Biogenic Reefs

Biogenic reefs, or more precisely mussel and oyster beds, in Denmark consist of large collections of dead and living blue mussels, horse mussels, and/or oysters. These form a persistent community that supports other characteristic hard substrate species.

Blue Mussel

The blue mussel (Mytilus edulis L.) is the most common mussel species in Denmark, and it occurs as an epibenthic species in all Danish waters. It thrives on a variety of substrates ranging from harbor piers, windmill foundations, and other fixed structures in the water column to soft muddy seabeds.

In fjords with favorable living conditions, blue mussels can occur in densities of up to approximately 10 kg per square meter, forming dense beds with varying spatial heterogeneity depending on the bed's age, mussel density, predator presence, and food availability in the area. The mussels are bound together within the beds by byssal threads, which can also anchor them to other hard objects like stones or empty shells. Often, these beds are not homogeneous throughout but contain areas without mussels or with very low densities. These gaps can vary in size from 10-30 cm to several meters between densely packed clusters of mussels.

Blue mussels can be found in dense beds or as more isolated individuals or small groups of 2-20 mussels. They typically live on substrates containing some hard material, but in many fjord areas, blue mussels also dominate soft muddy seabeds. Their distribution and individual size vary depending on factors such as salinity. The optimal salinity for blue mussels is between 18-28 PSU, found at depths of 0-15 m in Danish inland waters. However, they can also be found at greater depths, such as in Smålandsfarvandet and the Baltic Sea, where they occur in high densities on hard substrates in deeper waters.

Horse Mussel

The horse mussel (Modiolus modiolus), one of Denmark’s largest mussels (up to 20 cm in length), is common in Danish waters, typically found in the North Sea, the Limfjord, Kattegat, and the northern part of the Belt Sea. It lives down to approximately 40 meters, buried in seabeds of pebbles and coarse gravel or attached to stones or other mussels using byssal threads.

Horse mussels often form large beds on stone reefs and rocky substrates, consisting of both the shell structures of living mussels and the shells of deceased individuals.

Smaller horse mussels are highly susceptible to predation by sea stars, crabs, and whelks. Once the mussels reach a length of 5 cm, they largely escape the high predation rates. Consequently, horse mussel beds typically consist of larger, and thus older, mussels.

The European Flat Oyster

The European flat oyster, known as Ostrea edulis (with edulis meaning edible in Latin), is native to Denmark and has been known here since the Stone Age.

In the Limfjord, the European flat oyster is found in shallow waters at depths of 0.5-8 meters.

A mussel bed is defined as a biogenic reef when it exceeds a certain size. For blue mussels in Denmark, there must be contiguous areas of at least 2,500 m² with an average coverage of blue mussels of at least 30%.

The reef structure must be stable and permanent enough to allow for the establishment of a community of hard substrate species on the reef. The reef must therefore be identifiable in the same location over several years. For blue mussels, there must be at least three cohorts (age classes) present.

A horse mussel bed is defined as a biogenic reef when it covers at least 100 m² with a central coverage of 20% mussels and shells, of which 10% must be live mussels. The mussels must primarily consist of individuals over 4 cm in length.

Biogenic reefs create habitats for a wide range of benthic animal groups (epifauna), including other mussel species, shore crabs, snails, barnacles, and keel worms. Additionally, biogenic reefs serve as a food source for both fish and birds. The presence of macroalgae on blue mussel beds is primarily dominated by small species since both live mussels and empty shells are too unstable or poorly anchored to support larger species, such as many brown algae. Mussel and oyster beds thus play a significant role in marine flora and fauna, typically resulting in increased biodiversity on biogenic reefs compared to surrounding areas.

Mussels and oysters feed by filtering plankton from the water. As a result, biogenic reefs help remove plankton from the water, making it clearer so sunlight can penetrate deeper. This benefits attached macroalgae (seaweed) and eelgrass growing on the seabed, as they require sunlight for photosynthesis and growth.

Mussel and oyster beds (biogenic reefs) are typically found in shallow or moderately deep waters in fjords and along coasts with strong water flow. These areas often provide abundant food supplies and oxygen-rich water due to the moving currents. The biogenic reefs create a compact substrate on both hard and softer seabeds and typically rise only 20-30 cm above the sea floor. 

Denmark is obligated to protect biogenic reefs, as stipulated in the Habitats Directive, which states that member states must protect habitat type 1170 Reefs, which can be either stone reefs or biogenic reefs. Where mussel and oyster beds have disappeared, biogenic reefs can be reestablished by introducing juvenile mussels from aquaculture or material from other mussel and oyster beds. Depending on the placement of the new beds, they will filter water, support increased biodiversity, and have a stabilizing effect on the sediment.

By establishing new or restoring former mussel beds, ecosystem services can be reestablished and new habitats created. In Danish waters, the potential for creating blue mussel beds is the most favorable. However, methods for establishing beds of the European flat oyster and horse mussel are also expected to be developed.

Blue mussels thrive and grow best at salinity levels of 15-18 PSU. However, blue mussels generally exhibit high plasticity toward environmental factors and are found in all Danish waters to varying degrees. At depths of less than 1-2 meters, large populations of blue mussels are rare because ice, wave impact, and predation prevent the formation of stable beds. There are no definitive requirements for substrate type, and the beds can be placed on various seabed types.

Oxygen depletion and predation by starfish are the greatest threats to the survival of mussel beds, while mussel-eating birds can also impact their survival. In addition, mussel beds must reach a certain size to remain viable over extended periods. Nevertheless, mussel beds are not always stable, and there are instances where beds disappear without a clear explanation.