THE INFLUENCE OF TROPHIC LEVEL AS MEASURED BY DELTA-N-15 ON MERCURY CONCENTRATIONS IN FRESH-WATER ORGANISMS

The relationship between mercury (Hg) concentrations in freshwater bio ta and trophic position, as defined by stable nitrogen isotope ratios (delta(15)N), was examined in 6 lakes in northwestern Ontario. The hea vier isotope of nitrogen (N-15) increases an average of 3 parts per th ousand (parts per thousand) from prey to predator and is used as a mea sure of an organism's trophic position. Dorsal muscle from lake trout, burbot, walleye, northern pike, white sucker, lake cisco, lake whitef ish, and yellow perch was analyzed for Hg and delta(15)N using flamele ss atomic absorption and mass spectrometry respectively. Within each l ake, log Hg was significantly related to delta(15)N (r(2) ranged from 0.47 to 0.91, P < 0.01). For four species, yellow perch, northern pike , lake cisco, and lake trout, log Hg was positively related to delta(1 5)N (P ranged from 0.37 to 0.47, P less than or equal to 0.09) across all lakes. We also used delta(15)N measurements (assuming a 3 parts pe r thousand shift between an organism and its diet) and the developed w ithin-lake regression equations to calculate a prey Hg for each indivi dual fish. These food Hg values were then used to predict predator Hg using Norstrom et al's bioenergetics model. Predicted results were str ongly correlated to measured Hg concentrations (r = 0.91, P < 0.001), indicating that delta(15)N has potential to be used in modeling.