Potentially toxic epiphytic dinoflagellate assemblages on macroalgae in the NW Mediterranean

A potentially toxic epiphytic dinoflagellate assemblage on macroalgae was s tudied for I yr in a shallow protected rocky habitat in Palamos (Costa Brav a, NW Mediterranean). The assemblage was monitored on 4 macroalgae: Coralli na elongata (Rhodophyceae), Dictyota dichotoma, Dilophus fasciola and Halop teris scoparia (Phaeophyceae). The dominant dinoflagellates were Ostreopsis sp., and the accompanying species were Coolia monotis and Prorocentrum lim a, The diatom Coscinodiscus sp. was an abundant component of the assemblage . Ostreopsis followed the same seasonal pattern on the 4 macroalgae selecte d. Substrate was not significantly different for the dinoflagellate assembl ages. Ostreopsis was present both in the water column and in the sand conco mitant with maximal cell densities on macroalgae. Small-scale sampling reve aled that all the epiphytic organisms prefer slightly shaken habitats, Whil e Ostreopsis sp. prefers shaken to slightly shaken waters, Coolia monotis p refers slightly shaken to calm ones. The dinoflagellate assemblage follows a clear seasonal pattern, achieving maximum cell concentration during sprin g and summer without significant relative changes in the species compositio n. The epiphytic assemblage was widespread along the Catalan coast and Majo rca, although dinoflagellates were found to be more abundant in the Costa B rava. In Corsica, diatoms dominated the assemblage, whereas Ostreopsis sp. was a minor component.