Distribution and abundance of Ostreopsis spp. and associated species (Dinophyceae) in the northwestern Mediterranean: the region and the macroalgal substrate matter.

The range of Ostreopsis (Dinophyceae), a taxon harmful to both human health and ecosystems, has spread from a tropical and subtropical range of distribution to temperate areas, such as the Mediterranean Sea. This study has evidenced widespread summer occurrence in the northwestern Mediterranean, from French Catalonia to the French Riviera and Corsica. Ostreopsis spp. are usually associated with two other dinophycean taxa, Prorocentrum lima and Coolia spp. No obvious correlation (indicative of competition and/or facilitation) between the abundance of the three taxa was evidenced. In addition to local variability, we observed regional variability, with low abundance and local absence in French Catalonia and Languedoc, which contrasted with overall abundance and blooms in Provence, the French Riviera and Corsica, especially in late summer. Possible causes for this regional variability are discussed. Furthermore, the three taxa can grow on a variety of macroalgal substrates: 34 taxa belonging to the fleshy, bushy, flat and erect morpho-functional groups. Some macroalgal species were correlated with either high or low abundance of the studied dinophycean taxa and could therefore enhance or hinder their blooming.