Biodiversity & Conservation

Zostera marina provides shelter or substratum for a wide range of species including fish such as wrasse and goby species (also associated with kelp).

Leaves slow currents and water flow rates under the canopy and encourage settlement of fine sediments, detritus and larvae (Turner & Kendal,l 1999).

Seagrass rhizomes stabilize the sediment and protect against wave disturbance and favour sedentary species that require stable substrata and may, therefore, increase species diversity;.

The leaves are grazed by small prosobranch molluscs, for example, Rissoa spp., Lacuna vincta, Hydrobia spp. and Littorina littorea.

Zostera marina bed assemblages may include, in particular, Pipe fish (Syngnathus typhle, Entelurus aequoraeus), the sea anemones (Cereus pedunculatus, Cerianthus lloydii) and the neogastropod Hinia reticulatus.

Cuttlefish (Sepia officinalis) may lay their eggs amongst sea grass;

Beds on the south east cost of England may contain distinctive assemblages of Lusitanian fauna such as the hydroid Laomedea angulata, Stauromedusae (stalked jellyfish) and, rarely, sea horses Hippocampus guttulatus.

Zostera beds are naturally dynamic. The population is still recovering from loss of 90 percent of Zostera marina beds in 1920s and 1930s as a result of wasting disease. May show marked annual change, for example in the brackish conditions in the Fleet Lagoon (Dorset, UK) leaves die back in autumn and regrow in spring to early summer (Dyrynda, 1997).

Seagrasses provide shelter and hiding places. Leaves and rhizomes provide substrata for epibenthic species. These epibenthic species may be grazed by other species (Davison & Hughes, 1998). The sediment supports a rich infauna of polychaetes, bivalve molluscs and burrowing anemones. Amphipods and mysids are important mobile epifauna in seagrass beds. Cockle beds (Cerastoderma edule) are often associated with seagrass beds.

Seagrass meadows are considered to be the most productive of shallow, sedimentary environments (Davison & Hughes, 1998). The species richness of Zostera marina beds in the River Yealm, Devon, UK was significantly higher than that of adjacent sediment (Turner & Kendall, 1999). Zostera is directly grazed by ducks and geese. Epiphytes may be as productive as the seagrass they inhabit and are grazed by gastropods. Seagrasses are an important source of organic matter whose decomposition supports detritus based food chains. Seagrass detritus may make an important contribution to ecosystems far removed from the bed itself.

Zostera spp. are perennials but may act as annuals under stressful conditions. Seedlings rarely occur in seagrass beds except in areas cleared by storms, blow-out or excessive herbivory (Phillips & Menez, 1988). Seed mortality is very high (Phillips & Menez, 1988; Fishman & Orth, 1996). Seed may be dispersed through the gut of wildfowl (Fishman & Orth, 1996), or float long distances (up to 200 m in some cases) on attached gas bubbles. The generative stalk may be released and can also float long distances. If displaced pieces of shoot or rhizome float and may root if they settle on suitable substrata. However, vegetative reproduction probably exceeds seedling recruitment except in areas of sediment disturbance (Phillips & Menez, 1988; Reusch et al., 1998). Zostera marina provides refuges for some species and nursery areas for some, e.g. the two-spot goby Gobiusculus flavescens and the 15-spined sticklebacks Spinachia spinachia. Some commercially important species use seagrass beds as feeding ground, e.g. bass Dicentrachus labrax. Seahorses Hippocampus spp. reach their northern limit in seagrass beds along the south coast England (Davison & Hughes, 1998).

Zostera marina beds are unlikely to seed and establish rapidly. There has been little recovery of these beds since the 1930s. In Danish waters Zostera marina beds could take at least 5 years to establish even when near to established beds. Seeding over distances is likely to be slow.

Seagrass beds may act as corridor habitats for species moving from warm waters. Seasonal die back resulted in sediment destabilization as well as loss of cover for fish in the Fleet, Dorset, UK (Dyrynda, 1997).