Nine currently accepted species of the genus Sargassum (Phaeophyceae) have been recorded from the Mediterranean Sea: S. acinarium, S. desfontainesii, S. flavifolium, S. furcatum, S. hornschuchii, S. muticum, S. ramentaceum, S. trichocarpum and S. vulgare. Two of them, S. desfontainesii and S. furcatum, are Atlantic species, the Mediterranean range of which is restricted to the Alboran Sea, close to the Strait of Gibraltar. Three species, S. hornschuchii, S. ramentaceum and S. trichocarpum are endemic to the Mediterranean. The presence of S. flavifolium in the Mediterranean, a north-eastern Atlantic species, is at least in part based upon a misidentification and therefore requires confirmation. A taxon collected near Algiers and Annaba (Algeria), here referred to as ‘Sargassum sp. from Algeria’, although sharing most characters with S. flavifolium, also presents some distinctive characters. In the absence of genetic data, confirming or not its taxonomic value, and in the lack of studies on the herbaria containing the types of many species currently considered as synonyms of S. flavifolium, but which are possible candidates for accommodating the Algerian specimens, the authors prefer a conservative approach. They do not describe a new species, which would be premature, but draw attention to a taxon which may be more widely distributed in the Mediterranean rather than solely along the Algerian coasts.

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The composition and distribution of diatoms were studied using a biweekly sampling performed in three representative stations in the Mellah Lagoon (South Western Mediterranean, Algeria), during 2016. In total, 160 diatoms species-belonging to centric species (52 species) and pennate species (108 species)-were inventoried. The relative abundance of the pennate forms (67.50%) is clearly higher compared to the centric forms (32.50%). The blooms of diatoms are represented by 3 species: Chaetoceros constrictus Gran, 1897 with up to 15000 cells.L-1 (end of May), Chaetoceros sp. with up to 178320 cells.L-1 (early October), and Nitzschia longissima (Brébisson) Ralfs, 1861 with up to 200640 cells.L-1 (early September). Proliferations of three potentially toxic species: Pseudo-nitzschia group delicatissima (Cleve) Heiden, 1928 (2500 cells.L-1), Pseudo-nitzschia group seriata (Cleve) H. Peragallo, 1899 (700 cells.L-1) and Thalassionema nitzschoides (Grunow) Mereschkowsky, 1902 (210 cells.L-1), requires regular monitoring of this ecosystem where the exploitation of the shells is practiced. The diatom populations seem to be structured showing diversity (H') and equitability (J') indices reaching 4.56 bits.cell and 0.98 respectively, with the exception of springsummer and summer-autumn transition periods. high biological productivity and biodiversity (Knoppers, 1994). However, the lagoons are very sensitive environments, fragile and extremely vulnerable, in relation to the organization of their living and non-living components which depends on the interaction between the natural components of the marine and continental areas (Dufour et al., 1994). Due to the wide variability of their physical, chemical and biological parameters, coastal lagoons are generally unstable environments. The