Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Juliette L Smith

Committee Member

Kimberly S Reece

Committee Member

Marjorie AM Friedrichs

Committee Member

Roger L Mann

Committee Member

Kathi A Lefebvre


Harmful algal bloom (HAB) events are generally marked by the over-abundance of one particular HAB species. Co-occurrence of multiple HAB species or HAB toxins, especially at low cell or toxin concentrations, is common. While much research has been dedicated to understanding the detrimental effects of individual HAB species and toxins on human health and the environment, implications of HAB co-occurrence for seafood safety and shellfish health are poorly understood.Oysters support economically-valuable fisheries and aquaculture worldwide, however, oysters encounter co-occurring HAB species and toxins in their environment. Some HAB species and toxins are harmful to oyster health, harming the immune system, reducing feeding rates, or causing mortalities. Additionally, oysters co-accumulate HAB toxins; some associated with human health syndromes, such as diarrhetic shellfish poisoning. To support productive and safe oyster industries, the effects of co-occurring HAB species and toxins on larval oyster health, and the bioaccumulation of multiple toxins in adult oysters in the Chesapeake Bay, were investigated. The health and survival of larval oysters is paramount to shellfish productivity. Individual and combined effects of co-occurring HAB species and toxins were assessed using multiple series of 96-h bioassays with larval oysters; larval inactivity and mortality were measured throughout. Karlodinium veneficum and Prorocentrum cordatum are co-occurring HAB species associated with shellfish health issues. Independently, low cell concentrations of either species caused larval inactivity. Additionally, K. veneficum swarmed larvae and caused significant larval mortalities. The co-occurrence of P. cordatum did not alter the larval effects of K. veneficum. Separate bioassays examined co-occurring Alexandrium catenella and Dinophysis acuminata, and associated toxins: saxitoxin (STX), okadaic acid (OA), and pectenotoxin-2 (PTX2). Exposure to live A. catenella caused larval inactivity, while exposure to either species caused larval mortalities. Exposure to D. acuminata lysate or PTX2 also caused larval mortalities, with A. catenella lysate, STX, and OA exhibiting no significant larval inactivity or mortalities. Larval effects during lysate or toxin co-exposure were driven by D. acuminata lysate or PTX2, respectively. In both bioassays, the observed larval effects of co-exposure were driven by one HAB species or toxin. To inform seafood safety management, baseline HAB toxin data from Chesapeake Bay adult oysters were collected over two years. Azaspiracids (AZA1, AZA2), domoic acid (DA), OA, dinophysistoxin-1 (DTX1), PTX2, karlotoxins (KmTx1-1, KmTx1-3), goniodomin A (GDA), and microcystins (MC-RR, MC-YR) were detected in oysters. Regulated toxins were well below seafood safety limits, however, the presence of hepatotoxic, freshwater MCs in estuarine oysters reflects an urgent need for regulation of these toxins in seafood. Co-accumulation of toxins was common. Furthermore, solid phase adsorption toxin tracking devices (SPATTs) were co-deployed with oysters to assess additional methods of toxin monitoring. SPATTs provided additional toxin data that complemented, but could not replace oyster toxin data. As HAB species ranges shift and the need for sustainable shellfish aquaculture increases, so too does the need for understanding combined effects of HABs on shellfish, and the potential for toxin co-accumulation within shellfish. Regional and species-specific studies like these can inform and enhance HAB monitoring, mitigation, and management strategies.




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