Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

Intertidal seagrass beds:

OSPAR habitat: Zostera beds

Habitats Directive Annex I: Mudflats and sandflats not covered by water at low tide

Subtidal seagrass beds:

Habitats Directive Annex I: Coastal Lagoons

Description: This habitat includes both intertidal and subtidal seagrass beds. Seagrass beds develop in intertidal and shallow subtidal areas uusually on sands and muds but at Kearney in Co Down the Zostera marina forms dense beds in amongst cobbles and gravel. Dwarf eelgrass Nanozostera noltii is found highest on the shore, often adjacent to lower saltmarsh communities, the narrow-leaved eelgrass Zostera angustifolia on the mid to lower shore is currently synonymised with Z. marina. The eelgrass Zostera marina is predominantly found in the sublittoral.

The plants stabilise the substratum, are an important source of organic matter, and provide shelter and a surface for attachment by other species.

Eelgrass is an important source of food for wildfowl, particularly brent geese and widgeon which feed on intertidal beds. The intertidal eel grass beds are of international importance for overwintering Brent geese.

Where this habitat is well developed the leaves of eelgrass plants may be colonised by diatoms and algae, stalked jellyfish and anemones. The soft sediment infauna may include amphipods, polychaete worms, bivalves and echinoderms. The shelter provided by seagrass beds makes them important nursery areas for juvenile flatfish.

In Northern Ireland there are good examples of this habitat in Carlingford Lough, Dundrum Bay, Strangford Lough and Lough Foyle.

Associated biotopes:

Intertidal Seagrass beds

• LS.LMp.LSgr – Littoral seagrass beds
• LS.LMp.LSgr.Znol – Zostera noltii beds in littoral muddy sand

• EUNIS code: A2.61 and A2.6111

  Subtidal Seagrass beds

• SS.SMp.SSgr – Sublittoral seagrass beds
• SS.SMp.SSgr.Zmar - Zostera marina/angustifolia beds on lower shore or infralittoral clean or muddy sand

• EUNIS code: A5.53 and A5.5331

Priority species: Calvadosia campanulata, Haliclystus auricula

Threats:

• Disease: A wasting disease was responsible for die-back of large areas of seagrass in the UK in the 1930s. The fungus and slime mould which colonised the weakened seagrass have recently reappeared in seagrass beds around the Isles of Scilly.
• Natural cycles: The extent of seagrass beds may change as a result of natural factors such as severe storms, exposure to air, and freshwater pulses. Grazing by wildfowl can have a dramatic seasonal effect with more than 60% reduction in leaf cover reported from some sites. Warm sea temperatures coupled with low level of sunlight may cause significant stress and die back of seagrass.
• Physical disturbance, for example by trampling, anchoring, moorings, dredging, and use of mobile bottom fishing gear, land claim and adjacent coastal development through the construction of sea defences and potential for changes in the hydrological regime.
• Introduction of, and competition from, alien species such as Spartina anglica and Sargassum muticum.
• Increased turbidity reducing photosynthesis.
• Nutrient enrichment, at low levels, may increase production in Zostera while high nitrate concentrations have been implicated in the decline of mature Z. marina. Phytoplankton blooms, resulting from nutrient enrichment, have been shown to reduce biomass and depth penetration of eelgrass. Eutrophication can also result in a shift to phytoplankton epiphyte or macroalgal dominance.
• Marine pollution, eelgrass is known to accumulate Tributyltin and possibly other metals and organic pollutants. Several heavy metals and organic substances have been shown to reduce nitrogen fixation which may affect the viability of the plant, particularly in nutrient poor conditions. Accumulated pollutants may become concentrated through food chains.

References:

This habitat description has been adapted from the 1994 UK BAP Action Plan for Seagrass beds.