Deep-water Zostera marina meadows in the Mediterranean

In Morocco, Zostera marina Linnaeus has disappeared from many localities where it was historically reported. The only known remaining meadows along Mediterranean coasts of Morocco, though in North Africa, are those of Belyounech bay and Oued El Mersa bay, in the marine area of ‘Jbel Moussa’. An in-depth knowledge of these meadows is required for their effective conservation purpose. The Z. marina meadows of Jbel Moussa are deep, the lower limit being 17 m depth with patches extending down to 20 m depth. Seagrass cover of Belyounech bay meadow is continuous whereas that of Oued El Mersa is fragmented. Shoot density and aboveground biomass are higher in Belyounech meadow, with 745 ± 183 shoots.m−2 and 273 ± 40 gDW. m−2 of leaf biomass. During the survey, trawling scars and the invasive algae Caulerpa cylindracea Sonder were observed. Bioavailable Ni, As, Mo, Ag, Cd, Sn, Sb and U measured in the sediment are mainly accumulated in Z. marina roots. Nitrogen level is high in seagrass leaves and low in the sediment. Conversely, sediment is more enriched in phosphorus. Carbon levels and its isotopic ratio value are respectively higher and less negative in leaves when compared to the seagrass belowground compartments. All together, data collected during this survey allows defining the overall good health status of Z. marina meadows of Jbel Moussa. These Moroccan meadows, localized within the warm temperate-southern limit of the species, are well developed compared to many places worldwide. The exceptional presence of deep Z. marina meadows in the Mediterranean requires the implementation of measures as a major priority to ensure the conservation of these ecosystems, since seagrasses are being deeply threatened worldwide.