Environmental conditions and phytoplankton dynamics associated with Pseudo-nitzschia abundance and domoic acid in the Juan de Fuca eddy

The Juan de Fuca eddy, located off the coasts of Washington (USA) and British Columbia (Canada), has been identified as a region that frequently contains relatively high levels of domoic acid (DA), d toxin produced by some members of the marine diatom genus, Pseudo-nitzschia. This seasonal eddy provides a unique environment to study the influence of nutrients on Pseudo-nitzschia abundance and DA accumulation in the field. Vertical sampling in the Juan de Fuca region was conducted in May, July and September of 2001 in an effort to determine environmental conditions and phytoplankton dynamics within the eddy compared to surrounding waters. The eddy was characterized by high primary productivity and high biomass in May and September relative to surrounding waters and was dominated by diatoms in the >5 mum size-fraction. In May, nitrate (NO3-) concentrations and the corresponding NO3- assimilation rates by phytoplankton within the eddy surface waters were relatively low. In contrast, in September, NO3- was high and NO3- assimilation rates increased by 7 times relative to those in May. DA was below detection levels at all stations in May and July. In September, Pseudo-nitzschia reached highest cell densities (-2 x 10(4) cells l(-1)) and particulate DA (-30 ng DA equivalents l(-1)) was detected in surface waters of the eddy. The presence of DA in healthy growing phytoplankton communities indicates a need to examine other environmental conditions that induce DA production in natural Pseudo-nitzschia populations than have previously been reported in nutrient-stressed laboratory studies.