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Natural and Anthropogenic Stressors on the Coral Reefs of Guam

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 reef ecosystems, with loss of live coral cover, declining biodiversity, and reduced population abundances being potential outcomes. Harmful commercial fishing methods, such as bottom trawling, cause much damage to deep water coral ecosystems. Degradation in the structure and functioning of these fragile ecosystems also results in a concomitant decrease in the intrinsic value of the ecological system. Approximately eight percent of the global population lives within 62 miles (100 kilometers) of a coral reef, and many local communities and national economies are directly dependent upon them for tourism revenue, food, and coastal protection.

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

Climate Change and Coral Bleaching

Climate change refers to any change in climate over time, whether due to natural variability or human activities.  Over the course of the 20th century, mean near-surface air temperature over land and mean sea surface temperature (SST) increased with the 1990s being the warmest decade in recorded history. Elevated water temperatures caused corals to bleach, a process that is characterized by the loss of the symbiotic photosynthetic algae, zooxanthellae, from coral tissues. If prolonged, bleached corals may starve to death. The first large-scale bleaching event recorded in Guam was in 1994 and again in 1996; however, neither event resulted in significant coral mortality. Bleaching also was observed in 2006 and 2007 during a period of above average sea surface temperatures.

Predator Outbreaks

Crown of thorns seastar
Crown-of-thorns seastar (Acanthaster planci) in the Achang Reef Flat Marine Preserve, Guam (Photo: David Burdick)

An outbreak of tens of thousands of crown-of-thorns starfish (COTS) in the early 1970s had a devastating effect on corals along 22 miles (35 kilometers) of the northwestern coast. While coral cover did recover substantially by the 1980's, COTS outbreaks continued. Coupled with degrading water quality and reduced rates of coral recruitment, COTS outbreaks continue to serve as an important stressor and threat to coral reefs in Guam.

Coral Diseases

Coral diseases have become one of the major stressors affecting the health and resiliency of coral reef communities worldwide. 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. Very little is known about coral diseases in the Indo-Pacific. Diseases and syndromes affecting Guam's coral reefs are very similar to others reported in the Southern Marianas. However, only a few recent baseline surveys for disease have been conducted (2006 and 2007). It appears that coral diseases have not yet become a major problem in Guam, although minor outbreaks of white syndrome, black-band disease, brown-band disease, and ulcerative white spot disease have been reported.

Tropical Storms

Powerful cyclonic storms (typhoons) can dramatically disrupt entire ecosystems. Coral reefs, however, have shown resilience to storm-caused disturbances. In fact, such mechanical disturbances can be beneficial to a reef community by scouring the reefs to expose the substratum, thus providing more surface area for new coral recruits to settle. In Guam, tropical storms usually occur in the humid summer months. In the last half century, Guam has felt the impact of more than 50 major storms. Since 1994, it has been visited by four major storms with winds as high as 150 mph. When severe damage to Guamian reefs occurs, it is usually restricted to reefs that developed on unstable substrata in relatively protected areas of the coastline.

Coastal Pollution

It has been estimated that as much as 22% of the world's coral reef ecosystems are threatened by land-based pollution and soil erosion. Primary stressors are nutrients and other chemicals from fertilizers, pesticides, herbicides, sewage, as well as 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 sources of pollution originate from specific conveyances, such as pipes, tunnels, ditches, channels, wells, and fissures. Non-point source pollution refers to pollutants not introduced from a single, well-defined site. Examples of point source pollution include sewage outfalls, untreated wastewater from factories and other industrial plants, and heated water discharge from power plants. Other point sources include vessels that discharge their wastes in ports, marinas and other nearshore areas, dredging, and accidental spills of petroleum and other noxious chemicals, such as heavy metals.

Most pollutants in the nearshore areas of Guam are microbial pathogens, petroleum hydrocarbons, and sediments. The most obvious point source pollutants are from sewage treatment plant outfall pipes. Nonpoint source pollutants in the area include nutrients from septic tank systems, sewage spills, livestock and agricultural areas, and discharge of chemicals from urban runoff, farms, and illegal dumping.

Coastal Development and Runoff

Sediment plumes caused by soil erosion after heavy rainfalls
Sediment plumes caused by soil erosion after heavy rainfalls, such as this plume in Guam, lead to serious problems in coral reef ecosystems (Image courtesy of the Guam Forestry and Soil Resources Division, Department of Agriculture)

Increased sedimentation associated with runoff from coastal development and other causes of soil erosion are growing universal threats to coral reefs. In the past several decades, there has been a shift toward greater concentrations of human settlement in the coastal zones of many countries. This trend in the tropics and subtropics has greatly increased the stress on coral reef ecosystems. Freshwater runoff from landscape altering or clearing activities, such as the construction of houses, hotels, resorts, golf courses, marinas, other recreational facilities, piers, roads, bridges, and waste treatment plants has taken a terrible toll on some close-to-shore-reef areas. Sediment runoff settles on coral reefs, smothering them or increasing the turbidity of the water, which reduces both the amount of light reaching corals and the level of photosynthetic activity by corals's zooxanthellae. This, in turn, can cause diminished coral productivity and growth, enhanced macroalgal growth, and, ultimately, a communal shift on the reef from corals to macroalgae.

In addition to sediment loading, runoff may also carry high levels of nutrients from agricultural areas or septic systems, as well as petroleum products, pesticides, and other pollutants. Increases in the amount of nutrients on reefs enhance the growth of other organisms, such as sponges or macroalgae, which may outcompete corals for space. Outflows from water treatment plants greatly increase the nutrient levels surrounding their outflow pipes. Large power plants alter water temperatures by discharging extremely hot water into the coastal waters.

The rate of sedimentation of nearshore habitats in Guam has increased dramatically since the mid-seventies, primarily as a result of severe upland erosion, particularly in the southern part of the island. Erosion is caused by road construction and by the creation of “badlands”, which are a result of range fires (wildfires) set by poachers that denude the steep-sloping landscape. Another major cause of increased sedimentation on Guamian coral reefs is the construction of infrastructure to support the residential and tourist industries and the expansion of the military.

Tourism and Recreation

Scuba-diving tourists
Scuba-diving tourists (Photo: Dr. Anthony R. Picciolo)

Healthy coral reefs support thriving tourism and recreation industries. Millions of scuba divers, snorkelers, boaters, shell collectors, and fishermen visit U.S. coral reefs and nearby beaches annually. Local economies receive billions of dollars from visitors to reefs 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.

Studies have shown that divers and snorkelers can have a significant negative impact on coral reefs in terms of the physical damage they cause and a concomitant reduction in the aesthetic appeal of the reefs. Other tourist-related threats devastating to coral reefs include construction of hotels, resorts, and associated infrastructure, seafood consumption, beach replenishment, building of airports and marinas, and cruise ship operations. Impacts that can result from these activities include increased sedimentation, nutrient enrichment, pollution, exploitation of endangered species, and increased litter and waste.

Guam's economy depends on tourism, U.S. military installations, and locally owned businesses. However, water sports and activities such as snorkeling, scuba diving, jet skiing, and charter fishing can stress and damage coral reefs, the organisms that live on and associate with the reefs, and adjacent habitats. Residents, military personnel, and occasional tourists add to that reef destruction and stress by “harvesting” stony corals, black corals, sea fans, mollusk shells, etc. for decorations and souvenirs.

Stress to coral reefs from scuba diving and snorkeling activities is not well documented in Guam. The greatest volume of these activities occurs in only about five high-profile reef areas in Guam. Many inexperienced resort and student divers visit the same, easily accessible, shallow, popular dive sites within short periods of time. Reefs suffer when they are touched, scraped, grabbed, or sat and stood upon. Stirred-up sediment from snorkelers and divers increases turbidity. Feeding of reef fishes also alters their natural behavior. Additionally, jet skis utilized at these sites produce loud sounds, leak fuel, and damage corals and seagrass beds.

As a tourist destination, it is very important that the beaches of Guam be clean and stable. Beach restoration after major storm damage and almost daily mechanical beach cleaning occur frequently in Guam but can affect the biological communities in the intertidal areas.

Fishing Pressure

Reef fish for market
Reef fish for market

Healthy coral reefs support commercial, recreational, and subsistence fishing. Approximately half of all federally managed fisheries depend on coral reefs and related habitats for a portion of their life cycles. NOAA’s National Marine Fisheries Service (NMFS) estimates that the commercial value of U.S. fisheries from coral reefs is more than $100 million. Approximately 25% of all marine species of fishes are inhabitants of shallow water coral reef ecosystems. Other reef inhabitants, such as marine plants, non-coral invertebrates, and some turtles, are also exploited (and not always in a legal manner) for human use.

Guam's coral reef fisheries are very important to the island’s economy and native culture. Long before the Chamorus had any contact with Europeans, fishes were the primary source of proteins for the islanders. Even today, fishing remains an important part of their life and culture. However, the outlook for Guam's inshore fisheries is poor. A sizeable number of reef fishes and invertebrates have been targeted for commercial harvesting, resulting in a general decline in catch size and changes in species composition over the past half-century. Data also suggest that reef fisheries have not recovered from a steep decline in the mid-nineteen-eighties, the cause of which is still unclear.

Fishing methods used on Guam coral reefs include hook and line, cast nets, spear fishing, gill net, surround net, trolling, drag net, jigging, hooks and gaffs, spin casting, and bottom fishing. The use of scuba and underwater flashlights for spear fishing, as well as monofilament gill nets, has caused concern among Guamians and throughout other peoples in the region. These techniques seemed to have led to visible changes in species composition, including the near disappearance of large groupers and overall decline in a number of other fishes, including wrasses, parrotfishes, snappers, and small groupers. Abandoned gill nets also increase the damage to reefs.

To address the problem of declining fisheries, Guam established a system of five marine preserves in 1997, which vary in size and in permissible fishing activities and methods. Although illegal fishing remains a problem, the preserves appear to have provided some benefit in protecting existing stocks.

Trade in Coral and Live Reef Species

A spiny puffer made into a lamp
A spiny puffer made into a lamp

Many coral reef fishes and invertebrates are collected to supply a demand for seafood, ornamental aquarium specimens, live food for Asian markets, construction materials (e.g., coral block), jewelry, curios, pharmaceuticals, traditional medicines, and other products. Harvesting of coral reef ecosystem components can occur at unsustainable levels and lead to unhealthy changes in reef species composition, relative abundances, and biomass. Non-target species may undergo population explosions that alter the dynamics of a particular reef system. Reductions or disappearances of some herbivorous species may result in the replacement of stony coral-dominated reefs with algal-dominated reefs.

Guam currently does not permit the export of corals or live reef invertebrates and fishes. Corals and live rock (calcareous rock with living organisms attached, such as bacteria, coralline algae, sponges, worms, crustaceans and other invertebrates) for aquarium use are protected under local laws. Additionally, the University of Guam Marine Laboratory is permitted to harvest coral and live rock for research but only in areas not designated as marine preserves.

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 an organism 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 the native species occupying the same habitat or habitat type. In some cases, invasive organisms are successful predators that can decimate local or native populations. In Guam, the brown tree snake (an accidental introduction) is a prime example of an invasive, whereas in the Caribbean islands, the mongoose (a purposeful introduction) has devastated local areas. Invasives may outcompete native species occupying the same ecological niche. In some cases, invasive species have caused major alterations in ecosystems and even the extinction of other species.

Although Guam has spent considerable resources studying terrestrial invasive species, such as the brown tree snake, little work has been done on invasive marine species. The first systematic surveys of non-indigenous marine species in inshore waters were conducted in Apra Harbor, the Orote Penisula Ecological Reserve Area (ERA), and the Haputo ERA. Of the 85 non-indigenous species documented, about half were categorized as introduced and half were of unknown origin. The majority of these were sessile invertebrates that probably arrived in Apra Harbor in vessel hulls. These non-indigenous marine species do not appear to be negatively impacting native species yet, but there is concern that increased shipping activities in Apra Harbor may augment the risk to Guam's coral reef communities.

Ships, Boats, and Groundings

A ship grounded on a coral reef near Honolulu
A ship grounded on a coral reef near Honolulu (photo: Floyd Morris for the Honolulu Star Bulletin)

Vessel groundings and the impacts of boats and anchors are probably responsible for the most destructive physical damages caused by humans to coral reefs. Worldwide, more than 2,000 grounding accidents are reported annually, with over 400 vessels sinking each year. As recreational and commercial boating traffic increases in coral reef areas, sunken ships pose great hazards to coral ecosystem habitats for the short-term and the long-term. Damage to reefs from ship groundings may also be prolonged if the basic reef habitat and community structure are altered by the vessel. Other vessel impacts include accidental or purposeful disposal of sewage, garbage, ballast water, oil, and other hazardous materials; introduction of alien species in ballast water or on hulls; and lost ordinance.

Guam's Apra Harbor con­tains reefs with some of the highest coral cover on the island. It is also the largest U.S. deepwater port in the Western Pacific and the busiest port in Micronesia. Apra Harbor is shared by the Port Authority of Guam and the U.S. Navy. A large number and variety of commercial and military vessels are serviced annually and this number is likely to increase with a planned military expansion. Some of these reef areas may be dredged in the future to facilitate vessel passage and military operations. The coral reefs are also threatened by ship groundings, anchoring, and discharge of pollutants. Many vessels have run aground on Guam's coral reefs, causing minor to major damages to the reef communities. Navigational buoys dragged across reefs during major storms also cause significant damage to the reef habitats.

Marine Debris

A former coral reef in Guam destroyed by debris.
A former coral reef in Guam destroyed by debris. (Photo: D. Burdick/NOAA)

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 and ingestion by marine organisms and seabirds. The majority of marine debris comes from land-based sources, particularly urban centers, but a significant proportion comes from ships.

Marine debris is not categorized as a major stressor to Guam's coral reefs but the impact is felt nevertheless. Approximately 12 metric tons of mostly locally-generated coastal debris was removed from Guam's beaches in 2007. The most common items collected were beverage containers, cigarette filters, plastic bags and cups, plates, and food wrappers. Car batteries, appliances, tires, auto parts, and abandoned fishing gear were also collected. The majority of this debris is from land-based activities, such as picnics, sports, and beach visits. Litter washed from streets, parking lots, and storm drains also contributes to the debris found on Guam's shores. The major physical threat to the coral reefs is discarded fishing nets that can be found wrapped around coral colonies.

Security Training Activities

The U.S. Military has a strong presence on Guam
The U.S. Military has a strong presence on Guam (Photo:Kddeitrick)

U.S. military installations near coral reefs include operations in Hawaii, Johnston Atoll, Wake Atoll, Kwajelein Atoll, Guam, the Commonwealth of the Northern Mariana Islands, Key West and Panama City, Florida, Puerto Rico, the U.S. Virgin Islands, Cuba, and Diego Garcia in the Indian Ocean. Military bases and associated activities, including exercises, training, and operational procedures such as construction, dredging, and sewage discharge, have the potential for adverse ecological impacts on coral reefs. Impacts include 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.

A B-2 'Spirit' Stealth Bomber at Andersen AFB, Guam
A B-2 "Spirit" Stealth Bomber at Andersen AFB, Guam (U.S. AF photograph)

The Department of Defense conducts training activities in Guam, which include underwater demolition and landing craft exercises, and these have the potential to impact Guam's coastal waters and coral reefs negatively. The frequency of these activities has decreased since 2004, but is expected to increase with a proposed military base expansion that is expected to require some 20,000 construction workers. The expansion proposes to transfer more than 8,000 Marines and 9,000 dependents from Okinawa to Guam. Environmental impact studies are underway.

Offshore Oil and Gas Exploration

There are currently no oil or gas resources identified near Guam.

Ocean Acidification

A free-swimming planktonic mollusk
A free-swimming planktonic mollusk (Limacina helicina), an important part of the food web for pelagic fishes, forms a calcium carbonate shell that is negatively affected by increased ocean acidity (Photo: Russ Hopcroft, UAF/NOAA.)

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 increasing the hydrogen ion (H+) concentration in the ocean. There is a relationship between ocean acidification and corals’ and other calcifying organisms’ 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.

Stressed by warming surface temperatures and bleaching, sedimentation, predation, pathogens, predators, pollution, overfishing, etc., the world’s coral reefs may soon face their ultimate threat in rising ocean acidity.

At this time, however, no information is available on ocean acidification and Guam's coral reefs.

Other Threats

The crown-of-thorns starfish (COTS or Acanthaster planci) is a large, voracious predator of coral reefs throughout the Indo-Pacific. It feeds on the polyps of several species of stony corals. In moderate numbers, COTS play an important role in maintaining high biodiversity on coral reefs by keeping fast growing corals from overwhelming slower growing corals. However, periodic population explosions of COTS have been blamed for widespread reef destruction, particularly on Australia’s Great Barrier Reef.

A crown-of-thorns starfish feeding on coral polyps
A crown-of-thorns starfish feeding on coral polyps (Photo: Konrad Hughen, Woods Hole Oceanongraphic Institution)

Guam has been affected by widespread outbreaks of COTS since at least 2004. A local COTS population is considered in “active outbreak status” when densities reach or exceed 30 individuals/11,960 square yards (10,000 square meters). Manta tow surveys in some areas revealed COTS densities at greater than 1,000/11,960 square yards (10,000 square meters) with extensive COTS-related coral mortality. Persistent COTS outbreaks have had and will probably continue to have a severe impact on many of Guam's coral reefs.

In addition to COTS-induced damage to coral reefs, some genera of cold water starfishes also pose a threat to deep-water corals by feeding on their polyps. Parrotfish, butterflyfish, blennies, puffers, damselfish, and marine snails (Drupella) also feed on corals, although most, as natural inhabitants of coral reefs, do not pose a major threat to reef health and productivity. The short-term and long-term impacts of these threats on Guamian reefs have yet to be studied.

Other major threats to coral reef ecosystems include cable-laying operations, the scouring of sea floor by untethered cables, earthquakes, and volcanic eruptions – all of which have been found to interrupt and destabilize coral ecosystems.