Stable aggregations of unattached perennial vegetation on Baltic infralittoral coarse sediment

This habitat is distributed on Baltic bottoms in the photic zone with at least 90% coverage of coarse sediment. Coarse sediment has less than 20% of mud/silt/clay fraction (<63 μm), and the proportion of gravel and pebbles (grain size 2–63 mm) exceeds 30% of the combined gravel and sand fraction according to the HELCOM HUB classification. Stable aggregations of unattached perennial vegetation cover at least 10%, while perennial attached erect groups or Mytilus cover less than 10% of the bottom. The habitat is rare but can be found in the photic zone of most of the Baltic Sea area where the salinity is <10 or 5 psu (depending on the area), the exposure is moderate to sheltered, and the seabed is level over wide areas. 

Three associated biotopes with different dominant species of vegetation (at least 50% of the biovolume of the unattached perennial vegetation - Fucus spp. (typical or dwarf form) and Furcellaria lumbricalis) have been identified. ‘Baltic photic coarse sediment dominated by stable aggregations of unattached Fucus spp. (typical form)’ (AA.I1Q1) and ‘Baltic photic coarse sediment dominated by stable aggregations of unattached Furcellaria lumbricalis’ (AA.I1Q3) are encountered at 0.5 to 5 meters depth. Unattached Furcellaria lumbricalis may occur in specific, ball-shaped morphology adapted to soft bottom conditions, historically described as Furcellaria cf.aegagropila. ‘Baltic photic coarse sediment dominated by stable aggregations of unattached Fucus spp. (dwarf form)’ (AA.I1Q2) forms a characteristic biotope of shallow bays and lagoons between 0.25 and 2.5 m. This specific morphology of the Fucus spp. dwarf form lacks bladders and holdfasts; it is regularly dichotomous branched with branches of similar length resulting in a fan-shaped appearance of the thalli. The single plants can be loosely anchored in the sediment with its lower, dark brownish parts. The thalli are very fragile, break very easily into pieces and thus generate new thalli. 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 biotope exists in lower mesohaline salinities (7–10 psu) and moderately exposed to very sheltered conditions. 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 in between the loose lying thalli mobile gastropods, amphipods and insects look for shelter and food. However, 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 the dwarf form 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