Dinoflagellate rise and diatom retreat: functional shifts in phytoplankton assemblages along the central Algerian coast

Abstract Understanding the response of phytoplankton communities to environmental variability is essential to predicting ecosystem trajectories under climate change, particularly in ecologically sensitive coastal systems. This study investigates the composition, abundance, and ecological dynamics of phytoplankton communities along the central Algerian coast based on data collected during the SPHYTAL 2015 survey. A total of 55 species were identified, with dinoflagellates clearly dominating both qualitatively (70% of taxa) and quantitatively (77% of total cell abundance), followed by diatoms (25%). Prorocentrum and Ceratium were the most abundant dinoflagellate genera, while Coscinodiscus and Thalassiosira dominated among diatoms. Spatial variations in cell densities were influenced by gradients in sea surface temperature, salinity, and nutrient availability, particularly phosphates and nitrates. Multivariate analysis and diversity indices (Shannon, Simpson, Evenness) revealed a moderately structured and diversified community, indicative of a functional shift in phytoplankton assemblages. A climate-based simulation projected a tipping point by 2100, with dinoflagellates increasingly outcompeting diatoms under both RCP4.5 and RCP8.5 scenarios. These findings suggest a transition toward a more stratified and potentially harmful dinoflagellate-dominated regime, echoing patterns observed in other Mediterranean and Atlantic regions. The study underscores the need for integrating phytoplankton monitoring into coastal management and climate adaptation strategies.