To test how well use of discrete trophic levels represents pelagic trophic structure, we compiled dietary data from > 200 lake trout and pelagic forage fish populations and calculated a continuous (fractional) measure of trophic position for each population. Although discrete trophic levels qualitatively represent broad-scale patterns in trophic structure, pelagic food webs are characterized by complexity and omnivory, thereby limiting the ability of discrete trophic levels to quantitatively represent trophic structure in terms of mass transfer and energy flow. Lake trout trophic position, which ranged from 3.0 to 4.6, explained 85% of the between-lake variability in mean PCB levels in lake trout muscle tissue, providing a significant improvement over the use of discrete trophic levels as a predictor of contaminant levels. Having demonstrated the utility of trophic position, we developed a generalized "trophic position model" of lake trout food webs. This approach eliminates minor trophic linkages, calculates a fractional measure of each species' trophic position, and aggregates species of similar trophic position into trophic guilds. This "realized" model represents trophic structure in terms of mass transfer and accounts for the complexity and omnivory that characterize aquatic food webs. In our trophic position model, smelt (a species of pelagic forage fish) were designated a trophic guild separate from other pelagic forage fish, due to their elevated trophic position. Separate consideration of smelt was supported by elevated lake trout trophic position, PCB, and Hg levels in lakes containing smelt. Consideration of omnivory caused biomagnification factors (BMFs) to be many times higher than BMFs that ignored omnivory. These omnivory-corrected BMF estimates appeared to be more consistent with values calculated using stable nitrogen isotopes (δ15N), an alternative continuous measure of trophic position. δ15N provided trophic position estimates that generally corresponded with our diet-derived estimates.
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