Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL44 Communities on Baltic circalittoral clay and other hard substrata

Communities on Baltic circalittoral clay and other hard substrata

Quick facts

Red List habitat type code BAL44
Threat status
Europe Least Concern
EU Least Concern
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)


This is a Baltic Sea benthic habitat in the aphotic zone with at least 90% coverage of hard clay, marlstonerock, ferromanganese concretions and/or peat according to the HELCOM HUB classification. Hard clay substrates are known to occur mostly in high energy environments. Marlstone rock habitats have only been reported in the Baltic proper, Belt Sea and The Sound, and Ferromanganese concretions in the Baltic Proper, Gulf of Bothnia, Gulf of Finland and Gulf of Riga.

Sessile/semisessile epibenthic bivalves cover of at least 10% of the seabed and no perennial attached erect group has more than 10% coverage in this habitat. In some cases there may be no macrofauna but two associated biotopes with different dominant species of macrofauna have been identified: ‘Baltic aphotic hard clay dominated by Mytilidae’ (AB.B1E1) and ‘Baltic aphotic hard clay dominated by Astarte spp.’ (AB.B1E4). The latter is characterised by species preferring cold and saline water with Astarte spp. often making up between 70–90% of the total biomass.The near bottom water exhibits a salinity range between 10 and 15 psu, a temperature between 3 and 8°C and relatively good oxygen conditions. For ecological purposes, hard clay can be considered to be a hard substrate. Very few macrofauna species have the capacity to burrow into the substrate. 

Indicators of quality:

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time. There are no commonly
agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis. Diversity, abundance and biomass of the dominant species and associated fauna are potential indicators of quality of this habitat.
species and associated fauna are potential indicators of quality of this habitat

Characteristic species
For full habitat description, please download the habitat factsheet.

Threat status

Synthesis of Red List assessment

Current status and trends in quality of this habitat are mostly unknown although some areas of hard clay dominated by Astarte species have shown a significant decline in abiotic environmental quality due to an increasing exposure to oxygen depletion over the last 50 years.
The overall assessment for this EUNIS level 4 habitat has been based on the HELCOM (2013) assessments for the associated HELCOM HUB biotopes. Draft assessments were derived using a weighted approach whereby the HELCOM assessment outcomes were assigned a score. This was averaged across the relevant biotopes. The outcomes were reviewed by Baltic experts to reach a final conclusion. HELCOM (2013) assessed three relevant Baltic associated biotopes (AB.B1E1, AB.C and AB.F) as Least Concern (A1), and Baltic aphotic hard clay dominated by Astarte spp. (AB.B1E4) was assessed as Endangered (B2c(ii)). Three other biotopes were not evaluated.
Due to general rarity of areas dominated by Astarte spp. and no evidence of a significant decrease in quantity over the last 50 years for areas dominated by Mytilidae, this habitat has been assessed as Least Concern for both the EU 28 and EU 28+.
Red List Category Red List Criteria
Least Concern -
Red List Category Red List Criteria
Least Concern -

Confidence in the assessment

Red List of habitat categories and criteria descriptions

Pressures and threats

  • Mining, extraction of materials and energy production
    • Mining and quarrying
  • Pollution
    • Pollution to surface waters (limnic, terrestrial, marine & brackish)
    • Nutrient enrichment (N, P, organic matter)
  • Natural System modifications
    • Human induced changes in hydraulic conditions
    • Removal of sediments (mud...)
    • Extraction of sea-floor and subsoil minerals (e.g. sand, gravel, rock, oil, gas)
  • Climate change
    • Changes in abiotic conditions
    • Temperature changes (e.g. rise of temperature & extremes)
    • Flooding and rising precipitations

Habitat restoration potential

Where the dominant species is Mytilus edulis recovery may be possible within 5-10 years. Astarte spp. have a life span of 20-40 years and the colonies on hard clay are subject to seasonal oxygen depletion throughout its range. This means that recruitment may fail due to the higher sensitivity of larvae and juveniles against oxygen depletion, or need significantly more time for a fully recovery of associated communities.

Trends in extent

Average current trend in quantity

Decreasing Decreasing
EU28 EU28+

Trends in quality

Average current trend in quality

Unknown Unknown
EU28 EU28+

Conservation and management needs

All actions that reduce the level of eutrophication in the Baltic Sea will benefit this habitat. These include measures to reduce the diffuse run off of nutrients from agriculture and tackling point source pollution by installation of waste water treatment plants. Restoring/improving water quality and establishing protected areas can benefit this habitat as can introducing controls on activities such as bottom trawling, sand, gravel and mineral extraction which cause direct damage to the substrate and the associated communities.

List of conservation and management needs

  • Measures related to wetland, freshwater and coastal habitats
    • Restoring/Improving water quality
  • Measures related to spatial planning
    • Establish protected areas/sites
  • Measures related to hunting, taking and fishing and species management
    • Regulation/Management of fishery in marine and brackish systems
  • Measures related to special resouce use
    • Regulating/Managing exploitation of natural resources on sea


For each habitat a distribution map was produced from a wide variety of sources indicating known and potential occurrences of the habitat in 10x10 km grids within Europe. Occurrences in grid cells were given in two classes: actual distribution from relatively reliable sources (surveys, expert knowledge), and potential distribution based on models or less reliable indicators. Please download the fact sheet to see the map.

Geographic occurrence and trends

Seas Present or presence uncertain Current area of habitat (Km2) Recent trend in quantity (last 50 years) Recent trend in quality (last 50 years)
Baltic Proper Present 86,343 Unknown Decreasing
Belt Sea
Gulf of Bothnia
Gulf of Finland
Gulf of Riga
The Sound

Extent of Occurrence, Area of Occupancy and habitat area

Extent of Occurrence (EOO) (Km2) Area of Occupancy (AOO) Current estimated Total Area Comment
EU28 >50,000 >50 86,343 The area estimate for this habitat has been derived from a synthesis of EUNIS seabed habitat geospatial information for the European Seas but is recognised as being an underestimate.
EU28+ >50 86,343 The area estimate for this habitat has been derived from a synthesis of EUNIS seabed habitat geospatial information for the European Seas but is recognised as being an underestimate.
AOO = the area occupied by a habitat measured in number of 10x10 km grid cells.
EOO = the area (km2) of the envelope around all occurrences of a habitat (calculated by a minimum convex polygon).

Characteristic species

The full list of characteristic species and genus are available above from the Summary. The species available in the EUNIS database are shown here.
Invertebrates Astarte borealis
Invertebrates Astarte elliptica
Species scientific name English common name Species group
Astarte borealis Invertebrates
Astarte elliptica Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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