CS34 River Dynamics
Date: Wednesday, June 12, 2002
Time: 4:45:00 PM
Location: View Royal
 
deBruynAM, Dept. of Biol., McGill Univ., Montreal, QC, Canada, adrian.debruyn@mail.mcgill.ca
McCann, K, S, Dept. of Biol., McGill Univ., Montreal, QC, Canada, 
Rasmussen, J, B, Dept. of Biol., McGill Univ., Montreal, QC, CANADA, 
 
VARIABLE TROPHIC CONTROL IN A SEWAGE-ENRICHED RIVER FOOD WEB
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The classic dialectic of top-down vs. bottom-up control in food webs is slowly being replaced by a recognition that consumer and resource effects jointly determine trophic level biomass. A central feature of this synthesis has been the observation that food chains are rare in nature: trophic linkages are complex because prey are not uniform in their vulnerability to predation. We studied the effect of prey heterogeneity on trophic control in the littoral food web of the St. Lawrence River by exploiting a natural sewage enrichment experiment across a gradient of habitat structure. At sewage-enriched sites, we observed a dramatic increase in abundance of piscivores (top predators) and large suckers (invulnerable intermediate consumers), but no change in smaller benthivorous fishes (vulnerable intermediate consumers). These effects cascaded to the level of benthic invertebrates (primary consumers), permitting an increase in the prey of benthivores, but no increase in the prey of suckers. Furthermore, the relative magnitudes of these increases in abundance were related to habitat structure. Our results are consistent with the predictions of a simple food web model that recognizes heterogeneity within each trophic level.