Toxic or not? Modeling Pseudo-nitzschia Consumption and Domoic Acid Cycling in Washington's Intertidal Bivalves

The cosmopolitan diatom genus Pseudo-nitzschia includes a number of species that are capable of producing the neurotoxin domoic acid (DA). When blooms of toxin-producing Pseudo-nitzschia are advected over intertidal shellfish beds, suspension-feeding clams, including mussels and oysters, feed on the diatom and bio-accumulate DA in their soft tissues. This results in a potential threat for humans, marine mammals, and sea birds that consume DA-tainted clams. On the outer coast of Washington State, DA has been responsible for numerous harvest closures for razor clams (Siliqua patula). In Puget Sound, however, only three harvest closures have occurred because of DA. This disparity raises the question: Why has Pseudo-nitzschia only caused three harvest closures in Puget Sound, while the coastal razor clam fishery has been plagued by frequent closures? In this dissertation, I hypothesized that: 1) clam competition in Puget Sound reduces Pseudo-nitzschia concentrations in seawater, thus DA toxicity in all clams; and, 2) the physiology of Puget Sound clams reduces DA uptake or retention relative to Washington outer coast razor clams. I explore several factors that affect DA concentrations in clams after a Pseudo-nitzschia bloom has already been established, using integrated oceanographic and food web models. Model inputs include: clam assemblage composition and abundance, beach slope profiles, tidal advection, suspension-feeding rates, DA assimilation efficiency, and depuration rates. These parameters were obtained through field studies and laboratory experiments. Comparison of sand/cobble beaches in Puget Sound to the Washington outer coast indicates that Puget Sound beaches are steeper, have a greater variety of suspension-feeding clam species, and have higher clam standing stock than outer coast beaches. Comparison of clam physiology between Washington outer coast and Puget Sound species indicates that outer coast razor clams feed on Pseudo-nitzschia and depurate DA at a slower rate than Puget Sound species. Model results reveal several factors that determine clam DA concentrations: tidal height of clams, beach gradient, and clam standing stock, especially at low tidal elevations. In addition, DA uptake and depuration rates play a major role in regulating DA concentrations in clams, thus affecting harvest closure regimes on the outer Washington coast and in Puget Sound.