Elucidating Marine Biogeography with Macrophytes: Quantitative Analysis of the North Atlantic Supports the Thermogeographic Model and Demonstrates a Distinct Subarctic Region in the Northwestern Atlantic

Abstract 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, tabular, and graphic descriptions of the seaweed assemblages within the Subarctic and their interactions with the major grazer, the sea urchin Strongylocentrotus droebachiensis. Three of the four most abundant seaweeds in the North Atlantic Subarctic Region (Agarum clathratum, Desmarestia viridis, and Ptilota serrata) occur in mid-deep water and are protected from grazing by secondary chemicals. The kelp Alaria esculenta often forms a monocultural canopy in the shallow subtidal zone, and it is the second-most abundant seaweed in the Subarctic. Sea urchins form “fronts” that graze back the Alaria in spring and summer when wave action is moderate, but the wide Alaria zone is persistent on more exposed shores. Agarum clathratum, D. viridis, and P. serrata) in deeper water tend to form a savanna: dense patches of fleshy seaweeds separated by areas of coralline/urchin “barrens”. Unlike Arctic or estuarine barren zones, the coralline “barrens” of Subarctic rocky shores are calcitic, biostromal structures with a rich invertebrate infauna. These formations demonstrate great temporal stability, with longevities of decades to centuries, as shown through laser/ablation mass spectrograph analysis of Mg2+ content in the coralline Clathromorphum compactum. This research examines the relationship of the Subarctic to the eastern Atlantic Boreal and to the largely Boreal-dominated transition zone in the Gulf of Maine and Nova Scotia. 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. During the mid-Holocene's pre-rebound hypsithermal, due to a warmer and more open Canadian Northwest Passage, these species were likely supplemented by an injection of North Pacific Subarctic seaweeds. 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; invasion and change continue today, and likely represent a major anthropogenic bridging of an oceanographic barrier.