2.4: Ocean Resources

Most people know that the oceans are fished for seafood and used for transport and recreation, and that oil and gas are extracted from the seafloor. However, it is not widely recognized how valuable these resources are. For example, according to the U.S. Department of Transportation, U.S. ports handled more than $2.1 trillion in goods in 2024 with 465,000 vessel calls to these ports. The recreational cruise industry is estimated to have contributed $65 billion to the U.S. economy in 2023. The National Oceanic and Atmospheric Administration reported that, in 2020, the seafood industry supported 1.1 million jobs and generated $154.7 billion in sales, while recreational fishing contributed 595,000 jobs and $98 billion. The Congressional Research Service reports royalties from offshore oil and gas resources paid to the US Treasury were $5.6 billion in 2019 but fell to $3.7 billion in 2020. The National Economics Program also reports that in 2018, tourism and recreation based on the coasts and oceans supported more than 2.5 million jobs and generated $143 trillion in Gross Domestic Product in the U.S. There are also a number of other ocean resources or potential ocean resources that are not included in these figures. In the following sections, we will briefly survey the wide range of ocean resources. For this purpose, we grouped these resources into eight categories:

• Biological resources
• Transportation, trade, and military use
• Offshore oil and gas
• Methane hydrates
• Minerals and freshwater
• Recreation, aesthetics, and endangered species
• Energy
• Waste disposal 

 

Biological Resources

Fisheries

Fish and shellfish are probably the most valuable ocean resources. Seafood has a very high protein content and therefore contributes significantly to global dietary needs. In many coastal areas, seafood is the basic subsistence food because no other significant source of food protein is available. Iceland and many Pacific Ocean island nations, including Japan, are good examples of such seafood-dependent areas. Fisheries are also a major part of the U.S. economy.

Until the past century, seafood resources were, for all practical purposes, inexhaustible because they were replaced by reproduction faster than they were consumed. However, human populations have burgeoned in the last 100 years and the demand for seafood has increased accordingly. Consequently, the seafood resources of many parts of the oceans have been exploited so intensively that many species are overfished and can no longer reproduce fast enough to replace their populations. Overfishing, which has resulted from a variety of technical and socioeconomic factors, poses perhaps the greatest threat to the health of ocean ecosystems, other than climate change, ocean acidification, and deoxygenation. Most scientists feel that overfishing is a far greater threat than the oil spills, industrial waste, and domestic sewage discharges that often dominate media coverage of the oceans. Indeed, the increasing need for the greatest possible utilization of ocean fishery resources to feed a hungry and growing population, and the damage done to these resources by unwise exploitation and management, have been important factors in the development of the field of oceanography.

Historically, fisheries were, and many still are, resources not owned by any person, organization, or government. The fact that they were free for anyone to exploit led to a vicious cycle repeated in fishery after fishery. When a new fishery opens, a few fishing boat operators are able to catch large quantities of the resource species with little effort and make substantial profits. Other operators, aware of these profits, quickly enter that fishery. As the number of boats increases and the population of the target species is reduced, each operator must expend more effort to catch the same amount of fish. At the same time, the value of the catch may decline because the market is now flooded with this species. In response, each operator increases fishing efforts to catch more and recover profits lost in the price drop. Hunting and catching technologies are continuously improved, and the fishing pressure continues to rise. If unchecked, the continuous increase in fishing effort and effectiveness quickly leads to a collapse of the fish population as the maximum sustainable yield is exceeded (CC16). Many fishers then look for a new species to target, and the cycle begins anew.

In such instances, the fishery resource has declined, sometimes so precipitously that the commercial fishery has been essentially wiped out for decades. Perhaps the greatest known historical decline was the collapse of the anchovy catch off Peru from over 12 million metric tons in 1970 to less than 2 million metric tons in 1973. This decline constituted roughly 10% of the total world fishery catch at the time. A strong El Niño was certainly responsible for some of the decline, but many scientists believe that excessive fishing efforts contributed greatly to the problem. The California sardine fishery that supported the development of Cannery Row in Monterey, California during the 1940s (Fig. 2-11a) likely contributed significantly to the decline in sardine abundance that has not yet recovered. The number of king crab caught by fisheries in the Bering Sea (Fig. 2-11b) peaked and then declined rapidly in the 1980s, which was mirrored closely by abundance studies in the area. This decline occurred at a time of sustained increase of fishing pressure.The menhaden fisheries off the middle Atlantic coast of the U.S. and the cod fisheries off the coast of New England and Canada are more prime examples of such decimated resources, although environmental factors may also have contributed to the declines. Many other fisheries have also collapsed or may be close to doing so.

FIGURE 2-11 The effects of overfishing. (a) A history of changes in sardine stocks off California. Note that fishing pressure (landings data) remained intense despite the dramatic decline in abundance in the 1940s. California sardine stock declines have occurred periodically, so natural factors may have caused this decline. However, overfishing as the stock declined likely contributed and worsened the decline. (b) King crab abundance in Alaska declined dramatically in the early 1980s, when fishing pressure increased substantially. Neither sardine nor King crab populations have yet fully recovered.

Most major fisheries in the world are today being fished close to, or above, their maximum sustainable yield (CC16). When fishing is drastically reduced or halted for a species whose population has collapsed, the population of some species recovers. However, for other species, recovery never occurs because the species has been replaced in the food web by some other, often less commercially valuable, species. Evidence suggests that some fishes are steadily being replaced by jellies (jellyfish) in many parts of the world ocean. This replacement may be caused by global climate change, but it is very likely that overfishing is also largely or partly responsible for any such changes.

The United Nations Food and Agriculture Organization (FAO) monitors fisheries statistics worldwide. FAO reports that the share of fish stocks harvested unsustainably has grown significantly, climbing from 10% in 1974 to 37.7% by 2021. A further 50.5% were being fished at their maximum sustainable yield in 2021. FAO acknowledges that its data is less than perfect due to poor reporting of fishery statistics by a number of nations.

The rapidly growing recreational fishing industry also contributes to overfishing problems. For example, the recreational catch of striped bass on the east coast of the U.S. more than doubled between 2021 and 2022. The recreational catch in 2022 was estimated to be about 16 thousand metric tons, more than 8 times the amount caught by commercial fishers.

Most traditional fisheries are in coastal waters near the consuming population. However, as coastal fisheries have been depleted, fishers have exploited resources from the deeper parts of the oceans and from distant coastal regions. The movement to far-flung ocean fisheries was accelerated by the development of refrigeration, which allowed catches to be stored and transported long distances in fresh or frozen condition. In addition, deep-ocean fisheries were traditionally free and open for anyone to exploit because no nation owned or controlled the resources. Unfortunately, the species that were the easiest targets of deep-ocean fishers were also those most vulnerable to overexploitation because they reproduce and mature more slowly. The decimation of populations of many species of whales and seals is among the better-known examples of the overexploitation of ocean resources.

Overfishing is particularly serious in many developing countries surrounded by coral reefs. As modern medicine enables human populations in these countries to expand, the limited fishery resources of reefs near each village can no longer provide sufficient seafood for the growing population. Fishers are forced to exploit reefs in an ever larger area and must often resort to technological “improvements” in fishing techniques, such as dynamiting and spearfishing. Some of the fishes killed by dynamiting float to the surface for easy harvesting, but many of them sink and are lost to the fishers. Furthermore, dynamiting destroys the reef and habitat for the fishes that escape the blast and for future generations of fishes. Intensive spearfishing, particularly with scuba, can quickly remove breeding adults of a population and thus hinder reproductive replacement of the population. In these instances, technological advances in fishing have proven to be very destructive to the resources.

Fishing threatens not only the targeted species, but also other species in the food web. For example, harvesting and drastic reductions in sea lion and elephant seal populations in California during the Gold Rush era (mid to late 1800s) led to a sharp reduction in the population of their natural predator, the great white shark. The marine mammal population became protected by law in 1972 and has recovered substantially since the early 1980s. Correspondingly, the great white shark population has also slowly increased. Another example of this pattern is the precipitous decline of the now rare Pribilof fur seal in the Bering Sea, which was almost certainly caused largely by increased fishing pressure on its principal food species, including pollack.

Beyond altering food web dynamics, fishing can negatively affect non-target species through unintentional capture known as bycatch. Many fishing techniques, including nets and trawls, are not efficient in selecting the target species. For example, turtles were often unintentionally caught in shrimp nets, and many dolphins were caught and killed in nets set to capture large schools of tuna. Considerable effort has been put into developing nets that, while they may be less efficient at catching the target species, do not catch and kill endangered turtles or marine mammals. Dolphins, sharks, turtles, and many nontarget fish species are also caught in kilometer-long drift nets that form a barrier across the ocean photic zone and that capture, and usually result in the death of, anything large enough not to pass through the mesh. As in other fisheries, this incidental bycatch of “nonvaluable” species is simply discarded overboard. Such drift nets are now outlawed by most nations but, as with other illegal fishing methods, may continue to be used in areas far from surveillance by law enforcement officials. Enforcement of fishing regulations on the high seas is extremely difficult because no nation can afford to patrol the high seas adequately. 

Bottom trawling, a widely used fishing technique where a net is dragged across the sea floor to catch bottom dwelling species, has been identified as perhaps the most environmentally damaging fishing technique. These trawls destroy deep sea corals and other nontarget benthos in seafloor ecosystems. Many of the species in these seafloor ecosystems are extremely slow growing so trawling damage in some areas may not be repaired by re-colonization and re-growth for decades or centuries.

Many fisheries are now managed to avoid overfishing, but management is often ineffective because assessing the fishery stock size and its age composition is expensive and difficult. In addition, the maximum sustainable yield may vary dramatically from year to year because of the chaotic variations induced by natural factors (CC16, CC11). Consequently, managers have only a poor understanding of what maximum sustainable yield might be. Safe management requires that substantially less than the estimated maximum sustainable yield be caught each year to guard against errors inherent in the stock assessment data and against the inevitable years when stocks decline unexpectedly because of natural factors. However, managers are under pressure to allow the largest possible annual catch in order to provide adequate income to the owners and crews of fishing boats competing for the resource. Setting the catch “too low” would mean lost income and possibly jobs for fishers, as well as unnecessary “wasting” of some of the resource value.

A number of alternative management concepts are now applied to some fishery resources to address this problem. These include assigning catch quotas to individuals, individual boats, or communities, granting exclusive limited fishing access to defined geographic areas, or both. Each of these regulatory approaches embodies the principle that access to the fishery resources is a privilege and that access to these resources should no longer be open to any and all persons who choose to fish. Another conservation approach—establishment of fishing-free natural reserves where fish populations can reproduce freely—is also now widely applied.

To this day, humans are primarily hunters and gatherers in the oceans, much as Stone Age people were on land. Human development from ocean hunter-gatherer to ocean farmer is long overdue. Mariculture (ocean farming) historically has been used on only a small scale in very few locations. However, it has developed and continues to grow rapidly, particularly in China, where it has been practiced for thousands of years, and in several other developing nations of the Pacific Ocean basin.

Other Biological Resources

Apart from their aesthetic value, marine species are an important pool of genetic diversity. Many marine species have developed unique biochemical methods of defending themselves against predators, parasites, and diseases, and of detoxifying or destroying toxic chemicals. Therefore, marine species are a major potential resource for the development of pharmaceutical drugs and pollution control methods. The search for beneficial drugs and pollution-fighting organisms in the oceans is extremely tedious and has barely begun. However, a substantial number of potentially valuable pharmaceutical products have already been isolated from marine species and many are being tested for a variety of medical purposes. A number of pharmaceuticals derived from marine organisms have already become approved for human use. These include compounds isolated from marine sponges such as the antiviral acyclovir and the HIV/AIDS drug azidothymidine (AZT). Many of the compounds that show promise have come from rare ocean animals or algae found only in limited areas of certain coral reefs or other threatened ocean ecosystems.

Coral reefs are like the tropical rain forests of the oceans, in that they are the most promising sources of pharmaceuticals because of their extremely high species diversity. They sustain large numbers of candidate species, any one of which may contain numerous chemical compounds potentially valuable as drugs. Some such naturally occurring compounds have been used to design similar molecules that have similar drug properties but that can be industrially produced from widely available raw materials. Unfortunately, other pharmaceutically active compounds isolated from marine species may not be readily synthesized or redesigned. If this proves to be the case, conservation measures will need to be developed and enforced to prevent impoverished villagers who live near a reef from using destructive harvesting techniques to supply the pharmaceutical industry.

An example of destructive harvesting techniques is provided by the industry that provides fishes for tropical aquariums. A growing number of fishes bred in aquariums are entering the market. However, most tropical fishes sold in the aquarium trade in the U.S. are still captured from Philippine reefs. Some collectors in the Philippines and elsewhere capture fishes by releasing cyanide into cracks in the reef, even though this practice is illegal. When cyanide is released into the reef, many fishes and other species die, but some fishes are not killed immediately. They swim out of hiding to avoid the cyanide, but they are stunned by the chemical and, therefore, easily captured. As many as 90% of the fishes that survive the initial cyanide collection die of the cyanide’s effects during transport to the U.S. or after the buyer has placed the fish in a home aquarium. The ornamental (aquarium) fish market in the U.S. was estimated to be worth about $6.3 billion in 2022. Only a very tiny fraction of this money ever reaches the harvesters or is used to promote sustainable collection practices.

Transportation, Trade, and Military Use

Despite the rapid growth in air transport, surface vessels remain the principal and cheapest means of transporting cargo and people across the oceans. Large numbers of commercial, recreational, and military vessels enter or leave U.S. ports every day. The importance of the oceans for transportation is evidenced by the estimated $2.1 trillion in shipped goods handled by U.S. ports, and a passenger cruise industry that contributed $36.0 to the U.S. GDP in 2023. In addition to this value for commercial ocean transportation, the oceans have important military uses. The oceans are plied by many surface naval vessels, but submarines have become particularly important, especially to the U.S. Navy, as platforms to transport and deploy ballistic missiles. As a result, during the past several decades, intensive efforts have been made to improve ways to hide and find submarines. Very extensive oceanographic studies have been conducted to support these efforts, particularly studies of ocean surface and subsurface currents and acoustic properties of the oceans.

The many benefits of our use of the oceans for transportation, trade, military, and recreation do come at a cost to the ocean environment. Ships have become progressively larger, particularly oil tankers, container ships, and naval vessels (especially aircraft carriers and submarines). Larger vessels require deeper ports and harbors, which has increased the need for dredging navigation channels in many bays, estuaries, and rivers. Dredging damages benthos at the dredging site, and the dredged material, which is generally dumped at a site in the estuary or coastal ocean not far from the dredging site, can also have serious impacts (Chap. 16).

Offshore Oil and Gas

Oil and gas are extracted from beneath the seafloor in many parts of the world. Most of the undiscovered oil and gas reserves are believed to lie beneath the continental shelves and continental slopes. The search for the sedimentary structures most likely to yield oil or gas under the oceans, and the development of technologies to drill for and produce oil and gas safely and efficiently, have been intense throughout the past several decades. Oil and gas have been produced from wells drilled in shallow water for many years. However, technological developments have steadily extended capabilities for drilling in deeper and deeper waters, and in areas where weather, waves, currents, and sometimes ice conditions are progressively more demanding. The search for oil and gas, and the need to identify and control the environmental impacts of this search, are a consistent and important focus of recent oceanographic studies.

In the EEZ of the U.S., the most extensive known oil and gas deposits are in the Gulf of America (Golfo de México). However, substantial known reserves are present beneath the continental shelves of the northeastern and southeastern U.S., California, and Alaska (Fig. 2-12). There may also be many undiscovered deposits, particularly off Alaska. The offshore petroleum industry is among the largest natural resource development industries globally. Oil and gas are used primarily as fuel for vehicles, industry, and heating, but they are also the basic raw materials for plastics, pharmaceuticals, and other chemicals, cosmetics, and asphalt. Although fossil fuel burning may be reduced to prevent further buildup of atmospheric carbon dioxide (CC9), petrochemicals will still be needed in the future.

FIGURE 2-12 Offshore oil and gas deposits occur throughout much of the U.S. Atlantic, Pacific, and Arctic Oceans, the Gulf of America (Golfo de México), and the Bering Sea continental shelves. The deeper continental shelf and continental slope of the Gulf of America (Golfo de México) and the Arctic Ocean continental shelf are currently thought to hold the greatest potential reserves.

Drilling in relatively shallow water is usually done from offshore platforms supported by long legs anchored to the seafloor (Fig. 2-13). Floating platforms (Fig. 2-13a) are also used, especially in deep water, while drilling in the nearshore Arctic is done from artificial gravel islands. A production platform may tap into 100 or more wells drilled into the seafloor below. Directional drilling, a process in which the well pipe is drilled downward and then turned underground to drill at an angle or even horizontally away from the drill site, now allows a single well to drain oil from an area as much as 12 km in radius and to recover more than 20 times as much oil per drill site or platform. On an ocean production platform, the well pipes generally extend up through the platform’s legs. On the platform, oil, gas, and water are separated, and oil and/or gas are usually transported ashore through a pipeline laid on, or buried in, the seabed.

FIGURE 2-13 Structures of the offshore oil and gas industry. (a) These offshore exploration drilling rigs are under maintenance in Galveston, Texas. On the left is a typical jack-up drilling rig; on the right is a typical floating rig that rides on pontoons below the depth of most wave action. (b) Production oil rigs permanently anchored to the seafloor of Cook Inlet, Alaska.

 Methane Hydrates

The decomposition of organic matter releases methane. At low temperatures and high pressures, methane molecules can be trapped within the crystalline lattice of water ice crystals to form a combination called “methane hydrates” (on average, one molecule of methane for every five or six molecules of water ice). Methane hydrates were first observed several decades ago in samples of cores drilled on land and the ocean floor. These hydrates could be a potential source of methane, a clean-burning fossil fuel. (It is completely combusted to carbon dioxide and water with virtually no chemical waste by-products or products of incomplete combustion, such as are produced by the refining and burning of other fossil fuels). However, widespread exploitation of this resource is considered unlikely for several reasons. Most importantly, methane hydrates are widely dispersed, usually occurring in the pore spaces of sediments and rocks.

Methane is rapidly released from the hydrates at normal atmospheric pressures and temperatures, but the deposits are generally too deep to mine and bring to the surface. However, several methods are now being tested that release methane from the hydrate in the sediment or rock so that it could be collected in a drill hole and recovered in the same way that petroleum and natural gas are recovered from oil and gas deposits.

In 2000, interest in methane hydrates was revived when some fishers dragged their trawl net at a depth of 800 m in a canyon about 50 km east of the mouth of Puget Sound in the northeastern Pacific Ocean. The fishers were startled to see their net rise to the surface filled with 1000 kg of icy chunks that were fizzing and melting. They hauled the “catch” aboard their vessel but quickly shoveled it back overboard because they had no idea what it could be. They tried to save samples in a freezer, but the low temperature alone was not enough to stop the methane from escaping the hydrate. The gas, expanding as it was released from the hydrate, even broke the containers in which they tried to store the hydrate. 

A year later, an ROV discovered what were described as “glaciers” of frozen methane hydrates forming outcrops on the seafloor about 50 km from where the fishers had found hydrates. Since that time, deposits of methane hydrates have been found on the seafloor or buried in shallow sediments in several other areas of the oceans, and they are now thought to occur in many parts of the world’s oceans, especially on the continental slope.

The mechanism that forms methane hydrate deposits is poorly understood. Some hypotheses suggest that the methane is a product of the decomposition of organic matter buried in the sediments in marginal seas as the continents were pulled apart—the same source thought to be responsible for most of the world’s oil and gas deposits (Chap. 4). Instead of being contained in nonporous rocks and converted to oil and natural gas, decomposing organic matter released methane that migrated through porous sediments and rocks until conditions were right for the formation of methane hydrates.

Methane hydrates are unstable except at high pressures and low temperatures, so they occur only below a depth of about 500 to 600 m in the oceans. However, these conditions are also present at relatively shallow depths beneath the frozen tundra of the Arctic regions. Not surprisingly, methane hydrate deposits have also been found in these environments. Estimates of the extent of methane hydrate deposits suggest that the total world resource may be more than 100 times the total volume of natural gas estimated to be recoverable from world oil and gas deposits. Thus, although most methane hydrate is widely dispersed and probably unrecoverable, the economic value would be huge if only 1% of the total were contained in concentrated deposits that could be recovered. The methane would also be a very efficient and clean-burning fuel that could be readily adapted to most commercial and industrial energy uses. 

The first field test of the extraction of methane from methane hydrate deposits in the ocean was carried out in 2013 at a well drilled in the Nankai Trough, an ocean basin 80 km off the coast of Japan. Japanese researchers retrieved methane up to the surface over a period of one week from a water depth of 1000 m. Japan and China both successfully produced methane from test production wells in 2017, China drilling in the South China Sea and Japan drilling off Japan’s central coast. Significant efforts to establish a methane hydrate extraction industry are concentrated in a few nations, with Japan and China leading the way. The U.S. has identified the presence of methane hydrate deposits in the Gulf of America (Golfo de México) and Alaskan permafrost, but interest in these deposits has, for now, focused on research and mapping.

Minerals and Freshwater

Ocean sediments contain vast quantities of mineral and material resources other than just oil and gas. They include sand and gravel (Fig. 2-14b), manganese nodules, hydrothermal minerals, phosphorite nodules, and heavy minerals such as gold that are often present in sediments of current or ancient river mouths (Fig. 2-14a). Sand and gravel are currently mined in large quantities from the shallow seafloor and used as construction materials in locations where no local land resources are available. At present, few efforts have been made to exploit other marine mineral resources. Cassiterite, a tin mineral, is dredged from shallow waters offshore from Thailand and Indonesia, and gold-bearing sands are dredged from shallow river mouth deposits offshore of Alaska, New Zealand, and the Philippines. Despite the limited scope of current ocean mining activities, there is substantial interest in future development. Mineral deposits thought to be most likely exploitable include phosphorite-rich sands as potential sources of phosphorus for fertilizer, manganese nodules, and seafloor massive sulfide (SMS) deposits laid down by hydrothermal vent activity as potential sources of metals including iron, zinc, copper, nickel, cobalt, manganese, molybdenum, silver, gold, and platinum.

FIGURE 2-14 Some mineral resources of the oceans. (a) The principal mineral resources of the seafloor are manganese nodules that occur in areas of abyssal seafloor, phosphorite nodules and deposits that occur primarily on the continental shelf, and placer deposits (minerals containing gold and other heavy metals) that accumulate at river mouths. There are a number of commercial mining operations in different parts of the world, as well as other areas where potentially commercial deposits occur. (b) There are abundant sand and gravel resources on the continental shelves of the U.S. Atlantic and Pacific coasts in areas where fine-grained muddy sediments do not dominate.

Many mineral resources, particularly manganese nodules and hydrothermal minerals, are found primarily in the deep oceans far from land. The discovery of such mineral deposits has led to extensive oceanographic research to identify the processes that created them and to determine their distribution and abundance on the seafloor. Deep-ocean minerals are currently too expensive to mine compared to the dwindling, but still adequate, sources on land. However, the potential future value of those resources is considerable. If deep-ocean mining is developed, it may have significant environmental impacts, especially if large quantities of fine-grained sediment are released into the naturally clear waters of the open-ocean photic zone.

Until 1982, the mineral resources of the deep-ocean floor were not owned by any nation or individual and could legally be mined by anyone. During the 1960s and 1970s, a widespread fear arose in the international community that deep-ocean minerals might be exploited and depleted to the benefit of only one or two nations that commanded the technology to mine them. This fear was a principal driving force behind the negotiations that led to the Law of the Sea Treaty, described earlier in this chapter.

Table salt and freshwater are both produced from ocean waters. Salt is produced by evaporation in coastal lagoons (Fig. 2-15). Freshwater is produced by evaporation or by reverse-osmosis extraction units. The process generates high-salinity brine wastes that are discharged to the oceans. The high salinity may have adverse effects on the biota, especially if water temperature is also high and causes additional stress. Such situations occur in locations, such as the Persian Gulf, especially off Saudi Arabia, where freshwater production from seawater is practiced most intensively.

Figure 2-15 Solar evaporation ponds are used to produce common salt in South San Francisco Bay, California. (a) This satellite image shows the evaporation ponds in South San Francisco Bay. The bright red and green colors are due to blooms of different algae species that grow at different salinities as water evaporations. Levees separate the ponds. Many of these ponds have now been restored to wetlands. (b) The large white mounds are piles of salt collected from the ponds and awaiting processing, packaging, and sale.

Recreation, Aesthetics, and Endangered Species

Humans have probably always enjoyed living on the coast, not only for the ocean’s food resources, but also for its aesthetic qualities and its moderating effects on climate (Chap. 9, CC5). It is the aesthetic qualities of oceans that have inspired poets and artists for millennia. However, only within the past generation or two have the oceans become popular for a variety of recreational activities, including sunbathing, swimming, surfing, sailing, luxury cruises, snorkeling, and scuba diving. As these pastimes have developed, a wider range of people now see the ocean environment as a vital part of nature that deserves protection, unlike much of the despoiled land. Underwater photography and video have introduced a large percentage of the human race to the beautiful, strange, and alien world of marine life.

As these recreational uses have become more popular, there has also been a growing recognition of the effects that such activities have on the marine environment. For example, anchor damage by boats carrying recreational scuba divers has become so severe in some locations that anchoring is now illegal, and boats must tie up to permanent mooring buoys installed at carefully selected locations on the most heavily dived reefs. Scuba divers also can damage coral reefs by breaking coral with their fins or hands. Although incidental contact with the reef and some coral damage is inevitable, most divers are now careful to protect the reefs on which they dive. Damage to reefs by divers is a significant problem only on the most popular reefs, where many divers are in the water every day. Where divers are only periodic visitors, the reefs are able to recover from any minor damage they may sustain. Indeed, damage occurs naturally as a result of the feeding and other activities of large reef animals, such as parrot fishes, and as a result of periodic strong storms. Sometimes such damage is even beneficial to the health and species diversity of coral reef communities (CC17).

Energy

As fossil fuels have been depleted, climate change concerns have grown, and nuclear energy has lost favor, a major search has begun for alternate sources of energy. The oceans offer several potential sources of energy, including tides, waves, ocean currents, and thermal gradients. Full development of these energy resources, if possible, would allow substantial reduction in the use of fossil fuels. This, in turn, would decrease the emissions of carbon dioxide and air contaminants, such as sulfur dioxide, nitrogen oxides, and particulates. However, at present very little energy is generated from any ocean resource. Substantial engineering problems must be solved before ocean energy resources can contribute significantly to global energy demands. In addition, the possible environmental impacts of ocean energy generation technologies must be assessed.

The only ocean energy source currently exploited on a large commercial scale is the tides (Chap. 12). The technology used is exemplified by the tidal power plant at La Rance, France (Fig. 10-21). Because this technology requires a dam across the mouth of a bay or other inlet in which the tidal range is very large, relatively few locations are suitable (Fig. 10-20). To generate power during most of the tidal cycle and adjust to fluctuating energy demands during the day, water must be stored behind the dam during part of the tidal cycle and restricted from moving into the inlet during the flood. Consequently, tidal currents may be considerably reduced, residence time increased, and sedimentation altered in a way that can be detrimental to benthic biota, including valuable shellfish and bottom-dwelling fishes. The dam also hinders the passage of organisms and vessels.

A number of technologies have been proposed for energy recovery from currents. The most developed technology involves placement of huge fanlike turbines in a current. However, many huge turbines would need to be placed side by side across the current to generate significant power. Existing facilities are generally few and small, serving primarily isolated communities. 

The nature and scope of possible adverse environmental effects of the proposed technologies are unknown. The major concern may be that modifying currents could have other far-reaching effects. For example, if enough turbines were deployed across the Gulf Stream to generate the power equivalent of several nuclear power plants, energy withdrawn from the current and the eddy motion caused by the turbines could conceivably alter the direction and speed of the Gulf Stream to some small extent. Even the remote possibility of such adverse consequences will probably prevent investment of the large sums of money needed to develop this technology.

The energy potentially available from ocean waves and currents is truly enormous, but such energy is widely dispersed and will not be easy to harness. Coastlines where wave energy is greatest, and therefore the potential for generating energy from waves is highest, are shown in Figure 2-16a.

FIGURE 2-16 Harnessing energy from the ocean. (a) Waves could be harnessed for energy in areas where average wave energy is high. Such areas are primarily in high latitudes (where strong storms are more frequent), on west coasts of continents (because most storms move east to west with the westerlies), and on southern coasts of the Southern Hemisphere (because strong storms are frequent around Antarctica, and no continents limit the fetch or intercept the waves). (b) Various wave energy generators have been successfully used on some rocky coastlines. The design shown here has a chamber open to the sea just below mean sea level into which waves can force water. As water is forced in, the air in the chamber is compressed and drives a turbine. (c) One of several designs for wave energy generators that float on the ocean surface. This one is a “dam atoll.” Most of its mass is below the depth of wave motion, such that it does not rise and fall as waves pass. Instead, water enters the generator at the surface, spirals downward, and drives a turbine. (d) A wave energy generator that uses a field of mechanical pumps on the shallow seafloor to pump seawater into a reservoir, where it can be released through a generator. (e) OTEC power plant in Kona, Hawaii. You can see the long vertical pipe bringing in cold deep water and the discharge pipe extending into the ocean at the top of the photograph.

Several design approaches have been developed to extract energy from waves, although none has yet been tested on a large power plant scale. One relatively simple design uses the rise and fall of water at the shoreline to compress air within a boxlike structure or shaft, which is open to the ocean only below the waterline (Fig. 2-16b). The compressed air drives a turbine. A variant of this design uses the upward and downward surge of water within an enclosed box or shaft to drive the turbine directly. Although these designs are simple and proven, they have limited value because they can be effective only on coasts with persistent strong wave action. In addition, multiple generators must be placed along a coast if large amounts of power are to be generated. Such structures would have adverse effects on the aesthetics of the shore and alter sediment transport along the coast. Generators of this type have been built to serve small coastal communities in Norway. Ironically, one of the first such installations was destroyed by severe storm waves soon after it was built.

Several wave power extraction devices have been designed to be deployed offshore. For example, the “dam atoll” is designed to focus waves toward a central generator shaft (Fig. 2-16c). A more recent device design appears to overcome some of the disadvantages of earlier systems (Fig. 2-16d). In this design, a matrix of simple mechanical pumps would be placed just beyond the surf zone, where they would use wave energy to pump seawater up into a reservoir on the adjacent land. Power could then be produced, even when the wave energy was low, by releasing the seawater from the reservoir through a turbine. These pumps would be submerged, minimizing the aesthetic impact. However, they would need to cover very large areas of seafloor and fill large reservoirs to produce as much power as a typical power plant, and the pumps would have to withstand wave impacts during even the largest storms. 

Several wave energy generator designs have been tested with mixed success, but they would need to be deployed not far offshore in strings stretching along many kilometers of coast. They could be a navigation hazard and might interfere with movements of marine organisms, particularly marine mammals. Even more detrimental, a string of such wave generators along a coast would drastically reduce wave energy at the shore reducing longshore drift, changing sediment grain size, and altering both the sedimentation regime and the associated biology of the nearshore zone. On rocky coasts, the important supralittoral zone with its unique biota (Chap. 17) would be particularly affected.

The quantity of ocean thermal energy that potentially can be exploited is huge and may be easier to extract than wave or current energy is. Ocean thermal energy conversion (OTEC) systems exploit the temperature difference between deep and shallow waters in the oceans to drive a turbine and generate electricity. The process by which energy is extracted to run the turbines is analogous to a refrigerator running backward, or a power plant operating at unusually low temperatures. Conventional power plants use heat from burning fossil fuel or a nuclear reaction to vaporize water in a closed container. The resulting high-pressure steam drives a turbine and is then condensed by cooling water to be recycled.

In OTEC, water is replaced by ammonia or another suitable liquid that is vaporized at much lower temperatures. The ammonia is heated by warm surface waters flowing over heat exchanger tubes through which the ammonia passes. Ammonia evaporates, and the resulting high-pressure gas drives a turbine. Once through the turbine, ammonia is condensed as it passes through another heat exchanger cooled by cold water pumped up from below the permanent thermocline. The ammonia is then recycled. An OTEC power plant is essentially built around a wide pipe that reaches to depths from which cold water can be pumped. Such systems have already been tested successfully.

OTEC is very promising and is probably the form of ocean energy generation most likely to contribute significantly to world energy needs. However, OTEC can be used efficiently only where surface waters are warm and cold deep waters are accessible—conditions found year-round primarily in tropical and subtropical latitudes. Locations where deep water is found close to land are ideal because long power transmission lines across the seabed would not be needed. In the U.S., only a few coastal locations, such as Hawaii (Fig. 2-16e), where an operating prototype and research OTEC facility has been operational since the 1980s, and the east coast of South Florida, are suitable. Pacific island nations are perfect locations, but the considerable investment needed to develop and build OTEC power plants is difficult for such nations to afford. Several operating OTEC plants are currently located in Japan, China and, on the French island of Reunion in the Indian Ocean. The largest operating OTEC plant is a 100 Megawatt (enough to supply about 50,000 homes) plant in Okinawa, Japan. Continued development in the U.S. is uncertain but negotiations began in 2016 to design and build an OTEC plant on the island of St. Croix in the U.S. Virgin Islands. OTEC may become a more attractive technology if proposed floating OTEC platforms are fully developed. Floating OTEC platform designs could use the electricity they generate to produce hydrogen by electrolysis of water. The hydrogen could be either transported ashore in tankers to be used as a fuel or used to generate ammonia (for use in fertilizer and chemical manufacturing) from atmospheric nitrogen.

Waste Disposal

The oceans have been used for waste disposal for thousands of years and, for most of that time, without significant harm to the marine environment (Chap. 16). Indeed, use of the oceans for sewage waste disposal has historically proven to be one of the most effective advancements ever made in human health protection.

Oceanographic research has documented a variety of problems caused by the disposal of certain wastes in parts of the oceans. Although this research has led to the mitigation of many of the worst impacts, much remains to be learned. The oceans will undoubtedly continue to be used for waste disposal, and this may be the most environmentally sound management approach for some wastes. However, a much better understanding of the oceans is necessary to determine which wastes can be disposed of in this way, in what quantities, where, and how, without adversely affecting the environment or human health.

The oceans have a very great capacity to assimilate, safely and completely, large quantities of natural wastes if these materials are widely dispersed or released slowly enough that they are thoroughly mixed into the huge volume of the open oceans. Hence, the oceans suffered little from waste disposal practices until the past century, when human populations and modern industry began to grow explosively. Problems arose when cities and industries grew and became concentrated in large urban areas. This concentration caused the rate of disposal in many coastal and estuarine areas to exceed the rate at which the wastes could be dispersed and assimilated. Additional problems arose with the development of synthetic chemicals and materials, because the ocean ecosystem had no mechanisms to destroy or neutralize some of these substances.

Most of the wastes currently disposed in the oceans are liquids or slurries that are discharged through pipelines, called outfalls, into rivers, estuaries, or the coastal ocean. At one time, quantities of a variety of solid wastes, including chemicals, low-level radioactive wastes, construction debris, and trash, were transported out to sea on vessels and dumped with the assumption that they would simply fall to the ocean floor, most of which was thought to be lifeless, and not cause any harm. Growing understanding of ocean ecosystems and the effects of ocean dumping have now led to the elimination of almost all dumping of solid wastes in the oceans. Dredged material is now the only waste material dumped at sea from vessels in large quantities.

Waste disposal and its effects in the oceans are discussed in more detail in Chapter 16.