by: Steven Miller
Reproduced with permission. Copyright 1996 by Scientific American, Inc. All rights reserved.
Scuba divers joke that there are two ways to avoid decompression sickness, the rare but dreaded “bends”: don’t go down, or don’t come up. In a sense, and underwater habitat is a way of making the latter option possible, at least for a few weeks.
To understand how such an option becomes possible requires a little knowledge of physiology. Breathing air in the relatively high ambient pressure of the underwater environment causes a diver’s blood and tissue to accumulate excess inert gases-mostly nitrogen. The amount of excess gas absorbed by he diver’s body depends on the depth and time spent underwater. Thus, simple physics dictates how long a diver can remain at specific depths without risking the bends, which occurs when the excess inert gas absorbed during a dive bubbles out of a diver’s blood and tissues as he or she ascends and, consequently, the surrounding pressure declines (see “The Physiology of Decompression Illness,” by Richard E. Moon, Richard D. Vann and Peter B. Bennett; SCIENTIFIC AMERICAN, August 1995).
This buildup of gases typically restricts diving scientists who work deeper than about 20 meters to approximately one hour a day at depth, which seriously limits their experimental and observational capabilities. To get around these limitations, marine scientists use an array of undersea technology, including manned submersibles, underwater robots and sampling and remote observational equipment lowered from ships. Still, many tasks -particularly those in support of scientific research- require a prolonged human presence to observe with eyes instead of cameras or to touch with hands instead of robotics arms. And nothing can substitute for the advantages of having a brain to observe, learn and improvise when the need arises.
The only technique that allows this kind of sustained human presence is saturation diving. Saturation diving allows marine researchers to live and work at pressure for days, weeks or even months at a times. The technique is based on the fact that after about 24 hours at any working depth, a diver’s body becomes saturated with dissolved gas. Once the body is saturated, decompression -the period required to bring the diver gradually back to surface pressure without inflicting the bends- is the same regardless of how much time has been spent underwater. The main risk to divers is accidental, rapid ascension or surfacing, which could cause a life-threatening case of the bends if the surfaced diver is not quickly returned to pressure.
Saturation diving was developed in the 1960s, when dozens of systems were built for commercial or scientific purposes. They were designed to keep divers under pressure between dives, either in seafloor habitats or in shipboard vessels called deck decompression chambers; the latter also included pressurized diving bells to transfer divers to and from underwater work sites.
In habitats, the pressure was matched to the pressure of the depth at which the habitat was placed, enabling divers to come and go as they pleased. Such advantages notwithstanding, habitats gradually fell out of favor. Accidents, including fatalities, shut programs down; inefficient operations and insufficient funding were common; and programs were never designed to meet national research objectives.
Currently the only underwater habitat devoted to science is Aquarius. Since 1993 it has operated off Key Largo, Fla., as the centerpiece of a research program focusing on the state’s fragile and economically important coral reefs. The marine analogue of the terrestrial rain forest, coral reefs are home to between 20 and 40 percent of the 160,000 known marine species. (Knowledge of marine biodiversity is still relatively rudimentary, however, and these numbers are probably conservative.) Coral reefs are also among the earth’s most threatened ecosystems.
But the reefs are significant for more than their biodiversity and their status as endangered ecosystems -there are economic reasons for their study as well. For example, they are often an important barrier against shoreline erosion; they support commercial and recreational fisheries; and they are one of the primary attractions for millions of recreational scuba divers. Reefs also help to maintain beaches through cycles of growth and erosion and the development of seagrass and mangrove ecosystems are linked to reefs as well.
Aquarius accommodates six-person teams (five scientists and one habitat-support technician) during 10-day missions. Amenities include a hot water shower, unlimited freshwater, air-conditioning, various kitchen appliances and comfortable bunks-all of which help keep aquanauts rested, alert and productive. In contrast to its predecessors, Aquarius is really more a laboratory than a habitat. In addition to a comfortable living environment, Aquarius provides enough space to conduct experiments and includes computer and electronic capabilities to permit research that could not be accomplished any other way. Scientists spend six to nine hours a day in the water and often venture out at night as well.
With 21 missions off Key Largo, Aquarius has revolutionized the study of coral reefs. A few highlights attest to the habitat’s emergence as a mainstay of reef research:
Several years ago Daniel F. Gleason and Gerard M. Wellington of the University of Houston demonstrated that ultraviolet radiation from the sun can cause coral to "bleach." Bleaching is the term given to the occasional, massive and sometimes fatal coral maladies that have mystified and alarmed researchers for over a decade. Gleason and Wellington showed that solar ultraviolet radiation (which has become stronger because of losses of stratospheric ozone) can cause coral bleaching as far down as 25 meters. Researchers previously thought that ultraviolet radiation was filtered out in the top few meters of ocean water. Theories about bleaching must now consider ultraviolet light, as well as the higher ocean temperatures that have long been suspected, to play a role. Reefs near a densely populated region -such as the reefs that surround the Aquarius laboratory-face a more immediate threat: nutrient pollution from human sewage. Studies conducted from Aquarius, a mere 100 kilometers from Miami and just offshore form one of the more populated Florida Keys, have measured natural cycles related to nutrient chemistry and assessed the potential for sewage contaminated water to affect reef organisms. Reefs typically thrive where nutrients are sparse, so they are sensitive even to low levels of chronic pollution. Recently Gene A. Shinn of the U.S. Geological Survey and Francis J. Sansone of the University of Hawaii installed special monitoring wells to search for evidence near the reef of contaminated groundwater. Virtually all substantive knowledge about the voracious feeding habits of corals is deprived from work by Aquarius scientists, notably Kenneth P. Sebens of the University of Maryland. Aquarius scientists have also studied how corals reproduce and colonize reefs, which will have applications related to restoration of reefs damaged by ship groundings or other causes. Much of the three-dimensional structure of a reef comes from corals, but sponges are often found in greater numbers and diversity. Sponges filter and process vast quantities of plankton from water that passes around, over and through the reef. A recent Aquarius mission discovered that sponges are a net sink for some types of plankton, but, interestingly, they are also a net source for others (and for nutrients as well). These results have important ramifications for how reefs capture and cycle energy, especially in light of the fact that changes in water quality have been implicated in the demise of some reefs.
Aquarius benefits science in several less direct ways. The marine laboratory has captured public attention and fired imaginations about the value and wonders of the undersea realm. Even scientists have had their consciousness raised. Stolid aquanauts report feeling a sense of belonging in the world beneath the waves and a kind of empathy with its creatures. Such experienced can make a difference later, when former aquanauts go on to positions in which they write, teach or help to shape private-sector and government policy regarding out waters.
In comparison to other scientific outposts, Aquarius operating costs of about $1.2 million a year are modest. The laboratory is less expensive on a daily basis than an oceanographic cruise; the cost of a single space shuttle mission would cover the habitat’s expenses for the next 500 years. Nevertheless, federal budgets are being chopped, and the Aquarius program has been on the block every year since operations began. We hope this last underwater outpost will survive, allowing its users to continue expanding our knowledge of coral reefs, our oceans and our planet.