Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL61 Communities of Baltic lower circalittoral soft sediments (mud and sand)

Communities of Baltic lower circalittoral soft sediments (mud and sand)

Quick facts

Red List habitat type code BAL61
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 is a Baltic Sea benthic habitat in the aphotic zone, comprising areas of soft sediment, predominantly mud, below the halocline . The upper water layer is separated from the more saline deepwater layer by a permanent halocline located at depths of about 70-100 m (there is no halocline in the shallower areas in the northeastern Baltic). The strong permanent halocline and seasonal thermocline in summer limits vertical mixing of the water column leading to the formation of oxygen-depeted zones in the deep areas of the central Baltic. The pycnocline in the Baltic occurs around 80m of depth and below this hypoxia and anoxia occur almost permanently. Salinity has risen since early records at the beginning of the 20th century and increasing stability of the halocline restricts water exchange between the more saline bottom layer and overlying water masses. The environmental conditions of the deep zone of the Baltic are not uniform but vary widely in terms of salinity (14-21 ppt) and oxygenation (3-80% saturation). The conditions depend in the first instance on influxes, which are very irregular, which renew the deep waters with oxygen. Between 1948-52, for example, when there was frequent renewal of deep waters, worms such as Halicryptus spinulosus and Scoloplos armiger were present whereas during 1956-57 lack of influxes led to the extinction of the fauna in the deeper parts of the Bornholm and Gdansk Deeps. The three deep basins in the central Baltic are the Bornholm Basin, Gdansk Deep and Gotland Basin. In the Bornholm Basin, the Central Basin and the Gulf of Finland, the bottom layer is separated from the surface layer by a permanent halocline. During periods of stagnation this separation gives rise to an oxygen deficit and periodically to complete oxygen depletion and formation of hydrogen sulphide. The result is a total disappearance of macrofauna in the deepest part of these basins. In places there is a constant oxygen poor zone which is virtually devoid of macrofauna and has an extremely improvershed meiofauna, which generally consists of only a few thousand nematodes per square meter. Comparison of the persistence of the soft bottom macrofauna in the deeper parts of the Baltic between areas that the communities of the Gulf of Bothnia are the most stable, while those of the Bornholm and Gdansk Deeps vary strongly, mainly owing to the periodic oxygen deficit. Physical forcing can remedy deoxygenation effects of eutrophication through enhanced vertical mixing. The Slupsk furrow differs from other deep water areas of the Baltic Proper in both the salinity and oxygen content. All the remaining deep areas (the Bornholm, Gdansk and Gotland Deeps) are plagued by periodic oxygen depletion and/or the presence of H2S in the near-bottom water. At present the Slupsk furrow is the only part of the Baltic Proper where bottom areas below the isohaline water layer are inhabited by a number of marine species which take advantage of the relatively high salinity and acceptable oxygen content.

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.

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

Threat status

Synthesis of Red List assessment

There has been a marked decline in the quality of this habitat, which is apparent from the drastic changes in species and functional guild composition in central and northern sub-halocline areas including the gradual disappearace of Scoloplos armiger between the 1950s and 1990s. Former mollusc dominated communities that were still present in the southern Baltic in the early 1950s have been replaced by communities dominated by polychaetes. In the Gulf of Finland, mass occurrence of several macrobenthic animals at depths of 80-90 m and even deeper was not a rare phenomena. This has not been the case since 1989/90 despite some recolonisation. Demersal fisheries are widespread and also believed to have affected the quality of this habitat.
Habitat extent has also been affected. The earliest records of severe bottom water hypoxia and anoxia are from around 1950. Bottom water and sedimentary areas with low oxygen concentrations have been spreading during the latter part of the 20th century and the presence of H2S has become a permanent feature of the Gansk, Bornholm and Gotland Deeps. Naturally, hypoxia occurs intermittently, but the areas affected have expanded from <10,000 km2 before 1950 to >60,000 km2 since 2000. This is believed to be mainly caused by enhanced nutrient inputs.
On the basis of expert opinion, this habitat is assessed as Vulnerable for both the EU 28 and EU 28+ since there has been a substantial reduction in quality affecting more than 30% of its extent over the last 50 years. This is a consequence of expansion of the area permanently or temporarily affected by anoxia.
Red List Category Red List Criteria
Vulnerable C/D1
Red List Category Red List Criteria
Vulnerable C/D1

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
  • Pollution
    • Nutrient enrichment (N, P, organic matter)
  • Invasive, other problematic species and genes
    • Invasive non-native species
  • Climate change
    • Changes in abiotic conditions
    • Temperature changes (e.g. rise of temperature & extremes)
    • Water flow changes (limnic, tidal and oceanic)

Habitat restoration potential

Unknown. Recovery will be influenced by the frequency and longevity of anoxic periods. As these occur naturally, are intermittent and unpredictable it is not possible to estimate time scales for recovery. It should however be noted that some of the associated species are long lived and have not recolonised areas affected in the 1950s. Recovery may also be hindered or slowed down by the colonisation of opportunistic species such as polychaetes and depending on larvael supply from healthier areas with established populations.

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

Measures to reduce eutrophication can reduce the impact and duration of naturally occurring anoxic periods which affect this habitat. Limiting bottom trawling may be beneficial by allowing recovery of the habitat. The longer term effects of climate change are unknown, but changes in hydrographic conditions (e.g. water flow into the Baltic, salinity and temperature gradients) could potentially exacerbate the impact of anoxic periods on this habitat.

List of conservation and management needs

  • Measures related to wetland, freshwater and coastal habitats
    • Restoring/Improving water quality
  • 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 60,927 Decreasing Decreasing
Gulf of Finland

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 60,927 Based on EUSeaMap modelling. Mapped extent of this habitat is recognised as biased and underestimated.
EU28+ Unknown 60,927 Based on EUSeaMap modelling. Mapped extent of this habitat is recognised as biased and underestimated.
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 Amphiascoides dispar
Invertebrates Halicryptus spinulosus
Invertebrates Kliopsyllus constrictus
Invertebrates Pontoporeia femorata
Invertebrates Scoloplos armiger
Species scientific name English common name Species group
Amphiascoides dispar Invertebrates
Halicryptus spinulosus Invertebrates
Kliopsyllus constrictus Invertebrates
Pontoporeia femorata Invertebrates
Scoloplos armiger Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
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