1Fisheries and Oceans Canada, P.O. Box 5050, Burlington, Ontario L7R 4A6, Canada. E-mail:munawarm@dfo-mpo.gc.ca
2Plankton Canada, 685 Inverary Rd, Burlington, Ontario L7L 2L8, Canada
3Department of Zoology, University of Guelph, Guelph, Ontario N1G 2E6, Canada
4Department of Dentistry, University of Toronto, Toronto, Ontario Canada
5Austrian Academy of Sciences, Mondsee, Austria

From phytoplankton to microbial food web of the Laurentian Great Lakes: Status and synthesis

The North American Great Lakes represent an immense natural resource which has attracted an enormous concentration of human activities. Human populations have been taking advantage of the vast supply of fresh water and fertile land in the Great Lakes' watershed, as well as their relatively large resilience and capacity for resisting cultural eutrophication and contamination. Fisheries and Oceans Canada has conducted extensive phycological surveys during the past 30 years in the Great Lakes with standard techniques and protocol. These studies laid the foundation of the current microbial research being carried out in these large ecosystems.

It has become increasingly obvious in recent years that the "classical" concept of the phytoplankton being connected to solar radiation, inorganic carbon, nutrients for principal input, and to zooplankton as consumers is too simple to describe the planktonic food web adequately. Most of the algae is single celled. Links of algae to other unicellular groups are well known and their importance has been documented thoroughly. Having completed an overall picture of algal populations in the Great Lakes, we focused our research on other components of the microbial food web, which included autotrophic picoplankton, bacteria, heterotrophic nanoflagellates, and ciliated protozoa. The mean abundance of autotrophic picoplankton was compared for the summer of 1991. Lake Erie picoplankton was most abundant, reaching on average more than 100,000 cells ml-1; this was matched only in Lake Huron with more than 80,000 cells ml-1. The lowest mean value was found in the Detroit River. This could be attributable to the presence of toxic contaminants along the St. Clair River-Lake St. Clair-Detroit River corridor harbouring several petrochemical industries. Bacteria appear to vary the least across the Great Lakes ranging between 2.5 million cells ml-1 to 700,000 cells ml-1. The heterotrophic nanoflagellates differed substantially in various lakes. The ciliates showed an increasing trend with lowest abundance recorded in Lake Superior and highest in Lake Ontario.

Our recent results indicate that the smallest size group of phototrophs, the picoplanton (<2.00 µm), previously neglected, may represent a substantial part of phytoplankton biomass and production. These minute organisms are also known to be sensitive to pollution and contaminants. Recent research of the 1990s, by means of mean size-fractionated primary production results, suggest the changing size structure of the photosynthetic communities in Lakes Erie and Ontario which indicated dominance of picoplankton (2.00 µm) and nanoplankton (2-20 µm) and the elimination of net phytoplankton (>20.00 µm). The presentation will review the progress made in phycological and microbial research during the past 30 years with current emphasis on the microbial food web interactions of the Great Lakes.