Mussel beds on Pontic circalittoral terrigenous muds

This habitat is comprised of mixed circalittoral sediments – terrigenous muds -  mixed with variable amounts of recent or subfossil shells, most of them belonging to the blue mussel Mytilus galloprovincialis, occurring offshore, between depths of 20 and 45 m. At these depths environmental conditions are relatively constant year-round: low light, low temperature (6-9°C), and a constant salinity of 18 ppt. Mytilus galloprovincialis forms biogenic reefs through the accumulation of mussel shells in time and aggregation of the shells by byssal threads. Over time, a hard substratum higher than the surrounding sediment is formed, on which living mussel colonies attach themselves. The reef is formed of numerous elongated patches and/or continuous ridges, always transverse to the prevailing bottom currents (which bring food to the filter-feeders). Between these lay the organic-rich “Mytilus mud” formed by accumulation of mussels’ faeces and pseudofaeces. The biomass of Mytilus galloprovincialis may vary between 200 and 1,500 g/m².

Among the habitats which occur on sedimentary substratum in the Black Sea, the mussel beds have the highest biodiversity, due to both extending through a wide range of depths and to providing a multitude of microhabitats suitable for a large number of species. This biogenic reef is unique through the crucial ecological role played by the great biofiltration power of the mussel beds in, which ensures the benthic-pelagic coupling and provides enhanced ecosystem resilience. The ‘mussel mud’ formed by the blue mussels’ waste is an important source of food for deposit-feeding infauna living in the sediment around the mussel beds.

The high- biodiversity mussel beds harbour various threatened species and have socio-economical importance as a habitat (breeding grounds, nurseries) and fishing area for commercially valuable species (Psetta maeotica, Squalus acanthias, sturgeons, Rapana venosa). Mussels themselves are the most popular mollusc species for human consumption around the Black Sea, and mussel beds are a source of larvae and spat for aquaculture. The habitat is present all around the coasts of Bulgaria, Romania, Ukraine, Russia and Georgia as a discontinuous belt at variable depths between 20 and 45-80 m. Historically the habitat used to be present in front of the Turkish coast as well, but was completely destroyed due to intensive bottom trawling during the last 100 years.

Indicators of quality:

Biomass, density and cover are some of potential indicators of quality for this habitat.  In Romania the following thresholds have been established:

-Reduced habitat fragmentation – the area of enclaves of Melinna palmata muds occurring inside the habitat ≤10.5%

-Cover of living mussels inside patches ≥50%

-Median shell length of living Mytilus galloprovincialis inside patches ≥50 mm.

-Live biomass of Mytilus galloprovincialis ≥5,000 g/m²

In Russia and Ukraine the average biomass of macrozoobenthos in 1970-1980 was more than 450 g/m², with a density of 350 ind/m². Now some decline in the number and biomass is observed. In different regions average biomass varied from 220 to 300 g/m^2, and density 250-270 ind/m². Off the coast of Crimea average density and biomass of macrozoobenthos in the habitat were quite uneven as we can see from different authors: 3,700 ind/m² and 59 g/m^2, respectively and 844 ind/m² and 227.7 g/m², respectively.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats