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The study of the intraspecific variability is a crucial step for understanding the successful establishment and maintenance of invasive species. Alexandrium tamarense strains isolated in spring 2007 from a single bloom in Thau lagoon have been grown on three different media (ESNW based on natural seawater, and the artificial media, ESAW, ESAW+HA). A large diversity in the ability of strains to grow on these media was found. Irrespective of medium compo- sition, growth curves followed three models: (1) a classical shape, (2) a population crash followed in some cases (3) by growth recovery. Some strains were able to show significant growth in an environment completely artificial (ESAW). ANOVA indicated a significant difference between groups in growth rates allowing the distinction of contrasted categories among the strains studied in ESNW medium. These statistical tests also indicated the presence of distinct groups among the strains grown in the ESAW as well as for those on ESAW+HA medium. Lag phases were extremely variable between strains in all environments, suggesting a high variability of adaptation to the environment. The results revealed that wide fitness variations were exhibited by diverse conspecific A. tamarense individuals co-existing during a bloom.

Alexandrium tamarense (M. Lebour) Balech strains isolated in spring 2007 from a single bloom in Thau lagoon have been grown in nonaxenic artificial media. For three strains showing large oscillations in biomass (crashes followed by recoveries) on a scale of several days, a significant relationship was observed between changes in cell densities (as in vivo fluorescence) and changes in nitrate concentrations. Increases in cell densities were accompanied by decreases in nitrate, while decreases in cell densities corresponded to increases in nitrate, presumably due to nitrification. Net increases in nitrate could reach up to 15 mu mol N . L(-1) . d(-1) indicating a very active nitrifying archaeal/bacterial population. However, following population crashes, algal cells can recover and attain biomass levels similar to those reached during the first growth phase. This finding indicates that those archaea/bacteria do not compete for nutrients or do not hamper algal growth under those conditions. In contrast to diatoms, dinoflagellates such as A. tamarense do not excrete/exude dissolved organic matter, thus preventing excessive bacterial growth. This mechanism could help explain the recovery of this species in the presence of bacteria.