Warming and freshening coastal waters impact harmful algal bloom frequency in high latitudes

Harmful algal blooms contaminate seafood with toxins and poison humans and wildlife upon consumption. Toxic algae niches are projected to expand in high latitudes, but how the frequency of their blooms will evolve is still little known. Here we use climate models, 14 years of observations and probabilistic models of toxic algae, to assess the frequency of harmful algal blooms in a future warmer world. The warmer ocean temperatures increase the blooms in spring and autumn. However, the blooms reduce in summer as surface waters become excessively warm. Freshening reduces the blooms of species confined to high salinity ranges and has no effect on increasing the blooms. In a 3 °C warmer world, the blooms of D. acuta might increase by 50% and A. tamarense complex reduce by 40% along the Norwegian coast. Therefore, humans and wildlife are likely to become more exposed to diarrheic toxins and less to paralytic toxins. Warming and freshening of coastal waters is likely to increase the frequency of harmful algal blooms in autumn and spring and reduce blooms in summer, according to analysis of in-situ and satellite observations of toxic algae and ocean properties.