HISTORICAL LAND-USE, PHOSPHORUS LOADING, AND LOSS OF FISH HABITAT IN LAKE-SIMCOE, CANADA

Our purpose was to develop a case study linking human land-use activit ies, phosphorus (P) loading, hypolimnetic dissolved oxygen depletion, and loss of cold-water fish habitat in Lake Simcoe, Canada. Conversion from forested to agricultural land began during the 1820s and was lar gely completed by 1890. Erosional P inputs to Lake Simcoe sediments in creased from pre-agricultural loads of about 30-100 t . yr(-1) by the 1940s. Point-source loads of P first appeared about 1910, remained low at 2-3 t . yr(-1) until the 1960s, then increased to about 20 t . yr( -1) by 1990, as human population in the basin increased from about 50 000 to > 250 000 residents. Recruitment failure of lake trout (Salveli nus namaycush), lake whitefish (Coregonus clupeaformis), and lake herr ing (Coregonus artedii), occurred during the 1960s, 1970s, and 1980s, respectively, as hypolimnetic water quality deteriorated. The volume-w eighted, temperature-corrected hypolimnetic dissolved oxygen concentra tion from August 30 to September 19 declined from about 4.5 mg . L(-1) in 1975 to 2.0 mg . L(-1) by 1993. During late summer, 1975-1993, 60- 98% of the 6-12 degrees C thermal habitat volume had < 7 mg . L(-1) di ssolved oxygen, and 10-50% had < 3 mg . L(-1), the incipient lethal th reshold for lake trout. Reduction of P loads to surface waters and res toration of cold-water fish habitat are now the explicit management go als of the Lake Simcoe Environmental Management Strategy.