Physical restoration addresses the conditions in which the corals are growing to improve their health, growth rates, or fecundity (reproductive ability). For the most part, physical restoration of the reef is accomplished through passive measures in the ocean and proactive measures on land to control sources of pollution and stress.
However, there are some proactive measures that reef managers can take in the sea to address the physical factors of the ecosystem. These methods have generally been developed more recently, and some are still in the experimental stages.
Methods include mid-water coral nurseries, air curtains, shade cloths, algal removal, or mineral accretion devices such as Coral Aid and Biorock technologies.
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. 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 that the corals and their zooxanthellae are adapted to. Generally, corals growing in 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.
In some extreme cases, like the outbreak of Stony Coral Tissue Loss Disease in Florida and the Caribbean, it may be best to remove the corals from the ecosystem altogether. Land-based holding facilities can provide living gene banks, temporarily house corals during disturbance events, or be used for larval propagation. However, it should be noted that these systems take a high level of skill and funding and have a high risk of failure.
Air curtains and shade cloths have been trialed in areas such as the Great Barrier Reef to reduce the mortality of corals during bleaching events. While temperature is the main driver of coral bleaching, light also plays an important factor in contributing to bleaching events. Furthermore, bleached corals are easily stressed by UV and high levels or PAR, and, when bleached, lack the ability to produce mucous and fluorescent proteins to protect themselves. Both Air curtains and shade cloths act as temporary measures to reduce overall mortality.
Mineral accretion devices are an advanced reef restoration technology that uses low-voltage electrical energy to change the water chemistry directly around the structure. The process of electrolysis utilized by the technology increases seawater pH and causes carbonate salts to precipitate out of the water (CaCO3, MgCo3, etc.). In these conditions, corals can devote less energy to forming skeletal structures 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. This means that corals can thrive where temperature, water quality, or light levels would otherwise be outside the range for that specific coral’s survival.
Algal removal has been shown on small scales to improve the conditions for larval settlement by corals and other sessile reef organisms. Furthermore, in areas with low water turnover rates, algal removal can reduce nutrient levels in water without high grazer populations.
We also mention monitoring for resilience here, which means watching corals during disturbance events and tracking their health over time. This allows reef managers to identify the most resilient genotypes to utilize in later projects, such as larval culturing.
Generally, physical restoration is expensive and requires a high degree of regular maintenance compared with basic structural or biological restoration methods.
In many areas, though, focusing on only one type of restoration will not lead to success, and all the various types of restoration must be implemented together proportionally as needed. This is most effectively done through a strategy we call the three-part coral restoration system. This coral gardening system involves collecting a ‘feed stock’ of corals, rearing mature colonies, and transplanting them to natural or artificial structures.