Distribution and Status of the Zostera Biotope Complex
Overview of known UK distribution and extent
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
- 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
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 Britains 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.