First Record of Lepidodinium chlorophorum and the Associated Phytoplankton Community Responsible of the Green Tide South Western Mediterranean Sea (Hammam-Lif, Tunisia)

The bloom-forming dinoflagellates and euglenophyceae were observed in the coastal waters of Hammam-Lif (Southern Mediterranean), during a green tide event on 3 June 2023. The bloom was dominated by Lepidodinium chlorophorum, identified through ribotyping with densities reaching 2.3 × 107 cells·L−1. Euglena spp. and Eutrepsiella spp. contributed to the discoloration, with abundances up to 2.9 × 107 cells·L−1. Environmental data revealed significant depletion of nitrite and nitrate, coinciding with a rapid increase in sunlight duration, likely promoting the proliferation of L. chlorophorum and euglenophyceae. By 5 June, two days after the bloom, nutrient stocks were exhausted. Diatoms appeared limited by low silicate concentrations (<0.05 µmol·L−1), while dissolved inorganic phosphate and Nitrogen-ammonia were elevated during the bloom (0.88 and 4.8 µmol·L−1, respectively), then decreased significantly afterward (0.23 and 1.06 µmol·L−1, respectively). Low salinity (34.0) indicated substantial freshwater input from the Meliane River, likely contributing to nutrient enrichment and bloom initiation. After the event, phytoplankton abundance and chlorophyll levels declined, with a shift from dinoflagellates to diatoms. The accumulation of pigments (chlorophyll b and carotenoids) and the presence of Mycosporine-like amino acids (MAAs) during and after the bloom suggest that UV radiation and Nitrogen-ammonia were key drivers of this green tide.