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1. Harmful Algal Blooms in the Alaskan Arctic: An Emerging Threat as the Ocean Warms

   https://doi.org/10.5670/oceanog.2022.121  

   Anderson, Donald; Fachon, Evangeline; Hubbard, Katherine; Lefebvre, Kathi; Lin, Peigen; Pickart, Robert; Richlen, Mindy; Sheffield, Gay; Van Hemert, Caroline (January 2022, Oceanography)

   Harmful algal blooms (HABs) present an emerging threat to human and ecosystem health in the Alaskan Arctic. Two HAB toxins are of concern in the region: saxitoxins (STXs), a family of compounds produced by the dinoflagellate Alexandrium catenella, and domoic acid (DA), produced by multiple species in the diatom genus Pseudo-nitzschia. These potent neurotoxins cause paralytic and amnesic shellfish poisoning, respectively, in humans, and can accumulate in marine organisms through food web transfer, causing illness and mortality among a suite of wildlife species. With pronounced warming in the Arctic, along with enhanced transport of cells from southern waters, there is significant potential for more frequent and larger HABs of both types. STXs and DA have been detected in the tissues of a range of marine organisms in the region, many of which are important food resources for local residents. The unique nature of the Alaskan Arctic, including difficult logistical access, lack of response infrastructure, and reliance of coastal populations on the noncommercial acquisition of marine resources for nutritional, cultural, and economic well-being, poses urgent and significant challenges as this region warms and the potential for impacts from HABs expands. 

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

   https://doi.org/10.1029/2024JC021624  

   Lago, L S; Pickart, R S; Lin, P; Bahr, F; Fachon, E; Brosnahan, M L; Pathare, M; Muhlbach, E; Horn, K; Rajagopalan, A; et al (April 2025, Journal of Geophysical Research: Oceans)

   Abstract In summer 2022 the Chukchi Sea exhibited the highest concentrations of the toxin producing dinoflagellateAlexandrium catenellaever 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 ofAlexandrium catenellavegetative 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 the 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. 

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3. Fresh Phytoplankton Bloom Growth in the Fall and Its Control on Ocean Heating in the Pacific Arctic Region

   https://doi.org/10.1029/2025JC022895  

   Gaffey, Clare B; Frey, Karen E; Light, Bonnie; Cooper, Lee W; Grebmeier, Jacqueline M (May 2026, Journal of Geophysical Research: Oceans)

   Abstract Delayed sea ice freeze‐up and decreased sea ice extent in the Pacific Arctic region have altered light availability in marine waters, extending the ice‐free season and increasing opportunities for photosynthetic activity and energy exchanges. This study investigates phytoplankton bloom progression during the fall using chlorophyll pigments and examines the impact phytoplankton have on heat distribution within the water column. Measurements of chlorophyll‐aconcentrations and its associated degradation product pheophytin were made in conjunction with optical measurements of light transmittance within the upper water column during the Synoptic Arctic Survey cruise in the Central Arctic Ocean and northeast Chukchi Sea in September‐October 2022. Pheophytin proportions relative to chlorophyll‐agenerally increased over the course of the 2‐month cruise, indicating an overall seasonal decline in biological production. However, renewed phytoplankton growth indicated by reduced pheophytin proportions was observed within the surface ∼20 m of the water column on the Chukchi Shelf in late October. These subsurface blooms absorbed incoming solar radiation and enhanced ocean heating in the surface waters associated with the bloom while shading deeper depths from heating. Log‐linear regressions of heating rates associated each 1 μg L⁻¹increase of chlorophyll‐a(at the observed chlorophyll‐amaximum) with an approximate 15% decrease in heat input within the 10 m below the chlorophyll‐amaximum depth. The altered vertical heat distribution caused by fall blooms trapping heat at the bloom depth can influence upper ocean stratification. 

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4. Evidence for massive and recurrent toxic blooms of Alexandrium catenella in the Alaskan Arctic

   https://doi.org/10.1073/pnas.2107387118  

   Anderson, Donald M.; Fachon, Evangeline; Pickart, Robert S.; Lin, Peigen; Fischer, Alexis D.; Richlen, Mindy L.; Uva, Victoria; Brosnahan, Michael L.; McRaven, Leah; Bahr, Frank; et al (October 2021, Proceedings of the National Academy of Sciences)

   Among the organisms that spread into and flourish in Arctic waters with rising temperatures and sea ice loss are toxic algae, a group of harmful algal bloom species that produce potent biotoxins. Alexandrium catenella , a cyst-forming dinoflagellate that causes paralytic shellfish poisoning worldwide, has been a significant threat to human health in southeastern Alaska for centuries. It is known to be transported into Arctic regions in waters transiting northward through the Bering Strait, yet there is little recognition of this organism as a human health concern north of the Strait. Here, we describe an exceptionally large A. catenella benthic cyst bed and hydrographic conditions across the Chukchi Sea that support germination and development of recurrent, locally originating and self-seeding blooms. Two prominent cyst accumulation zones result from deposition promoted by weak circulation. Cyst concentrations are among the highest reported globally for this species, and the cyst bed is at least 6× larger in area than any other. These extraordinary accumulations are attributed to repeated inputs from advected southern blooms and to localized cyst formation and deposition. Over the past two decades, warming has likely increased the magnitude of the germination flux twofold and advanced the timing of cell inoculation into the euphotic zone by 20 d. Conditions are also now favorable for bloom development in surface waters. The region is poised to support annually recurrent A. catenella blooms that are massive in scale, posing a significant and worrisome threat to public and ecosystem health in Alaskan Arctic communities where economies are subsistence based. 

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5. Trends in Maumee River Nitrogen Loads and Their Complex Relationship to Harmful Algal Blooms in Western Lake Erie

   https://doi.org/10.3390/w18040465  

   Khan, Nusrat N; Paerl, Hans W; McCarthy, Mark J; Newell, Silvia E; Rudko, Noah; Muenich, Rebecca Logsdon (February 2026, Water)

   Nutrient runoff from agricultural activities in the watershed of Western Lake Erie (WLE) is a dominant driver of harmful algal blooms (HABs). While phosphorus (P) is a key factor causing these blooms and has been the focus for researchers and policymakers, the influence of nitrogen (N) on bloom dynamics has been overlooked. Total Kjeldahl N (TKN; organic N and ammonium N) has not been the focus of eutrophication research but was recently linked to bloom development in WLE. Here, monotonic and oscillatory statistical trend analyses were performed to interpret long-term (1982 to 2022) patterns of TKN in the Maumee River and were compared to algal biomass data as chlorophyll a. A predictive regression model used principal component analysis to estimate a chlorophyll-based index of HABs in WLE, and a systematic iterative process identified that TKN influences bloom dynamics along with soluble reactive phosphorus (SRP), total suspended solids (TSS) and flow. Although TKN loads exhibited a long-term decline, this decrease did not correspond to reduced HAB severity, reflecting the strong influence of flow-driven hydrologic variability on nutrient delivery and bloom response. The modeling results demonstrate that TKN, together with SRP, TSS, and flow, significantly contributes to predicting bloom magnitude. These findings highlight the need for dual-nutrient (N and P) management strategies and additional analyses of nutrient–hydrology interactions to improve HAB mitigation in WLE. 

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