Human impact on coral reefs is significant. Coral reefs are dying around the world.[1] In particular, coral mining, pollution (organic and non-organic), overfishing, blast fishing and the digging of canals and access into islands and bays are serious threats to these ecosystems. Coral reefs also face high dangers from pollution, diseases, destructive fishing practices and warming oceans.[2] In order to find answers for these problems, researchers study the various factors that impact reefs. The list of factors is long, including the ocean's role as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, biological virus, impacts of dust storms carrying agents to far flung reefs, pollutants, algal blooms and others. Reefs are threatened well beyond coastal areas.
General estimates show approximately 10% world's coral reefs are already dead.[3][4][5] It is estimated that about 60% of the world's reefs are at risk due to destructive, human-related activities. The threat to the health of reefs is particularly strong in Southeast Asia, where 80% of reefs are endangered.
Contents |
In the Caribbean Sea and tropical Pacific, direct contact between coral and 40–70% of common seaweeds cause bleaching and death of coral tissue via allelopathic competition. The lipid-soluble extracts of seaweeds that harmed coral tissues also produced rapid bleaching. At these sites bleaching and mortality was limited to areas of direct contact with seaweeds or their extracts. The seaweed ten expanded to occupy the dead coral's habitat.[6] However, as of 2009, only 4% of coral reefs worldwide had more than 50% algal coverage. Further, there is no recent global trend towards algal dominance.[2]
Competitive seaweed and other algae thrive in nutrient-rich waters in the absence of sufficient herbivorous predators. Herbivores include fish such as parrotfish, surgeonfish, tangs and unicornfish.[6]
Overfishing, particularly selective overfishing, can unbalance coral ecosystems by encouraging the excessive growth of coral predators. Predators that eat living coral, such as the crown-of-thorns starfish, are called corallivores. Coral reefs are built from stony coral, which evolved with large amounts of the wax cetyl palmitate in their tissues. Most predators find this wax indigestible.[7] The crown-of-thorns starfish is a large (up to one metre) starfish protected by long, venomous spines. Its enzyme system dissolves the wax in stony corals, and allows the starfish to feed on the living animal. Starfish face predators of their own, such as the giant triton sea snail. However, the giant triton is valued for its shell and has been overfished. As a result, crown-of-thorns starfish populations can periodically grow unchecked, devastating reefs.[8][9][10]
Although some aquarium fish species can reproduce in aquaria (such as Pomacentridae), most (95%) are collected from coral reefs. Intense harvesting, especially in South-East Asia (including Indonesia and the Philippines), damages the reefs. This is aggravated by destructive fishing practices, such as cyanide and blast fishing. Most (80–90%) aquarium fish from the Philippines are captured with sodium cyanide. This toxic chemical is dissolved in sea water and released into areas where fish shelter. It narcotizes the fish, which are then easily captured. However, most fish collected with cyanide die a few months later from liver damage. Moreover, many non-marketable specimens die in the process.[11] It is estimated that 4,000 or more Filipino fish collectors have used over 1,000,000 kilograms (2,200,000 lb) of cyanide on Philippine reefs alone, about 150,000 kg per year.[12] A major catalyst of cyanide fishing is poverty within fishing communities. In countries like the Philippines that regularly employ cyanide, more than thirty percent of the population lives below the poverty line.[13]
Dynamite fishing is another destructive method for gathering fish. Sticks of dynamite, grenades, or home-made explosives are detonated in the water. This method of fishing kills the fish within the main blast area, along with many unwanted reef animals. The blast also kills the corals in the area, eliminating the reef's structure, destroying habitat for the remaining fish and other animals important for reef health.[12] Muroami is the destructive practice of covering reefs with nets and dropping large stones onto the reef to produce a flight response among the fish. The stones break and kill the coral. Muroami was generally outlawed in the 1980s.[12]
Fishing gear damages reefs via direct physical contact with the reef structure and substrate. Gill nets, fish traps, and anchors break branching coral and cause coral death through entanglement. When fishermen drop lines by coral reefs, the lines entangle the coral. The fisher cuts the line and abandons it, leaving it attached to the reef. The discarded lines abrade coral polyps and upper tissue layers. Corals are able to recover from small lesions, but larger and recurrent damage complicates recovery.
Bottom dragging gear such as beach seines can damage corals by abrasion and fracturing. A beach seine is a long net about 150 meters (490 ft) with a mesh size of 3 centimeters (1.2 in) and a weighted line to hold the net down while it is dragged across the substrate and is one of the most destructive types of fishing gear on Kenya’s reefs.[12]
Bottom trawling in deep oceans destroys cold–water and deep–sea corals. Historically, industrial fishers avoided coral because their nets would get caught on the reefs. In the 1980s, "rock–hopper" trawls attached large tires and rollers to allow the nets to roll over rough surfaces. Fifty-five percent of Alaskan cold–water coral that was damaged by one pass from a bottom trawl had not recovered a year later. Northeast Atlantic reefs bear scars up to 4 kilometers (2.5 mi) long. In Southern Australia, 90 percent of the surfaces on coral seamounts are now bare rock. Even in the Great Barrier Reef World Heritage Area, seafloor trawling for prawns and scallops is causing localized extinction of some coral species.[12]
"With increased human population and improved storage and transport systems, the scale of human impacts on reefs has grown exponentially. For example, markets for fish and other natural resources have become global, supplying demand for reef resources."[14]
Reefs in close proximity to human populations are subject to poor water quality from land- and marine-based sources. In the Caribbean, approximately 80 percent of ocean pollution originates from activities on land. Pollution arrives from land via runoff, the wind and "injection" (deliberate introduction, e.g., drainpipes). Runoff brings with it sediment from erosion and land-clearing, nutrients and pesticides from agriculture, wastewater, industrial effluent and miscellaneous material such as petroleum residue and trash that storms wash away. Some pollutants consume oxygen and lead to eutrophication, killing coral and other reef inhabitants.[15]
An increasing fraction of the global population lives in coastal areas. Without appropriate precautions, development (e.g., buildings and paved roads) increases the fraction of rainfall and other water sources that enter the ocean as runoff by decreasing the land's ability to absorb it.[15]
Pollution can introduce pathogens. For example, Aspergillus sydowii has been associated with a disease in sea fans, and Serratia marcescens, has been linked to the coral disease white pox.[15]
Reefs in close proximity to human populations can be faced with local stresses, including poor water quality from land-based sources of pollution.[15] Copper, a common industrial pollutant has been shown to interfere with the life history and development of coral polyps.[16]
In addition to runoff, wind blows material into the ocean. This material may be local or from other regions. For example, dust from the Sahara moves to the Caribbean and Florida. Dust also blows from the Gobi and Taklamakan deserts across Korea, Japan, and the Northern Pacificto the Hawaiian Islands.[17] Since 1970, dust deposits have grown due to drought periods in Africa. Dust transport to the Caribbean and Florida varies from year to year[18] with greater flux during positive phases of the North Atlantic Oscillation.[19] The USGS links dust events to reduced health of coral reefs across the Caribbean and Florida, primarily since the 1970s.[20] Dust from the 1883 eruption of Krakatoa in Indonesia appeared in the annular bands of the reef-building coral Montastraea annularis from the Florida Reeftract.[21]
Sediment smothers corals and interferes with their ability to feed and reproduce. Pesticides can interfere with coral reproduction and growth.[15]
Excess nutrients, particularly nitrogen and phosphorus can cause eutrophication, upsetting the balance of the reef by enhancing algal growth and crowding out corals.This nutrient–rich water can enable blooms of fleshy algae and phytoplankton to thrive off coasts. These blooms can create hypoxic conditions by using all available oxygen. Biologically available nitrogen (nitrate plus ammonia) needs to be below 1.0 micromole per liter (less than 0.014 parts per million of nitrogen), and biologically available phosphorus (orthophosphate plus dissolved organic phosphorus) needs to be below 0.1 micromole per liter (less than 0.003 parts per million of phosphorus). In addition concentrations of chlorophyll (in the microscopic plants called phytoplankton) needs to be below 0.5 parts per billion.[22] Both plants also obscure sunlight, killing both fish and coral. High nitrate levels are specifically toxic to corals, while phosphates slow down skeletal growth.
Excess nutrients can intensify existing disease, including potentially doubling the spread of Aspergillosis, a fungal infection that kills soft corals such as sea fans, and increasing yellow band disease, a bacterial infection that kills reef-building hard corals by fifty percent.[23]
Marine debris is any solid object that enters coastal and ocean waters. Debris may arrive directly from a ship or indirectly when washed out to sea via rivers, streams, and storm drains. Human-made items tend to be the most harmful such as plastics (from bags to balloons, hard hats to fishing line), glass, metal, rubber (millions of tires!), and even entire vessels.[15]
Plastic debris kills several reef species. Derelict (abandoned) fishing nets and other gear—often called "ghost nets" because they still catch fish and other marine life despite being abandoned—can entangle and kill reef organisms and break or damage reefs. Even remote reef systems suffer the effects of marine debris. Reefs in the Northwestern Hawaiian Islands are particularly prone to the accumulation of marine debris because of their central location in the North Pacific Gyre. From 2000 to 2006, NOAA and partners removed over 500 tons of marine debris there.[15]
Rising sea levels due to climate change requires coral to grow to stay close enough to the surface to continue photosynthesis. Also, water temperature changes can induce coral bleaching as happened during the 1998 and 2004 El Niño years, in which sea surface temperatures rose well above normal, bleaching or killing many reefs. High sea surface temperature (SST) coupled with high irradiance (light intensity), triggers the loss of zooxanthellae, a symbiotic algae, and its dinoflagellate pigmentation in corals, turning coral white. Zooxanthellae provide up to 90% of their hosts' energy supply. Healthy reefs can often recover from bleaching if water temperatures cool. However, recovery may not be possible if CO2 levels rise to 500 ppm because concentrations of carbonate ions may then be too low.[24][25]
Warming seawater may also encourage an emerging problem: coral disease. Weakened by warm water, coral is much more prone to diseases including black band disease, white band disease and skeletal eroding band. If global temperatures increase by 2 °C during the twenty-first century, coral may not be able to adapt quickly enough.[26]
A 2010 report by the Institute of Physics predicts that unless the national targets set by the Copenhagen Accord are amended to eliminate loopholes, then by 2100 global temperatures could rise by 4.2°C and result in an end to coral reefs.[27][28]
Ocean acidification results from increases in atmospheric carbon dioxide. Oceans absorb around one–third of the increase.[3] The dissolved gas reacts with the water to form carbonic acid, and thus acidifies the ocean. This decreasing pH is another issue for coral reefs.[3]
Ocean surface pH is estimated to have decreased from about 8.25 to 8.14 since the beginning of the industrial era,[29] and a further drop of 0.3–0.4 units is expected.[30] Before the industrial age the conditions for calcium carbonate production were typically stable in surface waters since the carbonate ion is at supersaturated concentrations. However, as the ionic concentration falls, carbonate becomes under-saturated, making calcium carbonate structures vulnerable to dissolution. Corals experience reduced calcification or enhanced dissolution when exposed to elevated CO2.[31]
Bamboo coral is a deep water coral which produces growth rings similar to trees. The growth rings illustrate growth rate changes as deep sea conditions change, including changes due to ocean acidification. Specimens as old as 4,000 years have given scientists "4,000 years worth of information about what has been going on in the deep ocean interior".[32]
Within the last 20 years, once prolific seagrass meadows and mangrove forests, which absorb massive amounts of nutrients and sediment, have been destroyed. Both the loss of wetlands, mangrove habitats and seagrass meadows affect the water quality of inshore reefs.[33]
Coral mining is another threat. Both small scale harvesting by villagers and industrial scale mining by companies are serious threats. Mining is usually done to produce construction material which is valued as much as 50% cheaper than other rocks, such as from quarries.[34] The rocks are ground and mixed with other materials, like cement to make concrete. Ancient coral used for construction is known as coral rag. Building directly on the reef also takes its toll, altering water circulation and the tides which bring the nutrients to the reef. The pressing reason for building on reefs is simply lack of space.
Boats and ships require access points into bays and islands to load and unload cargo and people. For this, parts of reefs are often chopped away to clear a path. Negative consequences can include altered water circulation and altered tidal patterns which can disrupt the reef's nutrient supply; sometimes destroying a great part of the reef. Fishing vessels and other large boats occasionally run aground on a reef. Two types of damage can result. Collision damage occurs when a coral reef is crushed and split by a vessel's hull into multiple fragments. Scarring occurs when boat propellers tear off the live coral and expose the skeleton. The physical damage can be noticed as striations. Mooring causes damage which can be reduced by using mooring buoys. Buoys can attach to the seafloor using concrete blocks as weights or by penetrating the seafloor, which further reduces damage.[36]
Coral in Taiwan is being threatened by the influx of human population growth. Since 2007, several local environmental groups conducted research and found that much of the coral populations are being affected by untreated sewage, an influx of tourists taking corals for souvenirs, without fully understanding the destructive impact on the coral's ecological system. Researchers reported to the Taiwanese government that many coral populations have turned black in the southeast coast of Taiwan. Potentially, this could lead to loss of food supply, medicinal sources and tourism due to the breakdown of the food chain.[37]
The global standard for recording threatened marine species is the IUCN Red List of Threatened Species.[38] This list is the foundation for marine conservation priorities worldwide. A species is listed in the threatened category if it is considered to be critically endangered, endangered, or vulnerable. Other categories are near threatened and data deficient. By 2008, the IUCN had assessed all 845 known reef-building corals species, marking 27% as Threatened 20% as near threatened and 17% as data deficient.[39]
The coral triangle (Indo-Malay-Philippine archipelago) region has the highest number of reef-building coral species in threatened category as well as the highest coral species diversity. The loss of coral reef ecosystems will have devastating effects on many marine species, as well as on people that depend on reef resources for their livelihoods.[39]
The Great Barrier Reef is the world's largest coral reef system.[40][41][42][43] The reef is located in the Coral Sea. A large part of the reef is protected by the Great Barrier Reef Marine Park. Particular environmental pressures include runoff, salinity fluctuations, climate change, cyclic crown-of-thorns outbreaks, overfishing, and spills or improper ballast discharge.
Southeast Asian coral reefs are at risk from damaging fishing practices (such as cyanide and blast fishing), overfishing, sedimentation, pollution and bleaching. Activities including education, regulation and the establishment of marine protected areas help protect these reefs.
Indonesia is home to one third of the world's corals covering nearly 85,000 square kilometres (33,000 sq mi) and one quarter of its fish species. Indonesia's coral reefs are located in the heart of the Coral Triangle and have fallen victim to destructive fishing, tourism and bleaching. Data from 414 reef monitoring stations in 2000 found that only 6% are in excellent condition, while 24% are in good condition, and approximately 70% are in poor to fair condition.[44]
In 2007, Reef Check, the world's largest reef conservation organization, stated that only 5% of Philippines 27,000 square kilometres (10,000 sq mi) of coral reef are in "excellent condition": Tubbataha Reef, Marine Park in Palawan, Apo Island in Negros Oriental, Apo Reef in Puerto Galera, Mindoro, and Verde Island Passage off Batangas. Philippine coral reefs is Asia's second largest.[45]
Coral reefs in Taiwan are being threatened by human population growth. Many corals are affected by untreated sewage and souvenir-hunting tourists, not knowing that this practice destroys habitat and causes disease. Many corals have turned black from disease off Taiwan's southeast coast.[46]
|
||||||||||||||||||||||
|
|||||||||||||||||||||||||