Graduation Date

Spring 2023

Document Type



Master of Science degree with a major in Natural Resources, option Fisheries

Committee Chair Name

Dr. Eric Bjorkstedt

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Dr. Amy Sprowles

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Dr. Andre Buchheister

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Dr. Mark Henderson

Fourth Committee Member Affiliation

HSU Faculty or Staff


Harmful algal blooms, Northern California, Humboldt Bay, Pseudo-nitzschia, Domoic acid, Bivalves, Aquaculture

Subject Categories



Harmful algal blooms (HABs) result from outbreaks of any of several different species of toxin-producing phytoplankton and that can have major detrimental effects on marine ecosystems and pose severe health and economic threats to human communities. Of particular concern along the United States West Coast are HABs of pennate diatom genus Pseudo-nitzschia that produce the potent neurotoxin domoic acid (DA). The coastal ocean between Cape Mendocino, CA, and Cape Blanco, OR is a hotspot for Pseudo-nitzschia spp. HABs. Such blooms impact coastal fisheries and pose a potential threat to aquaculture operations in Humboldt Bay, California’s second largest estuary and largest producer of oysters. Yet, despite evidence that tidal exchanges carry Pseudo-nitzschia spp. from the ocean into the Bay, regular assays rarely detect high uptake of domoic acid in cultured oysters and sentinel mussels in upper reaches of the Bay. This study examined the gradient to which ocean-origin DA and Pseudo-nitzschia spp. enter Humboldt Bay using naturally occurring bivalves as an integrated measure of exposure. Bivalves were collected along ocean to upper estuary transects and processed for DA concentrations in their soft tissues. These samples were augmented with water samples collected to characterize the concentrations of Pseudo-nitzschia spp. and DA in the water column and to relate to DA concentrations in bivalves. Results demonstrate that DA concentrations in bivalves decline with increased distance from the mouth of the Bay in a manner that varies over time, and that this variability is linked to the variability and intensity of DA concentrations in the environment. These results provide strong support for the hypothesis that bivalves in the upper regions of the Bay experience less exposure to ocean-origin Pseudo-nitzschia spp. HABs. This study lays the foundation for understanding the dynamics and distribution of HABs in Humboldt Bay and warrants the development of future studies to map this risk in greater detail to support hypotheses regarding mechanisms that control HAB distributions and exposure.

Citation Style

Transactions of the American Fisheries Society


Thesis/Project Location


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