The waters of western and northern Alaska are warming, increasing the risk of blooms of the toxic dinoflagellate Alexandrium catenella. This species produces a dormant cyst that accumulates in sediments and germinates to initiate surface blooms. This talk synthesizes eight years (2018–2025) of observations, focusing on cyst beds documented in U.S. waters of the Bering, Chukchi, and Beaufort seas that represent the largest and densest Alexandrium cyst deposit globally.  These areas and cyst concentrations are 15–20 times greater than similar features elsewhere. The massive cyst bed in the Chukchi Sea reflects repeated deposition from blooms originating in the northern Bering Sea that are transported northward through the Bering Strait, with minimal germination in colder years; inputs often exceed losses, leading to high cyst abundances.  During 2022 this dynamic was documented when a massive Alexandrium bloom entered Bering Strait, leading to a mass encystment and deposition event, while low bottom temperatures suppressed germination in the area. This 2022 bloom presumably originated from unidentified cyst beds in the Russian waters of the Gulf of Anadyr, northwestern Bering Sea.  Similar blooms initiate in the southeastern Bering Sea, where another large cyst bed was recently characterized. A fourth cyst bed south of Nome was first observed in 2022 and has persisted since.  It is now clear that cyst beds in the Bering, Chukchi, and Beaufort seas operate on different time frames, with bloom initiation and impacts varying dramatically annually and regionally due to variable bottom temperatures and transport pathways. The latter affect marine ecosystems and coastal communities far from bloom origins. These unprecedented cyst beds represent a significant and growing threat to marine ecosystems and coastal communities dependent on marine resources for nutritional, cultural, and economic well-being.

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