Competition In the
Caribbean Sea and tropical
Pacific Ocean, direct contact between coral and common
seaweeds causes
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 seaweed or their extracts. The seaweed then expanded to occupy the dead coral's habitat. However, as of 2009, only 4% of coral reefs worldwide had more than 50% algal coverage which means that there are no recent global trend towards algal dominance over coral reefs. Competitive seaweed and other
algae thrive in nutrient-rich waters in the absence of sufficient
herbivorous predators. Herbivores include fish such as
parrotfish, the urchin
Diadema antillarum,
surgeonfishes, tangs and unicornfishes. The crown-of-thorns starfish is a large (up to one meter) 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 over fished. As a result, crown-of-thorns starfish populations can periodically grow unchecked, devastating reefs. File:Charonia tritonis a1.jpg|The overfished
giant triton eats the
crown-of-thorns starfish. File:Crown of Thorns Starfish.jpg|The crown-of-thorns starfish eats
coral.
Fishing practices Although some
marine aquarium fish species can reproduce in aquaria (such as
Pomacentridae), most (95%) are collected from coral reefs. Intense harvesting, especially in
maritime Southeast 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. It is estimated that 4,000 or more Filipino fish collectors have used over of cyanide on Philippine reefs alone, about 150,000 kg per year.
Dynamite fishing is another destructive method for gathering fish. Sticks of dynamite,
grenades, or homemade 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. 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 with a mesh size of 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. 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. 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. though these studies showed significant flaws in methodology and did not attempt to replicate the complex environment found in coral reefs.
Nutrient pollution off the southern coast of England, though not in a coral region, shows what a bloom can look like from a satellite
remote sensing system.
Nutrient pollution, 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. 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.
Air pollution A study released in April 2013 has shown that air pollution can also stunt the growth of
coral reefs; researchers from Australia, Panama and the UK used coral records (between 1880 and 2000) from the western Caribbean to show the threat of factors such as coal-burning coal and volcanic eruptions. The researchers state that the study signifies the first time that the relationship between air pollution and coral reefs has been elucidated, while former chair of the Great Barrier Reef Marine Park Authority Ian McPhail referred to the report as "fascinating" upon the public release of its findings.
Marine debris Marine debris is defined as any persistent solid material that is manufactured or processed and directly or indirectly, intentionally or unintentionally, disposed of or abandoned into the marine environment or the Great Lakes. 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
waste tires), and even entire vessels. There are two different classes of plastics, macro and
microplastics and both types can cause damage in a number of ways. For example, macroplastics such as derelict (abandoned) fishing nets and other gear—often called "
ghost nets" can still catch fish and other marine life and kill those organisms and break or damage reefs. Large items such as abandoned fishing nets are known as macroplastics whereas microplastics are plastic fragments that are typically less than or equal to 5 mm in length and have primarily been found to cause damage to coral reefs though corals ingesting these plastic fragments.Fig. 1. Video frame sequence of capture and ingestion of a microplastic particle by a polyp of Astroides calycularis. Obtained from Savinelli et al. (2020) Some researchers have found that ingestion of microplastics harms coral, and subsequently coral reefs, because ingesting these fragments reduced coral food intake as well as coral fitness since corals waste a lot of time and energy handling the plastic particles. Unfortunately, even remote reef systems suffer the effects of marine debris, especially if it is
plastic pollution. 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. Fortunately, there are solutions to protect corals and coral reefs against the harmful effects of plastic pollution. However, since little to no research exists regarding specific medicinal ways to help corals recover from plastic exposure, the best solution is to not let plastics enter the marine environment at all. This can be accomplished through a number of ways, some of which are already being enacted. For example, there are measures to ban microplastics from products like cosmetics and toothpaste as well as measures that demand for products that contain microplastics to be labeled as such so as to reduce their consumption. Additionally, newer and better detection methods are needed for microplastics and they must be installed at waste water treatment facilities to prevent these particles from entering the marine environment and causing damage to marine life, especially coral reefs. Many people are realizing the problem of plastic pollution and other marine debris though, and have taken steps to mitigate it. For example, from 2000 to 2006,
NOAA and partners removed over 500 tons of marine debris from the reefs in the Northwestern Hawaiian Islands. In order to avoid cigarette butt litter, some
solutions have been proposed, including possibly banning
cigarette filters and implementing a deposit system for
e-cigarette pods.
Dredging Dredging operations are sometimes completed by cutting a path through a coral reef, directly destroying the reef structure and killing any organisms that live on it. Operations that directly destroy coral are often intended to deepen or otherwise enlarge
shipping channels or
canals, due to the fact that in many areas, removal of coral requires a
permit, making it more cost-effective and simple to avoid coral reefs if possible. Dredging also releases plumes of suspended sediment, which can settle on coral reefs, damaging them by starving them of food and sunlight. Continued exposure to dredging spoil has been shown to increase rates of diseases such as
white syndrome,
bleaching and sediment
necrosis among others. A study conducted in the
Montebello and
Barrow Islands showed that the number of coral colonies with signs of poor health more than doubled in
transects with high exposure to dredging sediment plumes.
Sunscreen .
Sunscreen can enter the ocean indirectly through wastewater systems when it is washed off and from swimmers and divers or directly if the sunscreen comes off people when in the ocean. Some 14,000 tons of sunscreen ends up in the ocean each year, with 4000 to 6000 tons entering reef areas annually. There is an estimate that 90% of snorkeling and diving tourism is concentrated on 10% of the world's coral reefs, meaning that popular reefs are especially vulnerable to sunscreen exposure. In addition to oxybenzone, there are other sunscreen ingredients, known as chemical UV filters, that can also be harmful to corals and coral reefs and other marine life. They are: Benzophenone-1, Benzophenone-8, OD-PABA, 4-Methylbenzylidene camphor, 3-Benzylidene camphor, nano-Titanium dioxide, nano-Zinc oxide, Octinoxate, and Octocrylene. In
Akumal, Mexico, visitors are warned not to use sunscreen and are kept out of some areas to prevent damage to the coral. In several other tourist destinations, authorities recommend the use of sunscreens prepared with the naturally occurring chemicals
titanium dioxide or
zinc oxide, or suggest the use of clothing rather than chemicals to screen the skin from the sun. In 2020,
Palau enacted a ban on sunscreen and skincare products containing 10 chemicals including oxybenzone. The US state of
Hawaii enacted a similar ban which came into effect in 2021. Care should be taken to protect both the marine environment and your skin, as sun exposure causes 90% of premature aging and could cause skin cancer, and it is possible to do both. can induce
coral bleaching, as happened during the 1998 and 2004
El Niño years, in which
sea surface temperatures rose well above normal, bleaching and killing many reefs. Bleaching may be caused by different triggers, including high sea surface temperature (SST), pollution, or other diseases. SST coupled with high irradiance (light intensity), triggers the loss of
zooxanthellae, a
symbiotic single cell algae that gives the coral its color and the coral's
dinoflagellate pigmentation, which turns the coral white when it is expelled, which can kill the coral. Zooxanthellae provide up to 90% of their hosts' energy supply. In summary, ocean warming is the primary cause of mass coral bleaching and mortality (very high confidence), which, together with ocean acidification, deteriorates the balance between coral reef construction and erosion (high confidence). Warming seawater may also welcome the emerging problem of 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 to survive. 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. Even a temperature rise of just 2 °C, currently very likely to happen in the next 50 years (by 2068), there would be a more than 99% chance that tropical corals would be eradicated. Warm-water coral reef ecosystems house one-quarter of the marine biodiversity and provide services in the form of food, income and shoreline protection to coastal communities around the world. These ecosystems are threatened by climate and non-climate drivers, especially ocean warming, MHWs, ocean acidification, SLR, tropical cyclones, fisheries/overharvesting, land-based pollution, disease spread and destructive shoreline practices. Warm-water coral reefs face near-term threats to their survival, but research on observed and projected impacts is very advanced. Anthropogenic climate change has exposed ocean and coastal ecosystems to conditions that are unprecedented over millennia (high confidence), and this has greatly impacted life in the ocean and along its coasts (very high confidence).
Ocean acidification '') is an early harbinger of ocean acidification
Ocean acidification results from increases in atmospheric
carbon dioxide. Oceans absorb around one–third of the increase. 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. and a further drop of 0.3–0.4 units is expected. This drop has made it so the amount of hydrogen ions have increased by 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. This causes the skeletons of the corals to weaken, or even not be made at all.
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". Rising carbon dioxide levels could confuse brain signaling in fish. In 2012, researchers reported on their results after studying the behavior of baby
clown and
damselfishes for several years in water with elevated levels of dissolved carbon dioxide, in line with what may exist by the end of the century. They found that the higher carbon dioxide disrupted a key brain
receptor in the fish, interfering with
neurotransmitter functions. The damaged central nervous systems affected fish behavior and diminishing their sensory capacity to a point "likely to impair their chances of survival". The fishes were less able to locate reefs by smell or "detect the warning smell of a predator fish". Nor could they hear the sounds made by other reef fish, compromising their ability to locate safe reefs and avoid dangerous ones. They also lost their usual tendencies to turn to the left or right, damaging their ability to
school with other fish. Although previous experiments found several detrimental effects on coral fish behavior from projected end-of-21st-century ocean acidification, a 2020 replication study found that "end-of-century ocean acidification levels have negligible effects on [three] important behaviors of coral reef fishes" and with "data simulations, [showed] that the large effect sizes and small within-group variances that have been reported in several previous studies are highly improbable". In 2021 it emerged that allegations of some of the previous studies being fraudulent have been raised. Furthermore,
effect sizes of studies assessing ocean acidification effects on fish behavior have declined dramatically over a decade of research on this topic, with effects appearing negligible since 2015.
Ocean deoxygenation which can lead to bleaching and mass coral die-offs There has been a severe increase in mass mortality events associated with low oxygen causing mass hypoxia with an increase in reported events noted in the 1990s, 2000s and 2010s. The rise in water temperature leads to an increase in oxygen demand and the increase for
ocean deoxygenation which causes these large coral reef
dead zones. For many
coral reefs, the response to this hypoxia is very dependent on the magnitude and duration of the deoxygenation. The symptoms can be anywhere from reduced
photosynthesis and
calcification to
bleaching. Hypoxia can have indirect effects like the abundance of algae and spread of coral diseases in the
ecosystems. While coral is unable to handle such low levels of oxygen, algae is quite tolerant. Because of this, in interaction zones between algae and coral, increased hypoxia will cause more coral death and higher spread of algae. The increase mass coral dead zones is reinforced by the spread of coral diseases. Coral diseases can spread easily when there are high concentrations of
sulfide and hypoxic conditions. Due to the loop of hypoxia and coral reef mortality, the fish and other marine life that inhabryter, and some enter a phase of metabolic and ventilatory depression. Invertebrates migrate out of their homes to the surface of
substratum or move to the tips of arborescent
coral colonies. Around 6 million people, the majority who live in developing countries, depend on
coral reef fisheries. These mass die-offs due to extreme hypoxic events can have severe impacts on reef fish populations. Coral reef ecosystems offer a variety of essential ecosystem services including shoreline protection,
nitrogen fixation, and waste assimilation, and tourism opportunities. The continued decline of oxygen in oceans on coral reefs is concerning because it takes many years (decades) to repair and regrow corals. These diseases have different effects on the corals, ranging from damaging and killing individual corals to wiping out entire reefs.
Recreational diving During the 20th century recreational scuba diving was considered to have generally low environmental impact, and was consequently one of the activities permitted in most marine protected areas. Since the 1970s diving has changed from an elite activity to a more accessible recreation, marketed to a very wide demographic. To some extent better equipment has been substituted for more rigorous training, and the reduction in perceived risk has shortened minimum training requirements by several training agencies. Training has concentrated on an acceptable risk to the diver, and paid less attention to the environment. The increase in the popularity of diving and in tourist access to sensitive ecological systems has led to the recognition that the activity can have significant environmental consequences. Scuba diving has grown in popularity during the 21st century, as is shown by the number of certifications issued worldwide, which has increased to about 23 million by 2016 at about one million per year. Scuba diving tourism is a growth industry, and it is necessary to consider
environmental sustainability, as the expanding impact of divers can adversely affect the
marine environment in several ways, and the impact also depends on the specific environment. Tropical coral reefs are more easily damaged by poor diving skills than some temperate reefs, where the environment is more robust due to rougher sea conditions and fewer fragile, slow-growing organisms. The same pleasant sea conditions that allow development of relatively delicate and highly diverse ecologies also attract the greatest number of tourists, including divers who dive infrequently, exclusively on vacation and never fully develop the skills to dive in an environmentally friendly way.
Low impact diving training has been shown to be effective in reducing diver contact to more sustainable levels.
Other issues from a beach on
Aruba,
Caribbean Sea. 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.
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. 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, and because of this, some of the areas with heavily mined coral reefs have still not been able to recover. Another pressing issue is coral collecting. There are bountiful amounts of coral that are deemed so beautiful that they are often collected. The collected coral are used to make a handful of things, including jewelry and home decorations. The breakage of coral branches is unhealthy for the reefs; therefore, tourists and those who purchase such items contribute greatly to the already devastating coral reefs and climate change. 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. Also,
reef docks can be used to move over goods from large, seagoing vessels to small, flat-bottomed vessels. 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. == Oil ==