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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

This is the first study on the dinoflagellate cysts inAlgerianwaters and in Mellah Lagoon (SouthWesternMediterranean), located within a protected reserve. In total, 42 species of dinocysts belonging to 7 orders, 12 families and 23 genera, were identified in the 26 superficial sediment samples from Mellah Lagoon. The distribution of dinocysts in the sediment of this lagoon is heteroge-neous. Indeed, their abundance oscillates between 1 and 315 cysts g−1 dry sediment (DS). Cyst morphotype assemblages were dominated by a few numbers of species: Alexandrium minutum (15.87%), Gonyaulax verior (9.81%), Protoperidinium spp. (7.74%), Alexandrium affine (7.05%), Scrippsiella trochoidea (6.67%), and Alexandrium pseudogonyaulax (6.19%). There is a positive correlation between the density of cysts and the depth (r = 0.61; p < 0.05), organic matter (r = 0.70; p < 0.05), water content (r = 0.71; p < 0.05), and the fine fraction of sediment (r = 0.74; p < 0.05). Surprisingly, although the Mellah Lagoon is almost semi-closed, it holds an important specific richness in dinocysts (42 species) higher than others observed in Mediterranean lagoons. However, cyst abundances are low compared to other lagoons in the Mediterranean Sea. Finally, the presence of dinocysts of Alexandrium catenella/tamarense, A. minutum, and Gymnodiniumcatenatum associated to paralytic shellfish toxins, A. pseudogonyaulax which produces goniodomin A, also Protoceratium reticulatum and Gonyaulax spinifera complex which produce yessotoxins, needs to implement a monitoring programto prevent a potential human intoxication due to the consumption of contaminated sea products by these potent neurotoxins.