Biodiversity & Conservation

The following algal species have been recorded only from seagrass leaves: Halothrix lumbricalis; Leblondiella densa; Myrionema magnusii; Cladosiphon zosterae; Punctaria crispata and Cladosiphon contortus, which is larger and found primarily on Zostera sp. rhizomes. The decline of Zostera marina beds in Europe and North America due to wasting disease in the 1920s -30s caused Brent geese (Branta bernicla) to shift their preferences to Zostera noltii, which is now their preferred food (Davison & Hughes, 1998; Jones et al., 2000). In Europe, the decline in the dark-bellied Brent geese population to about 25% of its pre-1930s level strongly paralleled the decline in Zostera sp. (Ogilvie & Matthews, 1969). In the 1950s and 60s dark-bellied Brent geese on the Essex coast fed primarily on Zostera noltii and Ulva sp. (Burton, 1961). Wigeon numbers have declined dramatically in recent years, presumably due to the shift from Zostera marina to Zostera noltii. Wigeon tend to graze lower on the shore, switching to Zostera noltii once the supplies of Zostera marina or Zostera angustifolia (Percival & Evans, 1997). The majority of species associated with the biotope are not specific to the community (Asmus & Asmus, 2000b). However, loss of the biotope would engender a reduction in the material flux within the local ecosystem and loss of a major source of primary production and detritus. For example, Asmus & Asmus (2000b) estimated that loss of intertidal seagrass beds would probably result in a 9% reduction in aerobic microbial turnover, a 7% reduction in anaerobic microbial turnover, a reduction in predatory macrobenthos such as Carcinus maenas, and a 30 % reduction in the average biomass of benthic grazers such as Hydrobia ulvae and Littorina littorea. They estimated that secondary productivity, by macrobenthic infauna, was not significantly reduced.

• Seagrass beds are often associated with cockle beds. Suction dredging for bivalves, such as cockles and Mercenaria sp., has been reported to damage eelgrass beds (see 'extraction'; Davison & Hughes, 1998).
• The decline in seagrass beds in the 1920s and1930s together with their importance in coastal processes has resulted in scientific research use and experimental management studies.
• Seagrasses are considered to be of great ecological and economic importance (Asmus & Asmus, 2000a, b) and act a nutrient and sediment sinks.
• Seagrass promote the accumulation of sediment and their rhizomes bind the sediment. Therefore, they absorb a proportion of incident wave energy and may form a natural coastal defence.
• In the past seagrasses have been put to a number of uses world-wide, for example, sound-proofing, insulation, roofing thatch, binding soil, packaging, basket weaving and in the manufacture of 'coir' matting (see Kuelan, 1999 for review).

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