Coral reefs are complex and highly diverse ecosystems whose health is affected by both water and substratum quality. Watershed characteristics and estuarine state can greatly influence coastal coral reef community structure and function. Mangrove forests, wetlands and tidal flats can buffer the impacts of land-based activities on coral reefs, and hence their ecological role is critical in reef protection. Human activities within watersheds that increase erosion, sedimentation, runoff, nutrient input, and coastal pollution are among the greatest threats to coastal coral reefs.
Reef-building corals get most of their needed daily energy through photosynthetic products produced by single-celled, symbiotic algae living within their tissues, called zooxanthellae. As such, corals are truly solar powered, and anything that clouds the water column or coats their surfaces, such as sediments, negatively affects the colony. Additionally, many of the critical interactions on a coral reef are controlled by chemical signals between individual organisms, and between the organisms and their environment. For example, many corals only reproduce during limited periods each year, and the success of these spawning events depends on clean water. A variety of pollutants, including pesticides, oil and gasoline residues, heavy metals, sediment, and even fresh water from runoff, can interfere with egg and sperm production, fertilization, and larval development. Even if healthy larvae are produced in relatively pristine coastal areas, developing larvae drifting past an estuary can be killed or damaged by passing through areas of reduced water quality. Finally, coral larvae preparing to settle attach themselves to the reef and grow into adult colonies, are unable to recruit in areas of high sedimentation or sediment buildup. Without good water and substratum quality, both coral reproduction and larval recruitment fail, and coastal reefs go into decline.
There has been a great deal of interest in coral reef restoration within and adjacent to estuaries. Millions of dollars have been spent on such efforts, with limited positive results. The types of activities undertaken range from more passive rehabilitation efforts that focus on pollution and erosion control within adjacent water catchments, to the more interventional transplantation of corals from a healthy reef to one that has suffered damage. A number of restoration efforts have failed, with damage from collection of transplanted stock occurring at the source area, as well as death of all of the transplants when the underlying causes of coral stress and mortality were not fully addressed, or corals acclimated to one type of habitat or depth were transplanted to an area with different characteristics of light and water motion.
Restoration activities off Florida had a degree of success, where mitigation efforts consisted of stabilizing damaged corals and rubble, 'rebuilding' topographic relief by moving large coral heads and dislodged reef material into areas scraped clean by a grounded vessel, and transplanting both hard and soft corals to the site. In one study, a total of 11 scleractinian corals representing 8 species, as well as 30 soft coral colonies representing 12 species were transplanted. The hard coral transplants initially did well, but storm damage later caused a 50% loss of soft corals. There was recruitment of coral larvae but the coral cover remained low after 5 years. In a transplantation effort off Maui, Hawaii, 100% mortality of corals occurred over a 6 year period due to shifting sands and sedimentation. Cultivated corals and larvae are presently being produced for transplantation efforts (as well as for the aquarium trade) with the benefit of protecting natural stocks that would normally be harvested as seed material. Using cultivated material protects wild stock and also allows for the determination of whether or not the conditions have been improved enough to allow recovery to continue.
Transplantation of coral colonies or fragments, and seeding with cultivated larvae, are cosmetic solutions that may be of limited practical use over very small areas, perhaps of high economic value to diving tourism. However, genetic diversity in such instances will be low, setting the stage for future losses from environmental variability. And just as a giant redwood tree cannot be replaced by a sapling, a 200-year-old coral, killed as a result of poor land-use practices, cannot be replaced by a few fragments.
The death of corals within or adjacent to a degraded estuary is general proof that the area cannot sustain such populations, and transplanting additional colonies to replace those that were lost, without first addressing the causes of mortality, will only result in further losses. Freshwater and terrestrial sediments are carriers for a number of pollutants that are water soluble or can 'stick' to sediment particles. Following efforts at integrated watershed management, erosion control, and runoff prevention, limited transplantation efforts can be used to test if conditions have improved. The most effective efforts at coral reef restoration to date have been those that have focused on restoring those conditions that allow natural recovery to occur.
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