Ostreopsis ovata bloom along the Conero Riviera (northern Adriatic Sea): relationships with environmental conditions and substrata

The spatial and temporal distribution of the toxic benthic dinoflagellate Ostreopsis ovata was investigated along the Conero Riviera (northern Adriatic Sea) during summer 2007 to evaluate the role of substratum, depth and hydrodynamic conditions on bloom dynamics. Sampling was carried out from April to November on different substrata (seaweeds, mollusc shells and rocks) and in the water column. O. ovata bloom started in September, reaching maximum values in the first week of October (1.7 106 cells g1 fw corresponding to 21.0 106 cells g1 dw and 6.2 104 cells cm2) and completely disappearing at the beginning of November. On average, the abundances of O. ovata on rocks were significantly higher than those on seaweeds. Seaweeds with branched thalli supported higher values of epiphytes than non-branched ones. The abundances of O. ovata were significantly higher in sheltered than in exposed sites indicating a major role of hydrodynamism in regulating bloom dynamics. O. ovata abundances showed a marked decrease with depth with significantly lower values at depths >3 m. In the northern Adriatic, bloom development did not show any relationship with temperature, as the peak occurred when temperature values were decreasing.