Upwelling and flood-driven regulation of coastal diatom assemblages along the southern Moroccan Atlantic margin: environmental forcing, community structure, and amnesic shellfish poisoning risk within the Canary Current Upwelling System.

Eastern boundary upwelling systems sustain among the highest marine primary productivity globally, yet diatom assemblage responses to persistent upwelling and episodic hydroclimatic disturbances remain critically undercharacterised along the North African Atlantic margin. This study examines diatom spatiotemporal dynamics along the Aglou-Sidi Ifni coastline (~29°N, Moroccan Atlantic), a transitional sector of the Canary Current Upwelling System, over two annual cycles (2014-2016) at three coastal stations. A total of 126 diatom taxa were identified among 233 phytoplankton species (54% of total richness). Taxonomic revision against WoRMS and AlgaeBase yielded 20 nomenclatural updates. Total phytoplankton cell density exhibited a pronounced seasonal pattern, peaking in spring 2015 (65,890 cells L-1), exceeding spring 2014 by 24-31%. This amplification coincided with elevated nitrate (up to 390.6 μg L-1) and phosphate (up to 147.8 μg L-1), consistent with a flood-driven nutrient priming hypothesis, although single-event design precludes causal attribution. Molar N:P ratios ranged 2.58-6.32 (station means: 4.17-4.28), remaining below Redfield (16:1), evidencing chronic nitrogen limitation. Absent dissolved silicate data preclude complete bloom stoichiometry characterisation. Multivariate analysis explained 58.6% variance, with temperature and nitrate as primary community drivers. A southward gradient in diatom dominance (46-52%) and richness was consistent with spatial upwelling intensity differences. Six Pseudo-nitzschia morpho-groups were recorded, including P. multiseries near Massa Marine Protected Area. Confirmatory transmission electron microscopy (TEM), quantitative PCR (qPCR), and shellfish domoic acid assays are required before regulatory conclusions; this study provides a preliminary precautionary baseline signal.