Pseudo-nitzschia is a diatom genus known to produce the neurotoxin, domoic acid (DA), which causes Amnesic Shellfish Poisoning (ASP) and Domoic Acid Poisoning (DAP). Field studies were conducted in the Chesapeake Bay to determine which species were present, their toxicity and their spatial and temporal distribution. Strains were isolated from the Chesapeake Bay region and growth and toxin content were studied. The effect of rapid increases in light at low temperatures on toxin production physiology was investigated.

Toxic Pseudo-nitzschia is present in the Chesapeake Bay; however, abundance and toxin production are highly variable. Six species of Pseudo-nitzschia were identified: P. pungens, P. calliantha, P. subpacifica, P. cuspidata, P. fraudulenta and P. multiseries. The most abundant species was P. calliantha. Pseudo-nitzschia abundances were associated with low temperature (2-21°C) and high salinity (6-32) and were highest in winter and spring. Compared to other diatom species, Pseudo-nitzschia abundances were low, rarely present above 1000 cells mL-1 and they did not occur as monospecific blooms. Low Pseudo-nitzschia abundances and low, irregular domoic acid concentrations may partially explain the lack of documented toxic events in the Chesapeake Bay.

Growth rate and toxin content of strains of Pseudo-nitzschia exposed to different nitrogen sources and irradiances varied significantly, even among strains of the same species isolated from the same water sample. Strain-level differences were responsible for most of the variability in growth rate and toxin content. Sequences of the internal transcribed spacer (ITS) and large subunit (LSU) rRNA matched morphological species definitions, but offered no explanation for the physiological variability. Populations of Pseudo-nitzschia in the mid-Atlantic coastal zone appear to be comprised of numerous ecotypes that require sorting in the future.

The hypothesis that DA is produced as an energy modulation strategy when the light and dark reactions of photosynthesis are decoupled was tested by exposing exponentially growing P. multiseries to a rapid increase in irradiance at a low temperature. High light and low temperature conditions increased nitrate (NO3‾) uptake, nitrite (NO2‾) and ammonia (NH4+) release and decreased DA production by the cells. These results could have important implications for natural populations of Pseudo-nitzschia at times of low temperature and high light fluctuations, such as during spring blooms and upwelling events.

This thesis answered several questions about Pseudo-nitzschia populations in the Chesapeake Bay area and their ecophysiology, but raised many more. Physiological adaptations and biogeography of Pseudo-nitzschia and DA content of Chesapeake bivalves should be studied further to contribute to the development of predictive models for Pseudo-nitzschia bloom formation and toxin production.