Résumé
The bloom-forming dinoflagellates and Euglenophyceae taxa were observed in the coastal waters of Hammam Lif, a town located 12 km south of Tunisia’s capital (Southern Mediterranean), during a green tide event on June 3, 2023. The bloom was attributed to the dinoflagellate Gymnodinium chlorophorum with up to 23,226,088 cells L−1, which was identified through ribotyping. Euglena spp. and Eutrepsiella spp. also responsible of the discoloration and belonging to Euglenophyceae group were observed with densities up to 29,899,691 cells L−1.The environmental results indicated a significant depletion of nitrite and nitrate, with a rapid increase in sunlight duration, which likely promoted the proliferation of G. chlorophorum and the Euglenophyceae taxa. These green phytoplankton species presumably consumed the available nutrient stocks, leading to a clear depletion observed two days after the bloom (June 5, 2023). Diatoms growth appeared to be limited by a shortage of silicates, with concentrations falling below 0.05 µmol/L. In contrast, concentrations of dissolved inorganic phosphate (DIP) and ammonium (NH4+) were elevated during the bloom (0.88 and 4.8 µmol/L, respectively), but declined markedly by June 5 (0.23 and 1.06 µmol/L, respectively). The low salinity recorded on June 3 (34.0) suggests significant freshwater input from the Meliane River, likely contributing to nutrient enrichment and the initiation of the bloom. Following the bloom, phytoplankton abundance and chlorophyll concentrations declined, accompanied by a community shift from dinoflagellates to diatoms. The findings suggest that ultraviolet radiation (UVR) and anthropogenic nutrient inputs particularly ammonia were the primary drivers of this green phytoplankton bloom. During the bloom, G. chlorophorum accumulated pigments such as carotenoids and chlorophyll b, which decreased sharply after the event. The presence of mycosporine-like amino acids (MAAs) during and after the bloom may reflect differences in phytoplankton composition and could play a crucial role in protecting these organisms from UVR.