Quantitative analysis of North Atlantic macrophyte (seaweed) abundance was used to test the thermogeographic model (TM) of marine biogeography. The TM uses coastal area and seawater temperature over Pleistocene time to reveal long-term climate/ area clusters that predict biogeographic regions. An earlier study with the TM predicted 20 of 24 classical biogeographic regions; the 4 omitted regions were either weak, disputed, or structurally (sandy/silty regions) inapplicable to the TM. The TM predicted a new western North Atlantic Subarctic Region, centered on the Strait of Belle Isle and lying between Newfoundland and Labrador. In contrast, Nova Scotia and the Gulf of Maine were found to be a Boreal/Subarctic transition zone. The predictions of the TM were earlier supported with coralline algal abundances, and here are supported by a test with macrophyte assemblages.

Seaweed assemblages were studied at 51 primary SCUBA stations with 4–7 standard depth zones at each station from southern Labrador to Cape Elizabeth, ME. Meter-square quadrats were taken at each station, followed by at-sea sorting, identification, and weighing of biomass by species. A permanent record of these studies is provided by archived underwater photography and voucher herbarium samples, including tissue in silica gel to support future DNA analyses. Comparisons of European seaweed assemblages were accomplished by re-analyzing semi-quantitative data available in the literature.

We demonstrate with the biomass data that a 3000-km stretch of coast (i.e., northern Gulf of St. Lawrence, northeastern Newfoundland, and southern Labrador) that is centered on the Strait of Belle Isle has a unique assemblage of seaweeds. In its location and marine climate, this coast closely matches the Subarctic Region predicted by the TM. Based on graphic demonstration, this Subarctic Region is radically different from that of Nova Scotia and the Gulf of Maine, and its dominant species derive from the North Pacific Ocean. Several different statistical approaches applied to the macrophyte data demonstrate the strength of these findings, while also finding that a subset of Subarctic species persists in deeper water to the south in the transition zone (i.e., southwestern Nova Scotia, Gulf of Maine). Only a very small proportion of the seaweed flora of the Subarctic Region is Arctic in origin (<4%). Earlier geographic analyses of biodiversity did not discover the Subarctic Region because rare species hide the strength of assemblages based on abundance.

Many of the dominant seaweeds in the transitional region of the Gulf of Maine and Nova Scotia occur widely in Europe, and are Boreal in origin. Such Boreal species form 47–80% of the macrophyte biomass at depths shallower than 5 m in the northwestern Atlantic transition zone southwest of the Subarctic Region. In deeper water (>5 m), however, European Boreal species are only 22–25% of the macrophyte biomass. Although many of the dominant, shallow sublittoral species of the European Boreal flora have made the North Atlantic passage, only about 10% (by biomass) of mid-depth species (2.5–5 m) have crossed and even fewer of the deeper European species are found in the northwestern Atlantic. A few European “Boreal” seaweeds reach further north into the Subarctic Region, but only at low levels of biomass and primarily at mid-depths (2.5–5 m). The Subarctic is inhospitable for establishment of such species in shallow water because of low winter temperatures and sea ice, and in deeper water by low summer temperatures (<5 °C).

Few ecologists have had the opportunity to work in the Subarctic Region; instead, most of the recent descriptive and experimental studies of northwestern Atlantic ecosystems are based on studies in the smaller transition zone in the Gulf of Maine and southwestern Nova Scotia. Consequently, we provide detailed photographic,