Migration supports uneven consumer control in a sewage‐enriched river food web

Abstract

Summary We studied the response of the St Lawrence River food web to enrichment by primary‐treated sewage from the city of Montreal, Canada. Despite the richness and complexity of this food web, the openness of the system and the limited production possible in this temperate river's short growing season, we found that biomass densities of consumers responded to enrichment according to the qualitative predictions of a simple bioenergetic model. All consumer trophic levels responded to sewage enrichment, but not all members of each trophic level: top piscivores seemed to control small, benthivorous fish but were unable to control larger fish, so the intermediate consumer level became dominated by Catastomidae (suckers). Consequently, the epiphytic prey of small fish were free to respond to sewage enrichment, but the benthic prey of suckers were not. The important elements of complexity seemed to be the factors that determine the realized pattern of feeding linkages: heterogeneity in prey vulnerability (predator–prey size ratios) and predator efficiency (vegetation cover for ambush predators, inaccessibility of epiphytic prey to bottom‐feeding suckers). Because of the openness of the system, the community was able to respond to enrichment by a combination of demographic (production) and behavioural (migration) mechanisms. We argue that the food web model described an energetically favourable state, and that migration provided a ‘shortcut’ for the system to approach this state within a short growing season.