Some members of the diatom genus Pseudo-nitzschia produce the toxin domoic acid (DA), which through trophic transfer causes mass mortalities of wildlife, shellfish harvesting closures, and risks to human health. Nutrient and micronutrient limitation have been shown to regulate DA production. This study tested the hypothesis that changing partial pressure of CO 2 (pCO 2 ) can interact with nutrient limitation to help determine cellular DA levels, an environmentally relevant issue in light of current increases in atmospheric pCO 2 . Cultures of the toxic species Pseudo-nitzschia multiseries were incubated using semicontinuous methods under a matrix of three pCO 2 conditions: , 22 Pa (220 ppm), , 41 Pa (400 ppm), and , 74 Pa (730 ppm), and two phosphate concentrations: 20 mmol L 21 , P-replete; and 0.5 mmol L 21 , P-limited. DA production was regulated by both pCO 2 and phosphate availability. DA concentrations were 30-50 times higher in P-limited cultures compared to Preplete ones, at the same pCO 2 levels. Increasing CO 2 levels stimulated DA production under both nutrient conditions, but especially in P-limited cultures, where DA levels increased approximately four times over the pCO 2 range examined. Growth rates, primary productivity, photosynthesis vs. irradiance parameters, and cellular elemental ratios also responded interactively to the availability of both CO 2 and phosphate. Our results raise the possibility that growth rates and toxicity of the diatom Pseudo-nitzschia multiseries could increase substantially in the future high-CO 2 ocean, suggesting a potentially escalating negative effect of this harmful algal bloom species on the future marine environment.

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Reports of toxic harmful algal blooms (HABs) attributed to the diatom Pseudo-nitzschia spp. have been increasing in California during the last several decades. Whether this increase can be attributed to enhanced awareness and monitoring or to a dramatic upswing in the development of HAB events remains unresolved. Given these uncertainties, the ability to accurately and rapidly identify an emerging HAB event is of high importance. Monitoring of HAB species and other pertinent chemical/physical parameters at two piers in southern California, Newport and Redondo Beach, was used to investigate the development of a site-specific bloom definition for identifying emerging domoic acid (DA) events. Emphasis was given to abundances of the Pseudo-nitzschia seriata size category of Pseudo-nitzschia due to the prevalence of this size class in the region. P. seriata bloom thresholds were established for each location based on deviations from their respective long-term mean abundances, allowing the identification of major and minor blooms. Sixty-five percent of blooms identified at Newport Beach coincided with measurable DA concentrations, while 36 % of blooms at Redondo Beach coincided with measurable DA. Bloom definitions allowed for increased specificity in multiple regression analysis of environmental forcing factors significant to the presence of DA and P. seriata. The strongest relationship identified was between P. seriata abundances 2 weeks following upwelling events at Newport Beach.

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