Eutrophication of aquatic systems can induce silica (Si) depletion through enhanced production and sedimentation of diatoms. This may shift the phytoplankton assemblage to include increasing proportions of non-siliceous taxa such as potentially nuisance Cyanobacteria. I examined the role of Si depletion in the eutrophication of Lac la Biche, a large lake in northern Alberta. Paleolimnological analyses indicate that while Lac la Biche has undergone eutrophication since the mid-20 th century, there is no evidence for either Si depletion or Si mediated shifts in the relative proportion of diatoms and Cyanobacteria. However, I show through nutrient amendment experiments that Si depletion can be induced seasonally under specific nutrient regimes. Further, Si amendment significantly increased the proportion of diatoms relative to Cyanobacteria. I conclude that with ongoing eutrophication and anticipated declines in Si loading to aquatic systems associated with drought and climate warming, Si depletion may become increasingly important in structuring phytoplankton communities.