Ascophyllum nodosum is distributed in the western Atlantic Ocean from the Arctic Circle (66° 33'N) to Long Island (40° 42'N) in the south (Taylor, 1957; Gosner, 1978; Keser et al., 2005). The SST in this current distribution ranges from -2.1°C to 23°C (Keser et al., 2005). Fucus vesiculosus extends from Ellesmere Island (80° 53'N) to North Carolina (34° 48'N) (Taylor, 1957). The SST in this range of distribution varies from -2.1°C to 27.8°C.
Ascophyllum nodosum is probably one of the most studied macroalgae in the north western Atlantic, mostly because of its ecological importance as a primary producer (Baardseth, 1970; Josselyn and Mathieson, 1978, 1980; Carlson and Carlson, 1984; Cousens, 1984, 1986; Vadas et al., 2004), as a habitat for invertebrates and vertebrates (Johnson and Scheibling, 1987; Black and Miller, 1991; Rangeley and Kramer, 1998), and for its economic importance as a raw material for alginate, agricultural products, and livestock feeds (Sharp, 1986; Ugarte and Sharp, 2001). As a result, its physiological and ecological responses to environmental variables in all life stages are reasonably well documented. Its lower intertidal limits appear to be controlled by grazing pressure in sheltered environments and by competition with Chondrus crispus in exposed locations (Lubchenco, 1980). The upper limits of A. nodosum distribution on the shore are regulated by its tolerance to desiccation (Schonbeck and Norton, 1978). Exposure to long periods of drying results in visible tissue damage of A. nodosum after 21-28 days (op. cit.), and cell death occurs at 70% water loss (MacDonald et al., 1974). Ascophyllum nodosum loses 70% of its fresh weight excluding vesicles after 7.5 h at 22°C and relative humidity of 40-45% (Dorgelo, 1976). Severity of winter conditions and amount of seasonal rainfall do not seem to be critical factors for this species. A 24-year study in the environs of a thermal power plant in eastern Long Island Sound demonstrated that the growth rate of Ascophyllum is very sensitive to seawater temperature (Keser et al., 1998, 2005). The highest growth rate was observed during spring and mid-summer during the study period. A rapid decrease in growth occurred above 25°C, with total mortality occurring above 27°C (Keser et al., 2005).
However, the germlings and macrorecruits are rockweed's most vulnerable stages. Among the factors shown to reduce recruitment are wave action
(Vadas et al., 1990), canopy of adult plants (Vadas and Elner, 1992), and grazing pressure (Lazo et al., 1994).
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