The hypothesis that silica (Si) depletion in Lake Michigan and the severe Si depletion that characterizes the lower Great Lakes were induced by increased phosphorus (P) inputs was supported by bioassay experiments showing increased Si uptake by diatoms with relatively small P enrichments. The authors propose that severe Si depletion (Si concentrations being reduced to less than 0.39 mg SiO L prior to thermal stratification) results when P levels are increased to the extent that increased diatom production reduces Si concentrations to limiting levels during the thermally mixed period. The model proposed for biogeochemical Si depletion is consistent with previous findings of high rates of internal recycling because, under steady-state conditions for Si inputs, any increase in diatom production will produce an increase in permanent sedimentation of biogenic Si provided some fraction of the increased biogenic Si production is not recycled or unless there is a compensating increase in dissolution of diatoms.