Response of potentially toxic Pseudo-nitzschia (Bacillariophyceae) populations and domoic acid to environmental conditions in a eutrophied, SW Mediterranean coastal lagoon (Tunisia)

The population dynamics of Pseudo-nitzschia species from Bizerte Lagoon, northern Tunisia, were examined at six stations sampled monthly from March 2006 to February 2007. These are potential producers of the neurotoxin domoic acid (DA), cause of amnesic shellfish poisoning (ASP). Six morphospecies were counted: Pseudo-nitzschia brasiliana, Pseudo-nitzschia calliantha, Pseudo-nitzschia delicatissima complex species, Pseudo-nitzschia multistriata, Pseudo-nitzschia pseudodelicatissima and ‘Nitzschia seriata’ group species. Canonical correspondence analysis was used to correlate species with the physical and chemical properties of the seawater, and the statistical significance of species-variable associations was tested using the Van Dobben method. The P. delicatissima complex species abundance was significantly positively correlated to turbidity, and negatively associated with temperature, nitrate, silicate and DA. P. brasiliana was more abundant in samples with higher salinity, nitrate, silicate and DA, and with lower nitrite, phosphate and turbidity. P. multistriata was negatively linked to salinity and nitrate, and positively correlated to temperature and turbidity. Abundances of the P. calliantha, P. pseudodelicatissima and ‘N. seriata’ group species were not statistically related to any of the variables examined. We document, for the first time, the incidence of DA in the Bizerte Lagoon (at four stations) and at one offshore station in Bizerte Bay (Mediterranean Sea). Total DA ranged from 0.5 to 2.0 μg DA l−1 and was detected only in October 2006, when lagoon waters were rich in nitrate and silicate and the salinity was elevated. The beginning of autumn was the highest potential risk period for ASP events, linked to elevated salinity and P. brasiliana growth. These findings allow local jurisdictions to prepare for Pseudo-nitzschia bloom events, and provide data for eventual predictive models of Pseudo-nitzschia bloom formation and toxin production in Bizerte Lagoon, in comparison with models elsewhere in the world.