Allelopathic interactions between the HAB dinoflagellate Ostreopsis cf. ovata and macroalgae

Intense blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata have been a recurrent phenomenonalong several Mediterranean coasts during summer in the last few years. These blooms are oftenassociated with noxious effects on humans and deaths of benthic invertebrates. Previous studies carriedout on the Conero Riviera (northern Adriatic Sea) highlighted that Ostreopsis abundances recorded onrocks were significantly higher than on the surface of seaweeds, suggesting that some allelopathicinteractions might occur between Ostreopsis and macroalgal substrates. In this study we investigatedunder experimental conditions the interactions between O. cf. ovata and three of the most commonmacroalgae in this area: Dictyota dichotoma (brown alga), Rhodymenia pseudopalmata (red alga) and Ulvarigida (green alga). Three different experiments were set up: O. cf. ovata was grown (i) together withfresh macroalgal tissues, (ii) in media in which macroalgae were previously cultured, and (iii) in mediawith the addition of dry macroalgal powder at different concentrations. The results indicated that allthe investigated seaweeds exerted negative effects toward the benthic dinoflagellate O. cf. ovata.D. dichotoma inhibited the growth of O. cf. ovata in all tested experimental conditions; U. rigida hadinhibitory effect both in form of fresh thalli and dry powder but not as growth medium filtrate,suggesting that either Ulva does not release any allelopathic compound in the medium in absence ofO. cf. ovata or the alleged released allelochemicals are rapidly degradable. Neither the fresh thalli ofR. pseudopalmata or the filtrate of its culture medium showed any inhibitory effects, while a negativeeffect was only observed at high concentrations of dry thallus powder. With the exception ofD. dichotoma co-culture experiment, a complete algicidal effect was never observed partly becauseO. cf. ovata produced a large amount of resting stages, which permitted its survival.