Winter sediment resuspension in a submarine canyon drives the offshore export of toxigenic Alexandrium catenella cysts

Harmful algal blooms (HABs) caused by the toxigenic dinoflagellate Alexandrium catenella recur regularly in the Lower St. Lawrence Estuary (LSLE, eastern Canada). Using quantitative polymerase chain reaction (qPCR) to detect A. catenella DNA as a proxy for cyst abundance in the Pointe-des-Monts canyon system, we show that resting cysts are concentrated in nearshore sediments and on the canyon fan, but are depleted along the canyon axis. Sediment traps reveal that major cyst resuspensions occur exclusively during winter storms under ice-free conditions, with peaks in A. catenella DNA fluxes coinciding with turbidity currents and high total particulate matter export. These events effectively flush coastal cysts offshore, creating a negative feedback loop that reduces subsequent local bloom potential. Conversely, climate change-driven increases in summer-to-autumn storm frequency and intensity could resuspend cysts under optimal proliferation conditions. Sedimentary ancient DNA in a ²¹⁰ Pb-dated sediment core also demonstrates that qPCR reliably records known historical A. catenella bloom events. These results highlight the critical, yet underappreciated, role of submarine canyon systems and storm-driven sediment dynamics in modulating A. catenella bloom risks, and suggest that ongoing climate-driven shifts in sea-ice cover and storm seasonality are likely to influence future HABs in the LSLE and similar high-latitude systems.