The biodiversity of copepods as examined in the present study is based on an oceanographic survey conducted on two ecosystems along the Algerian coast (central and eastern) in the early autumn of 2014. During that investigation, the thermocline was present at shallow depth, extending between 30 and 80 m. A total of 49 species were recorded in the samples, belonging to 20 families, among which six families, Calanidae, Eucalanidae, Metridinidae, Oithonidae, Paracalanidae, and Temoridae, appeared to be dominant. The total abundance of zooplankton at the 13 stations was less than 600 ind/m³. Two geographic groupings have been distinguished: stations in the central sector (group 1) are in opposition to the other stations in the eastern sector (group 2). The quantitative levels of copepods are less abundant in the east (234.15 ind/m³) than in the centre (334.99 ind/m³). In the central regions, the trophic organization is dominated by omnivores (73.02%), with an inferior co-dominance of carnivores (14.02%) and herbivores (12.95%). In the east, copepods with omnivorous behaviour are strongly represented (82.14%), followed by phytophagous (13.85%) and carnivorous (4.01%) species. The distribution of the abundances over the copepod communities was found to be linked to the temperature, salinity, North Atlantic Oscillation (NAO) index, and phytoplankton biomass.

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.

Put-M.L. discussed the results. H.A. Scientific manager of the study, conceptualisation writing, proposal of treatments and indices used and final correction of the text, H.F. Sampling and data processing. Z.A. Performed the study. K.GEY. Statistical processing of results relating to PCA and CA. Instead-M.L. discussed the results. H.A. Performed the study, con