Compensatory Response of the Lower Food Web and Larval Fish Growth and Survival to Multiple Stressors in Lake Michigan and Lake Huron

Reductions in nutrient loads and impacts from invasive species may lower recruitment potential of Great Lakes fishes. Filtration by invasive quagga mussels has reduced phytoplankton biomass, shifted energy flow from pelagic to benthic pathways, and restricted phosphorus transport from nearshore to offshore.  Biomass of large cladocerans and adult planktivorous fishes has declined from impacts of quagga mussels and the predaceous cladoceran, Bythotrephes. These multiple stressors have altered zooplankton distributions and densities, and may lower larval fish growth and survival. Yet, in Lakes Michigan and Huron, some fish species still grow and recruit well in nearshore and offshore areas. To understand what drives fish recruitment, we sampled nutrients, fish larvae, and lower and upper food webs on nearshore-offshore transects from 2010-2013 in Lakes Huron and Michigan. A compensatory shift from large phytoplankton to the microbial food web—utilizable by both copepods and small cladocerans— may have mitigated its loss to zooplankton. In high recruitment years, larvae consumed small cladocerans and copepods, and grew well. Larval foraging efficiency may have increased due to increased light penetration and co-occurrence with zooplankton in the metalimnion. Our results suggest compensatory responses to multiple stressors by some zooplankton species may explain recent trends in fish recruitment.