Artificial reefs are human-made structures, ranging from sunken ships to purpose-built concrete modules, intentionally placed on the seafloor. They are deployed to serve various functions, such as enhancing local fisheries, providing new habitats, or restoring damaged marine ecosystems. These underwater installations quickly become focal points for marine life, introducing complex, hard substrate into areas that might otherwise be flat and barren. Studying these structures helps determine the extent of the marine biodiversity they are capable of supporting.
The Range of Species Richness
The question of how many species inhabit an artificial reef lacks a single, definitive answer, as the number is highly variable and location-dependent. Studies have documented species richness ranging from a few dozen to several hundred different species. For instance, a single artificial reef complex off Hawaii recorded sixty-eight fish species, while structures in the Texas Gulf waters have hosted nearly 200 marine fish species.
This richness includes both sessile and mobile organisms, which together form a complex new ecosystem. Sessile invertebrates, such as sponges, barnacles, and hard corals, colonize the reef surface, providing food and shelter. Mobile species, including commercially important fish like snapper and grouper, are attracted to the shelter and foraging opportunities the structure provides. The exact count depends heavily on the reef’s age, geographical location, and the availability of larval supply in the surrounding water column.
Key Factors Determining Biodiversity
The final species count and community composition on an artificial reef are determined by a few quantifiable variables.
Substrate Material and Complexity
The first factor is the substrate material and complexity of the structure itself. Stable, non-toxic materials that provide a textured surface, such as concrete or corroded steel, are better for larval settlement and colonization. High structural complexity, featuring numerous holes, crevices, and varied surfaces, significantly increases the number of available ecological niches, supporting a greater diversity of species.
Location and Depth
The location and depth of the deployment are major influences. Reefs placed in nutrient-rich waters with strong currents, which deliver a steady supply of plankton and fish larvae, exhibit faster colonization and higher species richness. Water depth also dictates the types of communities that form, as light availability affects the growth of photosynthetic organisms like algae and corals.
Size and Aggregation
The size and aggregation of the artificial reef structures play a role in determining the supported biodiversity. Larger, more numerous structures support a greater total number of species and a more stable population by offering a wider range of habitats. When multiple structures are deployed as a complex, they function as a larger habitat patch, sustaining fish populations more effectively than a single, isolated unit.
The Process of Ecological Succession
A new artificial reef undergoes a predictable process of ecological succession rather than immediately hosting a mature marine community. This process begins almost immediately after deployment with the formation of a microbial biofilm on the structure’s surface. Within weeks to months, this initial layer is followed by the colonization of fouling organisms, primarily algae and encrusting invertebrates like barnacles and sponges.
As these primary colonizers establish themselves, they alter the substrate and provide the first food sources and microhabitats for the next wave of life. This stage sees the arrival of motile invertebrates, such as mollusks and crustaceans, which graze on the algae or prey on the smaller sessile organisms. The presence of these organisms, combined with the growing structural relief, then attracts various fish species seeking shelter, food, and protection from predators.
The community matures over a timeline that spans several years, with significant development often occurring in the first two to three years. Long-term evolution involves the establishment of complex, three-dimensional habitats created by slower-growing organisms like hard corals and large sponges. Researchers note that the ecological succession can still be actively progressing even after five years, as the community moves toward a persistent or stable stage.
Artificial Reefs Versus Natural Habitats
Artificial reefs are compared to natural habitats, such as coral or rocky reefs, to gauge their ecological effectiveness. Studies suggest that artificial reefs can support fish density, biomass, and overall species richness at levels comparable to natural reference reefs. The success depends heavily on the reef’s material and the geographic context.
In some instances, artificial reefs may temporarily exhibit higher species richness than nearby natural areas, particularly when the natural habitat is degraded. However, a distinction exists between species richness (the number of different species) and species diversity (which accounts for the evenness of population sizes). While an artificial reef may attract a high number of different fish species, the community structure can sometimes be numerically dominated by only a few species. This results in lower overall diversity compared to a well-established natural reef. Artificial reefs function as localized ecological enhancements, successfully adding hard substrate habitat to featureless areas and contributing to local marine biodiversity.