Artificial reef


An artificial reef is a human-created freshwater or marine benthic structure.
Typically built in areas with a generally featureless bottom to promote marine life, it may be intended to control [|erosion], protect coastal areas, block ship passage, block the use of trawling nets, support reef restoration, improve aquaculture, or enhance scuba diving and surfing. Early artificial reefs were built by the Persians and the Romans.
An opportunity artificial reef is built from objects that were intended for other purposes, such as sinking oil rigs, scuttling ships, or by deploying rubble or construction debris. Shipwrecks may become artificial reefs when preserved on the seafloor. A conventional artificial reef uses materials such as concrete, which can be molded into specialized forms. Green artificial reefs incorporate renewable and organic materials such as vegetable fibres and seashells to improve sustainability and reduce energy consumption, pollution, and greenhouse gas emissions. In some cases, artificial reefs have been developed as artworks.
Artificial reefs generally provide hard surfaces where algae and invertebrates such as barnacles, corals, and oysters attach and spaces where different sizes of fishes can hide. The accumulation of attached marine life in turn provides intricate structures and food for assemblages of fish. The ecological impact of an artificial reef depends on multiple factors including where it is situated, how it is constructed, and the ages and types of species involved. While the artificial reefs allow for coral growth, it changes the ecosystem as the relative growth for different species is not always the same. Studies have found that macroalgal, cyanobacterial groups, and coral that are fast growing, grow in artificial reefs at different rates than they would grow in natural reefs.
Considerable research is being done into construction methods and the effects of artificial reefs. Many of the materials used early on are now considered undesirable. A 2001 literature review suggested that about half of the reefs studied met their objectives. Long-term planning and ongoing management were identified as essential factors in success.
A more recent analysis of reefs world wide between 1990 and 2020 concludes that artificial reefs can be useful tools for restoring marine ecosystems if they are strategically designed to suit their specific location and its resource needs.

History

The construction of artificial reefs began in ancient times.
According to historian Diodorus Siculus, the Romans blocked the harbor of Lilybaeum during the First Punic War against the Carthaginians around 250 BC. They built an artificial reef "with stones and construction material" and put poles in the channels using "large timbers and anchors".
Persians blocked the mouth of the Tigris River to thwart Arabian pirates by building an artificial reef.
Artificial reefs to increase fish yields or for algaculture began no later than 17th-century Japan, when rubble and rocks were used to grow kelp. The earliest recorded artificial reef in the United States is from the 1830s, when logs from huts were used off the coast of South Carolina to improve fishing. In the Philippines a traditional native fishing technique known as fish nests, is basically an artificial reef. It uses rocks and waterlogged wood to build mounds inside excavated trenches on shallow tidal waters that attract fish and crustaceans. The mounds are then harvested every few weeks during low tide by surrounding them with nets and dismantling them piece by piece. They are rebuilt after every harvest. Fish nests are often used to capture grouper fingerlings to be used as seeds for aquaculture. Fish nests were in common use since before 1939.
Beginning before the 1840s, US fishermen used interlaced logs to build artificial reefs. More recently, refuse such as old refrigerators, shopping carts, ditched cars and out-of-service vending machines replaced the logs in ad hoc reefs. Officially sanctioned projects have incorporated decommissioned ships, subway cars, battle tanks, armored personnel carriers, oil drilling rigs and beehive-like reef balls.

Purposes

Artificial reef structures have a variety of intended uses, ranging from the protection, enhancement and restoration of marine ecosystems to the support of human activities like fishing, recreational diving and surfing. Artificial reefs can be used as active restoration tools to mitigate environmental damage and habitat loss, restore degraded ecosystems such as kelp forests and coral reefs, and promote biodiversity.
In fisheries management, artificial reefs may be intended to increase production of species of recreational and commercial interest, enhance fishing yield, and support recreational, artisanal or commercial fisheries. They may be designed to protect benthic habitats from illegal trawling and restore fish stocks.
They may be placed to protect against coastal erosion.
They may also be developed to support eco-tourism, promote recreational activities like scuba diving and surfing, and mitigate tourism pressure on corals.
The design and construction of an artificial reef may be very different depending on its proposed location and intended goals. A reef that is designed for one purpose may be unsuitable for others. Early attempts to create artificial reefs frequently failed, or at best, met with mixed results. More recent reviews of work from 1990–2020 suggest that a correctly implemented artificial reef, designed to fit its target ecosystem, can be useful as a tool for the restoration of marine ecosystems. Reviewers call for better before/after and control comparisons of artificial and natural reefs, increased monitoring of reefs over their lifespan, and attention to the spatial orientation, complexity, and shape of reef substrate, among others.

Artificial reef communities

On artificial reef structures intended for ecosystem enhancement, reef communities tend to develop in more or less predictable stages. First, where an ocean current encounters a vertical structure, it can create a plankton-rich upwelling that provides a reliable feeding spot for small fish such as sardines and minnows, which draw in pelagic predators such as tuna and sharks. Next come creatures seeking protection from the ocean's lethal openness—hole and crevice dwellers such as grouper, snapper, squirrelfish, eels and triggerfish. Opportunistic predators such as jack and barracuda also appear. Over months and years the reef structure becomes encrusted with algae, tunicates, hard and soft corals and sponges.
File:Manta ray Biorock reef.jpg|thumb|right|alt=|A newly constructed electrified reef set up by Gili Eco Trust in Indonesia.
An electrified reef is an artificial reef where a small low voltage electric charge is applied to a sub-sea metallic structures that causes limestone to precipitate onto a metal frame onto which coral planulae can then attach and grow; the process also speeds up post-attachment growth.
3D printing technology has been employed both to create molds to optimize the environment for target species, and to directly create cast ceramic and concrete artificial reefs. Work has also been done to develop environmentally friendly materials. For example, Archireef has designed 3D-printed terracotta Reef Tiles, which are nontoxic, biodegradable, and have a pH-level that is compatible with coral. The tiles are small enough to be handled and installed by a diver. An installation in Hong Kong reported a 95 percent coral survival rate after three years, more than four times the survival rate of more traditional restoration methods.
Restoration and mitigation actions on artificial reefs can include activities such as coral transplantation, larval resettlement, and gardening.
For example, the Coral Restoration Foundation in the Florida Keys raises keystone species such as elkhorn and staghorn in coral tree nurseries and replants the corals onto degrading coral reefs. Application of such technologies to artificial reefs could help to restore marine ecosystems. A 2023 review article states: "The implementation of artificial reefs to restore marine ecosystems can be well done, investing resources in studies specifically aimed at determining the appropriate characteristics of ARs for each location."

Carbon sequestration

There is interest in the possibility that artificial reefs can be used to support carbon sequestration and counter climate change. Coastal vegetation ecosystems, algal beds, and phytoplankton have been identified as potential carbon sinks. It is hoped that increasing biomass at artificial reefs can provide another form of blue carbon storage.
RGV Reef, a 1,650-acre artificial reef created in 2017 in the Gulf of Mexico off the coast of Texas, is being studied to assess its potential for carbon capture. Another study area is located off Juehua Island in the Bohai Sea. Biological, physical, social and technological factors must all be considered in calculating carbon capture flow in aquatic systems. Near Juehua Island, M-shaped artificial reefs improved hydrodynamic conditions for creating a carbon sink, but local marine species had limited availability. Development of active marine management strategies and the introduction of appropriate biological species were suggested as ways to increase carbon capture potential.
In the Caribbean, researchers have found that the placement of breeze blocks as artificial reefs near tropical seagrass meadows can create a positive feedback loop. The reef structures attracted fish by providing shelter, and the fish in turn fertilized the seagrass and increased its productivity, providing both food and shelter. The combination of seagrass and reef structures provided added protection from fish nets as well as increasing biomass in the seagrass meadow. Estimates suggest that Caribbean seagrass beds can provide substantial pools for global carbon.