The microbial loop is now recognized as a component of planktonic food webs. To what extent it serves as a source or sink to the classic food web of algae, zooplankton and fish is debated. Studies of the effect of enrichment on community structure and fish stocks in oligotrophic British Columbia lakes suggests that large cladoceran zooplankton are "keystone" species determining the linkage of picoplankton production to higher trophic levels. If suitable algal food is present in a lake to support large Daphnia populations, they may utilize some of the picoplankton production and provide a short, direct food chain to fish. This research will develop radioisotope methods for tracing and quantifying links between trophic levels which can be used subsequently to investigate these suggestions.
