Biodiversity means the variety of life, encompassing levels of complexity from within species to across ecosystems. Biodiversity therefore includes several dimensions: evolutionary scale (genetic, species, higher taxonomic levels and phylogenetic diversity), functional scale, organizational scale (patch, ecosystem, landscape/seascape diversity), spatial scale (from sample to local, regional and global richness) and heterogeneity diversity. Biodiversity can therefore be measured in different and complementary ways, thus involving the use of at least 200 different metrics, which can suggest different and contrasting conclusions. It is worth emphasizing that species richness may be the least appropriate metric, despite its popular acceptance. There is a growing tendency for stakeholders, managers, government officials, environmentalists, scientists, politicians and the media to focus, as concerns threats to biodiversity, on species richness and climate change. However, focusing on climate warming can mask other stressors that, today, and perhaps for decades to come, may have more impact on ecosystems than global warming. In the Mediterranean Sea, the overall impact of Non-Indigenous Species (NISs) and overfishing on species diversity, ecosystem diversity and ecosystem functioning exceeds to a greater or lesser degree the direct impact of warming. Drastically altered functioning patterns, and even new ecosystems are spreading throughout the Mediterranean Sea. This trend is likely to become more pronounced over the next decades. Ecosystem goods and services are also being profoundly altered, generally towards a decline, as illustrated by the overgrazed barren grounds of the eastern basin, which no longer support fisheries, by the impact of the Caulerpa meadows on the scuba diving business and the economic value of the fisheries of the western basin, by the brackish lagoon ecosystems and by the blooms of the introduced comb jelly Mnemiopsis leidyi in the Black Sea, before the arrival of its predator Beroe ovata. Here, we draw attention to the fact that, at this moment and probably also in the future, the huge flow of NISs and overfishing constitute worrying issues, although largely ignored by stakeholders and political authorities. Take care: Un train peut en cacher un autre (one train can hide another; i.e. one danger may hide other unsuspected dangers), that is to say the impact of warming may contribute to hiding other effects, of at least equal gravity, such as biological invasions and overfishing.

Cystoseira granulata C. Agardh var. turneri Montagne was described by Montagne (1838) from Algeria (southern Mediterranean Sea). Subsequently, Agardh (1842) renamed the taxon as C. montagnei J. Agardh, on the basis of specimens from France and the northern Adriatic (northern Mediterranean Sea) which he believed to be identical to Montagne’s taxon. Finally, Sauvageau (1912) described C. spinosa Sauvageau and C. adriatica Sauvageau as nomina nova, to partly accommodate Agardh’s and some other authors’ concept of C. montagnei. These taxonomic treatments caused confusion regarding the delineation of these taxa and doubts have been raised regarding the taxonomic value of Montagne’s taxon, which today is often listed as taxon inquirendum in updated checklists and floras. Since 2014, we have collected near Algiers (Algeria) a species of Cystoseira that formed sparse forests between 10 and 25 m depth. Our specimens perfectly match the original description as well as the syntype material of Montagne’s taxon. They are well characterized and distinct from C. montagnei J. Agardh and from all the Cystoseira taxa hitherto described in having: a single axis with young tophules spinose becoming smooth-tuberculate when older; primary branches either slightly compressed with an inconspicuous rib and irregularly alternate in one plane, or cylindrical and branched in all directions, with spaced short spine-like appendages; and conceptacles both intercalary basal, just above the tophule, and terminal on branchlets. Here, we propose Cystoseira michaelae Verlaque et al. nom. et stat. nov. to accommodate Montagne’s taxon (Cystoseira granulata C. Agardh var. turneri Montagne) and the lectotypification of the species on the basis of both Montagne’s protologue and of an original specimen of Montagne (out of the syntype housed at the Muséum National d’Histoire Naturelle de Paris, PC). Cystoseira michaelae seems to be an endemic species restricted to Algeria and northern Tunisia. In addition, C. montagnei J. Agardh is lectotypified on the basis of an original specimen of J.G. Agardh (out of the syntype housed at the Botanical Museum of Lund University, LD). The study of lectotypes of C. spinosa Sauvageau and of its synonym C. adriatica Sauvageau confirmed that they are junior synonyms of C. montagnei J. Agardh.

A conceptual model was constructed for the functioning the algae-dominated rocky reef ecosystem of the Mediterranean Sea. The Ecosystem-Based Quality Index (reef-EBQI) is based upon this model. This index meets the objectives of the EU Marine Strategy Framework Directive. It is based upon (i) the weighting of each compartment, according to its importance in the functioning of the ecosystem; (ii) biological parameters assessing the state of each compartment; (iii) the aggregation of these parameters, assessing the quality of the ecosystem functioning, for each site; (iv) and a Confidence Index measuring the reliability of the index, for each site. The reef-EBQI was used at 40 sites in the northwestern Mediterranean. It constitutes an efficient tool, because it is based upon a wide set of functional compartments, rather than upon just a few species; it is easy and inexpensive to implement, robust and not redundant with regard to already existing indices.

The CARLIT method for the assessment of the ecological quality of European Mediterranean waters: relevance, robustness and possible improvements.

Where seaweed forests meet animal forests: the examples of macroalgae in coral reefs and the Mediterranean coralligene ecosystem

The sea level rise and the collapse of a Mediterranean ecosystem, the Lithophyllum byssoides algal rim.

Connectivity of populations of the seaweed Cystoseira amentacea within the Bay of Marseille (Mediterranean Sea): genetic structure and hydrodynamic connections