In the late summer of 1999, an extensive mortality of gorgonians and other epi‐benthic organisms was observed in the Ligurian Sea (Mediterranean Sea) from the Tuscan Archipelago to Marseille. Quantitative data from Tino Island and Portofino Promontory indicated that the proportion of affected gorgonians ranged from 60% to 100% in populations having a density of 9–27.8 colonies m−2, suggesting that millions of sea fans died along the coast of Liguria. This mass mortality episode coincided with a sudden increase of sea water temperature down to more than 50 m depth. Laboratory analyses showed that the colonies stressed by high temperature also underwent extensive attack by microrganisms (protozoans and fungi), which are interpreted as opportunistic pathogens.

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Harmful benthic microalgae blooms represent an emergent phenomenon in temperate zones, causing health, ecological and economic concern. The main goal of this work was to compile records of Ostreopsis at large temporal and spatial scales, in order to study the relationship between cell abundances, the periodicity and intensity of the blooms and the role of sea water temperature in 14 Spanish, French, Monegasque and Italian sites located along the northern limits of the Mediterranean Sea. General trends were observed in the two considered basins: the north-western Mediterranean Sea, in which higher cell abundances were mostly recorded in mid-summer (end of July), and the northern Adriatic Sea where they occur in early fall (end of September). The sea-water temperature does not seem to be a primary driver, and the maximal abundance periods were site and year specific. Such results represent an important step in the understanding of harmful benthic microalgae blooms in temperate areas, and provide a good base for policy makers and managers in the attempt to monitor and forecast benthic harmful microalgae blooms.

Canopy-forming macroalgae of the genus Cystoseira are being lost in several areas of the Mediterranean Sea. Cystoseira amentacea var. stricta and C. compressa are common species in the Ligurian Sea; they are distributed in patches, but their abundance and distribution is locally variable. We investigated changes in relative cover, biomass and morphology of Cystoseira amentacea var. stricta and C. compressa with increasing urbanisation, and effects of the presence or absence of Cystoseira species on the composition and structure of understorey assemblages. C. amentacea var. stricta habitats were lost close to urban areas, while C. compressa slightly increased. The morphological characteristics of these 2 species were very variable in space and time, and did not vary with urbanisation. Assemblages lacking canopy differed markedly from Cystoseira-dominated assemblages, particularly assemblages dominated by Corallina elongata, the species most responsive to changes in habitat structure and urbanisation, which forms dense turfs in urban habitats lacking canopies. Marked species-specific differences between the assemblages dominated by the 2 Cystoseira species were detected. C. amentacea var. stricta is a key species maintaining habitat complexity and species diversity in Mediterranean rocky shores and we recommend additional conservation actions, such as habitat restoration by transplantation of this endangered species.