Physical Drivers of a Massive Harmful Algal Bloom in the Northern Bering and Chukchi Seas in Summer 2022

In summer 2022 the Chukchi Sea exhibited the highest concentrations of the toxin producing dinoflagellate Alexandrium catenella ever recorded in the Arctic, documented by two back-to-back cruises. Here, we use the shipboard hydrographic and velocity data, together with ocean reanalysis fields, to investigate the physical factors that helped dictate the initiation and evolution of the bloom. High concentrations of Alexandrium catenella vegetative cells were first detected west of St. Lawrence Island, within Bering Summer Water, and were subsequently advected poleward. A backward trajectory calculation indicates that the water transporting the bloom originated from the Gulf of Anadyr, then passed through Anadyr Strait and the US side of Bering Strait, reaching 71°N roughly a month and a half later. A strong southerly wind event diverted part of the bloom into Kotzebue Sound and caused it to mix with warmer Alaskan Coastal Water, further promoting cell growth. We also investigate the possibility that part of the bloom was generated locally in the Ledyard Bay region due to germination from the large cyst bed there, as was observed in summer 2018. While such local germination may have occurred in early August, as was the case in 2018, considerably colder near-surface temperatures in 2022 would have slowed vegetative cell growth relative to conditions in 2018. Using mooring data from Bering Strait we demonstrate that the heat flux through the strait largely dictates the timing of cyst germination in the Ledyard Bay region, and also shapes subsequent growth and accumulation of vegetative cells there.