A study of the survival of Alexandrium cysts in natural sediments and their utility to reconstruct harmful algal blooms in the Pacific Northwest

Abstract

Harmful algal blooms (HABs) produced by the potentially toxic dinoflagellate genus Alexandrium are a natural phenomenon that have been present in the Pacific Northwest since at least the late 1700s. Alexandrium can produce a suite of toxins that if concentrated in shellfish, can be lethal within hours of consumption. Detections of Alexandrium toxins in shellfish have increased in the Pacific Northwest since the Washington Department of Health began monitoring for paralytic shellfish toxins (PST) in the 1950s but, it is unclear whether the increase is an artifact of increased monitoring efforts or if blooms are expanding geographically and becoming more frequent. It has been hypothesized that Alexandrium blooms have become more common in the Pacific Northwest as sea surface temperatures warm. However, the existing PST records in Washington are difficult to standardize or do not have the temporal resolution needed to statistically assess historical trends of Alexandrium detections relative to environmental drivers like sea surface temperature. The focus of this research was to develop a novel method to reconstruct Alexandrium bloom patterns using sediment cores and the benthic stage of its life cycle to better understand historical Alexandrium variability in relation to the environment. For most of the year, Alexandrium can be found on the sediment floor in a dormant stage known as a cyst. The long-term preservation potential, survival and/or degradation of Alexandrium cysts in the sediments are not well-understood. Cysts of the non-toxic dinoflagellate genus Scrippsiella commonly co-occur in the sediments with Alexandrium in the Pacific Northwest. The primary objectives of this body of work were to (1) better understand the survival and visual appearance of Alexandrium and Scrippsiella cysts over time in a sediment core (Chapter 3); (2) investigate the relationship between Alexandrium cyst densities in sediment cores and the available, nearby historical PST records (Chapters 2 & 4); (3) statistically assess correlations between the cyst record and potential environmental drivers (Chapters 2 & 4); and (4) develop a rapid method to estimate the percentage of Alexandrium and Scrippsiella cysts able to germinate from a sediment sample (Chapter 5). Results from this research suggest that Alexandrium cysts are able to survive in the benthic environment for >50-80 years and show little evidence of physical degradation as measured by visual appearances and ability to germinate. Cyst records developed from sediment cores are correlated to PST records and can help to illuminate past trends and long-term drivers of HABs. Thus, Alexandrium cysts in sediment cores may be a useful proxy of historical HABs and can be used to reconstruct historical bloom patterns at various locations. A better understanding of the historical Alexandrium cyst record could help inform predictions of HABs in light of climate change.