Modern diatom assemblages in surface sediments from the Maritime Estuary and the Gulf of St. Lawrence, Quebec (Canada)

The Gulf of St. Lawrence located in the northeastern part of North America is a transitional environment from estuarine to marine conditions. Diatom a nalysis of 41 surface sediments samples showed great diversity with the ide ntification of 51 genera and 135 species. Diatom concentrations in surface sediments varied between 9 x 10(3) and 1.9 x 10(6) valves per gram of dry s ediment, and mean diatom flux in the Gulf of St. Lawrence is about 53 x 10( 3) frustules/cm(2)/year. Q-mode factor analysis was performed upon 25 taxon omic categories that are defined by similar ecology. Eight factors that exp lain 93.8 percent of the variance were defined. From these, 6 diatom assemb lage zones that were closely related to surface water hydrography could be delimited. The Saguenay Fjord assemblage (I) is mainly composed of fresh to brackish water species Achnanthes spp., Tabellaria spp., Cocconeis placent ula var. euglypta and by a marine associated taxon Thalassiosira cf. pacifi ca. The Maritime Estuary assemblage (II) is principally dominated by estuar ine taxa, including Cyclotella meneghiniana. The Northwestern Gulf assembla ge (III) is characterized by Aulacoseira spp. The Northeastern and Central Gulf assemblage (IV) is dominated by the resting spores of Thalassiosira an tarctica. The North Atlantic assemblage (V) is marked by vegetative cells o f marine Chaetoceros spp. The last assemblage (VI) corresponds to nutrient rich and high productivity coastal areas with common Paralia sulcata. Attem pts to calculate quantitative estimates or to generate paleoecological tran sfer functions, in order to characterize the relations between the modern d iatom assemblages and the hydrographic conditions of the Gulf of St. Lawren ce, were not successful, in part due to the limited number of samples analy zed in this study to represent the high variability of the whole area.