SILICIC ACID LIMITATION IS NOT A TRIGGER FOR DOMOIC ACID PRODUCTION BY PSEUDO-NITZSCHIA BLOOMS IN THE PACIFIC NORTHWEST

William P. Cochlan¹, Mark L. Wells², Charles G. Trick³, Vera L. Trainer⁴, Evelyn J. Lessard⁵, and Barbara M. Hickey⁵

¹Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920, USA 2School of Marine Sciences, University of Maine, Orono, ME 04469, USA
³Schulich School of Medicine and Dentistry, University of Western Ontario, London, ONT, N6A 5B7, Canada
⁴NOAA Northwest Fisheries Science Center, Seattle, WA 98112, USA
⁵School of Oceanography, University of Washington, Seattle, WA 98195, USA

A central paradigm in the study of toxigenic diatoms is that concentrations of cellular DA (particulate DA per cell) become elevated as silicic acid (silicate) concentrations become limiting for growth and/or uptake by these diatoms. We have tested this hypothesis during numerous ECOHAB-PNW cruises from 2003-2006 in the coastal waters off Washington State, U.S.A. and British Columbia, Canada where toxigenic blooms of Pseudo-nitzschia typically occur. During 2004, we studied a massive toxic bloom measuring up to 48 km in diameter and reaching cell concentrations of 11-13 million cells/liter of P. cuspidata - the overwhelmingly dominant Pseudo-nitzschia species present. Results from this bloom event demonstrate that the highest levels of cellular toxin (5-64 pg DA/cell) correlate poorly with ambient silicate concentrations, and typically occur where dissolved silicate concentrations were 5-50 µM. None of the ~400 particulate DA (pDA) analyses conducted in 2004 (determined using cELISA) showed elevated cellular toxin concentrations when ambient concentration of silicate were < 4 µM, rather elevated pDA was generally associated with ambient silicate levels well above those considered limiting for its uptake and growth by most neritic diatoms. Increased cellular toxin levels also did not correlate with decreased ambient concentrations of nitrate or orthophosphate, indicating that toxin production in this natural Pseudo-nitzschia bloom was not governed by macronutrient availability. A similar result was found during the intense toxic Pseudo-nitzschia bloom in Monterey Bay, CA in 1998. The most established correlate for elevated domoic acid concentrations in 2004 was low dissolved iron concentrations; a finding consistent with laboratory culture experiments and our field incubation studies in the ECOHAB-PNW study region. These findings provide perhaps the most detailed insight to date into the environmental triggers for toxin production in natural assemblages of Pseudo-nitzschia, and demonstrate that commonly implicated macronutrient factors such as silicate limitation are poor predictors of either Pseudo-nitzschia dominance or toxicity in the Pacific Northwest.