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The current state of the oceans
Marine Pollution Bulletin, Volume 25, 1-4, pp. 28-31, 1992. Printed in Great Britain.
0025 326X/92 S5.00+0.(10 © 1992 Pergamon Press Ltd
The Current State of the Oceans A. D. MclNTYRE
Department of Zoology, Universityof Aberdeen, TillydroneAvenue, Aberdeen AB9 2TN, Scotland, UK
The open oceans are in good condition. They receive contaminant inputs mainly from the atmosphere and from shipping, and effects on the biota are minimal. In contrast, coastal zones around the world are significantly at risk from human activities. In addition to domestic and industrial inputs, both directly and via rivers, the margins of the seas are at risk from developments along the coast which destroy natural habitats, and from activities in the hinterland including dam building and land-use practices, which have major consequences in shallow waters. Recent assessments suggest that at the present time public health risks from sewage, eutrophication from excessive nutrients, and the toxic effects of persistent organic compounds are the topics of major concern.
To review the state of the oceans in a few thousand words may seem something of a tall order, but the topic was dear to the heart of Mike Waldichuck to whom this volume is dedicated, and he has published a number of particularly relevant contributions. In addition, the task is greatly facilitated today because several international bodies (UNEE OECD, IUCN, WRI) have recently produced assessments of the health of the oceans, some of which were designed as input to the 1992 United Nations Conference on Environment and Development. Given this background it is possible to comment briefly but authoritatively on the condition of the marine environment in the early 1990s. The review which follows draws heavily on the publications referred to above which are detailed in the Reading List. Given the extent and diversity of the oceans, any single generalization about their condition would be facile, but at least it is reasonable to draw a clear distinction between open seas on the one hand, and coastal areas on the other, a distinction which is valid particularly because of the differing nature of the potentially contaminating inputs to the two realms. T h e Coastal Z o n e Marine pollution is defined by GESAMP (Pravdic, 1981) in terms of inputs by humans which cause deleterious effects. Since most of such inputs originate from the land, it is only to be expected that the immediate impact would be in coastal zones. The current concensus is that three categories of input are of priority concern--sewage, nutrients, and synthetic organic compounds. Sewage has long been recognized as a local problem 28
in the sea, but with the continual increase of coastal populations around the world, and the consequent multiplication of sewage discharges, mostly untreated, to shallow water, the threat to public health is now on a global scale, and it has two aspects. First is the contamination of seafood, particularly filter feeding molluscs, by human pathogens from sewage, causing hepatitis, cholera, and a range of gastro-enteric diseases. Second is the risk to recreational users of beaches and coastal areas from sewage-contaminated sea water. Swimmers, wind surfers, water skiers, and even those using beaches only for sunbathing can be exposed to sewage micro-organisms and may suffer gastric upsets as well as ear, nose, throat, and eye illnesses. The need for proper treatment and disposal of sewage is now widely recognized, and monitoring of shellfisheries and bathing water is mandatory in some parts of the world. Unfortunately current monitoring techniques using faecal coliforms as indicators of sewage contamination are less than satisfactory and earlier work on water quality criteria in this connection is being questioned (Fleisher, 1991). Nutrients, particularly nitrogen and phosphorus, are now also seen as of greater concern than previously. These biostimulants which enhance plant growth, macroalgal seaweeds on the shore, and phytoplankton in the pelagic zones, were accepted in the past as a local problem confined to the vicinity of specific inputs. However, with the greatly increased use of agricultural fertilizers, the expansion of intensive stock rearing, and the many additional sewage and industrial discharges, the occurrence of excessive algal blooms is being increasingly reported from around the world, leading to deoxygenation of the water column, mortalities of fish and invertebrates, and reduction of coastal amenities from banks of sludge and scum from decaying algae. The southern bight of the North Sea, the inner part of the Adriatic Sea, coastal bays of Japan, and industrial sites from many parts of the world have all been implicated in recent years, and indeed any urbanized coastal area with reduced flushing from the open ocean is likely to be at risk from eutrophication. The problems are exacerbated when algal blooms are composed of toxic species, and records of these, causing problems to public health, fisheries, and to natural communities, have greatly increased. The third pollutant category of major current concern is synthetic organic compounds, which reach coastal waters particularly via rivers, direct discharges, and spills. Pesticides, herbicides, and industrial compounds such as chlorinated hydrocarbons and organometals are found in water, sediments and biota. Some,
Volume 2 5 / N u m b e r s 1 - 4
such as PCBs are extremely persistent and toxic. Public health risks are now reasonably well-controlled by monitoring, at least in the developed world, and fisheries in several parts of the world have been closed because of unacceptable residues in edible species (NOAA, 1988). The threat to marine life is less well understood. The main problem seems to arise from accumulation up the food chain resulting in high concentrations in top predators. In particular, decline of some seal populations may be due to the effects of PCBs on reproductive processes, and it is suggested that seals are more susceptible to disease because of interference with their immune systems by PCBs. The production and use of the most dangerous synthetic compounds have been banned or reduced by many nations, and some with relatively short half-lives, such as tributyltin, are now more or less under control, but others, because of their persistence will be widely distributed via the atmosphere and in the tissues of migratory species. They will remain in marine sediments and be available for recirculation to the biota for many years to come. Also, their great value in public health and agriculture to the developing world results in their continued application, particularly in the tropics. Thus, the adverse effects now well documented in industrialized countries must now be expected to spread. If the three categories of pollutants discussed above are regarded as of particular concern today, several others are now seen as less threatening in the sea: metals, radionuclides, and oil being prominent among these. Heavy metals were rightly a significant issue in the 1950s following the Minamata incident in Japan when consumption of mercury-contaminated seafood caused 43 deaths and many illnesses. However, this led to a general awareness of heavy metal dangers, to careful monitoring of such elements as mercury, lead, cadmium, and copper in marketable species, and to associated regulations on maximum permissible concentrations. As a result, heavy metals can no longer be regarded as a significant public health threat. Their more general impact on marine communities is evident only at the very high concentrations found in the immediate vicinity of metal-rich discharges or mine tailing effluents. Radionuclides were also near the top of the priority list of marine pollutants in the 1950s, but the Partial Test Ban Treaty of 1964 virtually stopped atmospheric testing of nuclear weapons, so that, accidents apart, marine inputs were confined to dumping (now halted) and to the relatively small number of discharges from power stations and reprocessing plants, all of which are rigorously controlled by international agencies. The third pollutant which has recently been reassessed is oil. Dramatic incidents such as the wrecks of the Torrey Canyon and the Amoco Cadiz, and the blowout of the Ixtoc well made worldwide headlines that suggested global catastrophe. But there now exists a well-documented body of knowledge on extensive investigations of large oil spills and on detailed followup studies of recovery, which highlights the great resilience of the marine environment in this context.
Damage can be severe but it is localized in space and time. The worst situations occur when oil contaminates coastal wetlands or is buried in sandy beaches which can remain contaminated for decades, but most spilled oil quickly disperses and degrades so that long-term effects on commercial fish stocks have not been recorded. Indeed, it now appears that the main global concerns from oil arise from operational discharges from ships, producing slicks which damage seabirds and degrade to form inert but persistent tar balls which circulate on ocean currents and affect beach amenity far from the original source, and from used oils from landbased sources. A possible new addition to the priority list of pollutants is plastics. The increasing use of synthetic compounds to replace natural materials for many types of manufactured goods has resulted in the proliferation of light, persistent debris in the form of fishing nets, straps, bands, containers, sheeting, and even fine particles, which float in the sea and, although chemically inert, interfere with and often kill marine life, and present a significant amenity problem on beaches. The coastal zone is clearly at risk from the various inputs discussed above, but an additional threat is posed more directly by some human activities. Coastal development associated with harbour construction, the building of industrial installations, and the demands of tourism including hotels, marinas, and other facilities tend to involve the draining of wetlands and the concreting of the coastline, so that natural habitats are destroyed, often irrevocably. In the Mediterranean for example, 42% of the coast of Spain is now taken up by tourist construction while on the French Riviera and round Alexandria, Athens, Istanbul, and Naples 90% of the coast is developed (Halim, 1991). While the effects of these changes at the land-sea interface are obvious, activities in the hinterland, sometimes hundreds of kilometres from the sea, can be equally devastating at the coast. The manipulation of hydrological cycles by dams and irrigation schemes interferes with the natural flow of water to the sea, altering the hydrographic regime, increasing the salinity of normally brackish waters and reducing the sediment input. In the 1960s there were as many as 55 dam-building schemes completed each year and the number is increasing, so that now at least 20% of the freshwater run-off from Africa and North America originates from impoundments. The opposite effect is created by land-use practices such as deforestation which result in soil erosion and the consequent silting-up of marine habitats which have evolved in situations of clear water and thus require low turbidity for survival. There is evidence, for example, of the death of coral reefs off the Philippines caused by deforestation in the mountains well inland.
Shelf Seas The bulk of the contaminating material entering the marine environment via rivers and direct discharges is 29
Marine PollutionBulletin retained in estuaries and coastal waters, where it sediments out on to the bottom. Very little is carried out to be distributed on the shelf and even less reaches the open ocean beyond the shelf edge at 200 m depth. The shelf, however, is the site of various human activities which do have environmental impact, including waste disposal and the exploitation of living and non-living resources. Until recently waste disposal was a significant factor. Sewage sludge, dredge spoils, chemical wastes, and radioactive materials were dumped or incinerated at sea, usually at specially designated sites. In some areas, for example in the southern bight of the North Sea and in the New York Bight, the rate of disposal operations was such that extensive stretches of the seabed were adversely affected. However, thanks to the London Dumping Convention there is now an international framework for the control of such activities and in many countries they are being significantly reduced. The shelf is also the site of exploitation of non-living marine resources. These include sand, gravel, shells and various heavy metals but at present the major activities relate to hydrocarbons--oil and gas, which are being increasingly extracted in marine areas around the world. The extension of these operations to any new region tends to be greeted with apprehension but in practice experience, for example in the North Sea, has shown that such fears can be greatly exaggerated. Assuming that major blowouts or other incidents can be avoided, the main pollution effect is confined to a few kilometres round the installations, and indeed the greatest concern probably arises from interference with fishing operations and with the disposal of the installations when their useful life is over. Finally, the shelf is the location of the world's major fisheries, yielding more than 80% of the global catch, and the pressure on stocks from intensive fishing is now a global problem. In most regions of the world stocks of commercial importance are fully exploited and many are overfished to the extent that closures are occurring or required. These intensive operations with heavy trawl gear have physical effects on the seabed and on the bottom-living species, while the excessive extraction of organisms from a community causes inbalance and alters the structure of the ecosystems. Most fisheries around the World are now fully exploited (FAO, 1990), and a number of important stocks have collapsed due to overfishing so the impact of fishing is far greater than that of any recorded pollution in the sea. This impact is greatly increased by the recent rise of mariculture which in some coastal regions has caused significant ecological changes (GESAMP, 1991). The establishment of coastal ponds for shrimp and fish cultivation, particularly in the tropics (Rosenberry, 1990), is leading to the significant loss of coastal habitats on a scale that must cause global concern.
human activities--the atmosphere and shipping. Atmospheric input is a diverse mixture of most known pollutants. Heavy metals, artificial radionuclides, synthetic organic compounds, nutrients, and hydrocarbons, including the products of evaporation, incineration, and combustion all mingle in the air, are carried round the world and eventually fall out by wet or dry deposition. However, because of the dilution and the long residence time often associated with atmospheric transport, the oceanic fallout, although measureable, leads to only very low concentrations in surface waters, so that no significant impact on marine biota has vet been detected. Nevertheless, quantification of atmospheric input to the oceans is not satisfactory since the dynamics of exchange at the air-sea interface makes the net flux difficult to measure. Further research in this field is required, so that adequate monitoring and assessment programmes can be developed. The second major source of contamination of the open ocean is shipping. This constitutes a more concentrated input, but is confined to traffic lanes and is usually subject to rapid dispersion and dilution, although some components such as tar balls and plastics are persistent and can be carried great distances, eventually accumulating on beaches far from the original source. However, these problems are now well addressed by international agreements such as M A R P O L 73/78, which, if ratified by all ocean states and properly implemented, will go a long way to containing the damage. Another threat to the condition of the open ocean, is from resource exploitation. For non-living resources, the concern must at present be regarded as only potential, for, although there are minerals of commercial interest in the deep seabed, metalliferous muds, oozes and nodules, at present the cost of extraction is so high in relation to their economic value that they are unlikely to be utilized. The use of energy from the deep sea--ocean thermal energy conversion (OTEC)--is currently being investigated and could lead to environmental problems, but this also is not likely to be operated on a big scale in the near future. The exploitation of living resources, on the other hand, is now a pressing current issue in the open oceans. For a century or more the problem was confined largely to sealing and whaling, particularly in the Antarctic, where some species were brought near to extinction and the balance of the marine food chain was altered by the reduction of large predators. Other oceanic resources, fast swimming species like tuna, salmon and squid, were not thought to be vulnerable. However the introduction of large purse seines and nylon drift nets provided the means of harvesting these species on a scale previously unthought of, and they also are now at risk. Fortunately, the threat is well recognized, even if not adequately documented, and international action is underway.
The Open Ocean As indicated above, pollutants from the land scarcely reach the seas beyond the continental shelf, but there are two major sources of input to the open ocean from 30
Conclusions As we move through the final decade of the Twentieth Century, contaminating inputs from human
Volume 25/Numbers 1-4
activities can be detected from the poles to the tropics in all parts of the marine environment, the water, the sediments, and the biota. In the open oceans the concentrations are not sufficient to cause detectable problems. In contrast, shallow waters are extensively degraded, and the vulnerable interface between land and sea, encompassing ecologically important brackish water and nursery grounds for commercial species is seriously polluted, while coastal constructions related to the demands of industry, tourism, and mariculture are progressively destroying natural habitats. The problems are recognized and some are addressed by international agreements and treaties, but much more must be done if the degradation of the marine environment is to be halted and reversed.
FAO (1990). Fisheries Circular No. 710, Revision 7. Rome. Fleisher, J. M. (1991). A reanalysis of data supporting US federal bacterial water quality criteria governing marine recreational waters. Research Journal, Water Pollution Control Federation 63(3), 259265. GESAMP (1991). Reducing Environmental Impacts of Coastal Aquaculture. Reports and Studies No. 47. Halim, Y. (1991). Manipulations of Hydrological Cycles. Annex V1 of UNEP Regional Seas Reports and Studies No. 115. IUCN/UNEP/WWF (1991). Caring for the Earth. Gland, Switzerland. NOAA (1988). PCB and Chlorinated Pesticide Contamination in US Fish and Shellfish: A Historical Assessment Report. NOAA Technical Memorandum NOS OMA 39. OECD (1991). Report on the State of the Environment. Paris. Pravdic, V. (1981). GESAMP, The FirstDozen Years. UNEP, Nairobi. Rosenberry, B. (1990). World Shrimp Farming 1989. European Aquaculture Society--Quarterly Newsletter No. 55. UNEP (1990). The State of the Marine Environment. UNEP Regional Seas Reports and Studies No. 115. Nairobi. WRI (1990). World Resources 1990-91. New York.
31
0025 326X/92 S5.00+0.(10 © 1992 Pergamon Press Ltd
The Current State of the Oceans A. D. MclNTYRE
Department of Zoology, Universityof Aberdeen, TillydroneAvenue, Aberdeen AB9 2TN, Scotland, UK
The open oceans are in good condition. They receive contaminant inputs mainly from the atmosphere and from shipping, and effects on the biota are minimal. In contrast, coastal zones around the world are significantly at risk from human activities. In addition to domestic and industrial inputs, both directly and via rivers, the margins of the seas are at risk from developments along the coast which destroy natural habitats, and from activities in the hinterland including dam building and land-use practices, which have major consequences in shallow waters. Recent assessments suggest that at the present time public health risks from sewage, eutrophication from excessive nutrients, and the toxic effects of persistent organic compounds are the topics of major concern.
To review the state of the oceans in a few thousand words may seem something of a tall order, but the topic was dear to the heart of Mike Waldichuck to whom this volume is dedicated, and he has published a number of particularly relevant contributions. In addition, the task is greatly facilitated today because several international bodies (UNEE OECD, IUCN, WRI) have recently produced assessments of the health of the oceans, some of which were designed as input to the 1992 United Nations Conference on Environment and Development. Given this background it is possible to comment briefly but authoritatively on the condition of the marine environment in the early 1990s. The review which follows draws heavily on the publications referred to above which are detailed in the Reading List. Given the extent and diversity of the oceans, any single generalization about their condition would be facile, but at least it is reasonable to draw a clear distinction between open seas on the one hand, and coastal areas on the other, a distinction which is valid particularly because of the differing nature of the potentially contaminating inputs to the two realms. T h e Coastal Z o n e Marine pollution is defined by GESAMP (Pravdic, 1981) in terms of inputs by humans which cause deleterious effects. Since most of such inputs originate from the land, it is only to be expected that the immediate impact would be in coastal zones. The current concensus is that three categories of input are of priority concern--sewage, nutrients, and synthetic organic compounds. Sewage has long been recognized as a local problem 28
in the sea, but with the continual increase of coastal populations around the world, and the consequent multiplication of sewage discharges, mostly untreated, to shallow water, the threat to public health is now on a global scale, and it has two aspects. First is the contamination of seafood, particularly filter feeding molluscs, by human pathogens from sewage, causing hepatitis, cholera, and a range of gastro-enteric diseases. Second is the risk to recreational users of beaches and coastal areas from sewage-contaminated sea water. Swimmers, wind surfers, water skiers, and even those using beaches only for sunbathing can be exposed to sewage micro-organisms and may suffer gastric upsets as well as ear, nose, throat, and eye illnesses. The need for proper treatment and disposal of sewage is now widely recognized, and monitoring of shellfisheries and bathing water is mandatory in some parts of the world. Unfortunately current monitoring techniques using faecal coliforms as indicators of sewage contamination are less than satisfactory and earlier work on water quality criteria in this connection is being questioned (Fleisher, 1991). Nutrients, particularly nitrogen and phosphorus, are now also seen as of greater concern than previously. These biostimulants which enhance plant growth, macroalgal seaweeds on the shore, and phytoplankton in the pelagic zones, were accepted in the past as a local problem confined to the vicinity of specific inputs. However, with the greatly increased use of agricultural fertilizers, the expansion of intensive stock rearing, and the many additional sewage and industrial discharges, the occurrence of excessive algal blooms is being increasingly reported from around the world, leading to deoxygenation of the water column, mortalities of fish and invertebrates, and reduction of coastal amenities from banks of sludge and scum from decaying algae. The southern bight of the North Sea, the inner part of the Adriatic Sea, coastal bays of Japan, and industrial sites from many parts of the world have all been implicated in recent years, and indeed any urbanized coastal area with reduced flushing from the open ocean is likely to be at risk from eutrophication. The problems are exacerbated when algal blooms are composed of toxic species, and records of these, causing problems to public health, fisheries, and to natural communities, have greatly increased. The third pollutant category of major current concern is synthetic organic compounds, which reach coastal waters particularly via rivers, direct discharges, and spills. Pesticides, herbicides, and industrial compounds such as chlorinated hydrocarbons and organometals are found in water, sediments and biota. Some,
Volume 2 5 / N u m b e r s 1 - 4
such as PCBs are extremely persistent and toxic. Public health risks are now reasonably well-controlled by monitoring, at least in the developed world, and fisheries in several parts of the world have been closed because of unacceptable residues in edible species (NOAA, 1988). The threat to marine life is less well understood. The main problem seems to arise from accumulation up the food chain resulting in high concentrations in top predators. In particular, decline of some seal populations may be due to the effects of PCBs on reproductive processes, and it is suggested that seals are more susceptible to disease because of interference with their immune systems by PCBs. The production and use of the most dangerous synthetic compounds have been banned or reduced by many nations, and some with relatively short half-lives, such as tributyltin, are now more or less under control, but others, because of their persistence will be widely distributed via the atmosphere and in the tissues of migratory species. They will remain in marine sediments and be available for recirculation to the biota for many years to come. Also, their great value in public health and agriculture to the developing world results in their continued application, particularly in the tropics. Thus, the adverse effects now well documented in industrialized countries must now be expected to spread. If the three categories of pollutants discussed above are regarded as of particular concern today, several others are now seen as less threatening in the sea: metals, radionuclides, and oil being prominent among these. Heavy metals were rightly a significant issue in the 1950s following the Minamata incident in Japan when consumption of mercury-contaminated seafood caused 43 deaths and many illnesses. However, this led to a general awareness of heavy metal dangers, to careful monitoring of such elements as mercury, lead, cadmium, and copper in marketable species, and to associated regulations on maximum permissible concentrations. As a result, heavy metals can no longer be regarded as a significant public health threat. Their more general impact on marine communities is evident only at the very high concentrations found in the immediate vicinity of metal-rich discharges or mine tailing effluents. Radionuclides were also near the top of the priority list of marine pollutants in the 1950s, but the Partial Test Ban Treaty of 1964 virtually stopped atmospheric testing of nuclear weapons, so that, accidents apart, marine inputs were confined to dumping (now halted) and to the relatively small number of discharges from power stations and reprocessing plants, all of which are rigorously controlled by international agencies. The third pollutant which has recently been reassessed is oil. Dramatic incidents such as the wrecks of the Torrey Canyon and the Amoco Cadiz, and the blowout of the Ixtoc well made worldwide headlines that suggested global catastrophe. But there now exists a well-documented body of knowledge on extensive investigations of large oil spills and on detailed followup studies of recovery, which highlights the great resilience of the marine environment in this context.
Damage can be severe but it is localized in space and time. The worst situations occur when oil contaminates coastal wetlands or is buried in sandy beaches which can remain contaminated for decades, but most spilled oil quickly disperses and degrades so that long-term effects on commercial fish stocks have not been recorded. Indeed, it now appears that the main global concerns from oil arise from operational discharges from ships, producing slicks which damage seabirds and degrade to form inert but persistent tar balls which circulate on ocean currents and affect beach amenity far from the original source, and from used oils from landbased sources. A possible new addition to the priority list of pollutants is plastics. The increasing use of synthetic compounds to replace natural materials for many types of manufactured goods has resulted in the proliferation of light, persistent debris in the form of fishing nets, straps, bands, containers, sheeting, and even fine particles, which float in the sea and, although chemically inert, interfere with and often kill marine life, and present a significant amenity problem on beaches. The coastal zone is clearly at risk from the various inputs discussed above, but an additional threat is posed more directly by some human activities. Coastal development associated with harbour construction, the building of industrial installations, and the demands of tourism including hotels, marinas, and other facilities tend to involve the draining of wetlands and the concreting of the coastline, so that natural habitats are destroyed, often irrevocably. In the Mediterranean for example, 42% of the coast of Spain is now taken up by tourist construction while on the French Riviera and round Alexandria, Athens, Istanbul, and Naples 90% of the coast is developed (Halim, 1991). While the effects of these changes at the land-sea interface are obvious, activities in the hinterland, sometimes hundreds of kilometres from the sea, can be equally devastating at the coast. The manipulation of hydrological cycles by dams and irrigation schemes interferes with the natural flow of water to the sea, altering the hydrographic regime, increasing the salinity of normally brackish waters and reducing the sediment input. In the 1960s there were as many as 55 dam-building schemes completed each year and the number is increasing, so that now at least 20% of the freshwater run-off from Africa and North America originates from impoundments. The opposite effect is created by land-use practices such as deforestation which result in soil erosion and the consequent silting-up of marine habitats which have evolved in situations of clear water and thus require low turbidity for survival. There is evidence, for example, of the death of coral reefs off the Philippines caused by deforestation in the mountains well inland.
Shelf Seas The bulk of the contaminating material entering the marine environment via rivers and direct discharges is 29
Marine PollutionBulletin retained in estuaries and coastal waters, where it sediments out on to the bottom. Very little is carried out to be distributed on the shelf and even less reaches the open ocean beyond the shelf edge at 200 m depth. The shelf, however, is the site of various human activities which do have environmental impact, including waste disposal and the exploitation of living and non-living resources. Until recently waste disposal was a significant factor. Sewage sludge, dredge spoils, chemical wastes, and radioactive materials were dumped or incinerated at sea, usually at specially designated sites. In some areas, for example in the southern bight of the North Sea and in the New York Bight, the rate of disposal operations was such that extensive stretches of the seabed were adversely affected. However, thanks to the London Dumping Convention there is now an international framework for the control of such activities and in many countries they are being significantly reduced. The shelf is also the site of exploitation of non-living marine resources. These include sand, gravel, shells and various heavy metals but at present the major activities relate to hydrocarbons--oil and gas, which are being increasingly extracted in marine areas around the world. The extension of these operations to any new region tends to be greeted with apprehension but in practice experience, for example in the North Sea, has shown that such fears can be greatly exaggerated. Assuming that major blowouts or other incidents can be avoided, the main pollution effect is confined to a few kilometres round the installations, and indeed the greatest concern probably arises from interference with fishing operations and with the disposal of the installations when their useful life is over. Finally, the shelf is the location of the world's major fisheries, yielding more than 80% of the global catch, and the pressure on stocks from intensive fishing is now a global problem. In most regions of the world stocks of commercial importance are fully exploited and many are overfished to the extent that closures are occurring or required. These intensive operations with heavy trawl gear have physical effects on the seabed and on the bottom-living species, while the excessive extraction of organisms from a community causes inbalance and alters the structure of the ecosystems. Most fisheries around the World are now fully exploited (FAO, 1990), and a number of important stocks have collapsed due to overfishing so the impact of fishing is far greater than that of any recorded pollution in the sea. This impact is greatly increased by the recent rise of mariculture which in some coastal regions has caused significant ecological changes (GESAMP, 1991). The establishment of coastal ponds for shrimp and fish cultivation, particularly in the tropics (Rosenberry, 1990), is leading to the significant loss of coastal habitats on a scale that must cause global concern.
human activities--the atmosphere and shipping. Atmospheric input is a diverse mixture of most known pollutants. Heavy metals, artificial radionuclides, synthetic organic compounds, nutrients, and hydrocarbons, including the products of evaporation, incineration, and combustion all mingle in the air, are carried round the world and eventually fall out by wet or dry deposition. However, because of the dilution and the long residence time often associated with atmospheric transport, the oceanic fallout, although measureable, leads to only very low concentrations in surface waters, so that no significant impact on marine biota has vet been detected. Nevertheless, quantification of atmospheric input to the oceans is not satisfactory since the dynamics of exchange at the air-sea interface makes the net flux difficult to measure. Further research in this field is required, so that adequate monitoring and assessment programmes can be developed. The second major source of contamination of the open ocean is shipping. This constitutes a more concentrated input, but is confined to traffic lanes and is usually subject to rapid dispersion and dilution, although some components such as tar balls and plastics are persistent and can be carried great distances, eventually accumulating on beaches far from the original source. However, these problems are now well addressed by international agreements such as M A R P O L 73/78, which, if ratified by all ocean states and properly implemented, will go a long way to containing the damage. Another threat to the condition of the open ocean, is from resource exploitation. For non-living resources, the concern must at present be regarded as only potential, for, although there are minerals of commercial interest in the deep seabed, metalliferous muds, oozes and nodules, at present the cost of extraction is so high in relation to their economic value that they are unlikely to be utilized. The use of energy from the deep sea--ocean thermal energy conversion (OTEC)--is currently being investigated and could lead to environmental problems, but this also is not likely to be operated on a big scale in the near future. The exploitation of living resources, on the other hand, is now a pressing current issue in the open oceans. For a century or more the problem was confined largely to sealing and whaling, particularly in the Antarctic, where some species were brought near to extinction and the balance of the marine food chain was altered by the reduction of large predators. Other oceanic resources, fast swimming species like tuna, salmon and squid, were not thought to be vulnerable. However the introduction of large purse seines and nylon drift nets provided the means of harvesting these species on a scale previously unthought of, and they also are now at risk. Fortunately, the threat is well recognized, even if not adequately documented, and international action is underway.
The Open Ocean As indicated above, pollutants from the land scarcely reach the seas beyond the continental shelf, but there are two major sources of input to the open ocean from 30
Conclusions As we move through the final decade of the Twentieth Century, contaminating inputs from human
Volume 25/Numbers 1-4
activities can be detected from the poles to the tropics in all parts of the marine environment, the water, the sediments, and the biota. In the open oceans the concentrations are not sufficient to cause detectable problems. In contrast, shallow waters are extensively degraded, and the vulnerable interface between land and sea, encompassing ecologically important brackish water and nursery grounds for commercial species is seriously polluted, while coastal constructions related to the demands of industry, tourism, and mariculture are progressively destroying natural habitats. The problems are recognized and some are addressed by international agreements and treaties, but much more must be done if the degradation of the marine environment is to be halted and reversed.
FAO (1990). Fisheries Circular No. 710, Revision 7. Rome. Fleisher, J. M. (1991). A reanalysis of data supporting US federal bacterial water quality criteria governing marine recreational waters. Research Journal, Water Pollution Control Federation 63(3), 259265. GESAMP (1991). Reducing Environmental Impacts of Coastal Aquaculture. Reports and Studies No. 47. Halim, Y. (1991). Manipulations of Hydrological Cycles. Annex V1 of UNEP Regional Seas Reports and Studies No. 115. IUCN/UNEP/WWF (1991). Caring for the Earth. Gland, Switzerland. NOAA (1988). PCB and Chlorinated Pesticide Contamination in US Fish and Shellfish: A Historical Assessment Report. NOAA Technical Memorandum NOS OMA 39. OECD (1991). Report on the State of the Environment. Paris. Pravdic, V. (1981). GESAMP, The FirstDozen Years. UNEP, Nairobi. Rosenberry, B. (1990). World Shrimp Farming 1989. European Aquaculture Society--Quarterly Newsletter No. 55. UNEP (1990). The State of the Marine Environment. UNEP Regional Seas Reports and Studies No. 115. Nairobi. WRI (1990). World Resources 1990-91. New York.
31