HEATH, R.,1 AND M. MUNAWAR2

1Department of Biological Sciences, Kent State University, Kent, OH, USA.
2Department of Fisheries and Oceans, Environment Canada, Burlington, ON, Canada

Importance of the microbial shunt in food webs of the Laurentian Great Lakes

Rational management of Great Lakes ecosystems depends on an accurate understanding of those factors that influence uptake and efficient transport of material and energy through the base of the food web. There are two major pathways by which matter (e.g. C and P) and metabolic energy flow from phytoplankton to microcrustaceans: the traditionally investigated grazing food chain, where phytoplankton are directly grazed, and the more recently recognized microbial shunt. The microbial shunt depends on bacterial production using autochthonous and allochthonous dissolved organic matter; bacterial production is then grazed by bacterivorous protists, rotifers and microcrustaceans. We have compared the relative importance of the grazing food chain and the microbial shunt in communities of differing trophic status in Lake Erie and Lake Superior. Here we report a general pattern that the microbial shunt is more important in oligotrophic communities than in eutrophic communities and more important in offshore communities than in coastal and nearshore communities. Our findings are useful in constructing an assessment of ecosystem health based on efficiency of energy and material flow.