Nous présentons ici, pour la première fois, un inventaire des dinoflagellés responsables des blooms toxiques ou non de la baie d'Annaba (Algérie) durant l'année 2002. Les analyses qualitative et quantitative des peuplements phytoplanctoniques révèlent l'existence de 11 taxons, parmi lesquels deux espèces, Alexandrium catenella et Gymnodinium catenatum , connues pour produire des toxines, ont provoqué des blooms. Sept autres espèces, Dinophysis caudata , D. fortii , D. rapa , D. rotundata , D. tripos , Lingulodinium polyedrum et Protoperidinium crassipes sont toxinogènes, mais n'ont jamais contribué, ici, à des floraisons phytoplanctoniques. Enfin, les deux derniers taxons, à savoir, Prorocentrum triestinum et Scrippsiella trochoidea , ont été associés à des eaux colorées, mais n'ont pas de potentialité toxique reconnue. L'étude physicochimique réalisée parallèlement à celle du phytoplancton a montré le caractère eutrophe des stations 1 et 2. La station 3, en revanche, est oligotrophe.

Environmental conditions ultimately leading to blooms of the toxic dinoflagellate Alexandrium catenella/tamarense were investigated at two Mediterranean sites (Annaba Bay, Algeria and Thau lagoon, France). Three years were examined in details: 1992 (a pre- Alexandrium period), 2002 (a year with the first bloom in Annaba) and 2010 (a year with a major bloom in Annaba). Most conditions were similar, but ammonium concentrations were much higher in Annaba (up to 100 μM) than in Thau (up to 10 μM). First records of A. catenella/tamarense were in 1995 for Thau and 2002 for Annaba, and coincided with soluble reactive phosphorus (SRP) decreasing below a concentration of about 1 μM. No other environmental variable could be related to those blooms. Thus, it is likely that the large reductions in SRP at both sites led to phosphorus limitation of a certain number of phytoplankton species and favored the development of A. catenella/tamarense .

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.

For the first time in the bay of Annaba (Southern Mediterranean), we studied the spatiotemporal distribution of potentially toxic benthic dinoflagellates: Ostreopsis cf. ovata, Prorocentrum lima, Coolia monotis, and Amphidinium carterae, hosted by the dominant macrophytes Posidonia oceanica, Padina pavonica, Codium fragile, and Halopteris scoparia. Sampling of these macrophytes was conducted weekly during spring and summer as well as bi-weekly in autumn and winter, from October 2022 to November 2023, at contrasting sites within Annaba Bay. The measured environmental parameters included temperature, pH, dissolved oxygen, salinity, ammonium, nitrate, nitrite, dissolved organic nitrogen, dissolved inorganic nitrogen, phosphate, dissolved organic phosphorus, silicate, and chlorophyll a. A proliferation of O. cf. ovata was recorded in July 2023, coinciding with a marked increase in temperature, with a maximum abundance exceeding 40 × 10³ cells g⁻¹ of fresh weight (FW) on H. scoparia and C. fragile. The maximum abundance of P. lima reached 8700 cells g⁻¹ FW on H. scoparia during July and August 2023. Coolia monotis exhibited a peak of 2800 cells g⁻¹ FW on H. scoparia. The abundance of A. carterae increased with temperature, reaching a maximum of 980 cells g⁻¹ FW on P. pavonica. The distribution of epiphytic dinoflagellates varied according to the macrophyte substrate. Overall, statistical analyses indicate that benthic dinoflagellate community structure is shaped by the combined effects of temperature, nutrient availability, and ecological niche differentiation, with temperature emerging as the dominant driver. This suggests that climate-driven increases in Mediterranean Sea surface temperatures are likely to extend the seasonal window of harmful benthic algal blooms, thereby enhancing ecological disturbances and potential risks to human health. This study provides the first assessment of BHAB dynamics along the Eastern Algerian coast, highlighting the role of ongoing regional warming in shaping future bloom patterns.