Sediment stabilization

Primary productivity

Associated fauna and flora

The extensive rhizome networks and above-ground leaf meadows of eelgrass create a complex biotope that significantly affects the functioning of the local coastal ecosystem and also provides a habitat for a diverse range of organisms. Significant ecosystem-level effects include the stabilization of coastal sediments and the production of organic detritus.

As previously mentioned, dense meadows of eelgrass leaves increase rates of sedimentation, and the rhizome and root networks bind the substratum together, thereby reducing sediment erosion. The roots also allow oxygen to penetrate into otherwise impermeable sdiments. The penetration of Zostera roots into the sediment aerates the upper layers and provides a more favourable habitat for burrowing animals.

Seagrass meadows are considered to be the most productive of shallow, sedimentary environments. Seagrass primary production supports a rich, resident fauna and as a result, the beds are used as refuge and nursery areas by many species, including commercial fish species (discussed further below). The decomposition of dead seagrass tissue by bacteria drives detritus-based food chains within the Zostera bed. High numbers of heterotrophic protists are found in the water column over seagrass meadows and take up both the dissolved organics leaching from the seagrasses and the rapidly multiplying bacteria. Seagrass detritus is also very rich in micro-organisms. 1 g (dry weight) has been calculated to support, on average: 109 - 1010 bacteria, 5 x 107 - 108 heterotrophic flagellates and 104 - 105 ciliates, yielding a total biomass of some 9 mg of bacteria and protists.

In addition to supporting detritus-based food chains within the seagrass bed, dead seagrass leaves can be transported by currents into other coastal biotopes. They can be deposited on the shore as dense drifts and enrich the upper littoral zone (den Hartog, 1987). Exported seagrass material can also enter the food webs of areas distant from the coastal zone. Seagrass leaves have been recorded at depths of nearly 8000 m, and after hurricanes mats of leaves up to 50 m across have been reported from the Florida Current.

The community composition of an eelgrass bed will depend upon a combination of factors, including the species of seagrass, the stability of the bed, the substratum type, salinity, tidal exposure and location. The richness of the community will reflect the variety and density of microhabitats and the local ecological conditions. The three Zostera species are found on similar substrata but in different tidal zones. Species diversity tends to be highest in the subtidal, fully marine, perennial populations of Z. marina and tends to be lowest in the intertidal, estuarine, annual beds of Z. angustifolia and Z. noltii (Jacobs & Huisman, 1982).

Detailed species lists for a number of the major British eelgrass beds have been compiled, including those in the Salcombe Estuary (Gardener, 1934), Helford Passage (Turk, 1990), Isles of Scilly (S. Hiscock, 1986) and Skomer (K. Hiscock, 1980, 1987). The characteristic and representative plant and animal species found in UK Zostera beds are listed in Appendix 2. Three major components of the eelgrass bed community are discussed below: epiphytes and non-epiphytic alage, invertebrates and fish living amongst the eelgrass, and wildfowl.

Living Zostera leaves provide a suitable substratum for numerous epiphytic algae, while other algae live between the seagrass shoots and within the surface layers of the underlying sediment. Whelan & Cullinane (1985) identified 60 algal species in a Z. marina bed in Ventry Bay, Ireland. A number of species (eg. the brown algae Halothrix lumbricalis and Leblondiella densa) are found only on Zostera leaves, while the large brown alga Cladosiphon contortus occurs principally on Zostera rhizomes.

Zostera beds are generally rich in epiphytes but poor in associated macroalgae owing to the shading effect of the dense eelgrass swards. In sandy habitats Chorda filum is often found with Z. marina. On mixed substrata, a layering of flora can be observed, with Zostera plants protruding up through stones colonized by macroalgae such as Halidrys siliquosa and Laminaria saccharina, often with Cystoseira sp. at the margins of the eelgrass bed (Whelan & Cullinane, 1985).

The algae found within Zostera beds are more digestible than the eelgrass itself and support the majority of the abundant grazers found within seagrass communities. In relatively open stands, the benthic algae may account for 70% of the total primary production of the bed. However, in dense beds, the thick carpets of Zostera leaves can reduce light availability for the algal understorey and as a result productivity is lower. Estimates of epiphytic productivity are relatively scarce but biomasses of the same order as those of the leaves to which they are attached are known.

A wide variety of invertebrate species occur on and among the plants of an eelgrass bed. Small gastropods grazing the algal epiphytes on the Zostera leaves include Hydrobia spp., Rissoa membranacea and Littorina littorea. The sediments underlying the beds support large numbers of polychaete worms (eg. Arenicola marina, Lanice conchilega) , bivalve molluscs (eg. Cerastoderma edule, C. glaucum) and burrowing anemones (eg. Cereus pedunculatus). Amphipod and mysid crustaceans are among the most abundant and important of the mobile fauna living amongst the eelgrass leaves.

Eelgrass beds are widely recognized to be important spawning and nursery areas for many species of fish, including commercial species. Smaller fish species include two-spot gobies Gobiusculus flavescens, and 15-spined stickelbacks Spinachia spinachia. Larger, commercially-important species using eelgrass beds as feeding grounds include bass Dicentrarchus labrax. Seahorses, Hippocampus spp., reach their northern limits in eelgrass beds along the south coast of England.

Eelgrass beds may act as corridor habitats for species migrating north from warmer water. The first (as yet unconfirmed) British record of the green wrasse, Labrus turdus, comes from eelgrass beds in the Isles of Scilly. The species is normally associated with seagrass beds in the Mediterranean (Fowler, 1992).

Wildfowl (ducks and geese) are among the few animals which graze directly upon Zostera and are able to digest its leaves. In Britain, Zostera is an important constituent of the diet of two sub-species of Brent geese Branta bernicla, wigeon Anas penelope, mute swans Cygnus olor, and whooper swans C. cygnus. Teal Anas crecca are reported to consume eelgrass seeds (Tubbs & Tubbs, 1983).

Since the occurrence of wasting disease and the consequent decline of Z. marina beds, the relative importance of the different Zostera species in Brent geese diet has shifted. Zostera noltii has replaced Z. marina as the preferred food and currently provides the main source of energy for Brent geese overwintering in Britain.

Ogilvie & Matthews (1969) reported that in Europe, the decline of the population of dark-bellied Brent geese (to approximately 25% of its pre-1930s level) strongly paralleled the decline in Zostera following the wasting disease epidemic. Since it appears that the intertidal Zostera species were not as severely affected by the wasting disease as Z. marina, it can be assumed that Z. marina must have been the preferred food species prior to the epidemic (Charman, 1977). 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. angustifolia and Z. noltii. Burton (1961) studied 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. Both he and Ranwell & Downing (1959) suggested that Z. angustifolia was not the preferred species because it had shed most of its leaves before the migrant geese arrived in Britain. Charman (1975) found that when Brent geese had exhausted the Zostera stock along the Essex coast, they had to move onto less preferred food sources, including Enteromorpha and saltmarsh plants, and then onto less traditional food sources such as inland pastures and winter cereals.

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 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).

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