Two representatives of the planktonic diatom genus Thalassiosira, commonly associated together in the moderate to high latitude North Atlantic Ocean, are demonstrated to be significantly different in both morphology and autecology. Thalassiosira antarctica Comber var. borealis Fryxell, Doucette et Hubbard (mostly resting spores) is a dominant species of the early spring phytoplankton bloom around Greenland and in the Arctic. In most paleoclimatic/paleoenvironmental studies of the last decades, Thalassiosira kushirensis Takano resting spores (r.s.) have probably been included in T. antarctica var. borealis. The present study of phytoplankton (from CTD probes and a single surface sediment sample, from a multi core) and comparative analyses of piston cores from the West Greenland margin show these taxa to be good indicators of environmental and hydrological changes related to the distribution of water masses in the studied area. T. antarctica var. borealis is interpreted as an indicator of northern cold water influx (especially increased meltwater input during spring bloom), whereas T. kushirensis r.s. appears to be related to temperate waters of Atlantic origin, and probably reflects reduced meltwater influx and increased ocean forcing. An abundance of T. kushirensis r.s. during the Little Ice Age (LIA) is interpreted to reflect an enhanced influence of the West Greenland Current (WGC) in this region.