Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL39 Infaunal communities in Baltic infralittoral muddy sediment - bivalves

Infaunal communities in Baltic infralittoral muddy sediment - bivalves

Quick facts

Red List habitat type code BAL39
Threat status
Europe Near Threatened
EU Near Threatened
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 photic zone where at least 90% of the substrate is muddy sediment according to the HELCOM HUB classification. Sessile/semi-sessile epibenthic macrofauna are absent and infaunal bivalves dominate the biomass at depths of below approximately 20 m. It is a habitat that is present in conditions of low to moderate exposure to wave action and currents.

Three associated biotopes with different dominant species of bivalves (at least 50% of the infaunal bivalves) and slightly different distributions have been identified. ‘Baltic aphotic muddy sediment dominated by Baltic tellin (Macoma balthica)’ (AB.H3L1) is commonly found all parts of the Baltic Sea. ‘Baltic aphotic muddy sediment dominated by ocean quahog (Arctica islandica)’ (AB.H3L3) can only be found in the southwestern parts in the Belt Sea where the salinity is > 15 psu and has an optimum depth range of between 25 and 80 m. ‘Baltic aphotic muddy sediment dominated by Astarte spp.’ (AB.H3L5) is only found in areas where the near bottom water exhibits a salinity range between 10 and 15 psu, a temperature between 3 and 8oC and relatively good oxygen conditions. It is encountered in the southern and western Baltic Sea, in the southern Baltic Proper, in the Belt Sea and the Sound. As an arctic-boreal species, Astarte borealis appears in these Baltic biotopes at its southern limit.

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 fauna are potential indicators of quality.

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

Threat status

Synthesis of Red List assessment

The presence of this habitat type in the Baltic is well established and the occurrence of some of the biotopes has been mapped, but there is a lack of comprehensive quantitative data on the area covered. There have been declines in the extent of two of the four associated biotopes with overall decline is considered to be more than 25%. There have also been some declines in quality of this habitat in the Belt Sea (more than 10% in for A.islandica dominated areas).
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 the two biotopes AA.H3L1 and AA.H3L8 as LC(A1). AA.H3L3 and AA.H3L8 were assessed as NT(A1). Given the severity of decline of some of the associated biotopes over the last 50 years, the current expert opinion is that this habitat should be assessed as Near Threatened (A1).
Red List Category Red List Criteria
Near Threatened A1
Red List Category Red List Criteria
Near Threatened A1

Confidence in the assessment

Red List of habitat categories and criteria descriptions

Pressures and threats

  • Biological resource use other than agriculture & forestry
    • Fishing and harvesting aquatic resources
    • Professional active fishing
    • Benthic or demersal trawling
    • Benthic dredging
  • Pollution
    • Pollution to surface waters (limnic, terrestrial, marine & brackish)
    • Nutrient enrichment (N, P, organic matter)
  • Natural System modifications
    • Siltation rate changes, dumping, depositing of dredged deposits
  • Climate change
    • Changes in abiotic conditions
    • Temperature changes (e.g. rise of temperature & extremes)
    • Changes in biotic conditions
    • Habitat shifting and alteration

Habitat restoration potential

The most threatened biotopes, those dominated by ocean quahog (Arctica islandica) and Unionidae are characterised by species with a slow growth rate and a life span >>50 years so natural recovery times to re-establish communities with mixed age populations of this species are likely to be substantial.
Adult individuals of Arctica islandica can tolerate periods of anoxia by burrowing deeper into the sediment and remaining inactive. The larvae settling on the surface and younger specimens have not got this potential. Astarte borealis is resistant to anoxic conditions, however recurring and long lasting anoxia is fatal

Trends in extent

Average current trend in quantity

Decreasing Decreasing
EU28 EU28+

Trends in quality

Average current trend in quality

Decreasing Decreasing
EU28 EU28+

Conservation and management needs

All actions that reduce the level of eutrophication in the Baltic Sea will benefit this habitat.These actions include measures to reduce the diffuse run off of nutrients from agriculture and tackling point source pollution by installation of waste water treatment plants. Some consecutive years when the oxygen level remains at a good level is needed for the recruitment to be successful.

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
    • Legal protection of habitats and species
  • Measures related to hunting, taking and fishing and species management
    • Regulation/Management of fishery in marine and brackish systems


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 Unknown Decreasing 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 Unknown Unknown Unknown This habitat is present in all the Baltic sub-basins however there is insufficient information for accurate calculation of EOO and AOO.
EU28+ Unknown Unknown This habitat is present in all the Baltic sub-basins however there is insufficient information for accurate calculation of EOO and AOO.
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

Not available

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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