Ups and Downs in the Life of a Toxic Pseudo-nitzschia Bloom in the Juan de Fuca Eddy off the Washington Coast

Evelyn J. Lessard¹, Brady Olson¹, Vera L. Trainer², William B. Cochlan³, and Barbara Hickey¹

¹School of Oceanography, University of Washington, Seattle, WA 98195, USA
²NOAA Fisheries, Northwest Fisheries Science Center, Seattle WA 98112, USA
³Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920, USA

In September 2004, we followed the population dynamics of a highly toxic Pseudo-nitzschia cuspidata bloom in the Juan de Fuca Eddy for ten days. This was an unusual bloom, as it was co-dominated by P. cuspidata and a vertically migrating small (ca. 15 µm) flagellate. Dilution growth and grazing experiments were performed daily at the depth of the 50% light level (2-4 m) while following a drifter. At the beginning of the drift, P. cuspidata abundance was already relatively high (3700 ml^-1) and cells were toxic (3 fM domoic acid cell^-1), indicating the bloom was already in progress. Over the course of the drift, P. cuspidata abundances oscillated up and down, but there was generally an increase followed by a decline. P. cuspidata abundance, as well as total chlorophyll, varied inversely with nitrate in the surface water, indicating nitrate utilization, and nitrate was depleted by the end of the drift as the drifter left the eddy. The up and downs of cell numbers and nutrients were in part due to physical processes in the eddy mixing nitrate into the surface waters, as well as growth. P. cuspidata growth rates were moderately high (0.5 - 0.94 d^-1) and did not appear to be nitrogen limited until the drifter left the eddy, where growth rate was very low (0.02 d^-1). Grazing rates on P.cuspidata were very low, even though microzooplankton biomass was relatively high, which contributed to the accumulation of P.cuspidata, but could not account for the declines. Particulate domoic acid (DA) reached >40 nM and followed cell numbers closely. Cellular DA therefore remained relatively constant throughout the bloom, but showed a modest increase (from 3 to 5 fM cell^-1) as nitrate fell below 2 µM. Towards the end of the bloom, as cell numbers dropped from their peak, dissolved DA increased dramatically, from ca. 2 to 15 nM; the release may have been related to micronutrient availability. Neither particulate nor dissolved DA was correlated with grazing rate, nor was grazing rate affected by additions of dissolved DA. Therefore, DA did not appear to be the cause of low grazing on P.cuspidata.