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.

This study investigates modern dinoflagellate cyst assemblages and their relationships with environmental parameters as indicators of eutrophication in the Gulf of Annaba, Algeria. In 2018, surface sediment and water samples were collected from three coastal stations, including a commercial port, to evaluate spatial variations in cyst abundance, diversity, and nutrient enrichment. A total of 874, 544 and 150 dinoflagellate cysts were recorded at stations 1 (Port), 2, and 3, respectively, reflecting strong ecological gradients linked to anthropogenic pressure and sediment characteristics. Species richness and diversity were the highest in muddy, organic-rich sediments at the port station, with dominant taxa such as Lingulodinium machaerophorum, Alexandrium pacificum indicating eutrophic conditions and potential harmful algal bloom (HAB) risks. Analyses of physico‑chemical parameters indicated significantly elevated concentrations of dissolved inorganic nitrogen (DIN), phosphate (PO₄), and silicate (SiO₄) in the bottom layers, with Station 1 showing the highest values. Ammonium (NH4) was the predominant nitrogen form, comprising over 50% of total DIN in surface and bottom waters at the port station, highlighting localized organic enrichment likely driven by urban, industrial, and port-related discharges. The distribution of dinoflagellate cysts correlates strongly with these nutrient and sedimentary patterns, validating their use as bioindicators of eutrophication. These findings underscore the ecological sensitivity of the Gulf of Annaba to anthropogenic impacts and advocate for the integration of cyst monitoring in coastal environmental.