Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL57 Infaunal communities of Baltic upper circalittoral muddy sediment dominated by bivalves

Infaunal communities of Baltic upper circalittoral muddy sediment dominated by bivalves

Quick facts

Red List habitat type code BAL57
Threat status
Europe Vulnerable
EU Vulnerable
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)


This habitat occurs on Baltic Sea aphotic bottoms with at least 90% coverage of muddy sediment according to the HELCOM HUB classification. Sessile/semi-sessile epibenthic macrofauna is not present and the biomass of infaunal bivalves dominates. The habitat generally occurs below a depth of approximately 20 m in locations of energy exposure. Three associated biotopes have been identified with different species of bivalves dominating (more than 50% of the biomass). These are: ‘Baltic aphotic muddy sediment dominated by Baltic tellin (Macoma balthica)’ (AB.H3L1); ‘Baltic aphotic muddy sediment dominated by ocean quahog (Arctica islandica)’ (AB.H3L3); and ‘Baltic aphotic muddy sediment dominated by Astarte spp.’ (AB.H3L5). These biotopes have slightly different distributions in the Baltic because of different temperature and salinity preferences of the associated dominating species. For example as an arctic-boreal species, Astarte borealis appears in these Baltic biotopes at its southern limit. It is resistant to anoxic conditions, however recurring and long lasting anoxia is fatal.The biotope dominated by the ocean quahog (Arctica islandica) can only be found in the southwestern parts in the Belt Sea where the salinity is high. Compared to shallow bottoms, the deep muddy bottoms are structurally relatively monotonous therefore the large shells of Arctica islandica increase the complexity of the habitat. It plays an important role as a biomass producer, enhancer of benthopelagic coupling, reducer of water turbidity, and ecosystem engineer as well as being among the longest-lived and slowest growing marine bivalves. 

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 and, in the case biotopes dominated by M. baltica, the presence of a full size range of individuals in the population. 

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 it is known to occur in all the sub-basins. There have been declines in extent over the last 50 years particularly where Astarte spp. and Arctica islandica dominate (by more than 50% and an estimated 20%, respectively). Further reduction in quantity is predicted (more than 80% in the next decades in the case of areas dominated by A. islandica). Habitat quality is also considered to have declined 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 the biotope AB.H3L1, Baltic aphotic muddy sediment dominated by Baltic tellin as Least Concern (A1), AB.H3L3, Baltic aphotic muddy sediment dominated by ocean quahog as Critically Endangered (A2) and the biotope AB.H3L5, Baltic aphotic muddy sediment dominated by Astarte spp as Endangered (A1). Given the past and predicted future decline in extent of this habitat, differences in vulnerability to anoxic conditions between the associated biotopes and some envisaged decline in quality of all the associated biotopes current expert opinion is that this habitat should be assessed as Vulnerable (A1) for both the EU 28 and EU 28+
Red List Category Red List Criteria
Vulnerable A1
Red List Category Red List Criteria
Vulnerable 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
  • Pollution
    • Pollution to surface waters (limnic, terrestrial, marine & brackish)
    • Nutrient enrichment (N, P, organic matter)
  • Climate change
    • Changes in abiotic conditions
    • Temperature changes (e.g. rise of temperature & extremes)

Habitat restoration potential

In areas dominated by the ocean quahog (Arctica islandica) recovery is likely to take many decades because of the long generation time of this species. No information exists on likely recovery rates for the other associated biotopes

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

The main anthropogenic threat of the habitat is eutrophication and the anoxia of the bottoms that follows. Even for species such as Astarte borealis which are resistant to anoxic conditions, longer and repetitive periods can cause mortality.
Action to reduce the level of eutrophication, which will benefit oxygen levels on the deep muddy bottoms, is urgently needed because successful recruitment of the characteristic bivalves requires a few consecutive years of good oxygen levels. Restricting bottom trawling in the areas where this habitat occurs may also improve the potential of the Arctica islandica to recolonize the seabed.

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


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 >50,000 Unknown Unknown This habitat is present in all the Baltic sub-basins therefore EOO is likely to exceed 50,000km2
EU28+ Unknown Unknown This habitat is present in all the Baltic sub-basins therefore EOO is likely to exceed 50,000km2
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 Arctica islandica
Invertebrates Astarte borealis
Invertebrates Macoma balthica
Species scientific name English common name Species group
Arctica islandica Ocean quahog Invertebrates
Astarte borealis Invertebrates
Macoma balthica Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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