In late spring 2015, the West Coast of North America experienced one of the most toxic algal blooms on record. The bloom affected wildlife, including anchovies, sea birds, and sea lions, and led to the closure of commercial fisheries from California to Washington. Scientists quickly learned that the bloom consisted of diatoms in the genus Pseudo-nitzschia, but they couldn’t tell why these algae had become so toxic. A new paper in Geophysical Research Letters shows that, at least in Monterey Bay, the diatoms in this bloom became particularly toxic because of an unusually low ratio of silicate to nitrate in the waters of the bay.