Evaluating the evidence for top-down control of the Lake Huron zooplankton community in 2007

Between 2003 and 2007, several trophic level indicators in Lake Huron underwent declines, including chlorophyll, Diporeia, cladoceran and cyclopoid zooplankton, planktivorous fishes, and piscivores.  Using a 2007 monthly field sampling and bioenergetics modeling approach, we focused on the planktivore/zooplankton trophic interaction, seeking to determine whether planktivory by vertebrates (rainbow smelt, bloater) and invertebrates (Mysis relicta, Bythotrephes longimanus) could explain the zooplankton declines.  Between May and September, we compared planktivory to standing stock biomass and daily production of zooplankton.  Overall, vertebrate planktivores consumed 5-11% of the total zooplankton production, and never exceeded 0.2% of standing stock biomass.  Invertebrate planktivores had a far greater impact.  Mysis planktivory ranged 2-18% of zooplankton production, similar to fish.  Bythotrephes planktivory, however, ranged 71-510% of zooplankton production and 1-10% of zooplankton biomass, between June and September.  Given that planktivory exceeded zooplankton production during July-September, we found evidence that predation (primarily by Bythotrephes) limits zooplankton biomass during late summer.  With the changing Lake Huron food web, future research should focus on Bythotrephes as a key planktivore, including work on its diet composition.  Curiously, concurrent stable isotope analyses revealed Bythotrephes to be δ¹⁵N-depleted relative to Diporeia and Mysis, suggesting feeding at a lower trophic level than expected.