Why Are Scientists Exploring the Coral Reefs of Bonaire?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
Coral reefs are familiar examples of biodiversity in the ocean. Most people have seen photographs and video images of shallow-water coral reefs, typically showing multitudes of brightly colored fishes and other animals that provide a variety of benefits including value for recreation and tourism industries, protecting shorelines from erosion and storm damage, supplying foods that are important to many coastal communities, and providing promising sources of powerful new antibiotic, anti-cancer and anti-inflammatory drugs (for more information about drugs from the sea, visit the Ocean Explorer Web site for the 2003 Deep Sea Medicines Expedition (http://oceanexplorer.noaa.gov/explorations/03bio/welcome.html). Much less familiar is the fact that many of Earth’s coral reefs appear to be in serious trouble due to causes that include over-harvesting, pollution, disease, and climate change (Bellwood et al., 2004). In the Caribbean, surveys of 302 sites between 1998 and 2000 show widespread recent mortality among shallow- (< 5 m depth) and deep-water (> 5 m depth) corals. Remote reefs showed as much degradation as reefs close to human coastal development, suggesting that the decline has probably resulted from multiple sources of long-term as well as short-term stress (Kramer, 2003; for additional information about threats to coral reefs, see ‘More About the Coral Reef Crisis,’ below).
Despite these kinds of data and growing concern among marine scientists, visitors continue to be thrilled by the ‘abundance and diversity of life on coral reefs.’ This paradox is an example of ‘shifting baselines,’ a term first used by fishery biologist Daniel Pauly. A baseline is a reference point that allows us to recognize and measure change. It’s how certain things are at some point in time. Depending upon the reference point (baseline), a given change can be interpreted in radically different ways. For example, the number of salmon in the Columbia River in 2007 was about twice what it was in the 1930s, but only about 20% of what is was in the 1800s. Things look pretty good for the salmon if 1930 is the baseline; but not nearly as good compared to the 1800’s. The idea is that some changes happen very gradually, so that we come to regard a changed condition as ‘normal.’ When this happens, the baseline has shifted. Shifting baselines are a serious problem, because they can lead us to accept a degraded ecosystem as normal - or even as an improvement (Olson, 2002). So, people who have never seen a coral reef before may still find it to be spectacular, even though many species have disappeared and the corals are severely stressed.
One of the few coral reefs that seems to have escaped the recent coral reef crisis is found in the coastal waters of Bonaire (part of the Netherlands Antilles in the southwestern Caribbean). A 2005 survey of the state of Bonaire’s reefs (Steneck and McClanahan, 2005) found that they were among the healthiest reefs in the Caribbean, even though dramatic changes have occurred among corals and other reef species. This means that Bonaire’s reefs have unique importance as baselines for comparison with other Caribbean coral reef ecosystems. Detailed mapping of Bonaire’s shallow- and deep-water coral reefs is a top priority for protecting these ecosystems, as well as for defining a baseline for investigating and possibly restoring other coral reef systems. This mapping is the focus of the Bonaire 2008: Exploring Coral Reef Sustainability with New Technologies Expedition.
[For more information, activities, and lessons about coral reefs, visit the Cayman Islands Twilight Zone Expedition Education Module at http://oceanexplorer.noaa.gov/explorations/07twilightzone/background/edu/edu.html, and the National Ocean Service Coral Reef Discovery Kit at http://oceanservice.noaa.gov/education/kits/corals/welcome.html]
The overarching goals of the Bonaire 2008: Exploring Coral Reef Sustainability with New Techologies Expedition are to:
- Produce comprehensive maps of the sea bottom environment of Bonaire over a substantial depth range;
- Describe physical and chemical conditions near healthy bottom ecosystems; and
- Investigate biodiversity of deep sea bottom ecosystems, with particular attention to new types of communities and new invertebrate species.
Autonomous Underwater Vehicles
Autonomous Underwater Vehicles (AUVs) are the technological centerpiece of the Bonaire 2008: Exploring Coral Reef Sustainability with New Technologies Expedition. AUVs are underwater robots that operate without a pilot or cable to a ship or submersible. This independence allows AUVs to cover large areas of the ocean floor, as well as to monitor a specific underwater area over a long period of time. Typical AUVs can follow the contours of underwater mountain ranges, fly around sheer pinnacles, dive into narrow trenches, take photographs, and collect data and samples.
Two kinds of AUVs will be used to map the reefs of Bonaire. The Fetch1 AUV was developed by Mark Patterson (Co-Principal Investigator of the Bonaire Expedition), and carries sensors to measure dissolved oxygen, pH, and chlorophyll, as well as an underwater video camera and side-scan sonar. The Gavia AUVs were developed by Hafmynd ehf, a company based in Iceland. The Gavia AUVs have similar sensors and also carry a type of sonar called multibeam. For more information and images of these AUVs, visit http://oceanexplorer.noaa.gov/explorations/08bonaire/welcome.html
Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a ‘transducer.’ The transducer may be mounted on the hull of a ship, or may be towed in a container called a ‘towfish.’ If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an ‘echo’ to the sonar transducer. The system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and an experienced operator can use the strength of the echo to make inferences about some of the object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. This is a general description of ‘active sonar.’ ‘Passive sonar’ systems do not transmit sound pulses. Instead, they ‘listen’ to sounds emitted from marine animals, ships, and other sources. Subbottom profiler systems are another type of sonar system that emits low frequency sound waves that can penetrate up to 50 meters into the seafloor. Visit http://ocean.noaa.gov/technology/tools/sonar/sonar.html for more information about sonar systems.
Side-scan sonar systems use transducers housed in a towfish, usually dragged near the seafloor, to transmit sound pulses directed toward the side of the ship, rather than straight down. Return echoes are continuously recorded and analyzed by a processing computer. These data are used to construct images of the seafloor made up of dark and light areas. These images can be used to locate seafloor features and possible obstructions to navigators, including shipwrecks (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information). Multibeam sonar system are used to make bathymetric maps and create three-dimensional images of the seafloor. Multibeam sonars send out multiple, simultaneous sonar beams in a fan-shaped pattern that is perpendicular to the ship's track. This allows the seafloor on either side of the ship to be mapped at the same time as well as the area directly below (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information). Sonar technology is also capable of detecting objects that are in the water column, so it may also be possible to obtain information about how fishes are using Bonaire’s coral habitats at the same time as the reefs are being mapped.
Deep-water Diving Technology
Selected portions of the Bonaire coral reefs will be surveyed by divers to ‘groundtruth’ (verify) data from video and side scan sonar surveys. In depths less than 30 m, divers will breathe ordinary air using conventional SCUBA equipment. As anyone knows who has ever tried, it is impossible to breathe underwater through a snorkel, pipe, or hose that is more than a few feet long. This is because the surrounding water exerts pressure on a diver’s body that is equal to one atmosphere (about 14 pounds per square inch) for every 33 feet of depth. To breathe through a long snorkel underwater, the muscles we use to fill our lungs have to overcome this water pressure - and they are not designed to do that.
Conventional SCUBA technique overcomes this problem by using a device called a demand regulator (invented in 1943 by Jacques Cousteau and Emile Gagnan). A demand regulator supplies air at a pressure that matches the pressure in the water (ambient pressure), so there is no additional pressure for breathing muscles to overcome. A diver using this type of regulator breathes gas at higher pressures than at the surface. Instead of a normal pressure of one atmosphere, a diver at 33 feet is breathing air pressurized to two atmospheres; at 66 feet, the pressure is three atmospheres, and so on. But there are several problems with this system.
One problem is that if a diver takes a lungful of air at a depth of 66 feet and then ascends to 33 feet, the external pressure drops from 3 atmospheres to 2 atmospheres. Since the volume of a gas is inversely proportional to the pressure of the gas, as the pressure drops the volume will increase. So the air in the diver’s lungs would expand, and if the diver was holding his breath the expanding air could rupture his lungs. When this happens, air bubbles can enter the bloodstream causing a condition known as ‘air embolism.’ If the bubbles block important blood vessels, the result can be paralysis or death.
Another problem is that as the pressure of a gas increases, the solubility of that gas in a liquid increases as well (Henry’s law). So if a diver breathes air from a demand regulator at 66 feet for a while, her blood will contain twice the amount of dissolved gases from the air than it did at the surface. Again, the problem comes when the pressure is reduced. Anyone who has ever opened a can of soda knows what happens when you suddenly release the pressure on a liquid containing dissolved gas: bubbles form in the liquid. If the diver rapidly ascends from 66 feet, the dissolved gases in her blood may form bubbles, creating a problem that is somewhat similar to an air embolism in that critical blood vessel may become blocked. This condition is called ‘decompression sickness’ or ‘the bends,’ and was first seen in miners working in pressurized coal mines (it was also a problem for workers constructing the Brooklyn Bridge, who spent hours working underwater in pressurized iron boxes called caissons, so yet another name for the condition is ‘caissons disease’). Since air is about 70% nitrogen, more nitrogen is dissolved in the blood than other gases and the bubbles of decompression sickness are bubbles of nitrogen gas. Oxygen isn’t believed to be involved, since much of the oxygen dissolved in a diver’s blood is quickly bound by hemoglobin, and normal metabolism reduces blood oxygen concentration.
Divers avoid decompression sickness by closely monitoring their dive time and depth, since they both affect the amount of gas that dissolves in the blood. Decompression tables and dive computers show how long a diver may stay at a particular depth without having a high risk of decompression sickness. If they stay longer than this time, then they have to return to the surface in stages, stopping for a specific amount of time at shallower depths (‘decompression stops’) to allow the nitrogen to diffuse out of their blood without forming bubbles.
More problems arise when divers descend to depths greater than about 40 m. As the pressure of a gas increases, the physiological effects of that gas may change; and some of these changes are bad news for divers. Oxygen, for example, becomes toxic at high pressure and can cause convulsions. Nitrogen can cause a condition called ‘nitrogen narcosis’ or ‘rapture of the deep’ which is similar to alcohol intoxication. To overcome these problems, divers on the Bonaire Expedition will not breathe ordinary air when they explore reef areas at depths between 40 m and 100 m. Instead, they will use technical breathing mixtures.
The easiest way to deal with the problem of oxygen toxicity is to reduce the proportion of oxygen in the breathing gas mixture. But this means that the proportion of some other gas would have to be increased, and increasing the proportion of nitrogen would increase problems with nitrogen narcosis. So gas mixtures for deep diving substitute helium for nitrogen. Helium is not toxic, even under the high pressures needed for deep diving. Helium also has another advantage: it is a much smaller molecule, and therefore less dense than nitrogen. As the pressure of a gas increases, so does its density; and as gas density increases so does the work needed to move that gas around. Since the density of helium is less than nitrogen, it is easier to inhale and exhale under high pressure.
Some breathing gas mixtures for deep diving consist of oxygen and helium alone, and are called Heliox. Others contain nitrogen as well, and are called Trimix. The problem with mixtures containing helium is that the small helium molecules dissolve into the blood much more quickly than nitrogen, so longer decompression times are needed. To deal with this problem, divers may turn to a ‘Nitrox’ mixture that contains nitrogen and oxygen but with less nitrogen and more oxygen than ordinary air. Nitrox mixtures can be used at moderate depths without risking oxygen toxicity, and allow divers to greatly decrease the time needed for decompression. For more information on technical diving, visit the Cayman Islands Twilight Zone Expedition Web page (http://oceanexplorer.noaa.gov/explorations/07twilightzone/background/techdive/techdive.html).
More About the Coral Reef Crisis
Coral reefs are exposed to stress from a variety of natural events, as well as from human activities. Natural sources of stress include severe storms and tsunamis, predators, and diseases. In the absence of other stresses, coral reefs are often able to recover from damage caused by storms and tsunamis, and healthy reefs can act as natural wave barriers that help protect shorelines. Predators such as the crown-of-thorns starfish can cause severe damage when starfish populations become unusually large. In recent years, outbreaks of various diseases have been reported from an increasing number of coral reefs around the world. The causes of these outbreaks are not known.
Stresses resulting from human activity include:
* Invasive species, particularly large algae, often accidentally introduced in the ballast water of cargo ships or by aquarium keepers who dump unwanted specimens into coastal waters;
* Mechanical damage resulting from ship groundings, boat anchors, and careless tourists who stand on or hold onto living coral;
* Collection of living coral for aquaria or souvenirs;
* Ocean acidification resulting from increased atmospheric carbon dioxide dissolving into the ocean to form carbonic acid; increased acidity makes it more difficult for marine organisms to form body parts from calcium carbonate;
* Pollution from land runoff contaminated with sediment, fertilizers and pesticides;
* Marine debris that can smother corals and injure or entrap many marine species; and
* Increased ocean temperatures that can cause corals to expel their zooxanthellae (symbiotic algae that live within the tissues of many corals), resulting in a ‘bleached’ appearance.
Many scientists believe that the widespread decline of coral reefs is the result of accumulating stresses from multiple sources. In the Caribbean, for example, increased harvesting of herbivorous fishes coupled with nutrient-rich runoff from land created conditions favorable for the growth of large algae. The algae were kept in check by the black spined sea urchin (Diadema antillarum), which were present in huge numbers on most reefs in shallow water - until a disease outbreak reduced the sea urchin populations by about 99%. Without herbivorous fishes or the sea urchin, large algae grew rapidly, shading and overgrowing reef building corals (Bellwood, et al., 2004).
For more information, visit http://oceanservice.noaa.gov/education/kits/corals/welcome.html.
More About Sustainability
The modern use of ‘sustainability’ as it pertains to environmental issues and human communities originated with the United Nations World Commission on Environment and Development. In the Commission’s report, Our Common Future, ‘sustainability’ is defined as “..development that meets the needs of the present without compromising the ability of future generations to meet their own needs" (World Commission on Environment and Development, 1987). The key concepts underlying this definition are that sustainability involves economic and social qualities in addition to environmental quality, and that if something is sustainable it persists over a relatively long period of time.
Similarly, ‘reef sustainability’ involves the health of many species (not just corals), as well as an overall community structure that is more or less stable over a relatively long period of time. ‘Community structure’ refers to the ecological functions of the various species that inhabit a community (e.g., small algae, large plants, herbivores that eat small algae, herbivores that eat large plants, carnivores that eat large animals, animals that eat detritus). In general, the structure of coral reefs is defined by the species of corals, algae, and fishes that inhabit a reef community, and the ecological functions , such as primary production, herbivory, that they perform within the community. The coral reef crisis has resulted from dramatic changes in community structure and functions, probably caused by a combination of stresses. So the decline of herbivores and corals accompanied by a dramatic increase in large algae described above would indicate a loss of sustainability in the reef systems where these changes occurred, and has been called a ‘phase shift’ (Steneck and McClanahan, 2005).Put another way, ‘reef sustainability’ implies resilience to environmental stress. This resilience includes the ability to resist a phase shift, as well as the ability to recover from environmental disturbances such as severe storms or unusually high temperatures. When multiple stresses reduce this resilience, sustainability declines and reefs are more vulnerable to major changes in community structure that we consider to be degradation. Detailed information on the structure of these reefs from the Bonaire 2008: Exploring Coral Reef Sustainability with New Technologies Expedition may help explain why Bonaire’s coral reefs appear to be unusually resilient (and more sustainable) than many other Caribbean coral reefs. This understanding is critical to efforts to protect and restore other reef systems impacted by the coral reef crisis.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.