Building shellfish reefs

Lithosphere (solid materials, soil and rocks) – shellfish

Morphology: The dimensions of an oyster reef will determine its impact on the surrounding environment. The area influenced by an oyster reef will depend on reef height, water level, wave conditions (height, direction), slope of the tidal flat and sediment grain size.

Fig. 8. Habitat requirements lithosphere

Substrate type: Which substrate is the most suitable for the creation of an oyster reef depends on whether the site is exposed or sheltered. Literature reveals significant relationships between the natural occurrence of oyster reefs and the variables exposure time, bottom shear stress, wave action and flow velocities (Lenihan, 1999; Schellekens et al., 2011). Preferably the substrate is inundated during 40-50% of the time and does not move after settlement of oyster larvae (Powers, Peterson et al., 2009). At sheltered sites, most (natural and artificial) hard substrates, ropes or even plant materials can create the necessary substrate for settlement (Brumbaugh and Coen, 2009). Dutch oyster farmers, for instance, successfully collect oyster larvae on loose lying mussel shells in sheltered areas with low sediment deposition rates. Most artificial reefs are however built with oyster shells. Oyster shells are bigger and heavier than mussel shells, making them less prone to displacement and smothering. At exposed sites like the North Sea coast, it is only possible to use fixed substrates. Otherwise currents, wave action and sediment displacements will toss and turn the substrate and damage or smother oyster larvae.

Fixing the substrates can enhance oyster settlement and survival, but fixation is expensive and may be unnecessary when sufficient substrate is provided (Brumbaugh and Coen, 2009). When fixation is needed, stabilizing structures such as gabions (Dutch: schanskorven) ust last until the growth of young oysters has created a fixating crust. It is best to test locally whether and how long substrates for oyster larvae must be fixed (Sluis and Ysebaert, 2012). This is also done in the Building with Nature Case Shellfish reefs.

Suspended particles/sedimentation: By growing upwards, oyster reefs have the potential to grow keep up with sedimentation, but in case of excessive sedimentation oysters will get smothered and die (Powers et al., 2009; Sluis and Ysebaert, 2012). Lower reefs can die if they are smothered by sediment or exposed to hypoxia or anoxia (Lenihan, 1999; Piazza, Banks et al., 2005). High sedimentation rates can also be a threat to the settlement of oyster larvae, as these need a bare hard substrate to settle on. Schulte et al. (2009) found that artificial oyster reef elevation above the soft sediment is positively correlated to adult oyster densities and larval settlement and survival. When sedimentation rates are too high, oyster reefs shift from an healthy elevated reef configuration composed of abundant oysters to a degraded state where vertical growth and spat settlement are hampered (Schulte, Burke et al., 2009).

The balance between erosion and accretion is crucial for oyster reef survival. If erosion is too strong, the reef will be damaged or sink below the level at which it effectively reduces wave action. If sedimentation and erosion are suitable for oyster growth, oyster reefs have the potential to keep up with sea level rise.