Dense populations of the mixotrophic dinoflagellate Alexandrium tamarense can be found in coastal temperate waters with an almost globally distribution. In many cases such algal blooms are responsible for paralytic shellfish poisoning (PSP) events, which are mediated by saxitoxin and its derivatives. These toxins are produced by the dinoflagellates and accumulate in the food chain. However, not all populations of Alexandrium tamarense are capable of PSP toxin production. The role of additional biologically active chemical compounds for the development and persistence of plankton blooms by processes such as anti-grazing defence or inhibition of competitors is not yet clarified. In co-incubation experiments such biological interactions can be investigated and the magnitude of the observed effects can be determined. Studying populations from different regions that are more closely or more distantly related can elucidate the evolutionary significance of such properties. In this study, a combination of genotypic makers such as sequences of the 28S rDNA, microsatellites and amplified fragment length polymorphisms (AFLP) proofed to show the relatedness of populations of Alexandrium tamarense with a different degree of resolution. Phenotypic markers such as toxicity and toxin profile composition as well as the expression of the above mentioned allelopathic properties can then be analysed for the correlation with the genotypic markers. The expression of phenotypic characters and their ecological significance can then be seen from an evolutionary biological perspective.