In a 2001 publication in the Journal of Phycology, Walter Adey and Robert Steneck presented a mathematical model demonstrating that coastal biogeography should be determined by coastal area and seawater temperature over Pleistocene time. The model identified space-time regions, many of which were geographically similar to classical empirically developed biogeographic regions and others that were entirely new. The predictions of the Thermogeographic Model were quantitatively tested against coralline algal species abundances developed for the colder North Atlantic and published from 1964-1975.

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The current investigation uses seaweed standing crop data to further demonstrate the validity of the Thermogeographic Model. Using the research vessel Alca i, to obtain quantitative abundance and distribution data, seaweed assemblages were studied at 51 primary SCUBA stations with 4–7 standard depth zones at each station from Cape Elizabeth, ME to southern Labrador. Meter-square manual harvests of seaweed from quadrats dropped at each station were sorted at sea by species, followed by identification and weighing. Voucher herbarium sheets were produced and samples for DNA analyses were collected

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The biomass data demonstrated 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 and statistical 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. A subset of Subarctic species persists in deeper water to the south. Notably, less than 4% of the seaweed flora of the Subarctic Region is Arctic in origin. Earlier geographic analyses of biodiversity did not discover the Subarctic Region because rare species hide the strength of species assemblages based on abundance.

 

Many of the dominant shallow water seaweeds in the transitional region of the Gulf of Maine and Nova Scotia occurs widely in Europe and are Boreal in origin. Such Boreal species

constitute 47–80% of the seaweed 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 represent only 22–25% of the macrophyte biomass, the remainder being southward penetrating Subarctic species. By analyzing the TM and its relationship to the history of deglaciation after the Last Glacial Maximum (LGM), we hypothesize the western Atlantic rocky shore was occupied primarily by Subarctic glacial relict seaweeds in the early Holocene. In the latest Holocene, and perhaps only in historical times, a surge of eastern North Atlantic Boreal seaweeds entered and came to dominate the southern, climatically transitional half of this coast. This ongoing invasion likely represents a major anthropogenic bridging of an oceanographic faunal barrier.