Expansion of the Asiatic green alga Codium fragile subsp tomentosoides in the Gulf of Maine [Review]

摘要

The invasive Asiatic green alga Codium fragile subsp. tomentosoides, which was introduced to the northwestern Atlantic in eastern Long Island Sound, New York during 1957, has recently colonized several parts of the Gulf of Maine via two invasion sites: (1) mid-coastal Maine at Boothbay Harbor after transplantation of Long Island oysters (1964); and (2) southern Massachusetts as a result of the expansion of attached plants from the Cape Cod Canal (1969) into nearby Cape Cod Bay (1972). Because of its warm-water affinities, southern New England populations initially expanded more rapidly than northern ones, becoming particularly invasive in shallow subtidal habitats. By contrast, the Boothbay Harbor populations were rather "quiescent," long-lived, and circumscribed until the early 1970s. After a buildup of significant biomass, extensive fragmentation and drifting occurred via strong south-flowing currents; thereafter, Codium colonized mid-coastal Maine, New Hampshire, and northern Massachusetts. Currently, the Casco Bay area just south of Boothbay Harbor has the highest number of invasion sites, particularly at offshore islands. Attached populations are limited to a few contiguous sites just north of Boothbay Harbor, with only drift specimens occurring in "Downeast" Maine. Recent introductions have also been recorded in the Canadian Maritime Provinces, both within the Atlantic (1989) and the Gulf of St. Lawrence (1996) shorelines. Thus, the species' expansion in the northwestern Atlantic has involved multiple introductions, different vectors (shellfish and vessels), dispersal by major south-flowing currents, and differential viability/abundance of in situ populations due to varying hydrographic and exposure conditions. In comparing a series of permanent study sites in southern Maine and New Hampshire during 1982 to 2001, Codium increased from one to 26 sites during 19 years, with the most evident expansion between 1996-1998. A comparison of several demographic features (length, weight, density, biomass, and percent occurrence) for six Codium populations ranging from mid-coastal Maine to New Hampshire showed that outer estuarine and nearshore open coastal plants were smaller and had more limited densities and biomass than those found at warmer offshore insular sites like Star Island, New Hampshire. Codium is now the dominant canopy species in some southern Maine and New Hampshire locations, extending to similar to8 m below mean low water; it occurs in both disturbed sites (e.g., former urchin barrens) and established kelp beds at sheltered and exposed locations. The Asiatic red alga Neosiphonia (Polysiphonia) harveyi, which is the dominant epiphyte on Codium, is also exhibiting a rapid expansion in this same geography. Drifting populations of Codium may be a good vector for its introduction, as well as for several other epiphytes. Another aspect of the invasion of Codium into the Gulf of Maine is the spread of the introduced bryozoan Membronipora membranacea, which is common on kelps but appears to be exhibiting enhanced survival on Codium because of its perennial growth pattern and lack of epibiontic sloughing.