Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL9 Communities on Baltic infralittoral clay and other hard substrata

Communities on Baltic infralittoral clay and other hard substrata

Quick facts

Red List habitat type code BAL9
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 with at least 90% coverage of hard clay, marlstone rock, ferromanganese concretions and/or peat according to the HELCOM HUB classification. Sessile/semi-sessile epibenthic bivalves cover of at least 10% of the seabed and no perennial attached erect group has more than 10% coverage. In some cases macrovegetation or macrofauna may be absent. The habitat is typically encountered in high energy exposure areas. 

Seven associated biotopes have been identified. Four of these are associated with areas of hard clay and may be dominated (at least 50% of the biomass) by Mytilids (Mytilus spp., Modiolus modiolus), have a mixed epibenthic macrocommunity, a sparse epibenthic macrocommunity or have no dominant macrocommunity (AA.B1E1,  AA.B1V, AA.B2T and AA.B4U). The marlstone rock habitat  'AA.C: Baltic photic marl' is only present in the Baltic proper, Belt Sea and the Sound and  'AA.F: Baltic photic ferromanganese concretion bottoms' which is typically found below 10m occurs in Baltic proper, Gulf of Bothnia, Gulf of Finland, Gulf of Riga. 'AA.G: Baltic photic peat bottoms' which occurs in the Baltic proper and Belt Sea, has developed where marine erosion processes along the German and Danish coastline have laid these subfossil substrates bare. It is found salinity ranges between 7 and 18 psu, at all exposure classes and at depths from 0-20m. Knowledge about the latter biotope is scarce but the surface can be covered by filamentous annual algae and single juvenile Fucus spp. or Chorda spp. specimens. Normally peat bottom lacks epibenthic communities and only some specialised burrowing bivalves like Barnea candida or Zirfaea crispata may penetrate into the peat.

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 dominate 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

This habitat is present in all sub-basins of the Baltic Sea and therefore EOO exceeds 50,000 km2. There is a lack of data on quality and extent of this habitat although there has been an estimate of decline of the peat biotope of more than 25% over the last 50 years and overall a significant decline (>25%) over the last 50 years. The lack of quantitative data on extent, quality and trends over time means that accurate calculations of EOO and AOO are not possible at the present time. This Red List assessment has therefore been based on expert opinion.
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 associated biotopes (AA.B1E1, AA.C and AA.F) as Least Concern (A1), Baltic peat bottoms (AA.G) (which is rare and restricted to a comparably small area of the Baltic) was assessed as Vulnerable (B2b) and three other biotopes were not evaluated (AA.B1V, AA.B2T and AA.B4U). All the associated biotopes are considered to have declined to some extent over the last 50 years therefore expert opinion is that overall the habitat is Near Threatened for the EU 28 and EU 28+.
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
    • Benthic or demersal trawling
    • Benthic dredging
  • 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...)
    • Estuarine and coastal dredging
    • Extraction of sea-floor and subsoil minerals (e.g. sand, gravel, rock, oil, gas)
    • Other human induced changes in hydraulic conditions

Habitat restoration potential

Not where the underlying substrate (peat, marlstone rock, ferromanganese concretions) have been removed, however this habitat is not well studied therefore capacity to recover is largely unknown.

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

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 Unknown 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 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

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 Barnea candida
Invertebrates Modiolus modiolus
Invertebrates Zirfaea crispata
Species scientific name English common name Species group
Barnea candida Invertebrates
Modiolus modiolus Invertebrates
Zirfaea crispata Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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