Community composition will depend upon a combination of factors, including the
stability of the bed, the substratum type, salinity, tidal exposure and location. Of the
three species of Zostera, diversity tends to be lowest in the intertidal,
estuarine, annual beds of Z. noltii (Jacobs & Huisman, 1982). Wildfowl (ducks
and geese) are among the few animals which graze directly upon Zostera and are able
to digest its leaves. Eelgrasses provide shelter and hiding places. The leaves and
rhizomes provide substrata for the settlement of epibenthic species which in turn may be
grazed upon by other species.
Eelgrass beds are widely recognised to be important spawning and nursery areas for many
species of fish, including commercial species.
Dense meadows of eelgrass leaves increase rates of sedimentation, and the rhizome and
root networks bind the sediment, thereby reducing erosion. The roots also allow oxygen to
penetrate into otherwise impermeable sediments. The penetration of Zostera roots
into the sediment aerates the upper layers and provides a more favourable habitat for
Eelgrass primary production supports a rich, resident fauna and as a result, the beds
are used as a refuge and nursery area by many species. The decomposition of dead eelgrass
tissue by bacteria drives detritus-based food chains within the Zostera bed. High
numbers of heterotrophic protists are found in the water column over eelgrass meadows and
take up both the dissolved organics leaching from the eelgrasses and the rapidly
Keystone (structuring) species
Importance of habitat for other species
Since the occurrence of the wasting disease which led to the widespread loss of Zostera
marina beds throughout Europe and North America in the 1920s-30s, the relative
importance of the different Zostera species in Brent geese diet has shifted. As a
result of the decline of Z. marina and its slow recovery, Brent geese were forced
to migrate to other feeding areas and to switch their feeding to intertidal beds of Z.
angustofolia and Z. noltii. Zostera noltii has replaced Z. marina
as the preferred food and currently provides the main source of energy for Brent geese
overwintering in Britain. Burton (1961) studied the dark-bellied Brent geese on the Essex
coast in the late 1950s and early 1960s and found that they fed almost entirely on Z.
noltii and the alga Enteromorpha sp. This shift in eelgrass abundance from Z.
marina to Z. noltii has also affected wigeon. Wigeon numbers have declined
dramatically in recent years and the reduced availability of eelgrass is considered to be
one of the contributory factors. Grazing wigeon are very vulnerable to human disturbance.
Where wildfowling is popular, wigeon appear to avoid the Z. noltii beds near the
top of the shore and only begin to feed there when the Z. angustifolia and Z.
marina lower down the shore are exhausted (Percival & Evans 1997).
New leaves appear in spring and the eelgrass meadows develop over the intertidal flats
in the summer. Leaf growth ceases around September or October (Brown 1990), and leaf cover
begins to decline during the autumn and over the winter. Plants may experience a complete
loss of foliage, dying back to the buried rhizomes.
Time for community to reach maturity
In perennial populations, the rhizomes survive the winter to produce new leaves the
following spring, while in annual populations, both the leaves and rhizomes die.