Comparison of two sites with respect to HABs viability: a topographic eddy and a coastal upwelling region

B.M. Hickey¹, S. Geier¹, V. Trainer², W. Cochlan³, R. Thomson⁴, A. MacFadyen¹, N. Kachel¹, E. Lessard¹, C. Trick⁵ and M. Wells⁶

¹Box 355351, School of Oceanography, University of Washington, Seattle, Washington
²NOAA Fisheries, Marine Biotoxin Program, Northwest Fisheries Science Center, 2725 Montlake Blvd. East Seattle, WA, 98112, USA
³San Francisco State University, California, USA
⁴Institute of Ocean Sciences, Patricia Bay, British Columbia, Canada
⁵University of Western Ontario
⁶University of Maine

Results from ECOHAB Pacific Northwest studies show that the Juan de Fuca eddy, a topographically-trapped seasonal feature on the North American west coast, is a site where Pseudo-nitzschia (PN) frequently produce domoic acid. Highest toxin is generally located in the colder water in the eddy and along its edges. The location of toxin is similar in each survey, whereas the strength of the toxin has dramatic variability between cruises.

Although PN are found in the nearshore coastal upwelling zone as well as in the vicinity of the eddy, PN have not been found to be toxic in blooms associated with actively upwelling water. The working hypothesis is that toxic blooms are transported southward from the eddy region in the seasonal shelf break upwelling jet and then, if a toxic patch is favorably located, onshore to the coast during the downwelling conditions (onshore transport) that occur during storms. The eddy region has a number of characteristics that differentiate it from the coastal upwelling zone. In particular, particle residence times in the eddy region are several times those in an active upwelling zone. Also, nitrate time series (derived from temperature-nitrate survey data) suggest that the supply of macro nutrients to the eddy is much more persistent than to the coastal upwelling zone-- in the latter region nutrient supply is entirely cut off for several days following each reversal from upwelling (high salinity) to downwelling (low salinity) conditions. In contrast to the coastal upwelling zone, the eddy region is supplied by the comparatively constant, nutrient-rich near surface estuarine flow out of the strait. The source of this nutrient-rich water is coastal water which enters the strait at depth as an estuarine return flow, subsequently mixing vertically with the outflowing surface layers. Last, we present a new hypothesis-- that the strong density front which marks the offshore edge of the plume from the Columbia River may prevent HABs from reaching the coastal clamming beds.