Abstract
Lake Huron, the third largest freshwater lake in the world, has become increasingly oligotrophic since the early 1990s and is also currently experiencing substantial changes in species abundances and diversity. We used long‐term biomonitoring data to calculate whole‐body energy densities for Lake Huron rainbow smelt ( Osmerus mordax ), lake trout ( Salvelinus namaycush ), and also for herring gull ( Larus argentatus ) eggs collected from 1989–2011. Significant temporal declines in energy densities were observed for all three species, and significant declines were found for lake trout size‐at‐age and herring gull egg sizes. For lake trout and herring gulls, these declines occurred during a period of reduced abundance of pelagic forage fish and increased inclusion of terrestrial food in diets of herring gulls. Declining energy density in rainbow smelt occurred concomitant with declining phytoplankton and zooplankton resources, the principal energy source for this species. Reduced production capacity in lower trophic levels appears to be affecting upper trophic levels of the Lake Huron food web, emphasizing the cascading effects of ecosystem change.