BIOMASS SIZE SPECTRUM OF THE LAKE-MICHIGAN PELAGIC FOOD WEB

Biomass size spectra for the complete Lake Michigan pelagic food web from picoplankton to salmonids were constructed for nine sampling transects around the lake in May and in September 1987. Size spectra were typical for freshwaters, having distinct peaks corresponding to major size groups. Biomass concentration of algae, zooplankton, and planktivores conformed to particle-size model predictions, but piscivore biomass was lower than predicted because these species are stocked. Mean annual total pelagic biomass wa 72.3 g.m-2 compared with a predicted range of 78.8-85.3 g.m-2. Potential production of piscivores, Mysis, and Pontoporeia was in agreement with model predictions. No estimates of zooplankton or planktivore production were available, but we calculated that these could be 72.1-91.6 and 2.5-4.1 g.m-2, respectively. Our analyses suggest that piscivore production is constrained by food web structure. Bloater, which comprise 72% of planktivore biomass, make up less than 20% of salmon diets. We estimate that piscivore production could be double the current value of 0.27 g.m-2.yr-1 if the forage fish community changed to include species more available to salmon.