A conceptual model of annual Ostreopsis cf. ovata blooms in the northern Adriatic Sea based on the synergic effects of hydrodynamics, temperature, and the N:P ratio of water column nutrients

•O. cf. ovata bloom is triggered by a combination of optimal temperature and N:P ratio.•Synergic effects of hydrodynamics, temperature and nutrients affect the bloom onset.•Water temperature threshold (25 °C) plays a key role on cyst germination.•N:P ratio around Redfield value is a necessary condition to allow cell proliferation.•After the onset, bloom development can be sustained at temperature values below 20 °C.

The ecology of Ostreopsis cf. ovata blooms in the Conero Riviera (N Adriatic Sea) was investigated based on sampling carried out from 2007 to 2012, in order to assess the role of environmental factors associated with blooms. Generally, the temporal trend of blooms showed the first cell appearance at the end of July/early August, the maximum abundances in late-summer at end of September/early October reaching up to 106 cells g−1 fw on macrophyte samples, and the decline of the blooms at end October/early November. Calm conditions appeared to be a prerequisite for blooms. When suitable hydrodynamic conditions exist, O. cf. ovata blooms appear to be triggered by a combination of optimal temperature and available nutrients. A water temperature threshold of 25 °C plays a key role in the germination of O. cf. ovata cysts and therefore in bloom onset, and an N:P ratio around Redfield value is a necessary condition to allow cell proliferation. The synergy of higher temperatures and optimal N:P ratios resulted in a higher net growth rate of O. cf. ovata cells. After the onset, blooms can be maintained at temperature values even below 20 °C and at N:P ratios that are in excess of the Redfield ratio. Once the bloom has started it may be maintained not necessarily through high growth rates, but likely through other physiological mechanisms. Bloom decline occurred when temperatures dropped below 18 °C. The net effect of the synergy between local hydrodynamic conditions, temperature, and N and P availability may help to explain why blooms in the northern Adriatic Sea occur differently from those in other Mediterranean regions.