985 TP loadings to the Loch. As point source inp uts were proportionally richer in dissolved morybdate reactive phospho rus (MRP) than diffuse inputs, the MRP loading was reduced by 46%. Fro m 1974 to 1995, TP concentrations in Lough Neagh (NI) increased despit e 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 inf luenced 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 MR P loadings observed in the Lough Neagh catchment has been repeated els ewhere.