Karenia brevis blooms impair human health, marine ecosystems, and coastal economies in the Gulf of Mexico via their production of carbon‐based neurotoxins (brevetoxins), which contain no nitrogen (N) or phosphorus (P). N and P limitation of growth rate substantially increases brevetoxins in this dinoflagellate, consistent with predictions of the carbon nutrient balance (CNB) hypothesis. This hypothesis further predicts that an...