Summer dynamics of Alexandrium ostenfeldii (Dinophyceae) and spirolide toxins in Cork Harbour, Ireland

Proliferations of toxic Alexandrium spp. have adversely affected the shellfish aquaculture industry worldwide. A. ostenfeldii can produce several biotoxins, including the recently characterised fast-acting toxins spirolides (SPX). A dual labelling fluorescent in situ hybridisation (FISH) assay was developed for discriminating simultaneously between the closely related taxa A. ostenfeldii and A. peruvianum. Surveys were undertaken throughout the summers of 2006, 2007 and 2008 in Cork Harbour, Ireland, where a mixed community of Alexandrium spp. develops annually. A. peruvianum was not detected but the presence of A. ostenfeldii was confirmed by FISH and morphological analysis. The species never reached high concentrations (max. ∼200 cells l^-1) and contributed on average to only 0.4% of the Alexandrium community, usually dominated by A. minutum and A. tamarense (Group III). Although cell concentrations were several orders of magnitude lower, the dynamics of A. ostenfeldii were similar to those of other Alexandrium spp. during the 3 consecutive summers, suggesting a common response to environmental forcing. Analytical chemistry performed on extracts from passive solid-phase adsorption samplers identified lipophilic toxins dominated by okadaic acid, but also 13-desmethyl SPX C and 20-methyl SPX G, with dynamics generally congruent with those of A. ostenfeldii. The passive samplers enabled the quantification of background toxin levels at very low A. ostenfeldii concentrations, showing potential for forecasting of toxic events. The ability to quantify toxic A. ostenfeldii cells within high density microalgal populations of morphologically similar species makes the dual FISH assay valuable for phytoplankton monitoring programs and future biogeographical and population dynamics studies.