4.2 Materials

The kind of materials used can affect the design life and longevity of the reef, the colonization of artificial substrates by benthic organisms and, consequently, the composition of the fish assemblage that will inhabit the reef.

First of all, the materials should be inert in order to avoid pollution and bioaccumulation of contaminants in the environment and in aquatic organisms.
The choice of the reef materials should also consider aspects such as resistance to the chemical and physical forces that are in constant action in the marine waters, lifetime and suitability for colonization by benthic communities.
With regard to stability, a general rule is that the weight of the materials used for the construction of the reef units should be at least double the specific gravity of seawater or, alternatively, that the structure should actually be anchored to the seabed (OSPAR Commission, 1999). In terms of durability, the materials should assure a minimum lifetime of 30 years. As for functionality, the materials have to be suitable to colonization by benthic organisms, based on field verification conducted for a minimum of one year. Lastly, for economic reasons, the materials should be cost?effective (Grove et al., 1991).

Numerical modelling should be applied to verify that the reef will remain structurally intact and “on?station” over at least 30 years, including in case of exceptional marine and weather events.
A wide range of natural and man?made materials have been used in artificial reef construction. Natural materials include rocks, shells and wood, the latter being less durable over time due to the action of burrowing organisms. Rocks can be scattered on the seabed, deployed in chaotic piles or assembled inside frames that are made of steel, iron, plastic or wood. Concrete, iron, steel and plastic are the most often used artificial materials worldwide. Fiberglass, coal ash by?products, ceramic and ferro?cement have also been employed. These materials facilitate the pre?fabrication of specifically designed modules prior to water transport to the deployment site.
A number of ecological considerations should also be made as some materials can be selective towards benthic organisms. For example, a greater abundance of benthic species has been recorded on concrete and plywood compared to fiberglass or aluminium (Anderson and Underwood, 1994). Moreover, Bombace et al. (1997) have found a selective settlement of the burrowing bivalve, Pholas dactylus, on the horizontal surfaces of coal?ash blocks. A list of potential materials for artificial reef construction, including their advantages and disadvantages is reported in Table 1.