We measured the concentration of diatom frustules and amorphous silicon per gram of dry sediment at 5-m intervals in the upper water column (0–40 m), in sediment traps placed at 37 and 60 m below the surface, and in a sediment core. The average concentration of frustules per gram of dry suspended sediment in the water column was 3.14 × 108. The 37 and 60 m trap sample averages were 1.16 × 108 and 5.03 × 107 frustules/g dry sediment, respectively. Subsamples from the sediment core averaged 6.31 × 106 frustules/g dry surficial sediment. The average proportion of amorphous silicon/g dry sediment was 8.6% in the water column, 7.7% in the 37 m traps, 6.0% in the 60 m traps, and < 2.0% in subsamples from the sediment core. The major fraction of amorphous silicon produced annually as diatom frustules was decomposed before incorporation in the permanent sediment. A comparison of the annual silicon requirement for diatom production and silicon inputs showed that the watershed contributes < 5.0% of the dissolved reactive silicon required for annual diatom production. These observations suggest that recycling of biogenic silicon provides the major source of soluble reactive silicon required for diatom blooms in Lake Michigan. Key words: Lake Michigan, diatoms, sedimentation, sediment traps, dissolution, deposition, silicon, sources, sinks, recycling