Morphometric analysis of Ostreopsis cf. ovata cells in relation to environmental conditions and bloom phases

Blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata are associated with noxious effects on human health and mortality of benthic organisms, due to the production of palytoxin-like compounds. We investigated the O. cf. ovata bloom along the Conero Riviera (northern Adriatic Sea) during summer 2009 and 2010, in order to analyse morphometric characteristics in relation to bloom phases and environmental conditions. Up to 1000 cells were measured in each study year in natural samples. The morphometric analysis showed a marked variability in cell size (DV: 18.75–75μm; W: 12.5–60μm), and a relationship between cell dimensions and bloom phases was shown: cells in the final phase had a significantly longer DV than those in both the initial and in the proliferation phases. O. cf. ovata showed significantly lower DV in sheltered sites compared with exposed ones, suggesting that the former represent favourable sites for intense O. cf. ovata proliferation and that turbulence can affect O. cf. ovata growth rate and consequently its cell size. The dimensions of Ostreopsis cells in the water column were significantly higher than those of epiphytic ones, suggesting that resuspended cells in the water column probably derived from a mature benthic population. O. cf. ovata developed from August to November, with the highest abundances on 9th September in 2009 (1.3×10⁶cellsg⁻¹ fw, corresponding to 1.6×10⁷cellsg⁻¹ dw and 63.8×10³cellscm⁻²) and on 19th August in 2010 (1.2×10⁶cellsg⁻¹ fw corresponding to 1.6×10⁷cellsg⁻¹ dw and 47.8×10³cellscm⁻²). The abundances of O. cf. ovata were significantly higher in sheltered than in exposed sites, but such differences were significant only during the proliferation phase, suggesting that hydrodynamics affect abundances mainly during the proliferation bloom phase when the highest abundances are reached.