PELAGIC FOOD-CHAIN STRUCTURE IN ONTARIO LAKES - A DETERMINANT OF MERCURY LEVELS IN LAKE TROUT (SALVELINUS-NAMAYCUSH)

The trophic structure of pelagic communities in glaciated regions is highly variable due to restricted dispersal of glacial relict taxa and recent species introduction. Much of the between-lake variation in Hg in lake trout (Salvelinus namaycush) flesh from the St. Lawrence system (non-point-source contaminated lakes), which spans more than two orders of magnitude (0.03-3.96 mug/g), results from differences in the length of pelagic food chains. Hg levels from the longest food chains where pelagic forage fish and the crustacean Mysis relicta were present were about 3.6-fold higher than those from the shortest food chains where these last two trophic components were missing. A lack of correlation between Hg levels in small-mouth bass (Micropterus doiomieu), which do not exploit the pelagic food chain, and the length of the pelagic food chain confirmed that the positive relationship observed in lake trout could not be attributed to the confounding effect of some physicochemical factors covarying with our pelagic food chain classification. Our results confirmed that the food chain biomagnification model developed by Rasmussen et al. (1990. Can. J. Fish. Aquat. Sci. 47: 2030-2038) to predict PCB levels in lake trout can also be applied to Hg biomagnification.