Submerged rooted plant communities on Baltic infralittoral mixed substrata (predominantly soft)

This benthic Baltic Sea habitat occurs in the photic zone with more than 10%, but less than 90% coverage of  hard and soft substrata according to the HELCOM HUB classification. Coverage of submerged rooted plants which also includes plants with rhizoids (i.e. Charales) cover at least 10% of the seabed, and more than other perennial attached erect groups. Mixed sediments covered by rooted plants are mainly distributed in moderate exposure levels but may also occur in sheltered conditions. The habitat covers the full salinity range of the Baltic Sea and is distributed from the Belt Sea up to the northern part of Bothnian Bay. Depending on the salinity and depth the dominant species (>50% of the biovolume), defining the associated biotope type, varies. 

’Baltic photic mixed substrate dominated by pondweed (Potamogeton perfoliatus and/or Stuckenia pectinata)’ (AA.M1B1) is found between 0.2-4 m depth in sheltered sites with up to 6 psu.  ‘Baltic photic mixed substrate dominated by watermilfoil (Myriophyllumspicatum and/or Myriophyllum sibiricum)’ (AA.M1B3) has a similar distribution but a more narrow depth range (0.2-2 m).  ’Baltic photic mixed substrate dominated by Charales’ is found in a wider range of salinity (2-15), depth (0.2-7 m) and wave exposure (low to moderate) (AA.M1B4).  ’Baltic photic mixed substrate dominated by Zannichellia spp. and/or Ruppia spp. and/or Zostera noltii’ is found at 0-4 m depth throughout the salinity gradient of the Baltic Sea and in low to moderate exposure (AA.M1B2).  ’Baltic photic mixed substrate dominated by common eelgrass (Zostera marina)’ (AA.M1B7) differs most strongly from the other sub-biotopes in distribution, occurring mainly at moderate exposure and in salinities of 5 psu or higher. It is typically found deeper than the other associated biotopes (1-6 m) and often marks the lower depth limit distribution of soft bottom vegetation. This biotope is absent from areas with low salinity in the inner part of Gulf of Finland and Gulf of Bothnia.

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 overtime. 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. 

The vertical depth limit of submerged rooted plants is used in several countries as a Water Framework Directive parameter for assessing ecological status. The overall quality and continued occurrence of this habitat is, however, largely dependent on the presence of the rooted plant species which create the biogenic structural complexity on which the characteristic associated communities depend. The density and the maintenance of a viable population of these species is a key indicator of habitat quality, together with the visual evidence of presence or absence of physical damage.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats