Littoral and Pelagic Energy Sources in Food Webs of Recovering Lakes

I studied how the flow of energy through benthic and pelagic pathways influence the biological recovery of whole lake food webs from acidification and metal contamination. Fish communities generally depend on littoral and pelagic energy pathways in roughly equal amounts. In Sudbury lakes the benthic invertebrate community is depauperate from decades of metal contamination which may disrupt the ability of fish to rely on littoral energy. I hypothesized that a depauperate benthic invertebrate community would negatively affect the omnivorous fish and higher trophic levels by limiting their diet choices to stronger pelagic pathways.;Previous studies had shown that the littoral benthos in Sudbury lakes lacked larger-bodied species, but my closer investigation of the littoral invertebrate community showed no total biomass difference between the impacted Sudbury lakes and reference systems. A multivariate analysis revealed strong differences in the factors influencing invertebrate biomass between the two areas, and suggested that Sudbury invertebrate communities were strongly influenced by fish predation and availability of organic matter, underlain by a still very strong toxicity effect. Since no difference in total biomass between Sudbury and a reference condition was observed, I suggest that Sudbury invertebrate communities are in an alternate stable state that is potentially as productive as a reference system, albeit with a simpler community that is adapted to higher levels of metals in water and the surrounding sediment.;Furthermore, I examined fish trophic positions in the food web as well as the use of littoral and pelagic food sources by fish using stable carbon and nitrogen isotopes. Trophic position, littoral reliance of fish, and the importance of littoral (benthic invertebrate) relative to pelagic (zooplankton and Chaoborus) resources were compared between Sudbury lakes and a set of reference lakes. In the reference lakes, the trophic position of fish increased with increasing benthic invertebrate biomass. In lakes with a depauperate littoral invertebrate community, yellow perch trophic position declined with increased use of littoral food sources, suggesting a lower food quality compared to the reference lakes. In Sudbury lakes a potentially poorer quality of especially smaller benthic invertebrates resulted in a stronger reliance by fish on pelagic resources, while maintaining the same trophic positions they held in the reference lakes. I suggest that impaired subsidizing effects from littoral energy pathways increase the use of pelagic pathways, wherein perch that normally specialize on benthic invertebrates rely on zooplankton (mainly Chaoborus) to maintain population biomass.;While the aquatic ecosystem in Sudbury lakes appeared to have similar productivities (standing crop biomasses) to reference systems, the factors shaping the different trophic levels functioned significantly differently. Even though the food web dynamics appeared stable, benthic invertebrate communities were strongly influenced by both direct and indirect effects of fish and underlying chemistry. Furthermore, differences in energy pathways leading towards the fish community suggested that these ecosystems are stable but not yet recovered and may still be strongly influenced by the direct and indirect effects of metal contamination and acidification.;Forage fish are also affected from the lack of top-down piscivores and another legacy of metal contamination in Sudbury lakes. I experimentally increased trophic complexity in one lake by adding smallmouth bass to study the effects of benthic-pelagic coupling. The hypothesis was that cascading predation effects would include yellow perch shifting from pelagic to littoral habitats as well as shifting their diet accordingly. A higher reliance on benthic invertebrates might also be expected as a consequence of yellow perch spending more time in the sheltered littoral habitat. I monitored density, habitat use and diet changes in the fish community as well as benthic invertebrate biomass. A predator introduction experiment showed a strong effect wherein perch inhabiting the littoral and pelagic areas of the lake had very different diets. While littoral perch remained strongly reliant on benthic invertebrates, pelagic perch fed more on open-water food sources, contradicting my prediction of a stronger overall littoral diet choice.