Spartina sp. (Cord-grass)

Sargassum muticum (Wire weed, strangle weed, Japanese weed)

There is increasing concern about the effects on marine ecosystems arising from the introduction of non-native species, this process often occurring accidentally as a result of human activities (Carlton, 1996). To date, the British Isles have been colonized by two plant species which may potentially affect the native Zostera beds.

The smooth cord-grass Spartina alterniflora was introduced to Southampton Water, probably via ship’s ballast water, from the east coast of North America prior to 1870 and was first found on mudflats near Hythe, Southampton. It subsequently interbred with the native small cord-grass, S. maritima to produce the sterile hybrid S. townsendii, and later the fertile hybrid S. anglica. Spartina anglica has rapidly colonized mudflats in England and Wales due to its fast growth rate and high fecundity. Deliberate planting to stabilize sediments accelerated its spread throughout Britain. Spartina anglica forms dense monospecific swards and is now the most common cord-grass species in Britain.

The spread of S. anglica has had a number of ecological consequences. It is an aggressive pioneer species and may have contributed to the demise of the native S. maritima. Butcher (1941a) raised concerns that its pioneering consolidation, raising mudbanks and affecting currents, may result in the removal of sediments from Zostera beds. Percival et al. (1997) reported a reduction in Zostera coverage at Lindisfarne, Northumberland, due a combination of change in sedimentation pattern and encroachment by S. anglica.

Fears have been expressed that the introduced brown alga Sargassum muticum may compete with and displace eelgrass (Druehl 1973). The species was first recorded in Europe in 1971 as drift material on Southsea Beach and by 1983 it occupied suitable habitats along approximately 360 km of the southern coast of Britain (Critchley 1983a, b). Within the British Isles, populations of S. muticum are currently found along the entire Channel coast from Kent to Cornwall, with one site in north Cornwall. There are also populations in the Isles of Scilly and Channel Isles. Currently the most northerly population is the most recently established, in Strangford Lough in Northern Ireland (Davison & Davison, 1995; Davison, 1996, 1997b).

The colonization of the UK by S. muticum has generated an intensive research and monitoring programme, creating one of the best documented case-histories of the spread of a non-indigenous marine organism in European waters. As a colonizer, S. muticum requires the availability of clear substratum. The presence of any existing canopy or algal turf can restrict or totally inhibit colonization (Deysher & Norton, 1982). Consequently, its spread is favoured where competition for space is reduced. A clean, hard, textured surface such as shell, rock or metal is preferred. Mature S. muticum plants can be also be dispersed when attched to small stones or shells carried by the current (Critchley, 1983a, b; Jephson & Gray, 1977; Nicholson et al., 1981).

The evidence that Sargassum competes significantly with Zostera is conflicting. In the San Juan Islands, Washington State, S. muticum and Z. marina were found to co-exist in different habitats with no evidence of competition or displacement (Norton, 1977). Fowler (1995) observed that despite the shading caused by the extensive S. muticum canopies in the Solent and Isle of Wight area, there did not appear to be any associated declines in Zostera. Covey and Hocking (1987) found no evidence of S. muticum replacing Zostera in the Helford, Cornwall.

At the Bembridge lagoons, Isle of Wight, S. muticum sporelings colonized newly exposed substratum only after the frost-induced die-back of Z. marina (Fletcher & Fletcher, 1975). However, the establishment of Sargassum may have prevented subsequent recolonization by the eelgrass (Critchley, 1980; Farnham et al, 1981). Tubbs (1995) reported that in the Solent, S. muticum and Z. angustifolia compete for space in lower shore lagoons. Replacement of Laminaria saccharina and Z. marina by S. muticum has been documented at Grandcamp on the French Atlantic coast. However, Givernaud et al., (1991) reported that S. muticum had replaced Z. marina mainly in parts of seagrass beds that had been damaged by human activity.

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