The shallow water, coastal flora and fauna from Cape Cod to southern Labrador in the northwestern North Atlantic have been biogeographically regarded as a single unit, although the northern half has been only weakly sampled. The recent “Adey/Steneck biogeographic model” for the subtidal has shown the northern half of this coast as a core Subarctic Region, while the southern half is mixed Boreal/Subarctic (the North Atlantic Boreal being centered in the British Isles). In this study, quantitative sampling, and statistical and graphic analyses of the dominant intertidal biota shows the two areas to be quite different based on species biomass or number of individuals/m2. Ascophyllum nodosum, highly dominant in the southern part of the area becomes an occasional in the north, with Fucus vesiculosus in part replacing it, while Fucus distichus, a minor species in the south, becomes a dominating element on the northern rocky shores. The ubiquitous, intertidal, understory of the bushy-red alga Chondrus crispus and its associated algae in the Gulf of Maine and Nova Scotia, virtually disappears in the northern half of the region, being replaced by the large, filiform, brown alga Chordaria flagelliformis and its complex of ecologically-associated species. The characteristic, intertidal mollusc fauna shows a parallel change, with the abundant Littorina littorea of the southern coast being replaced by L. saxatilis northwards. Those species dominating the intertidal coastal biota surrounding the Strait of Belle Isle (center of the Subarctic core) provide 85% of the number/area/biomass count, but are only 13% of that count in the Gulf of Maine. These results provide further support for the Adey/Steneck theoretical model and demonstrate the necessity for using quantitative area/biomass data, as opposed to only species presence/absence data, in biogeographic analyses.