I compared changes in offshore Lake Ontario major species-group biomass, production and diets before (1987-1991) and after (2001-2005) invasion-induced ecological change. I synthesized the observations into carbon-based mass-balanced food webs linking two pathways of energy flow; the grazing chain (phytoplankton-zooplankton-fish) and the microbial loop (autotrophic bacteria-heterotrophic protozoans) and determined how the structure and function of the food web changed between time-periods. I use the food web descriptions to simulate the reestablishment of native deepwater bloater. I developed empirical models describing spatial variation in temperature and applied them to investigate predator temperature distributions, bioenergetic consequences of alewife diet and distribution shifts, and zooplankton productivity. Primary production declined as did the biomass and production of all species-groups except Chinook salmon. Total zooplankton production declined by approximately half with cyclopoid copepod production declining proportionately more. Zooplankton species richness and diversity were unaffected. Alewife adapted to low zooplankton production by consuming more Mysis, increasing their trophic level. The increased prey-size and exploitation of spatial heterogeneity in resource patches and temperature may have allowed alewife to maintain their growth efficiency. The trophic level also increased for smelt, adult sculpin, adult alewife and Chinook salmon. Phytoplankton grazing rates declined and predation pressure increased on Mysis, adult smelt and alewife, and decreased on protozoans. Resource to consumer trophic transfer efficiencies changed; increasing for protozoans, Mysis, Chinook salmon and other salmonines and decreasing for zooplankton, prey-fish and benthos. The changes suggest both bottom-up and top-down influences on food web structure. The direct trophic influences of invasive species on the offshore Lake Ontario food web were minor. Carbon flows to Mysis indicated an important, and changing ecological role for this species and we hypothesize that Mysis may have contributed to Diporeia declines. Simulations suggest that only a small reestablished bloater population, limited by Diporeia production, could be sustained.

I compared changes in offshore Lake Ontario major species-group biomass, production and diets before (1987-1991) and after (2001-2005) invasion-induced ecological change. I synthesized the observations into carbon-based mass-balanced food webs linking two pathways of energy flow; the grazing chain (phytoplankton-zooplankton-fish) and the microbial loop (autotrophic bacteria-heterotrophic protozoans) and determined how the structure and function of the food web changed between time-periods. I use the food web descriptions to simulate the reestablishment of native deepwater bloater. I developed empirical models describing spatial variation in temperature and applied them to investigate predator temperature distributions, bioenergetic consequences of alewife diet and distribution shifts, and zooplankton productivity. Primary production declined as did the biomass and production of all species-groups except Chinook salmon. Total zooplankton production declined by approximately half with cyclopoid copepod production declining proportionately more. Zooplankton species richness and diversity were unaffected. Alewife adapted to low zooplankton production by consuming more Mysis, increasing their trophic level. The increased prey-size and exploitation of spatial heterogeneity in resource patches and temperature may have allowed alewife to maintain their growth efficiency. The trophic level also increased for smelt, adult sculpin, adult alewife and Chinook salmon. Phytoplankton grazing rates declined and predation pressure increased on Mysis, adult smelt and alewife, and decreased on protozoans. Resource to consumer trophic transfer efficiencies changed; increasing for protozoans, Mysis, Chinook salmon and other salmonines and decreasing for zooplankton, prey-fish and benthos. The changes suggest both bottom-up and top-down influences on food web structure. The direct trophic influences of invasive species on the offshore Lake Ontario food web were minor. Carbon flows to Mysis indicated an important, and changing ecological role for this species and we hypothesize that Mysis may have contributed to Diporeia declines. Simulations suggest that only a small reestablished bloater population, limited by Diporeia production, could be sustained.