by Marcia Collie and Julie Russo
The human imagination can inspire great scientific discovery. Such was the case when Captain Nemo and the crew of the Nautilus marveled at the abundance of animal life in the deep sea. It was novelist Jules Verne who came up with this idea for his submarine adventure Twenty Thousand Leagues Under the Sea published in 1870. Sixty years later, William Beebe's Half Mile Down described his dive sphere descents to the deep sea and provided glimpses of a strange new world of flashing lights and sea monsters. However, disproving the myth that the deep sea was an inhospitable desert took practically a whole century before an astonishing number of real animals from the seabed were unearthed by scientists Bob Hessler and Howard Sanders from the Woods Hole Oceanographic Institution (WHOI).
More recent studies have given scientists plenty of room to argue that the ocean bottom supports as diverse a habitat as any community on Earth. But between our limitless imaginations about the natural world and public recognition for its real value, can the resource be protected from exploitation, let alone be understood?
The discovery that the deep sea may be every bit as biodiverse as a tropical rainforest comes at a time when pressure is mounting to use every available square foot of land for development. A major problem of the 21st century will be deciding what to do with the vast increases in waste that a growing population—forecast to double from five billion to 10 billion in the next century—will produce. The oceans—which cover seven-tenths of the planet's surface—are likely to become an option for waste management in the future.
At present, ocean dumping is predominantly banned by international law. The motivation for banning ocean dumping was sparked by the shutdown of beaches due to contaminated wastes from sewage-derived micro-organisms, the closing of shellfish beds due to metal contamination, and infection of fish by lesion-causing parasites. Coastal oceans continually enriched by nutrients in waste products that run off the land suffer from eutrophication resulting in an increase in toxic algal blooms and decreased oxygen levels, both of which can kill fish populations.
With more than 80 percent of the ocean at depths of more than 3,000 m, the deep-sea floor may seem safe from the man-made disturbances that threaten terrestrial and coastal ocean environments. And yet most environmental litter from both natural and artificial waste—such as sewage sludge, mining tailings, fly ash from power stations, dredged spoils from harbors and estuaries, dangerous man-made organic compounds used for pesticides, weapons, and industrial uses, as well as packaged goods—makes its way to the sea floor over time.
The vast and remote deep-sea floor could make it appear like an attractive alternative for dumping. To determine the impact of waste disposal on bottom-living animals, the National Undersea Research Program (NURP) has recently supported numerous projects in the oceans and Great Lakes. Of particular concern to researchers are the effects of dumping on living resources and deep-sea biodiversity, as well as the transmission of contaminants back to the human population. In the most detailed study ever done related to the impacts of ocean dumping, NURP-funded scientists documented the impact of 42 million tons of wet sewage sludge dumped 2,500 m (8,000 ft) off the Mid-Atlantic coast between 1986 and 1992. One of the most significant environmental impacts detected at the "106-mile dumpsite," named for its location 106 nautical miles southeast of New York Harbor, was the restructuring of a community of deep sea organisms.
Two momentous developments laid the foundation for observations made at the 106-mile dumpsite. The first development was the invention of the box corer, a stainless steel trap that takes relatively undisturbed bites out of the seafloor, enabling biologists to count the number of species in each core and compare them to cores collected elsewhere in the deep ocean. The second development was an ecological survey conducted for the U.S. Minerals Management Service in the mid-1980s by Fred Grassle, a benthic ecologist (formerly of WHOI), who now serves as director of the Institute of Marine and Coastal Sciences, and Nancy Maciolek of Battelle Ocean Sciences.
In a series of 233 cores taken for the survey along a 176-kilometer track off the coast of New Jersey and Delaware during a two-year period, Grassle and Maciolek found an incredible diversity of animals, most of which were unknown. They picked out 798 species, 171 families, and 14 phyla at around 2,100 m (6,720 ft)—a sampling that revealed much richer life at those depths than earlier samples had hinted. They reserved their count to the tremendous diversity of tiny invertebrate mud dwellers too big to slip through their sieves.
As they sampled sites to the north and south, the number of new species they found had doubled without reaching a plateau, suggesting that species diversity was much richer than ever imagined. If a generalization were to be drawn for how many new species could be found every square kilometer of the sea floor beneath more than 1,000 meters of water—excluding the abyssal depths thought to have less species diversity—the researchers came up with a ballpark figure of 10 million species. "This sampling revealed that the deep-sea may, in fact, rival tropical rainforests in terms of the numbers of species present," Grassle said. "Thus the deep sea may physically resemble a desert, but in terms of species composition it is more like a tropical rainforest."
One aspect of the deep-sea biodiversity study was not apparent until sewage sludge dumping began at the 106-mile dumpsite around the same time period. A shallower site in the New York Bight Apex had shown unacceptably high pathogen levels and signs of fish disease, which led to its closure. As an alternative, roughly eight million tons of sludge a year began to be dumped at the 106-mile deepwater site on the continental rise adjacent to the New York Bight starting in 1986.
Researchers Grassle and Maciolek found themselves with a baseline of information on deep-sea organisms right around the dumpsite, which other scientists could use for comparison in determining whether damage might be caused to deep-living communities.
During the course of the next six years, NURP sponsored studies to determine the fate and effects of the sewage sludge at the sea floor. All of the evidence indicated that the sludge material dumped by barges did reach the ocean bottom slightly west of the area where it was discharged, and that it had significant effects on the metabolism, diet, and composition of organisms that lived there.
There was a presence of sludge in sediments at the dumpsite, and the level of silver was 20 times higher at the site relative to an unaffected reference area. This was confirmed by chemist Michael Bothner of the U.S. Geological Survey. The submersible Alvin used by Bothner and his colleagues helped them collect the silver samples in sediment cores to make the determination. They were also able to observe how contaminants introduced to the sediments from dumping penetrated to a depth of 5 cm below the sea floor as organisms living in the sediments burrowed through them. However, during a 10-month period of sampling, researchers observed seven occasions where the currents were strong enough to resuspend the contaminated sediments. During the same period, chemist Hideshige Takada of Tokyo University and Bothner reported elevated levels of linear alkylbenzenes (LABs), widely used as surfactants in synthetic detergents, and coprostanols, a fecal marker of animals, at the dumpsite.
The increased flux of sludge caused measurable changes in the benthic ecology near the dumpsite. Dr. Cindy Lee Van Dover (a biological oceanographer), Grassle, and colleagues observed a tenfold increase in the abundance of urchins, starfish and sea cucumbers at the dumpsite, and the ingestion of sludge-derived organic matter by sea urchins. By entering into the benthic food web, researchers believe that it is likely that long-term disposal programs would result in the restructuring of the benthic community favoring species that can exploit the organic material available in sewage sludge. Two species of polychaete worms, normally not abundant in the area, also increased at the dumpsite.
Sludge disposal at the 106-mile dump site was curtailed in July 1992. "This provided additional opportunities to examine the long-term dispersal and effects of waste material in the deep-sea environment," Bothner said. Bothner found that silver levels in sediment samples had begun to decline after the dumping stopped. In subsequent studies by Bothner, and chemical oceanographers Elizabeth Lamoureux and Bruce Brownawell of the State University of New York at Stony Brook, it was found that while LABs had tapered off, elevated levels of organic contaminants including PCBs and PAHs in surface sediments around the dumpsite could still be detected. Van Dover found that the density of benthic communities at the dumpsite was decreasing and ingestion of sewage-derived organic matter was also subsiding at the dumpsite.
While the effects of sludge dumping appeared to be abating in the vicinity of the dumpsite, an additional chapter to the story of the 106-mile site still remains to be written. Levels of silver appeared to be on the increase 50 nautical miles south of the dumpsite, as did the densities of sediment-dwelling organisms. This suggests that the recovery of the dumpsite had led to changes in other habitats as resuspended materials were transported to the south of the dumpsite, according to a 1993 study by Grassle, Paul Snelgrove, associate chair in fisheries conservation at Memorial University of Newfoundland, and Rosemarie Petrecca, a senior marine scientist at Rutgers University.
Many questions remain to be answered about the potential short- and long-term effects of toxic compounds found to accumulate in deep water sediments from ocean dumping. Are toxins diluted to acceptable concentrations? Do heavy metals present as serious an environmental problem to deep-sea benthic communities as they do in shallow seas?
Two arguments in favor of deep-ocean dumping are that the material dilutes during sinking and is stable on the sea floor. The present body of research suggests that dilution does not completely abate the effects of dumping, nor does the waste sit still once it gets to the bottom. By establishing a long-term observatory at the 106-mile dumpsite, Grassle hopes future research will be directed towards better understanding the interactions of deep-sea organisms with their natural environment, and towards monitoring the potential effects of pollution necessary for the future wise management of ocean resources.
"Almost nothing is known about the tolerances of deep-sea organisms to the gradual build-up of anthropogenic chemicals, and there is a potential for changes to be widespread if they do occur," Grassle said. "Measurement of pollutants, descriptions of deep-sea communities from many parts of the ocean, and in situ toxicity studies are urgently needed."
Real aquanauts have replaced the imaginary crew of the Nautilus descending by submarine into the abyss. Life persists in the absence of sunlight, and the aquanauts find a remarkable diversity of creatures including sea cucumbers with their porcupine bristles, the brilliant yellow and red star shaped crinoids, branched gorgonians, crabs, shrimp, and rat tail fish, and examples of the millions of mud-dwellers, most of which have never been viewed by the human eye. Along the journey, they also find the results of hundreds of years of waste disposal at sea including bottles, cans, pottery, cocktail sticks off the great luxury liners, and big chunks of clinker from coal-fired steam engines. But the more the aquanauts discover, the less they understand. The deep sea—the largest, most diverse environment on Earth—still awaits us as we enter the 21st century.