Poor water quality in Loch Leven (Scotland) in 1995 in spite of reduced phosphorus loadings since 1985: the influences of catchment management and inter-annual weather variation

Eight tonnes of phosphorus in all forms (total phosphorus, TP) entered Loch Leven from its catchment in 1995, compared to 20 t in 1985. Diffuse run-of f from the land, and waste from over-wintering geese contributed 59% of the total loading in 1995 cf. 42% in 1985. Point-sources of sewage, and waste- water from fish-rearing ponds, produced the rest. Inputs of phosphorus in s oluble reactive form (SRP) totalled 5 t, i.e. 63% of the TP loading in 1995 , as compared with 1985 values of 11.8 t and 59%. Point-sources of SRP cont ributed 54% of the total SRP input in 1995 cf. 69% in 1985. Loadings from t hree sewage treatment works (STW) totalled 3.1 t TP in 1995 as compared wit h 5.3 t in 1985; this included 2.6 t SRP (cf. 3.6 t). Daily per capita outp uts of the upgraded Kinross North and Milnathort STWs were 0.68 g and 0.81 g TP, respectively, compared with pre-upgrade values of 1.77 g and 2.03 g. Nett reductions in TP and SRP loadings between 1985 and 1995, are 55% and 5 9% respectively. These values are attributable as much to the lower rainfal l of 890 mm over the period of study in 1995, compared to 1250 mm in 1985, as to 'managed' elimination of P usage at a major industrial source, and up grades of STWs. In spite of these cutbacks, a combination of the lower rain fall and an extraordinarily hot summer in 1995 negated the expected reducti on in lake phosphorus and chlorophyll levels. The lowered specific areal lo ading of ca. 0.7 g P m(-2) estimated for 1995 still considerably exceeds th e ideal maximum for the loch. These statistics nevertheless ignore the sign ificance of a reduction of ca. 7 t in P entering the system in bio-availabl e form, a recently completed upgrade of a major STW and channelling of effl uent from a small works out of the catchment.