The Biodiversity of the Mediterranean Sea: Estimates, Patterns, and Threats

The Mediterranean Sea is a marine biodiversity hot spot. Here we combined an extensive literature analysis with expert opinions to update publicly available estimates of major taxa in this marine ecosystem and to revise and update several species lists. We also assessed overall spatial and temporal patterns of species diversity and identified major changes and threats. Our results listed approximately 17,000 marine species occurring in the Mediterranean Sea. However, our estimates of marine diversity are still incomplete as yet—undescribed species will be added in the future. Diversity for microbes is substantially underestimated, and the deep-sea areas and portions of the southern and eastern region are still poorly known. In addition, the invasion of alien species is a crucial factor that will continue to change the biodiversity of the Mediterranean, mainly in its eastern basin that can spread rapidly northwards and westwards due to the warming of the Mediterranean Sea. Spatial patterns showed a general decrease in biodiversity from northwestern to southeastern regions following a gradient of production, with some exceptions and caution due to gaps in our knowledge of the biota along the southern and eastern rims. Biodiversity was also generally higher in coastal areas and continental shelves, and decreases with depth. Temporal trends indicated that overexploitation and habitat loss have been the main human drivers of historical changes in biodiversity. At present, habitat loss and degradation, followed by fishing impacts, pollution, climate change, eutrophication, and the establishment of alien species are the most important threats and affect the greatest number of taxonomic groups. All these impacts are expected to grow in importance in the future, especially climate change and habitat degradation. The spatial identification of hot spots highlighted the ecological importance of most of the western Mediterranean shelves (and in particular, the Strait of Gibraltar and the adjacent Alboran Sea), western African coast, the Adriatic, and the Aegean Sea, which show high concentrations of endangered, threatened, or vulnerable species. The Levantine Basin, severely impacted by the invasion of species, is endangered as well

Relationship between primary producers and bacteria in an oligotrophic sea - the Mediterranean and biogeochemical implications

International audienceThe proverbial blue colour of the Mediterranean reflects some of the most extreme oligo-trophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satelhte data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is hrectly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionahty between bacterial and primary production of 0.22, whlch, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low bio-mass of benthic organisms in the region

Benthic-pelagic coupling: effects on nematode communities along southern European continental margins

Along a west-to-east axis spanning the Galicia Bank region (Iberian margin) and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m). Nematode standing stock (abundance and biomass) and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude) governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter). Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes