Common associations of mollusc and Foraminifera species were investigated in eastern Chaleur Bay, Gulf of St. Lawrence. Three depth-related biotopes are recognized. The distribution of species in this open bay environment appears to be controlled by substrate and (or) water mass characteristics. North–south differences in shallow water assemblages are related to summer bottom water temperature; calcareous Foraminifera and molluscs dominate in relatively high temperature environments. The influx of cold water (<1 °C) at intermediate depths (40–80 m) is reflected by an increase in the abundance of arenaceous Foraminifera species such as Reophax scottii, and by the absence of numerous mollusc species that are found at these depths elsewhere. A deep bay biotope (>80 m) can be recognized primarily on the basis of the mollusc species Yoldia limatula and Periploma fragile in association with the Foraminifera species Islandiella islandica. The observed mollusc–Foraminifera associations can be applied to paleoenvironmental and biostratigraphic studies of north temperate Holocene marine sediments.

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