Potentially toxic epiphytic dinoflagellate assemblages on macroalgae in the NW Mediterranean

A potentially toxic epiphytic dinoflagellate assemblage on macroalgae was studied for I yr in a shallow protected rocky habitat in Palamos (Costa Brava, NW Mediterranean). The assemblage was monitored on 4 macroalgae: Corallina elongata (Rhodophyceae), Dictyota dichotoma, Dilophus fasciola and Halopteris scoparia (Phaeophyceae). The dominant dinoflagellates were Ostreopsis sp., and the accompanying species were Coolia monotis and Prorocentrum lima, The diatom Coscinodiscus sp. was an abundant component of the assemblage. Ostreopsis followed the same seasonal pattern on the 4 macroalgae selected. Substrate was not significantly different for the dinoflagellate assemblages. Ostreopsis was present both in the water column and in the sand concomitant with maximal cell densities on macroalgae. Small-scale sampling revealed that all the epiphytic organisms prefer slightly shaken habitats, While Ostreopsis sp. prefers shaken to slightly shaken waters, Coolia monotis prefers slightly shaken to calm ones. The dinoflagellate assemblage follows a clear seasonal pattern, achieving maximum cell concentration during spring and summer without significant relative changes in the species composition. The epiphytic assemblage was widespread along the Catalan coast and Majorca, although dinoflagellates were found to be more abundant in the Costa Brava. In Corsica, diatoms dominated the assemblage, whereas Ostreopsis sp. was a minor component.