Zostera marina provides a habitat for a wide range of species to find shelter or a suitable substratum on which to live. Fish occur amongst the eelgrass and include the wrasse and goby species also found in kelp. The green wrasse Labrus turdus is normally associated with eelgrass beds in the Mediterranean and may be present in Isles of Scilly Zostera marina beds (Fowler 1992). Especially found in eelgrass beds are pipefish Syngnathus typhle and Entelurus aequoreus and, rarely, seahorses Hippocampus ramulosus. Cuttlefish Sepia officinalis are also found and lay their eggs amongst eelgrass. Small prosobranchs, especially Rissoa sp(p) and Lacuna vincta graze on the leaves. The mud snail Hydrobia ulvae is found on leaves in brackish conditions. At open coast sites, stauromedusae stalked jellyfish Haliclystus auricula and Lucernariopsis campanulata may be present on leaves. The hydroid Laomedea angulata and the algae Rhodophysema georgii, Halothrix lumbricalis, Leblondiella densa, Myrionema magnusii, Cladosiphon zosterae and Punctaria crispata have only been recorded attached to eelgrass leaves. The endophytic green alga Entocladia perforans is also host-specific to Zostera marina. Eelgrass rhizomes help to stabilise sediments and may thereby increase species diversity. Sea anemones Cereus pedunculatus, Cerianthus lloydii and the prosobranch Nassarius reticulatus are often common in the sediment. In the Isles of Scilly, the sea anemone Anthopleura ballii is unusually present.

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.

Zostera marina provides refuges for many species of fish and nursery areas for some.

The slowing of water movement by leaves encourages accumulation of sediments whilst the dense rhizome and root system stabilizes the sediment preventing or reducing sediment loss. The consolidation of the sediments enables the development of richer infaunal communities with higher densities of individuals than those in adjacent bare sediments (reviewed most recently in Boström & Bonsdorff (1997).

Eelgrasses have high rates of primary production and are an important source of organic matter whose decomposition provides a starting-point for detritus-based food chains. They also provide a substratum for other plant species.

Zostera marina, Labrynthula macrocystis

Intertidal and probably shallow subtidal Zostera marina beds provide a source of food for a variety of wildfowl, although not to the extent that intertidal Zostera noltii do. Studies of feeding on Zostera rarely differentiate which species is being referred to. Tubbs & Tubbs (1983) reported that brent geese-grazing contributed to the cover of Zostera marina and Zostera noltii being reduced from between 60-100% cover in September to between 5-10% cover between mid-October and mid-January. The observation that the decline in Zostera marina during the wasting disease of the 1930s was followed by very heavy losses of the Brent goose and the Canada goose (den Hartog 1977) suggests that they rely on Zostera marina for a large proportion of their food. However, it remains unclear and seems unlikely that wildfowl grazing affects subtidal Zostera marina beds.

Zostera marina beds are naturally dynamic, at least in open coastal areas. In the Isles of Scilly, beds have ‘advancing’ and ‘receding’ edges. The fungus Labrynthula macrocystis caused the loss of over 90% of Zostera marina beds in the 1920s and 1930s and a full recovery has not yet occurred (see Vergeer et al. 1995 for a recent review). Zostera marina beds may show marked annual changes. In brackish conditions, there is die-back of the leaves in the autumn and regrowth in the spring and early summer (Jacobs 1982; Dyrynda 1997). This die-back has been observed to be almost complete in The Fleet in Dorset, UK (Dyrynda 1997) and resulted in sediment destabilization as well as loss of cover for fish and substratum for invertebrates.

Zostera marina beds most likely do not seed and establish rapidly. There has been little recovery of Zostera marina beds following the wasting disease in the 1930s. Olesen & Sand-Jensen (1994) reported that, in Danish waters, new Zostera marina beds could take at least five years to become established and stable with small patches (<32 shoots) showing high mortalities. However, these observations are near to established beds and seeding over a distance particularly between isolated water bodies is likely to be slow. An extensive series of experiments has been undertaken to try to re-establish beds (see, for instance, Fonesca et al. 1994).

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