Dynamics and impacts of toxic Alexandrium catenella blooms in the Pacific Arctic

Abstract

Recent warming in the Pacific Arctic region is facilitating the emergence of harmful algal blooms (HABs) in polar waters, posing a significant threat to northern communities and ecosystems. Of particular concern is the dinoflagellate Alexandrium catenella, a cyst-forming species that produces a potent suite of paralytic shellfish toxins (PSTs). In this thesis, Pacific Arctic bloom dynamics are explored in the context of the meroplanktonic life cycle of Alexandrium, revealing the lasting impacts of bloom events upon this region. Building upon previous work that characterized a massive accumulation of resting Alexandrium cysts on the Chukchi shelf, the implications of cyst bed disturbance and resuspension for bloom initiation were investigated. Although there is currently a narrow seasonal window within which physiological and environmental conditions align to promote bloom initiation, this interval may become wider under continued Arctic borealization. In the summer of 2022, a large, concentrated, and highly toxic bloom of Alexandrium was tracked in real-time as it progressed northward through the Bering Strait region. The high resolution at which this bloom was observed provided the opportunity to initiate community event response, as well investigate the hydrodynamic drivers contributing to the extraordinary density, scale, and toxicity of this event. Little is currently known about the uptake and depuration of PSTs in Arctic taxa, introducing significant challenge into modeling trophic toxin transfer. Following the 2022 bloom, toxicity in a bivalve population of Macoma calcarea on the southern Chukchi shelf was measured over three years, showing that this species serves as a reservoir for PSTs in the food web by retaining significant toxin levels for over a year after a bloom event. Beyond the persistence of PSTs in the ecosystem, advected blooms have the potential to create lasting impacts by sustaining and shaping the Chukchi shelf Alexandrium cyst bed. A population study of Alexandrium isolates using microsatellite markers revealed high diversity and little spatial differentiation across the study region. However, a significant shift in population genetic structure was detected over time, likely resulting from new cyst inputs by advected blooms from the Bering Sea. These results highlight the dynamic nature of the Chukchi cyst bed, which serves as a terminal deposition site that is formed and maintained by sub-Arctic sources. This thesis adds significantly to our understanding of the relationship between bloom dynamics and algal toxin risks in warming Pacific Arctic waters, providing insights that can be used to shape future risk mitigation and response efforts.