The occurrence of a high biomass bloom of a vitamin auxotroph along with significant B12 uptake by picoplankton accounts for vitamin B12 turning over several times daily during blooms and limiting the growth of C. polykrikoides nearly as much as nitrogen, usually the limiting nutrient in these systems C. polykrikoides takes up B12 faster than microplankton outside of the blooms, and its large cellular demand and the more than daily B12 turnover during blooms likely leads to the limitation frequently observed since incubations were 24–48 h