Stable aggregations of unattached perennial vegetation on Baltic infralittoral mixed substrata (predominantly hard)

This is a Baltic Sea benthic habitat in the photic zone where more tham 10% of the seabed is a mix of both hard and soft substrata according to the HELCOM classification. Stable aggregations of unattached perennial vegetation covers at least 10%, while perennial attached erect groups or Mytilus cover less than 10% of the bottom. The habitat occurs in most of the Baltic Sea area where the salinity is <10 or 5 psu (depending on the area), the exposure is sheltered and the seabed is level over wide areas within the photic zone.

Four associated biotopes with different dominant species of vegetation (constituting at least 50% of the biovolume) have been identified. ‘Baltic photic mixed substrate dominated by stable aggregations of unattached Fucus spp. (typical form)’ (AA.M1Q1) and ‘Baltic photic mixed substrate dominated by stable aggregations of unattached Furcellaria lumbricalis’ (AA.M1Q3) are encountered at depths of 0.5 to 5 meters. The unattached Furcellaria lumbricalis may occur in specific, ball-shaped morphology adapted to soft bottom conditions, and was historically described as Furcellaria cf. aegagropila.; ‘Baltic photic mixed substrate dominated by stable aggregations of unattached rigid hornwort (Ceratophyllum demersum)’ (AA.M1Q4) is encountered at a depth of 0 to 2 meters.

‘Baltic photic mixed substrate dominated by stable aggregations of unattached Fucus spp. (dwarf form)’ (AA.M1Q2) forms a characteristic biotope of shallow bays and lagoons between 0.25 and 2.5 m. This specific morphology of the Fucus spp. lacks bladders and holdfasts and the single plants can be loosely anchored in the sediment. Under more exposed conditions plants form a ball-shaped form, able to roll over the sea bottom. The Fucus dwarf forms coexist with attached F. vesiculosus, unattached Furcellaria lumbricalis, higher plants like Ruppia spp., Zannichellia palustris, Stuckenia pectinatus (formerly known as Potamogeton pectinatus), Zostera spp. and several Charophytes. The unattached thalli can cover the sediment up to about 10 cm height and thus form a three-dimensional habitat comparable to the interstitial space in coarse sediments. Epifauna is seldom attached to the Fucus dwarf form, but gastropods, amphipods and insects look for shelter and food in between the loose lying thalli. If abundances of the unattached form are very high, the sediment below becomes deoxygenated and the associated infauna below the Fucus layer may die. Presently this biotope is only known to occur in Sweden and Germany. In Germany it exists only in very few coastal lagoons with low to moderate eutrophication pressures and salinities of around 7–10 psu.

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.

Density of unattached Fucus spp. (typical and dwarf forms), the lower limit of the Furcellaria belt, the amount of epiphytic algae, and density of Furcellaria are potential indicators of quality of this habitat.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats