Study of habitat complexity and biodiversity in artificial reefs

Artificial reefs are created for marine and coastal environments to increase habitat complexity and biodiversity.

Swansea University’s Dr Ruth Callaway recently conducted research into how “Interstitial Space and Trapped Sediment Drive Benthic Communities in Artificial Shell and Rock Reefs”.

This research tested the macrobenthic infauna collected in standardised artificial reef units made from four materials: cockle shells, mussel shells, oyster shells and limestone rocks of about 4cm diameter. The field study took place over five months on an intertidal sand flat in Swansea Bay.

The basic design of the reefs used in the field study was an adaptation of Salix’s river restoration product, Rock Rolls. Salix provided the polypropylene mesh bags used as reef netting for all of the artificial reefs created for the study. We also provided the experimental Rock Rolls used as rock reefs.

Research objectives

  • to measure the differences in macrobenthic species abundance and richness in the artificial reefs compared to the sandy intertidal surrounding areas.
  • to compare diversity properties and community structure in the four reef types.
  • to quantify the degree to which interstitial space and trapped sediment affect the benthic community.

Species diversity in artificial reefs

  • 45 invertebrate species were counted in the artificial reefs, compared to 12 in the surrounding reef-free areas. Most of the invertebrates were polychaetes, followed by crustaceans and bivalves.
  • 37 species were found exclusively in the reefs, such as amphipods.

Interstitial space in artificial reefs

The study demonstrated significant enhancement in infauna diversity in sandy coastal environments through the use of artificial reefs. The material seemed less relevant than the amount of interstitial space generated.

There was no significant difference in the amount of sediment trapped in each of the different reef materials.

The amount of interstitial space was significantly larger in the rock units than the cockle units, but there was no difference between the other materials.

Due to the larger size of the rocks, there was a smaller ratio of interstitial space to volume of material. This meant that there was less interstitial space per unit volume of the shell reefs. Rock reefs would need to be about double the size of shell reefs to produce the same amount of interstitial space.

Species richness in artificial reefs

Species richness and abundance were not significantly different between the four reef types.

Species richness was significantly correlated with the amount of interstitial space and volume of trapped sediment in the reefs. This indicates that the space created drives colonisation.

However, if the space becomes too large, the species may no longer use it in the same species-specific ways. Some of the uses of interstitial space include:

  • temporary shelter at low tide
  • refuge from predators
  • protection from challenging environmental conditions
  • provision of oxygen and food provided by water circulation
  • creatures becoming washed into and trapped in the reefs
  • use of the reefs as hunting grounds (more research is suggested for this last point to understand the temporary use of artificial reefs)

Biodiversity in artificial reefs

Bivalve shell reefs showed no greater benefit to biodiversity than limestone rock reefs.

However, rock reefs showed the lowest multivariate dispersion, reflecting more constancy. This potentially suggests that higher stress levels are possible in the marine communities populating shell reefs, although more research is required here.

The full research paper can be read on the Frontiers in Marine Science webpage.

Interstitial space in Rock Rolls improves invertebrate habitats

This exciting new research into artificial reefs in intertidal environments adds to Dr Callaway’s previous findings that habitats for invertebrates are enhanced by the use of Salix’s Rock Rolls in freshwater environments.

Dr Callaway’s presentation at a Salix River, Wetland and Sediment Conference explored how her field studies in freshwater environments, as well as marine environments, revealed that the factor that best improved biodiversity was the amount of interstitial space in Salix Rock Rolls.

Watch Dr Callaway’s talk on how Rock Rolls impact biodiversity and improve habitats for invertebrates.