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
