Origin and Fate of Harmful Algal Blooms in the Warming Chukchi Sea

The waters of the Alaskan Arctic (the northern Bering, Chukchi, and Beaufort seas) are undergoing rapid and profound environmental and ecological changes as a result of ocean warming. Many organisms are spreading and flourishing in Arctic waters as a result of climate warming, but few present as significant a threat to human and ecosystem health as harmful algal bloom (HAB) species.  HABs (sometimes called red tides) are accumulations of microscopic algae that cause harm in a variety of ways, with a key mechanism being the production of potent toxins responsible for illness and death in humans and wildlife.  In the Alaskan Arctic, the major concerns are diatom species in the Pseudo-nitzschia genus and one dinoflagellate species called Alexandrium catenella.  The former produce domoic acid (DA) and the latter saxitoxin (STX), both potent neurotoxins.  These toxins can enter the marine food web when the algae are eaten by filter-feeding animals such as clams or fish (e.g., herring), and the toxins can then be transferred to higher levels when these animals are consumed by predators like crabs, walrus, seals, seabirds, and whales.  Illnesses and death are thus possible not only for these animals, but for humans who consume them.  Shellfish have historically been considered the primary source of dietary exposure for these toxins globally, but Alaskan Arctic residents rely on a large diversity of marine resources for food, adding a new and poorly understood dimension to the threat from HABs.

When this project began, it was known that Alexandrium catenella occasionally occurred north of Bering Strait, but bloom events were rare and local residents were unaware of any danger to them or to the ecosystems they rely on for food.  Supported by data from one survey cruise supported by this project and multiple “ship of opportunity” cruises supported by other projects, we characterized an exceptionally large and dense A. catenella cyst accumulation zone (cyst bed) in the region.  A cyst is a dormant cell in the A. catenella life history that lies dormant in bottom sediments until temperatures are supportive of the germination (hatching) that inoculates surface waters with the swimming cells that form blooms.  As cells deplete the nutrients they need for growth, new cysts are formed that fall to bottom sediments, completing the cycle.  To highlight the threat that these cysts represent, the Arctic Alexandrium cyst bed is at least 15 times larger and denser than an equivalent feature along the US East Coast in the Gulf of Maine where large and dangerous toxic blooms are annually recurrent.  We have also shown that in warm years, climate-driven warming of surface and bottom waters in the region can now support complete life-cycle progression for Alexandrium – meaning that the cells can germinate, bloom, and form new cysts, all within the Arctic. 

We identified two mechanisms for blooms and toxicity in the Alaskan Arctic: The first is transport of blooms that originate in the Northern Bering Sea and travel through the Bering Strait to Arctic waters via natural currents, impacting communities and ecosystems far from the origin of those blooms.  Oceanographic analyses suggest that the source of these transported blooms is the western Bering Sea, in Russian waters.  The second mechanism for bloom formation is the germination of cysts from the Arctic cyst bed, leading to what are called locally initiating blooms.  The size of the huge Arctic cyst bed is thought to arise from repeated cyst formation and deposition events from transported blooms, with minimal losses because of suppressed germination during cold years.  This was observed in 2022 when the largest A. catenella bloom ever reported was transported from the south through Bering Strait and into northern waters, while cold bottom temperatures limited germination from the Arctic cyst bed. In other words, only one mechanism was observed that year, leading to deposition of many more cysts than germinated, increasing the size and density of that cyst bed.  In other field surveys during the warmer years of this project, both bloom mechanisms were observed to be in action. 

For the Pseudo-nitzschia HAB species, domoic acid was found to be present in animals at multiple levels of the food web, but concentrations were lower than for saxitoxins, suggesting that Alexandrium and saxitoxins are the most significant threat to wildlife and human health. 

Overall, the unique nature of the Alaskan Arctic, the lack of scientific understanding of HAB impacts on marine wildlife, and the reliance of coastal populations on subsistence harvesting for nutritional, cultural, and economic well-being poses new and significant challenges that need to be addressed as this region continues to warm and the potential impacts from HABs expand. This project has contributed substantially to the growing knowledge base needed to understand and manage these threats.