About coral reefs and current approaches to their restoration
Coral reefs are one of Earth's most biologically diverse ecosystems. Globally, they provide 60 billion dollars in ecosystem goods and services such as fisheries, coastal protection, tourism, and biodiversity, and the livelihoods of around half a billion people depend on them.
We need to preserve the environmental integrity of marine habitats, of which, coral reefs are one of the most structurally diverse habitats in Earth's oceans. Yet it is their three-dimensional complexity that also renders coral reefs vulnerable to damage by natural and man-made causes. In the lightless waters of the aphotic deep-sea zone, reefs formed by "cold-water" corals around the globe have sustained particularly chronic and devastating effects of bottom fishing (Roberts et al. 2006). This damage went largely unnoticed until the deployment of manned and unmanned underwater vehicles equipped with cameras became more routine. We now understand that vast tracts of cold-water coral reefs have been damaged around the globe, with trawlers leaving behind large swathes of broken, dispersed coral fragments. The recovery potential of a damaged cold-water coral reef is unknown, but is expected to take decades to centuries (Edwards, 2010).
Land development, destructive fishing methods and tourism cause physical damage to coral reefs both in the deep sea and in the shallow warm tropical regions we are more familiar with. Damaged reefs are not only dysfunctional, but their constituent corals become more susceptible to stressors such as disease, sedimentation, pollution and changing ocean dynamics such as warming and acidification.
However some coral species are pre-adapted for reef re-growth, able to survive as fragments and regenerate their colony form over time. Thus, the current method of reef restoration uses volunteer SCUBA divers to transplant loose fragments back onto the larger reef framework. This has very limited success due to SCUBA limitations on humans that restrict time spent underwater. These limitations also prohibit human intervention with cold-water coral reef restoration on continental shelves, slopes, seamounts and ridges in the deep sea (>200m depth) where bottom trawling has profoundly degraded the 3D complexity and thus the structure and functioning of these ecosystems. Yet the ability of some coral species to regenerate from damage and fragmentation is an evolutionary strategy that we can exploit to restore and conserve these valuable ecosystems, provided we can overcome the aforementioned challenges.
A scuba diver transplanting a tray of corals Habitat restoration of coral reefs provides a viable solution to mitigate such damage (Rinkevich, 2005; Hill et al,2007; Garrison & Ward, 2008; Forrester et al, 2011). Manual remediation of shallow tropical reefs is achieved by using human scuba divers to transplant healthy pieces of coral onto damaged areas of reef. The transplants can be derived from intact coral reefs or by rearing coral fragments in coral "nurseries" such as in laboratory aquaria or in situ on a sheltered area of reef. This method of "coral gardening" can be highly effective in areas that are actively managed and protected, or not subjected to significant levels of local anthropogenic activities (Edwards 2010). Until recently, the bathymetric remoteness of cold-water coral reefs had prevented coral gardening in deep sea habitats. However emerging cold-water coral rearing techniques (Str?mberg et al. 2010) alongside the now routine use of remotely operated vehicles to manipulate and sample these habitats (Roberts et al. 2009) now makes this conservation tool a possibility for deep-sea coral reefs. Aquaria-reared fragments of the cold-water coral Lophelia pertusa (Scleractinia) attached to plastic trays were transplanted in the Kosterfjord, Sweden at about 82m water depth in 2008/9. These corals continue to grow today, and offer a real solution to restoring these ecosystems at ecologically relevant spatial scales.
Although ROV-based transplantation methods are successful, these deployments are limited by both time and costs. A single ROV deployment can only deploy one tray at a time to the seafloor. Each ROV dive is also dependent on a ship-based pilot that must fly and control the actions executed by the ROV. The ROV also cannot interact with its environment to adjust its co-ordinate headings or (thrust) without the input of the pilot. The result is reef rehabilitation on a highly localised patchy scale that may not fully restore the ecosystem function of cold-water coral reefs.
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