A Review of Strategies of Active Coral Reef Restoration

Interest in active coral restoration has grown in recent years alongside marine conservation efforts typically encompassing reef and fish habitat protection. Increasingly, coral reef ecosystems have suffered an unprecedented loss of habitat-forming hard corals. These ecosystems have been subject to numerous disturbances including declining water quality, over-fishing practices, over-harvesting of reef species, outbreaks of coral predators and coral disease [1]. Furthermore, climate change has emerged as the primary threat to coral reefs which was emphasised during the recent 2016–17 global marine heat wave which led to the most extensive coral bleaching event in history [4]. The severity, intensity and frequency of mass coral bleaching and extreme weather events is increasing, diminishing the time for recovery between these events. These disturbances have resulted in a driving need for active reef restoration and the development of two primary methods has emerged [3]. These methods consist of structural and biological reef restoration.

Structural Reef Restoration:

The goal of structural restoration of a coral reef is to increase the amount of habitat and reef structure available for corals and other reef organisms to grow on and seek shelter. Typical construction involves the use of artificial reefs, sinking of wrecks, or relocation of rocks/dead coral heads. Structural restoration is required in areas where the natural reef has been lost due to disturbances. In addition to restoring damaged reefs, this technique can also be used to extend the reef boundaries, create new reefs in sand flats, improve fisheries, or create alternative dive sites to mitigate some of the negative impacts of tourism.

By adding artificial structures in these areas where the physical and biological conditions for coral growth are still adequate, the reef can quickly and effectively be restored. In areas that are struggling, artificial structures will need to be ‘seeded’ with coral transplants to facilitate and increase the rate of development. However, in dense reef areas structural restoration is not necessary, and may be a waste of resources and time.

A more advanced application of structural reef restoration includes the use of mineral accruement devices. Mineral Accruement devices are an advanced reef restoration technology which uses low voltage electrical current to alter the water chemistry directly around the structure. Electrolysis is utilised to increase surrounding seawater pH and encourage calcium carbonate precipitation. In these conditions, corals can devote less energy to forming skeletal structure and divert that energy into other processes such as tissue growth/repair, immune system, mucus production, lipid storage, or reproduction. Corals growing on these devices tend to grow 3-5 times faster than their natural counterparts and will survive in an extended range of physical conditions [1]. This means that corals can thrive in areas where temperature, water quality, or light levels would otherwise be outside the range for that specific coral’s survival. These devices also encourage more natural settlement from coral larvae than typical reef environments as a stronger foundation is available with less spatial competition.

Another example of advanced structural reef restoration is Mid-water coral nurseries. These nurseries have shown great results in coral growth and survival rates in many different regions. Mid-water nurseries are used as a staging area for rehabilitation of damaged or propagated corals before they are placed back out onto artificial or natural substrates [2]. By floating in mid-water, the nurseries can be placed in areas with high water quality (such as in the open ocean), but still maintain the same ambient light levels which the corals and their zooxanthellae are adapted to. Generally, corals growing on mid-water nurseries have very high survival rates and tend to grow faster than similar colonies on the natural reef due to decreased stress caused by sedimentation, eutrophication, predation, or pollution. Typically, coral nurseries require significant maintenance compared to some of the simpler types of structural reef restoration. In many areas though, focusing on only one type of restoration will not lead to success, and other more biological restoration methods should also be explored.

Biological Reef Restoration:

Biological restoration involves increasing the amount of living corals on the reef in areas where structure may already be available. This is generally achieved by implementing methods such as propagating coral colonies, collecting and rehabilitating naturally broken coral fragments, culturing coral larvae, or transplanting living coral colonies. These methods aim to regrow corals in areas where populations have been diminished or lost. Restoration of this kind is commonly required in areas which have been impacted by bleaching, disease, sedimentation, predation (COTs and Drupella snails) and algae overgrowth [3]. Proceeding these types of mass mortality events, small coral colonies can be used to ‘seed’ the reef, eventually growing large enough to become sexually reproductive and return the balance on the reef.

Biological restoration can also be used to assist in the development of artificial reefs by transplanting corals to areas where threats are reduced. To be successful, the restoration areas must still have ample light, temperature, and water quality to be favourable for coral growth. In areas where the threats to the corals are still present, or the ecosystem has completely collapsed, biological restoration may not be effective and could even have negative consequences for surrounding reefs. In these areas restoration techniques that address the physical conditions of coral growth must be employed.

Nurtured.Co Reef Restoration:

Through analysis of the strengths and limitations of the aforementioned reef restoration strategies, Nurtured.Co identified the need for conservation practices to diversify across each of the methods to best support coral reef restoration. Nurtured.Co’s technology leverages low-cost materials to provide structural restoration through its unlimited design potential for artificial reefs while simultaneously attaching corals to increase biological restoration. However, the technologies greatest strength lies within its ability to physically alter its own mirco-environmental conditions through the use of mineral accruement technology [1]. This advanced technology allows for increased settlement of coral larvae onto the structures which greatly enhances the biodiversity on each structure. When fully established, Nurtured.Co artificial reefs can act as a ‘seedbank’ and help replenish local surrounding coral colonies through an influx of healthy, biodiverse and resilient corals on each structure.

References

[1] T. Goreau, Marine Electrolysis for Building Materials and Environmental Restoration, In-Tech Publishing, 2012.

[2] I. Baums, "A Restoration Genetics Guide for Coral Reef Conservation," Molecular Biology, vol. 17, pp. 2798-811, 2008.

[3] Boström-Einarsson, L., Babcock, R. C., Bayraktarov, E., Ceccarelli, D., Cook, N., Ferse, S. C., ... & McLeod, I. M. (2020). Coral restoration–A systematic review of current methods, successes, failures and future directions. PloS one, 15(1), e0226631.

[4] Hughes TP, Kerry JT, Álvarez-Noriega M, Álvarez-Romero JG, Anderson KD, Baird AH, et al. Global warming and recurrent mass bleaching of corals. Nature. 2017;543: 373–377. 10.1038/nature21707 [PubMed] [CrossRef] [Google Scholar]