Infaunal communities of Baltic upper circalittoral muddy sediment dominated by bivalves

This habitat occurs on Baltic Sea aphotic bottoms with at least 90% coverage of muddy sediment according to the HELCOM HUB classification. Sessile/semi-sessile epibenthic macrofauna is not present and the biomass of infaunal bivalves dominates. The habitat generally occurs below a depth of approximately 20 m in locations of energy exposure. Three associated biotopes have been identified with different species of bivalves dominating (more than 50% of the biomass). These are: ‘Baltic aphotic muddy sediment dominated by Baltic tellin (Macoma balthica)’ (AB.H3L1); ‘Baltic aphotic muddy sediment dominated by ocean quahog (Arctica islandica)’ (AB.H3L3); and ‘Baltic aphotic muddy sediment dominated by Astarte spp.’ (AB.H3L5). These biotopes have slightly different distributions in the Baltic because of different temperature and salinity preferences of the associated dominating species. For example as an arctic-boreal species, Astarte borealis appears in these Baltic biotopes at its southern limit. It is resistant to anoxic conditions, however recurring and long lasting anoxia is fatal.The biotope dominated by the ocean quahog (Arctica islandica) can only be found in the southwestern parts in the Belt Sea where the salinity is high. Compared to shallow bottoms, the deep muddy bottoms are structurally relatively monotonous therefore the large shells of Arctica islandica increase the complexity of the habitat. It plays an important role as a biomass producer, enhancer of benthopelagic coupling, reducer of water turbidity, and ecosystem engineer as well as being among the longest-lived and slowest growing marine bivalves. 

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 fauna are potential indicators of quality and, in the case biotopes dominated by M. baltica, the presence of a full size range of individuals in the population. 

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats