Distribution des kystes du type Alexandrium excavatum dans les sediments recents et postglaciaires des marges est-Canadiennes

Abstract

Palynological analyses were performed in surface sediment samples from the North Atlantic and adjacent basins in order to document the distribution of organic-walled cysts of the toxic taxon belonging to Alexandrium excavatum. The major occurrence of the cysts along the south Scandinavian and southeastern Canadian margins reveals affinities of this taxon for neritic environments with cool-temperate (15-17°C in August) and relatively low salinity (-32‰) conditions in surface waters. In the Gulf of St. Lawrence area, cyst abundances increase from the Estuary to the outlet of the Gulf (Cabot Strait). The analyses of postglacial sedimentary sequences from this area show a maximum abundance of Alexandrium excavatum-type around 9000 BP, which corresponds to the beginning of the present interglacial. This acme is marked by particularly high concentrations (up to 104 cyst.cm-3) at the outlet of the Gulf of St. Lawrence (Cabot Strait area). Both the recent and postglacial distribution of the cysts of Alexandrium excavatum-type along the eastern Canadian margins suggest a regional seeding in the Gulf of St. Lawrence from the adjacent North Atlantic neritic zones.