Lower Food Web Dynamics in the Great Lakes: Implications for Changes in Prey Fish Production

The relative importance of resource driven (bottom-up) as opposed to predation driven (top-down) factors in structuring aquatic ecosystems has been a dominant concern in limnology for the past 30 years.  Some of the earliest support for top-down structuring of zooplankton communities came from the Great Lakes.  Recent changes in the lower food webs of Lake Michigan and Lake Huron, however, suggest that primary production might be playing an overriding role in structuring zooplankton populations, and possibly also impacting prey fish populations.  In Lake Huron, the magnitude of the spring phytoplankton bloom has declined dramatically in recent years, beginning in 2003.  This has been accompanied by dramatic population declines of the two dominant cladocerans in the lake, Daphnia mendotae and Bosmina longirostris.  Average non-predatory cladoceran biomass in the lake declined by over 90% between 1998-2002 and 2003-2006.  In addition, the reductions in spring chlorophyll have coincided closely with declines in deep-living populations of the benthic amphipod Diporeia, an important fish food item.  A similar situation has occurred in Lake Michigan, where reductions in chlorophyll have coincided with decreases in cladoceran populations.  Additionally, population densities of the large hypolimnetic calanoid Limnocalanus macrurus have increased in recent years, closely coinciding with increases in summer epilimnetic water clarity and suggesting a response to increased food resources in the deep chlorophyll maximum.  In the case of both lakes, the declines in lower trophic levels have corresponded with declines in prey fish densities, which points to potential bottom-up control of these fish populations.