Seasonal Periodicity of Diatoms, and Silicon Limitation in Offshore Lake Michigan, 1975

Diatom biomass maxima occurred in spring and fall and produced a bimodal bloom sequence at an offshore Lake Michigan station. The maximum in May was preceded by rapid growth as indicated by increased values of primary productivity and pigment concentration. As the spring bloom progressed, decreasing nutrient levels apparently slowed diatom growth. The diatom-biomass accumulation rate declined, assimilation quotients were minimal, and soluble reactive silicon was reduced from 13 to 6.8 μmol/ℓ. In summer, after the bloom, diatom biomass and silicon reached seasonal minima of < 250 mg C/m2 and ~ 1.0 μmol/ℓ, respectively. Diatom biomass increased again in October when silicon supplies were replenished and the concentration exceeded 6.6 μmol/ℓ. A critical silicon concentration of ~ 6.5 μmol/ℓ, may control the development and timing of offshore diatom populations in Lake Michigan. Key words: Lake Michigan, diatoms, growth, biomass, periodicity, silicon, uptake, limitation