Subtidal macrophyte dominated sediment

Underwater beds of pebbles, gravel, sand or mud, where seagrass or seaweeds grow

Underwater beds of pebbles, gravel, sand or mud, where seagrass or seaweeds grow © Gavin Black

Much of the seabed around the UK is made up of sediments, which can be anything from boulders to pebbles to sands and muds, or a mixture. 

Plant communities can develop on all these different sediments, occupying a wide range of underwater seascapes from exposed open coasts to sheltered inlets and coastal lagoons, which have only a limited connection to the open sea. 

Lagoons are one example of areas in which the seawater may become more or less salty depending on levels of evaporation or of freshwater inflow from rivers.  The marine plants that live there are adapted to these different levels of salinity, and include pondweeds and reeds as well as common green seaweeds.

Beds of eelgrass can also grow on mud and sand in calm, shallow water with varying levels of seawater salinity.   The size of eelgrass beds can expand by as much as 30m per year, and they may be important refuges for young fish and shellfish.

Maerl beds are another important plant seascape, providing other marine life with a surface to settle on and a refuge to hide in, within soft sediment seascapes that are otherwise featureless.  The maerl itself with its hard, knobbly skeleton, looks more like coral than a plant. 

Forests of kelp and other brown and green seaweeds can be a common underwater sight.  They grow where there are larger stones of shell fragments to attach to, or else some beds develop as mats on the sediment surface.

For the official habitat definition please see the documents listed below.


European distribution

These habitats are found around the south-west of England and on western coasts.  It is especially common on the west coast of Scotland.

Underwater beds of pebbles, gravel, sand or mud, where seagrass or seaweeds grow fact


Conservation Status

  • Important habitats within this group such as maerl and subtidal seagrass beds are UK BAP Priority Habitats (BAP habitats are now Habitats of Principal Importance/Priority Habitats) and OSPAR Threatened and/or declining habitats.
  • They are also included within different habitat types in Annex I of the Habitats Directive.


Official habitat definition

EUNIS habitat A5.5 Sublittoral macrophyte dominated sediment


Further information

JNCC Marine Habitat Classification

JNCC - UK BAP Priority Species and Habitats

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated July 2010)

JNCC EUNIS habitat correlations table