Distribution and Status of the Zostera Biotope Complex

European perspective

Overview of known UK distribution and extent

European perspective

Species of the genus Zostera are common on many coastlines throughout the world, with a distribution range from the Arctic zone to the equatorial tropics. The genus has not been recorded from West Africa or South America to date, and it occurs only sparsely in the Mediterranean and Black Sea (Dawes, 1981). This may reflect true distribution patterns or a lack of documented observations from these areas. The three species discussed in this report are distributed as follows:

  • Zostera marina is widespread throughout the Atlantic and Pacific. In the eastern Atlantic it extends from the Arctic Circle to Gibraltar, including the Mediterranean (Stace, 1997)
  • Zostera angustifolia has only been recorded around the British Isles, Denmark and Sweden (Cleator, 1993; C. Stace, pers. comm.). This apparently limited distribution is a reflection of the disputed taxonomic status of this form (as discussed previously).
  • Zostera noltii is more southerly than Z. marina and is restricted to the Atlantic, including the Mediterranean Sea. It extends from southern Norway to the tropic of Cancer (Cleator, 1993).

Overview of known UK distribution and extent

Historical context

Tubbs (1995) suggests that until the outbreak of wasting disease in the 1920s, the majority of intertidal and shallow subtidal mudflats in Britain and Europe were ‘clothed’ in eelgrass. The first Zostera distribution survey in England was undertaken by Butcher (1933a), reporting on the die-offs due to the wasting disease epidemic. He concluded that since 1917 Z. marina had become scarce and restricted to sheltered sites such as lagoons. Zostera angustifolia appeared to have become the most common Zostera species from this time. The distribution of Z. noltii remained stable, although this was still a relatively uncommon species (Butcher, 1933a,b, 1934). Two distinct periods of decline were identified, the first immediately after World War I, the second in the period 1931-32. Butcher (1941a, b) reported that recovery of the beds had begun by 1933 and was quite rapid, with some beds fully recovered within a few years of the 1930s epidemic.

However, Tubbs (1995) suggested that the disease continued to affect Zostera populations until the mid-1940s and that recovery did not really begin until the 1950s. The recovery has not been well documented but Tubbs considered that most Zostera beds have not yet fully recovered, and that only 20 of Britain’s 155 estuaries have eelgrass meadows more than 1 ha in extent. He reported that Z. marina has not recolonized the estuaries in southern and eastern England where it was once abundant, but that there are numerous small beds on the Channel coast from the Isles of Scilly to the Isle of Wight. He also reported that Z. marina beds on the west coast of Britain are extensive, dense and vigorous, particularly on the west coast of Scotland and around the Outer Hebrides.

Current distribution and extent in targeted geographic areas

The current distribution and known extent of the Zostera biotope in the UK are summarized in the linked table. The table is organized around the Annex I habitat features, providing information on both marine sites that have been designated as SACs as well as others that are not formally SACs, but which support Zostera. The Zostera sites recorded range in extent from 6.5 km2 to only 20-40 ha. Although brief descriptions of many of the sites with Zostera biotopes are provided in Davison (1997a), there is still a need for additional accurate estimates of extent. As discussed in Chapters VII-IX, obtaining this information is a high priority in the development of a conservation plan for Zostera biotopes in the UK.

Summary of Zostera distribution in the UK

The overall distributions of the three Zostera species in mainland UK are summarized in the following maps, based on those given in Stewart et al. (1994). The maps show post-1970 records of the three species. Additional up-to-date information can be found in the MNCR database, while Kay (1998) gives a detailed review of Zostera distribution in Wales.

Zostera marina is still widely, but patchily distributed around England, Scotland and Wales, with concentrations of post-1970 records in south-west England and along the west coast of Scotland, including the Hebridean islands.  Figure

The post-1970 records of Z. angustifolia are located mainly on the southern and eastern coasts of the UK, with major concentrations near the Isle of Wight, the Thames Estuary and the Moray and Cromarty Firths in Scotland.  Figure

Zostera noltii also has a predominantly eastern post-1970 distribution, with concentrations in the Thames Estuary area and the Moray and Cromarty Firths. There are also a number of records in the Argyll/Clyde area of western Scotland.   Figure

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