Addaia Bay is a rather small (85 ha), shallow (less than 12 m depth) and sheltered bay situated on the northern coast of Menorca (Balearic Islands, western Mediterranean), where the seabed is mostly covered by seagrass meadows. The seagrass Cymodocea nodosa dominates the inner parts of the bay, with densities up to 2325 shoots m~2, while Posidonia oceanica covers most of the outer part, with shoot densities ranging from 235 shoots m~2 in deep waters to 950 shoots m~2 at a depth of 2.5 m. Other widespread phytobenthic communities are dominated by Caulerpa prolifera (muddy bottoms), photophilic algal stands (rocks and dead Posidonia rhizomes), and Ruppia cirrhosa (very sheltered sites). The presence of a well preserved barrier reef made up of Posidonia oceanica in the eastern part of the bay is noteworthy, since these formations have disappeared or are in regression all over the Mediterranean coasts. The lagoon, situated at the inner side of the Posidonia reef, is covered by dense Cymodocea nodosa stands and free-living algae (Valonia aegagropila, Rytiphloea tinctoria and maerl). The only anthropic disturbances affecting the bay seem to be the construction of a marina and dredging. Considering the biomass values and the surfaces occupied by each community, the total phytobenthic standing crop of Addaia Bay in summer has been estimated as 12201 dw (4911 dw if the below ground biomass of Posidonia oceanica is not considered). Seagrasses account for nearly the 95% of the total biomass. Average primary production has been estimated to be around 530 g dw m~2 y" 1 , which emphasizes the important contribution of these bays to the total productivity of Mediterranean coastal areas and the predominant role played by seagrass meadows in these environments.