Influence of environmental factors on the bloom dynamics of the benthic dinoflagellate Ostreopsis cf. ovata in the Mediterranean Sea

During the last few decades, recurrent and intense blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata have been frequently reported during summer along several sections of the Mediterranean coast. In these areas, blooms have been associated with both noxious effects on human health and mortality of marine organisms, due to the production of palytoxin-like compounds. Ostreopsis grows on several types of benthic substrata (macrophytes, rocks, invertebrates, sands) forming a brownish, mucilaginous mat that can be easily resuspended in the water column. Blooms typically develop in sheltered, shallow coastal areas characterized by rocky bottom habitats. The role of environmental factors on the Ostreopsis bloom dynamics has been studied in the northern Adriatic Sea since 2006. Each year, maximum abundances are typically recorded in late summer-autumn with 106 cells g-1 fresh weight macroalgal thalli. Ostreopsis abundances show a significant decrease with depth, most likely related to light intensity. Substrate type and availability are also thought to influence Ostreopsis blooms; living substrata often host lower abundances of epibionts than other substrates, suggesting colonization is possibly limited by allelopathic interactions. The synergic effects of hydrodynamics, temperature nutrient availability, particularly both inorganic and organic phosphorus, are main factors triggering the bloom.