Marine harmful algal blooms (HABs) in the United States: History, current status and future trends

•The first effort to quantitatively analyze trends in U.S. marine HAB events.•Some HABs (PSTs and NSTs) show no trend over 30 years; DSTs show no trend over 12 years.•Others (AST, CTXs, Margalefidinium, brown tides, cyanotoxins) increased in frequency and/or geographic extent.•A statistically significant increasing trend is seen for all U.S. HAB events combined.•Part of the expansion is a realization of the true scale of the problem, long obscured by inadequate monitoring.•Increases also linked to species dispersion and stimulation from nutrient pollution, aquaculture, and ocean warming.