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 activities, 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 largely completed by 1890. Erosional P inputs to Lake Simcoe sediments increased 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 (Salvelinus namaycush), lake whitefish (Coregonus clupeaformis ), and lake herring (Coregonus artedii), occurred during the 1960s, 1970s, and 1980s, respectively, as hypolimnetic water quality deteriorated. The volume-weighted, temperature-corrected hypolimnetic dissolved oxygen concentration 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°C thermal habitat volume had < 7 mg·L -1 dissolved oxygen, and 10-50% had <3m g · L -1 , the incipient lethal threshold for lake trout. Reduction of P loads to surface waters and restoration of cold-water fish habitat are now the explicit management goals of the Lake Simcoe Environmental Management Strategy. Resume : Nous avions pour objectif une etude de cas faisant le lien entre l'utilisation humaine du territoire, la charge en phosphore, l'epuisement de l'oxygene dissous dans l'hypolimnion et la perte d'habitat du poisson d'eau froide dans le lac Simcoe au Canada. Le deboisement a des fins agricoles s'est amorce dans les annees 1820 et il etait pratiquement acheve en 1890. La charge en phosphore mesuree dans les sediments du lac Simcoe, et associee a l'erosion, s'est accrue pour passer de 30 t·an -1 a 100 t·an -1 dans les annees 1940. Les charges en phosphore d'origine ponctuelle sont apparues pour la premiere fois vers 1910 et elles sont restees assez peu importantes, soit a 2-3 t ·an -1 , jusque vers les annees 1960, pour ensuite s'elever a environ 20 t·an -1 en 1990, tandis que la population dans ce bassin hydrographique s'elevait, passant d'environ 50 000