The invasive canopy alga, Codium fragile ssp. tomentosoides, first observed at the Isles of Shoals in 1983, has become the dominant canopy species to 8 m throughout the islands. Codium populations are replac-ing themselves at most sites in what appears to be a new, climax, canopy species. However, Codium densities have declined in protected Gosport Harbor areas where it first became established. Codium has only slowly expanded its presence in adjacent nearshore subtidal habitats. Recent studies suggest a com-bination of factors that may be influencing the relative success of populations between habitats. The her-bivorous sea slug, Placida dendritica, may be reducing populations in protected areas in spite of predators such as the green crab, Carcinus maenas, while surge may inhibit herbivore buildup in exposed habitats. Temperature instability due to localized, wind-driven upwelling may be slowing the buildup of subtidal Codium populations in nearshore sites. The combination of Codium dominance and the acquisi-tion of increasing epibiont diversity are producing a new, potentially more complex community state than the previous kelp-dominated climax typical of the Gulf of Maine.
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