What are Artificial Reefs?
An artificial reef is a human-made underwater structure that substitutes as a natural reef to form a habitat for marine life. Artificial reefs are placed in areas where there is little bottom topography or near coral reefs to attract marine populations. These structures are made from a variety of materials including shipwrecks, construction debris, oil rigs, concrete, and any other man-made objects. Bridges, lighthouses, and other offshore structures can also act as artificial reefs. By providing shelter from predation and surfaces for encrusting organisms to grow on, the man-made structure creates a habitat. Lower-level food chain organisms attract predators increasing the biodiversity. As time passes the reef is colonized by sponges, hard and soft corals, algae, numerous fish species, crustaceans, and many other creatures. Typically, artificial reefs do not take as long to establish as natural coral reefs. Artificial reefs may be intentional or unintentional.
Why Artificial Reefs?
Artificial reefs serve to protect coral reefs form human-induced damages as well as supporting biodiversity and healthy ecosystems. In addition, they are used for mitigating coastal erosion, creating surf breaks, and in the past have been built to block ships from entering coastal waters. These structures create plankton-rich feeding spots that attract small animals and the reef soon becomes covered by encrusting coral and sponges. The collection of smaller marine organisms brings in predators and expands to establish a new habitat. The significance of this is that these spots can divert reef-enthusiasts, like divers and boaters, from endangered coral reefs to the artificial reef. Not only do artificial reefs serve as protectors, but they can be used to maintain fisheries management and promote ecotourism for Small Island Developing States.
Fisheries management allow for sustainable management by protecting the ecosystem and the economic reliance on fisheries. This means controlling our behavior, marine habitats, and marine resources to estimate fish mortality and produce optimum yield. To manage these components of fisheries, artificial reefs have been implemented to manipulate marine habitats. It is easily mistaken that artificial reefs are an encompassing solution to fisheries management since there are no adverse side effects, but there are very few applications of this, so there are limited examples to follow. Establishing artificial reefs creates two scenarios that promote sustainability of fish populations and fishing. In one scenario the reef increases the environmental carrying capacity by establishing an additional habitat. This therefore increases biomass. In the other scenario the artificial reef serves to aggregate fish and makes them more vulnerable to being harvested by fishermen. One demonstration in fisheries management has been the deployment of concrete modules in Algarve, Portugal to build an artificial reef complex as part of the Algarve Artificial Reef Program. This program studied the colonization process, the role of reefs as nursery grounds for juvenile protection, fish and benthic invertebrate assemblage, water chemistry, the use by the fishing community, and the socio-economic impact on the local community. The results of this program include an increase in the mean number of species, a high level of biomass, confirmation of the increased proportion of fish as juveniles, and demonstrated the multiple roles artificial reefs serve for recruitment, nursery grounds, and reproduction for adults.
Artificial reefs have been around since the Neolithic time period, and then more prominently in 17th century Japan. People noticed that putting simple structures made from bamboo and palm leaves in the water attracted fish. This logic was backed by science and looked more heavily into in the 1830’s in the United States. The idea of FADs, or, Fish Attraction Devices was fine tuned over the years and now looks like the artificial reefs that we see today. Throughout this page, we will cover the different types of artificial reefs you will see today, what works and what does not work, and the benefits and harms of artificial reefs.
Types of Artificial Reefs
Artificial reefs generally fall into two categories, manufactured and recycled. Though the purposes of artificial reefs may vary, effective reefs must be heavy, non-toxic, and resistant to erosion and other forms of physical and chemical wear.
Most modern manufactured reefs have a bamboo fiber, synthetic fiber, or metal framework, which is then covered in a mixture of neutral-pH concrete and calcium carbonate (limestone). A higher proportion of concrete creates a stronger structure. However, this increases the acidity of the structure, making it less habitable for coral polyps.
Electro Mineral Accretion (EMA)
EMA is a method of crystallizing limestone on a metallic structure, via an electric current within a solution that contains dissolved calcium carbonate (limestone). A low voltage current is run through the structure, which can be generated by floating solar panels on the surface or by a battery. This current prevents rust and promotes the crystallization of the limestone forming an attractive habitat for coral. The nooks and crannies of calcium carbonate formed this way are also highly appealing to small marine organisms.
Mass produced and custom concrete structures
Organizations such as Reef Ball are currently at the forefront of artificial reef design and production. Due to the expense of production and placement, many NGOs are at the forefront of artificial reef design, placement, and purchase. Reef Balls and other customizable structures undergo extensive testing to ensure their stability and resistance to the physical and chemical wear encountered in marine environments. The customizable aspects of these structures, such as variations in size, material, and other parameters can be utilized to provide an ideal habitat for a desired species in a specific region.
In St. Thomas, the Division of Fish & Wildlife and the University of the Virgin Islands received funding to partner and install an artificial reef in the territory. According to a press release, the project should be completed in July 2024, and "UVI is readying the strongest specimens out of roughly fifteen types of coral in nurseries for eventual propagation onto the reef, which will take the shape of an iconic petroglyph. The institution is collaborating with researchers from Woods Hole Oceanographic Institution on cutting-edge technology to create an environmentally sensitive design made from sourced materials ranging from carbon-friendly cement to natural chemicals to help stimulate coral growth. The permitting process for the project with the Division of Coastal Zone Management, and the Army Corps of Engineers is moving toward final approvals of location." 
3-D Printed Artificial Reefs
In recent years, 3D printers have become faster, cheaper, and more accurate. Now, companies like Design Lab have begun using a technique called Modular Artificial Reef Structure (MARS) to design and 3D print artificial reefs. This technique makes it possible for more intricate and unique structures to be created. 3D printing allows for easier modifications in designing structures with different cavity sizes, forms, and textures in order to cater to different marine species. This can be particularly helpful when trying to rebuild specific marine populations.
The re-purposing of materials for use in artificial reefs is growing in popularity and provides a generally cheaper option than custom manufactured reefs. Accidental recycled reefs are common in regions which have undergone significant storm activity, as man-made structures, boats, and other vehicles are drawn out and sunk by natural forces. If the wreckage remains stationary for a significant period, seaweed, algae, coral, barnacles, and other organisms will begin to populate the structure. Observation of these accidental reefs has led to private and governmental organizations depositing retired structures onto the seafloor, for environmental, economic, and entertainment benefits.
Recycled artificial reefs must be carefully planned. When materials are not properly secured, such as in Osborne Reef, Florida, the environmental and economic results can be costly. It is also essential that the recycled materials are extensively stripped and cleaned before placement, as remaining materials such as glass or rope can pose hazards to both wildlife and humans. Toxic coatings, such as lead paint, or other hazardous chemicals, such as oils or mercury, must also be carefully removed, so they do not leech into the surrounding ecosystem, harming its inhabitants. Many marine creatures, such as coral polyps, will also only occupy a structure that has a near-neutral pH.
An interesting photo gallery of the formation of many artificial reefs from The Atlantic can be viewed here.
Public Transit Vehicles
Recycled public transit vehicles are ideal for artificial reefs, as they are heavy, solid structures with a multitude of caverns and nooks that readily attract and provide shelter for sea life. In early 2000, the outdated New York City Subway Redbird Cars began to be phased out of use. These were sunk along the east coast of the United States, with the varied purposes of attracting more sea life and improving fisheries counts to providing new attractions for recreational scuba divers.
Retired Ships, Tanks, and Other Military Equipment
Outdated and retired heavy military equipment is ideal for artificial reef formation, due to their heavy framework and durability. Compared to the costly removal and systematic breakdown and recycling that would happen to old military ships on land, the cleaning, stripping and sinking of a ship is relatively cheap and provides ample environmental and economic benefits by providing a habitat for marine life and attracting recreational divers. One of the largest projects in recycling Navy ships into artificial reefs is Reef-Ex, which has organized many reef placements in US waters.
Additional Types of Artificial Reefs
There are several other unique artificial reefs that do not fit into the basic categories of manufactured and recycled artificial reefs. These reefs can range from purposeful installations of art to those providing additional ecosystem services that other reefs might not.
Art and sculpture reefs
Art and sculpture reefs are a subset of manufactured reefs and are usually constructed more with the purpose of attracting tourists rather than benefiting fisheries. These reefs are a recent phenomenon, the most famous of which is Silent Evolution by artist Jason de Caires Taylor. Silent Evolution was placed off the coast of Cancun, Mexico, with the intent of providing an eco-tourism option in a place more famous for its nightlife than its conservation efforts. The nearby Manchones Reef was under pressure from hurricanes and tourists, and researchers saw a steady decline in its health. Taylor sculpted hundreds of figures, mainly local residents from the area, out of neutral-pH clay, which is favorable to marine organisms. These sculptures formed the framework of the reef. The sculptures were then placed on the seafloor in 4 to 9 meters of water, so that they are accessible to both divers and snorkelers. Silent Evolution has since drawn a steady flow of tourists every year to view the reef, along with a growing population of coral and marine life.
Memorial reefs are artificial reefs which individuals can have their ashes mixed into postmortem. Several companies and organization arrange for the individual's ashes to be mixed into the concrete and limestone basis of a manufactured reef, which will then be placed in an underwater cemetery such as Neptune Memorial Reef. This underwater cemetery includes art and sculpture reefs as well as the individuals memorial reef providing additional structures for the aquatic life to inhabit.
Breakwater reefs are a type of artificial reef that function as normal coral reefs in providing biodiversity while also providing a sustainable way to prevent coastal erosion and protect from storm surges. These reefs do this by slowing the waves and lessening the amount of wave energy impacting the shoreline, which is important in large storm events where waves are much larger. In addition to protecting the coasts, breakwater reefs can provide recreational services. Surfing reefs are artificial reefs designed to cause a significant surf break and can double surfing days annually in some regions by redirecting the water and create larger and more frequent waves. They usually consist of large granite blocks or massive geotextile bags, which are strongly resistant to wear. Occasionally oysters can be used as breakwater reefs as well. Some of the more notable artificial surfing reefs are those at Cables, Perth, Australia, and at Boscombe, UK.
How Do Artificial Reefs Stack Up with Natural Reefs?
Note: Most studies on this topic are controversial and inconclusive. Some claim that they have greatly increased populations; others claim little benefit. The inconclusiveness and need for additional studies of this kind with more controlled variables should be noted. Also, in the case of heavily exploited areas, significantly more help is needed besides just adding reef structures.
Diversity of Artificial Reefs
A study on the Caribbean Island of Bonaire investigated the difference in diversity between artificial reefs and natural reefs. The research indicated that there was an equal level of diversity, however the composition of the diversity was much different. Some organisms, such as sergeant majors and bluehead wrasse, were more abundant in the artificial reefs. This indicates that artificial reefs can be used as a way to preserve biodiversity, but it can lead to a change in the composition of organisms within the reefs.
Assessing Ecosystem Functions of Reefs
A recent study looked at nutrient storage and found that artificial reefs performed better than nearby sand flats but were not as functional as a nearby coral reef. 
Advantages of Artificial
- Artificial reefs can be placed in convenient and easily accessible locations to promote tourism, or in distant locations to boost fisheries.
- Recycled reefs usually consist of cheap recycled materials, which is advantageous economically and indirectly for the environment.
- Manufactured reefs can be calibrated and designed specifically for their environment and to attract specific organisms to increase keystone populations.
- Reefs also provide a long term profit for tourism, as discussed in the below section.
- According to some studies, rapid colonization, high fish densities, and high catch rates were consistently present on artificial reefs a few months after placement.
- Artificial reefs have the ability to act as buffers for coastal erosion when installed upon coast lines. They are able to mitigate some of the damage created by crashing waves and water erosion as they absorb some of the impact.
Disadvantages of Artificial
- Manufactured reefs, and even some recycled reefs, involve expensive labor costs for production and for the cleaning and stripping of recycled materials.
- The cost of the transportation and actual placement of artificial reefs can also be very high, as they must be brought out by ships and are usually placed by the means of cranes and extensive floats to ensure that they are properly positioned.
- The possible introduction of toxins by artificial reefs must also be considered, specifically in the case of scuttled ships and other equipment. If cleaning and stripping of the structures is not done thoroughly, toxins such as lead, mercury, or heavy oils can harm the surrounding waters and marine life.
- The large initial investment of an artificial reef is also a large commitment. If the reef fails to become populated, there is little that can be done to rectify the initial investment.
- It has been suggested that only 50 percent of artificial reefs meet their established goals. However, additional studies are needed.
Eco-tourism and Artificial Reefs
In addition to increased marine life populations, new dive sites lead to new economic opportunities. Reefs that gain larger, healthier populations due to the placement of artificial reefs, and the addition of aesthetically interesting structures are a magnet for snorkelers, recreational scuba divers, and photographers. For many developing coastal nations, artificial reefs provide a cheap option to attract tourists in an otherwise poor market. The placement of artificial reefs leads to a long term profit, where tourists will continue to visit, and fisheries will see increased populations and production. If placed close to shore, in an easily accessible area, increased and easily located fish populations attract tourists interested in fishing. An increased volume of tourists attracted by the reefs will create a demand for hotels, restaurants, and other markets associated with tourism. When looking at how artificial reefs can increase eco-tourism, it is also important to look at whether this increased eco-tourism is having a negative impact on the natural or artificial reefs success. As dive sites increase and people are drawn towards artificial reefs, this can give natural reefs the opportunity to strengthen and grow, as they are given a break from human interaction. However, the increased number of divers drawn to an area can also have the opposite impact. More people in the water can lead to further damaging the existing reefs as the dive sites in an area may become more popular.
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