The seaweed flora of Bras d'Or Lake, Cape Breton Island, Nova Scotia is compared with those of the Baltic Sea in northern Europe and five Northwest Atlantic embayments ranging from Pomquet Harbour, Nova Scotia to Narragansett Bay, Rhode Island. Both Bras d'Or Lake and the Baltic Sea have experienced extensive geological and hydrographic changes during the past 15,000 years, which have resulted in limited connections to the open sea, negligible tides, low salinities, and reduced floras. The ecology of these two unique saltwater bodies differs from true estuaries like the York River of Maine, the Great Bay Estuary System of New Hampshire/Maine, and the Merrimack River of Massachusetts, which experience extreme hydrographic and tidal variability. Of the 309 total seaweed taxa recorded from the six Northwest Atlantic embayments, Bras d'Or Lake had the lowest number of red, brown, and green algae (i.e., 83) and shared taxa (mean 61.2 ± 4.9 SE), whereas Narragansett Bay had the highest numbers of total (203) and shared taxa (mean 111.6 ± 20.1 SE). Several taxa were restricted to single embayments, with a few glacial and limnic relicts occurring in Bras d'Or Lake and the Baltic Sea. Nine disjunct warm-water taxa were present in Bras d'Or Lake, with this representing the second highest numbers found in four embayments located north of Cape Cod, Massachusetts; this pattern reflects the characteristics of a shallow inland lake. Eleven introduced taxa were recorded from the various Northwest Atlantic embayments, ranging from two in Bras d'Or Lake to 3–10 in the other five embayments. The absence of an intertidal fucoid zone in the Lake (and the Baltic Sea) is probably associated with several physical factors, including a lack of tides, extensive ice-scouring, long-lasting atmospheric reductions of sea level, and reduced salinities. By contrast, the occurrence of subtidal fucoid algae within both brackish water habitats is probably due to a lack of strong competition from perennial kelp populations, which have limited tolerances to low salinities. Morphological features of attached submerged fucoids are analogous to those of loose-lying or entangled salt marsh forms, which are usually small, thin, profusely branched, and have extensive vegetative fragmentation. Unique aegagropilous (detached, ball-shaped) populations of Gracilaria tikvahiae also were found at some inland Bras d'Or Lake sites, presumably resulting from ice scouring and detachment.