This thesis focus on the influence of human-induced environmental changes on the physiology of Ostreopsis ovata; a toxic, benthic dinoflagellate, which form blooms in tropical and temperate marine waters. O. ovata produces palytoxin and ovatoxins and copious amount of mucilage, all of which have significant negative ecological impacts on the benthic ecosystems where they occur. Understanding the factors affecting O. ovata growth and toxin production is therefore of great importance if we are to understand how this species form such successful blooms. To accomplish this, growth, toxicity, carbohydrate accumulation and related physiological traits of two strains of O. ovata (one from Japanese waters and the other from Mediterranean waters) were studied in laboratory experiments. The cells were grown under the conditions simulating climate-induced changes (higher temperatures and acidification of marine waters) and eutrophication (increase of nitrogen and phosphorus in coastal ecosystems).

O. ovata showed strain-specific responses to the tested environmental changes. Higher temperatures increased growth and reduced the cell toxin quota of the Mediterranean strain while it decreased the growth and toxicity of the Japanese strain. Nutrient (nitrogen-N and phosphorus-P) sufficiency significantly increased O. ovata biomass accumulation. N deficiency markedly decreased the growth while it increased cell toxicity. Lower temperatures (20 - 25 °C) increased the particulate carbohydrate accumulation in both strains. Cell volumes, C (carbon), N and P cell quotas and atomic ratios of O. ovata were significantly higher when grown at 20 °C, than those grown at 30 °C. Acidification did not significantly affect growth and cell toxicity, but increased particulate carbohydrate accumulation. The combined effect of higher temperature and acidification delayed the onset of the blooms by 3 - 5 days, in relation to present temperature and pH conditions.

When extrapolating these results to natural marine ecosystems, it can be inferred that O. ovata occurring in lower-latitudes are growing optimally in the present climate conditions. On the other hand, the strains of O. ovata in higher-latitudes seem to have higher temperature optima than they have now at present conditions. Therefore, further expansion of O. ovata blooms is most likely possible in the warm, eutrophic coastal waters in higher-latitudes.