Upwelling-driven vertical segregation of co-occurring phytoplankton, including HAB species, revealed by niche analyses.

Recurrent toxic blooms of Alexandrium minutum, Pseudo-nitzschia spp. and Dinophysis acuminata, impact the European shellfish industry. In temperate latitude embayments with spring-summer upwelling, reset phytoplankton mini-successions develop during short-term (a few days) wind-driven events interspersed by relaxation or downwelling. High resolution observations were made at a reference station during cruise "REMEDIOS-TLP", 2-14 July 2018 to explore assembly dynamics (cluster analysis) and niche structuring (OMI and WitOMI) processes during a relaxation-upwelling event. A well-mixed "Relaxation habitat" (H1) co-occurred with a shallow warmer and fresher "TS stratified surface layer" (H2) (site of Alexandrium cell maximum) from the confined water mass on the shoreward side of the downwelling front. These two habitats preceded a new upwelling pulse of cold nutrient rich waters which raised the Chl-a rich waters and entrained populations of Pseudo-nitzschia through the new "Bottom upwelled water" (H4). Diatoms (central to pennate and back to central) peaked during spin up and spin down phases within the "Upwelling pycnocline" (H3), site of the subsurface chlorophyll maximum (SCML), and a short-lived thin layer (TLP). Leptocylindrus minimus, suspected threat to caged salmon in Canada and Chile, thrived at the "upwelling diatom bloom maximum" phase but spring 2018 anomalies excluded optimal environmental windows for D. acuminata development. Isolated patches of Dinophysis revealed decimetre-scale niche segregation with Alexandrium populations. Detailed biological parameterization presented here will contribute to improve prediction of assemblages and processes which favour harmful algal blooms.