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Natural and Anthropogenic Stressors on Palauís Coral Reefs

Shallow-water coral reef ecosystems are impacted by a wide range of physical, chemical, and biological threats and stressors that stem from both anthropogenic (caused by human activities) and natural causes. Threats are defined as environmental trends with potentially negative impacts. Stressors may be defined as factors or processes that harm ecosystem components, causing lethal or semi-lethal effects. Categories of stressors include chemical (e.g., pollution), physical (e.g., storm or boat damage), and biological (e.g., invasive species).

Human activities are major contributors to the worldwide deterioration and degradation of coral ecosystems, with loss of live coral cover, declining biodiversity, and reduced abundances in species populations. Certain commercial fishing methods, such as bottom trawling, can cause substantial damage to deep water coral ecosystems. Degradation in the structure and function of coral ecosystems also results in a simultaneous loss in intrinsic value of the ecological system. Approximately 8% of the global population lives within 62 miles (100 km) of a coral reef, and many local communities and national economies are directly dependent on coral reef ecosystems for tourism revenue, food, and coastal protection.

The most common and recognized stressors on coral ecosystems worldwide are: climate change and coral bleaching; diseases; ocean acidification; tropical cyclonic storms; coastal development and runoff (sedimentation); coastal pollution; tourism and recreation; fishing pressure; trade in coral and live reef species; ships and boat groundings; marine debris; aquatic invasive species; security training and military base activities; and offshore oil and gas exploration.

Climate Change and Coral Bleaching

Small patch of bleached coral
Small patch of bleached coral at a site monitored by the Palau International Coral Reef Center. (Photo: Jane Thomas, IAN Image Library)

Climate change over time can be driven by natural variability and human activities, working individually and in concert. In the 20th century, mean near-surface air temperature over land and mean sea surface temperature (SST) increased 0.6 ± 0.2°C, with the 1990s being the warmest decade in recorded history.  Elevated water temperatures cause corals to bleach, a process that is characterized by the loss of photosynthetic zooxanthellae (symbiotic algae) from coral tissues. If prolonged, corals may starve to death. Mass bleaching typically occurs when sea temperatures increase 1 - 2 degree above local seasonal maxima, although many colonies will bleach at lower temperatures. Although some corals may recover from brief episodes of bleaching, if ocean temperatures warm too much or remain high for an extended period, bleached corals often will die.

Prior to the 1997-1998 El Nino and subsequent coral bleaching, Palauan coral communities were in excellent condition with no signs of stress. In 1999, however, surveys found that bleaching was widespread and variable throughout Palau. Bleaching devastated Acropora spp., which had the highest mortality compared to other stony corals.  In fact, mortality of Acropora spp. was virtually 100% at all depths at some locations. Corals in estuaries closer to shore survived better than corals farther from shore.  Off-shore reefs saw mass mortalities of faviids, poritids, Fungia spp. and Acropora spp.

Since the 1997-1998 El Niño, Palau has not experienced any major bleaching events other than some localized bleaching in different parts of the archipelago, which may be related to human impacts. However, coral reefs throughout the archipelago are still displaying signs of stress and bleaching, which demonstrates the destructive effects that one major bleaching event can have on the recovery and resiliency of these marine ecosystems.

Coral Diseases

The increasing global prevalence of coral diseases has become one of the major stressors affecting the health and resiliency of coral reef communities. Since the early 1990s, scientists have documented a rapid emergence of coral diseases with increases in the number of diseases reported, coral species affected, geographic extent, prevalence and incidence, and rates of associated coral mortality. Diseases have played a prominent role in regulating coral population size, diversity, and demographic characteristics. Very little is known about coral diseases in terms of causative agents. Some diseases have already had widespread catastrophic effects. In the Caribbean, for example, two once-dominant Acropora spp., the staghorn and elkhorn corals, are now officially listed as endangered.

Little is known about coral diseases in the Indo-Pacific. Baseline information for coral diseases in Palau, first obtained in 2004, suggest that diseases are spreading in Palau. At individual study sites, initial surveys identified between five and nine diseases and syndromes, including black band disease, brown band disease, skeletal eroding band, white syndrome, patchy necrosis, yellow spot disease, other cyanobacteria-causing diseases, and various tumors.

Tropical storms

Cyclonic storms are an important process in the structure and dynamics of coral reef ecosystems. They constitute a powerful mechanism for change and can dramatically disrupt the ecosystem, associated communities, resource availability, and the physical environment. However, coral reefs have shown resilience to storm-caused disturbances. Mechanical disturbances from tropical storms can benefit a reef community by helping to maintain high species diversity, particularly when the reef substratum is scoured by a storm, which creates space for new coral recruits to settle. Massive and robust boulder-shaped corals (e.g., brain corals) are more capable of withstanding powerful wave action than those with more fragile branching structures, such as the Acropora spp.

Tropical storms are quite common in Palau. Corals on the outer reef slope are much more sus­ceptible to physical damage and removal from storm surge and large waves. Therefore, many of Palau’s forereefs are dominated by encrusting and massive forms of coral species. Data quantifying the effects of storms on Palau’s coral reefs are currently not available.

Coastal Development and Runoff

Mining coral rock for construction materials.
Mining coral rock for construction materials.

Increased sedimentation associated with runoff from coastal development is a growing threat to coral ecosystems worldwide. In the past few decades, there has been a shift toward greater concentrations of human settlement in the coastal zones of many countries. This trend has increased stress on coral ecosystems, especially in the tropics and subtropics. Runoff from landscape altering activities, including the construction of residential developments, hotels, resorts, golf courses, marinas and other recreational facilities, piers, roads, bridges, and waste or water treatment plants have taken a tremendous toll on some close-to-shore-reef areas. Runoff can carry high levels of nutrients, microorganisms, and pollutants, such as petroleum products or pesticides, which can cause diseases in coral colonies or outright kill them. Increased nutrients and/or pollutants over a reef can enhance growth of certain reef organisms, such as sponges, which can outcompete corals for space. Additionally, outflows from water treatment, waste, and other industrial plants, which can discharge extremely hot water into cooler surrounding coastal waters, may also greatly increase the local water temperature and induce bleaching or death of coral colonies.

Sediment runoff from cleared areas can also settle on coral reefs and smother them, resulting in the death of dominant hard coral communities and replacement of these communities with macroalgae. In addition, spikes in sedimentation can cause increased water turbidity. This in turn reduces the amount of sunlight reaching corals and greatly diminishes photosynthetic activity of a coral’s zooxanthellae, resulting in bleaching and starvation of the coral.

Forest cleared for the construction of the Compact Road in Babeldaob
Forest cleared for the construction of the Compact Road in Babeldaob (Photo: P.L. Collin/CRRF)

Recent demographic shifts and changes in permitting, construction, road building, sedimenta­tion, and land use have resulted in increased coastal development in Palau. This has, in-turn, greatly augmented the amount of runoff from the land into the surrounding marine environment. Much of the increased coastal development is a direct result of the con­struction of a paved, two-lane, 53-mile (85 km) highway ("Compact Road") around Palau’s main island, Babeldaob, in 2007. The Compact Road and its associated bridge connect Palau’s former national capitol in Koror with its new capitol, Melekeok in Babeldaob. Moving the central govern­ment from Koror to Babeldaob led to increased mining of corals for use as construction materials, escalated development in and around Babeldaob, and subsequent runoff from the island into local waterways and the marine environment. Additionally, access roads constructed out from the paved Compact Road were unpaved, which led to an upsurge in the amount of sediment washing into surrounding waters. It is expected that as communities, new airports, and resorts spring up around Babeldaob, farming and eventually commercial agricultural production will also increase, which could lead to intensified sedimentation, nutrient runoff, and pollution coming from the island.

Coastal Pollution

It has been estimated that as much as 22% of the world’s coral ecosystems are currently threatened by land-based pollution and soil erosion. Pollution affects a coral ecosystem by introducing microorganisms, excess nutrients or sediments, and toxic chemicals into the environment. These can cause either diseases and/or mortality in corals or they can depress the immunity, reproduction, or functionality of the corals, making it susceptible to other stressors such as climate change, disease, and invasive species.

The primary stressors from land-based sources of pollution are documented to be nutrients; chemicals from fertilizers, pesticides, herbicides, and sewage; and increased sedimentation from coastal development and stormwater runoff. Other noxious substances such as heavy metals and petroleum products also add to the pollution load at specific locations.

Sources of pollution may be either point source or non-point source. Point source pollution originates from specific conveyances, such as pipes, tunnels, ditches, channels, wells, and fissures.  Examples include sewage outfalls; untreated wastewater from factories and other industrial plants; heated water discharge from power plants; and vessels that discharge wastes in marinas and nearshore areas or accidentally spill petroleum or other noxious chemicals and heavy metals into the water. Non-point source pollution refers to pollutants not introduced from a single, well-defined site. Non-point sources include acid rain; excess fertilizers, herbicides, and insecticides from agricultural lands and residential areas; oil and toxic chemicals from urban runoff and energy production; sediment from improperly managed construction sites, agricultural lands, and eroding stream banks; salt from irrigation practices; and bacteria and nutrients from livestock, pet wastes, and faulty septic systems.

Major industries in coastal areas of Palau that contribute point source pollution include fishing companies and hotels. Fishing companies discharge brine, oil trash, and sewage from ships moored at their docks. Over time, the cumulated effects of small oil spills and other pollutants from fueling stations may damage nearby coral ecosystems. There are also challenges related to landfill operations, which increase the concentration of nitrates and phosphates in runoff, and a deteriorating public sewer system, in which frequent sewage overflows adversely affects water quality by contaminating the area with bacteria. Improper farming methods and unplanned road construction are also common sources of coastal pollution.

Tourism and Recreation

Healthy coral reefs support thriving tourism and recreation industries. Each year, millions of scuba divers, snorkelers, boaters, shell collectors, and fishermen visit U.S. coral reefs and nearby beaches. Local economies receive billions of dollars from visitors through diving tours, recreational fishing trips, hotels, restaurants, and other businesses based near reef ecosystems. Tourism is particularly significant in many Caribbean and Pacific islands. In the Florida Keys alone, over four million tourists purchase about $1.2 billion in services annually. Despite their great economic and recreational value, coral reefs are severely threatened by pollution, disease, and habitat destruction. Once coral reefs are damaged, they are less able to support a rich biodiversity, losing value as a tourist destination.

A live aboard scuba diving vessel in Palau
A live aboard scuba diving vessel in Palau

Studies have shown that divers and snorkelers can have a significant impact on coral reefs in terms of physical damage. For example, divers and snorkelers have caused significant damage to corals along underwater “sight-seeing” trails in the U.S. Virgin Islands and other parts of the world by touching the corals, removing chunks of the reef as “souvenirs”, and standing on the reefs.  Other tourism-related threats that have devastated coral reefs include hotel and resort development, construction of infrastructure to support such resorts, seafood consumption, beach replenishment activities, airport and marina construction, and cruise ship operations. The negative impacts resulting from these activities include increased sedimentation, nutrient enrichment, pollution, exploitation of endangered species, and increased litter and waste.

Palau is building a self-sustaining national economy consisting primarily of subsistence agriculture, fishing for local consumption and export, construction, and tourism.  In fact, the United Nations' 2007 Statistical Yearbook for Asia and the Pacific has placed Palau as the highest tourism earning country in the Asia-Pacific region in terms of contribution to Gross Domestic Product. Currently, the main industry in Palau is tourism. The majority of tourists come to dive, snorkel, and visit the famous Rock Islands. However, a number of people also come to experience Palau’s waterfalls, forests, and historical sites. From 1992-1997, the number of visitors to the archipelago doubled from nearly 30,000 to 60,000. In 1997, however, due to several factors, including a massive El Niño bleaching event, tourism began to decline.  As of 2001, though, the number of tourists in Palau has rebounded, and the industry is again booming. For example, many communities in Babeldaob have taken advantage of the opening of “Compact Road” to develop land-based tourism activities; this has substantially enhanced local tourism. In 2007, a river boat tour business also began operating throughout the archipelago opening up areas of Palau that previously were untouched to the ever-expanding tourism industry.

With increasing numbers of visitors annually and complete dependence of the economy on tourism, the potential harm to coral reefs is a major cause for concern. Therefore, many Palauans are taking action to prevent damage to their reefs.  For example, locals have placed mooring buoys at known dive sites to minimize anchor and dive boat damage to the coral reefs. In addition, legislation has been enacted by the government to help mitigate harmful impacts at sensitive tourist sites such as Jellyfish Lake.  The government also recently enacted legislation outlawing the collection of corals from its reefs. Diving tour operator education is also helping to diminish the negative impacts to corals and reefs from tourism and recreation.

Fishing Pressure

Recreational, subsistence, and commercial fishing on many reefs and coral ecosystems throughout the world play an important social, cultural, and economic role. NOAA’s National Marine Fisheries Service (NMFS) estimates that the commercial value alone of U.S. fisheries from coral reefs is more than $100 million. However, overfishing and destructive fishing practices caused by traps, bottom trawls (which are especially damaging to deep water coral ecosystems), anchor damage, and vessel groundings are threatening to destroy the resources and reef communities that the fishing industry relies on. Overfishing depletes a fishery by catching so many adult and juvenile fishes that not enough remain to breed and replenish the population. In addition to pollution and global climate change, overfishing is now considered one of the greatest global threats to the health and survival of coral reefs.

Approximately one-half of all federally managed fisheries depend on coral reefs and adjacent habitats for a portion of their life cycles. In addition, about 25% of all marine fishes (as well as numerous other marine organisms) are inhabitants of shallow-water coral ecosystems (including coral reefs, adjacent seagrass beds, mangrove forests, and other associated hard and soft bottom habitats) for their entire lives.  It is those marine organisms that must rely on reefs and their associated shallow-water ecosystems that are at the greatest risk from exploitation and the negative impacts from overfishing and destructive fishing practices.

The trochus or topshell, Trochus niloticus, is a conically-shelled marine snail that inhabits coral reefs.
The trochus or topshell, Trochus niloticus, is a conically-shelled marine snail that inhabits coral reefs. The inner or nacreous layer of the shell produces valuable mother-of-pearl products, and the large muscular foot is a popular food.

Fishing in Palau is a multi-species industry involving subsistence, recreational, and commercial fishers that supply fish to commercial markets, restaurants, and buyers for export. In addition to coral reef fishes and pelagic tuna and mackerels, the mangrove crab (Scylla serrata) and the trochus (Trochus niloticus) are the top commercial marine animals. Palau established the 1994 Marine Protection Act to better manage local fisheries resources. Specific management tools de­scribed in the Act include: bans on export of certain species; closed harvest seasons for rabbitfishes and groupers;size limits for certain wrasse species, humphead parrotfishes, crabs, and lobsters; mesh size limits for nets; and permit requirements for aquaculture and aquarium trade operations. However, overall, there continues to be a noticeable decline in Palau’s total maximum yield from several reef areas. This decline has been attributed to several factors, including the presence of large-scale pulse fishing operations, coral bleaching, habitat loss, and sedi­mentation from land-based activities.

Trade in Coral and Live Reef Species

A poacher illegally collecting fishes by squirting a sodium cyanide solution into a crevice in the reef.
A poacher illegally collecting fishes by squirting a sodium cyanide solution into a crevice in the reef.

Many coral reef fishes and invertebrates are collected to supply a domestic and international demand for seafood. In addition, marine organisms are also removed from reefs and coral ecosystems to serve as ornamental aquarium specimens (fishes, corals, other invertebrates, and live rock); construction materials (e.g., coral blocks); jewelry; curios (e.g., mollusk shells, coral skeletons, precious corals, dried starfishes, and lamps made from shells or porcupine fish); pharmaceuticals; traditional medicines; and other products. Overharvesting of reef species at unsustainable levels leads to unhealthy changes in the population structure and biomass of reef species. In the absence of specific predators, for example, non-target species may undergo a population explosion that alters the dynamics of the entire reef system. In addition, reductions in or the disappearance of certain herbivorous species may result in the replacement of stony coral-dominated reefs by algal-dominated reefs, as algae outcompete corals for space.

Additional harm or alteration of reef communities is caused by widespread use of destructive collecting techniques, such as the illegal use of cyanide to capture live reef fish. Cyanide poisoning kills many organisms on a reef, including non-target species, and causes substantial habitat damage. Although cyanide poisoning has not been reported in the U.S., with the possible exception of limited use in some of the Freely Associated States associated with the live reef fish food trade, cyanide use to collect reef species is practiced heavily in many parts of southeastern Asia and the Indo-Pacific.

A lamp made from a sea urchin
A lamp made from a sea urchin

In the last few years, there has been a significant reduction in the volume of live fish, coral, and other marine organisms exported from Palau for the aquarium trade. This is due in part to the ratification of the Regulations on the Collection of Marine Resources for Aquaria and Research in December 2004. These regulations were developed by Palau’s Ministry of Resources and Development in response to provisions cited in the 1994 Marine Protection Act, which required the Ministry to develop regulations “regarding the taking and export of fish for aquarium purposes”. Many Palauans had expressed concerns about the negative impacts associated with an aquarium fishery in the archipelago. For example, the aquarium fishery was viewed by local fishermen as unnecessary competition from foreigners for food resources, and divers and snorkelers saw it as a destructive practice, which devalued coral reefs and destroyed reef communities needed to support the tourism industry.

Invasive species

The stability of an ecosystem may be threatened by the accidental or purposeful introduction of non-indigenous species, also called “invasive” or “alien” species. An invasive species is a species outside of its native or historical habitat. For various reasons, including the absence of natural enemies, invasive species may grow and reproduce at the expense of native species occupying the same habitat or habitat type. In some cases, the invasive organisms are successful predators which can decimate local or native populations; for example, the brown tree snake in Guam (an accidental introduction) or the mongoose in the islands of the Caribbean (a purposeful introduction). Invasives may outcompete native species occupying the same ecological niche or even worse may cause major changes in the dynamics of an ecosystem, resulting in the extinction of another species.

Invasive organisms can spread rapidly in the open ocean and are very difficult to control or eradicate. Currents may act as dispersal highways between habitats. Many invasive organisms are also carried in the ballast water of vessels or attached to their hulls. Some were aquarium pets which were released into local marine waters or escaped specimens from an aquaculture operation. Quite frequently, scientists are not aware when new invasive species arrive in a new habitat. However, some are visibly noticeable such as the Pacific lionfish (Pterois volitans) in warm Atlantic coastal waters.

The invasive hydroid species,  Eudendrium carneum.
The invasive hydroid species, Eudendrium carneum.(University of Sao Paulo)

Invasive species are one of the greatest threats to island biodiversity. In Palau, invasive species of all types threaten local flora and fauna and their associated ecosystems, which in-turn threatens Palau’s economy, human health, agriculture, and even the traditional way of life. Terrestrial invasive species, such as macaque monkeys (Macaca fascicularis), cause harm as predators on native animals or their eggs, while other invasives, including parrots and cockatoos, destroy native plant populations. Certain invasive species such as pigeons, rats, and mosquitoes carry diseases, while others are noisy pests, such as the Caribbean coqui (a tree frog) or parrots and their relatives.

In some cases, a native species can become invasive when environmental conditions change. An example of this is the vigorous and fast-growing vine known as “kebeas” (Merremia peltata).  Kebeas naturally grows in forest clearings and was not a problem prior to substantial increases in development, road-building, and land-clearing. Currently, there are numerous opportunities for kebeas to proliferate and expand, threatening forests and communities all over the islands of Koror and Babeldaob.

In addition to kebeas, terrestrial invasive species of greatest concern include the crab-eating macaque monkey; wild pigs; rats; mice; Imperata cylindrica (a species of quite flammable grass); Chromolaena odorata, which is a native North American shrub but an invasive weed in Palau and other parts of Asia; the coral bean tree or red sandalwood tree (Adenanthera pavonina); the African freshwater cichlid fish; and tilapia (Oreochromis mossambicus), which was introduced for aquaculture. The tilapia has escaped captivity and proliferated excessively, becoming an invasive species throughout the archipelago.

Although several marine invasive species have been identified in Palau, according to the 2008 version of The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States, it appears that these invasives are not having a quantifiable effect on Palauan fisheries or the marine tourism industry; however, they do have the potential to become a serious problem.  Most marine invasive species in Palau belong to a small group of invertebrates that were most likely introduced as fouling organisms on ship’s hulls or from ballast water pumped out in harbors. These include sea squirts, hydroids and other cnidarians, mollusks, sponges, and bryozoans.

Presently, only one marine invasive species, the hydroid, Eudendrium carneum, is threatening to become a “pest” organism in Palau. It is a rapidly growing species and has been found on rocky bottoms, often forming intertwined tangles of branches that trap sediments and interfere with the feeding of grazers, such as parrotfishes and surgeonfishes, which scrape algae from rock surfaces, and the settlement of invertebrate larvae.

Of note, there are three known invasive species in Palau’s famous Jellyfish Lake: an anemone in the genus Aiptasia (glass anemone); its endosymbiotic strain of dinoflagellate algae (zooxanthellae) belonging to the genus Symbiodinium; and a sponge of the genus Haliclona.  The glass anemone appears to establish itself on any suitable hard substrate, which makes it a formidable competitor for space and a potential threat to benthic biodiversity.

Ships, Boats, and Groundings

Vessel groundings and impacts from boat anchors cause substantial and visible physical damage to coral reefs. More than 2,000 grounding accidents and 400 sunken vessels are reported annually. As recreational and commercial boating traffic increases in and around coral reefs, the grounding and sinking of ships poses great hazards to these ecosystems both immediately and for the long-term.

Even after a vessel is grounded or sunk, it may continue to move because of currents, wave action, and high winds, causing additional, significant reef damage. Diverse fish assemblages damaged by grounded ships may not recover, especially on a reef that has a spur and groove configuration. Pollutants, such as oil and other chemical contaminants, may be released as a result of the impact from a grounding or sinking. Loose chunks of coral rubble knocked free on impact may threaten adjacent undisturbed coral habitat. In addition, contamination by components of antifouling paints, which are used to reduce the growth of aquatic organisms on ship’s hulls, can significantly reduce coral recruitment in the area of the grounding and may hinder recovery of the community.

The dropping and dragging of large anchors, particularly from cruise ships, destroys large areas of reef habitat. Designation of anchorages in less sensitive areas, installation of mooring buoys, and identification of areas sensitive to anchor damage are necessary to reduce the destructive practice of unregulated anchor dropping and dragging.

Four major ship groundings occurred in Palau since 2005. One – a military vessel – caused such severe damage to the affected coral reef that the ecosystem has not shown signs of recovery. Not only was the reef physically damaged by the crushing impact, but thermal discharges from the ship’s energy sources (generators, engines) led to bleaching and subsequent death of corals, especially Acropora spp., and antifouling paint, scraped from the hull, inhibited growth and reestablishment of many reef organisms.

Marine Debris

Marine debris presents a serious and continuous worldwide threat to the marine environment. Debris adversely impacts marine life through the destruction of essential habitat as well as entanglement of and ingestion by marine organisms and seabirds. The majority of marine debris comes from land-based sources, particularly urban centers, but a significant proportion also comes from ships. Typical debris collected from beaches and other shorelines includes beverage cans and bottles, styrofoam containers, cigarettes, disposable lighters, plastic utensils, food wrappers, and fishing line. The most notable impacts of marine debris on coral ecosystems come from derelict fishing gear such as nets, fishing line, and traps. Netting and fishing line can roll across reef habitats, crushing corals and dislodging sessile organisms. Additionally, fishing gear frequently becomes snagged on corals and continues to trap ("ghost fishing") fishes, seals, and sea turtles, resulting typically in their death.

Marine debris is commonly found in Palau, particularly on beaches around the islands, and fishing lines and nets are sighted frequently in surrounding waters. However, impacts of this debris on nesting bird and turtle populations are not known. Palau recently initiated a marine debris prevention and removal program to reduce and remove debris from the shores and coral reefs.  This project is designed to improve the island's marine habitats and associated resources and educate local fisherman about sustainable solutions to pollution and recycling issues. In addition, the Koror State Government has a regular cleanup program around the Rock Islands to collect marine debris, as currents bring debris originating outside of Palau to the atolls of Ngeruangel and Helen Reef.

Security Training Activities

Military bases and associated activities including exercises, training, and operational procedures (i.e., construction, dredging, and sewage discharge) have the potential for adverse ecological impacts on coral reefs, including excessive noise, explosives and munitions disposal, oil and fuel spillage, wreckage and debris, breakage of reef structure, and invasive species introductions from ship bilge water or aircraft cargo. Currently, there are no security training activities being conducted in Palau; however, the long-reaching activities of U.S. military installations in Hawaii, Johnston Atoll, Wake Atoll, Kwajelein Atoll, Guam, and the Commonwealth of the Northern Mariana Islands have the potential to negatively impact reefs and coral ecosystems of Palau.

Offshore Oil and Gas Exploration

The release of oil and other noxious petroleum products into the marine environment can have serious consequences for reef ecosystems, such as smothering and causing the death of marine fauna or by inhibiting coral growth and recruitment. Cleanup of a coral reef following an oil spill is exceptionally difficult, and certain studies suggest that it may be most productive to let natural processes, including a combination of wind, solar radiation, currents, waves, and degradation by microorganisms, disperse and evaporate the oil. It should be noted, however that recovery of a coral ecosystem from an oil spill can require decades, in which time potentially substantial damage may have been done to the affected reef and surrounding ecosystems for hundreds of miles.

The processes of locating, recovering, transferring, and transporting petroleum resources also have the potential to have serious negative effects on coral ecosystems, especially those that are located in threatened areas. Initially, seismic surveys involving very loud sound (pressure) waves are sent into the sub-bottom rock layers to study their geological structure. Once oil and gas reserves are located, production requires platform installation, dredging, drilling, the discharge of wastes and drill cuttings, and polluted air emissions. These impacts, in addition to the physical effects related to the movement of ships and equipment, can all present significant threats to the organisms and environments where these activities occur.

Although there has not yet been any oil extraction in Palau, there has been interest from the private sector in exploring the northern Velasco Reef, a sunken atoll north of Ngaruangel Reef. Consequently, the Palau government recently passed a resolution that supported an agreement between the Kayangel State and Palau Pacific Energy Inc. (PPE) that gave the oil company exclusive rights to conduct exploration, drilling, and production operations for petroleum during the terms of the exploration license.

Ocean Acidification

Ocean acidification is the term given to the ongoing increase in acidity of the oceans, caused by their uptake of anthropogenic atmospheric carbon dioxide (CO2). Dissolved CO2 in seawater increases ocean acidity by elevating the hydrogen ion (H+) concentration in the ocean.  There is a relationship between ocean acidification and corals’ ability to build their calcium carbonate (CaCO3) skeletons. As the acidity of the oceans increases, many reef community inhabitants grow at slower rates. This is because higher CO2 levels in the ocean also reduce carbonate ion availability, which is critical for corals to build their skeletons.

The increase in acidity has numerous negative consequences for other marine calcifying organisms such as coccolithophores, foraminiferans, echinoderms, crustaceans, and mollusks. These, like corals, use the calcite or aragonite forms of calcium carbonate to construct calcareous coverings and skeletons. Should atmospheric CO2 reach levels 25% higher than present day, which certain studies have forecasted, some coral reefs may no longer grow fast enough to keep up with natural forces of erosion and dissolution. Stressed by warming surface temperatures, bleaching, sedimentation, predation, pathogens, pollution, overfishing, etc., the world’s coral reefs and coral ecosystems may soon face their ultimate threat in rising ocean acidity.

Currently, no information is available on ocean acidification and Palau’s coral reefs.

Other Threats

The Crown-of-Thorns Starfish (Acanthaster spp),also known as COTS, is a large voracious predator of coral reefs throughout the Indo-Pacific. It feeds on polyps of several stony coral species. In moderate numbers, COTS plays an important role in maintaining high biodiversity on a reef by keeping fast-growing corals from overwhelming slower growing corals. However, COTS also experiences periodic population explosions and has been blamed for widespread reef destruction, particularly on Australia’s Great Barrier Reef.

The Crown-of-thorns starfish is a voracious predator of coral reefs.
The Crown-of-thorns starfish is a voracious predator of coral reefs.

Damage to coral reefs due to outbreaks of other echinoderms, such as spiny cold water starfishes, which feed on deep-water coral polyps, and sea urchins, also occurs globally.  Sea urchins can erode the coral substratum, remove newly settled corals, and intensely feed on the corals (corallivory), greatly diminishing the population.

Other coral reef species such as parrotfish, butterflyfish, blennies, puffers, damselfish, and marine snails (Drupella) also feed on corals. Most native coral predators do not pose any major threats to reefs. However, explosions of any corallivore population can have disastrous effects on the growth and survival of a reef.

Additional stressors that have been shown to destabilize and disrupt the dynamics of coral ecosystems include cable-laying operations, which scour the sea floor with untethered cables, earthquakes, and volcanic eruptions.

Multiple stressors, acting sequentially and/or synergistically can be extremely harmful to coral reef ecosystems. In many parts of the Caribbean, for example, the effects of several stressors, such as pollution, decline of key herbivores, severe tropical storms, and coral bleaching have likely led to observed shifts in community structure from coral-dominated to macroalgal-dominated reefs. In general, the effects of multiple stressors are poorly understood, making it difficult to discern the cause of local or regional coral decline.

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