environmental issues. Mankind has continually abused the sea, regarding it as an inexhaustible source of food and minerals, and a dumping ground for rubbish. Now it is more widely accepted that the oceans are finite, and there is a need to use and manage them carefully, especially as globally environmental pressures are mounting because of the burgeoning human population.
Pollution is important enough to be discussed separately.
Carbon Dioxide.
Regarded by some people as a pollutant, this is a key substance in natural cycles and so cannot be treated like man-made substances. As a greenhouse gas, increasing concentrations of carbon dioxide in the atmosphere threaten
climate change globally, modify ocean circulation, and so reduce the viability of many marine species. Sharp reductions in emissions per capita (>60%) are needed to avoid irreversible changes, bearing in mind the rate at which the human population is growing and future industrial expansion by the developing world, especially China. No one of the suite of solutions, which range from improving energy efficiency and switching to renewable energy sources, to capturing carbon dioxide and storing it somewhere—for instance, in deep geological deposits that formerly contained
offshore oil and gas—is likely to be sufficient.
Over-exploitation of Living Resources.
This is now a growing and controversial problem.
Fish,
shellfish, and, in Asia,
seaweeds are important sources of food, particularly in developing countries. Improvements in technology for locating and catching fish, and the considerable post-war expansion of commercial
fisheries, resulted in fish catches peaking at over 100 million tonnes a year in 2000. But these catches are now declining, and the evidence is that the majority of fish stocks are either over-exploited, or close to being so. A seemingly logical solution is to develop farming techniques for fish and certain
Crustacea, such as shrimps. However, these place a high demand on coastal areas and lead to habitat destruction and degradation, notably of the
mangrove swamps in the Far East. Providing suitable feed for such farms is also a problem. A substantial proportion of the million tonnes of sand
eels (
Ammodytes sp.) caught by commercial fisheries in the North Sea gets used to feed not humans but poultry and farmed fishes.
Habitat Destruction and Alteration.
This is a major environmental issue in places. In the tropics the clearance of mangrove swamps not only destroys the nursery feeding grounds of many fishes, but also makes the coastline vulnerable to flooding and
tsunami. In developed countries coastal defences, sea walls and groynes, alter coastal habitats and can result in coastal erosion elsewhere. Dredging shipping channels for deep-draught vessels like
container ships and
ro-ro ships, the building of port facilities, and removal of hazards to
navigation, all lead to small pieces of habitat loss that globally lead to massive losses of coastal environments. The biggest problem is associated with the urbanization of the coastline. Most of the world's megalopolises (cities with more than 10 million people) are sited on coastlines and there is a general migration of people to coastal environments; over half of all people now live within 60 kilometres (37 mls.) of the sea, putting enormous environmental pressure on coastal habitats, with all the concomitant pollution problems.
Non-living Resources.
These include hydrocarbons, aggregates, and minerals such as phosphates and placers. The exploitation of hydrocarbons for fossil fuels has direct implications for the marine environment. Enormous quantities are shipped around the world, about 1,500 million tonnes annually. Accidents inevitably happen, and the big spills caused by the foundering of large
tankers such as the
Torrey Canyon,
Exxon Valdez, and
Prestige are amongst a number of notorious events that have resulted in major environmental catastrophes, often in highly sensitive marine habitats. The firing of the oil wells and terminals during the 1991 Gulf War had an immense impact on the Persian Gulf. But such spectacularly awful events only result in the discharge of relatively small quantities of oil compared to the minor spills during the day-to-day operations of ports and offshore oil and gas facilities. These have a negligible effect on deep ocean habitats but have a serious impact on coastal and inshore environments, and on charismatic animals such as
seabirds and
marine mammals like sea otters.
The exploitation of mineral and aggregate resources, both from the sea and on the neighbouring land, also has considerable impact on the coastal seas. Diamonds are dredged from marine sands off Namibia, and tin from Malaysia. These, and other developing countries, mining for gems, precious metals, and iron minerals, generate large quantities of spoils and their disposal is a real problem. Land is becoming more and more important to conserve; discharging into rivers is almost always unacceptable, which leaves marine disposal as the least damaging option.
Dredging marine aggregates for building can be very damaging to marine habitats, though its overall impact may be less than using land deposits, especially when the impact of transporting the material to the site is taken into consideration. Delivery of aggregates by ship to a building site in central London or New York can be achieved with less disruption and environmental damage than by road.
Introduction of Exotic Species.
This is an environmental issue that emerged in the 1990s, though the reasons for it go back much further. For example, the building of the
Suez Canal has resulted in the movement of many species from the Red Sea into the eastern Mediterranean, some of which, like large
sharks, are not popular newcomers. Another route is the expansion of shellfish culture, especially of oysters, in which the local stocks have been ‘improved’ by the introduction of larger, faster-growing species from far afield, often bringing with them pests and diseases that have subsequently run rampant. The growing popularity of keeping marine species in aquaria has also resulted in the importation of undesirable new species, the most notorious of these being the appearance of a vigorous spore of a highly invasive green seaweed,
Caulerpa, beneath the windows of the marine aquarium in Monaco. This male spore can only reproduce by fragments of it being carried around by ships. However, once established it overgrows the local seaweed communities, drastically reducing the diversity, and destroying local fisheries. Not being able to breed sexually has been no bar to its dispersal; in two decades it has become established all round the western Mediterranean and is now in the eastern Mediterranean, most probably carried on the anchors of pleasure craft. Even more surprising has been its recent appearance in the seas off Japan, Australia, and California, in each case probably as a result of establishing marine aquaria.
However, the most serious introductions of exotic species have arrived in the
ballast water discharged from large
bulk carriers. Almost all the animals now inhabiting San Francisco Bay are species native to the seas off Japan rather than the Californian coast. Similarly, around many Australian ports returning large bulk carriers delivering iron ore to Japanese steel mills have brought back hundreds of unwanted and undesirable marine species in their ballast tanks. Most worrying are the large numbers of dinoflagellates that are implicated in
red tides. But many of the other species are able to out-compete and displace the native species because they arrive free of disease and the predators that normally keep them in check.
Eutrophication.
The over-enrichment of environments with excessive amounts of organic matter and fertilizers coming from run-off from land and rivers. It results in excessive plant growth, and the plant and animal communities become dominated by just a few rampant species. These changes in ecology lead to reductions in the numbers of species present including many commercial fish species. They also lead to an increase in the frequency of red tides.
Coral reefs are particularly sensitive to the impacts of eutrophication, becoming overgrown with seaweeds and eventually dying. Eventually, under extreme conditions, over-enrichment results in all the oxygen in the sediments being used up, and hydrogen sulphide and then methane is generated in the sediments, and creates dead zones, which are devoid of normal life. In 2004 the United Nations Environment Programme warned that ‘dead zones’ are being created in many waters—including parts of the Baltic, Irish, and Adriatic seas. The areas of these zones had doubled in the previous fifteen years and were becoming a greater threat to some fish stocks than overfishing.
Water.
Water itself is the final environmental issue. In arid countries there is a proliferation of desalination plants to provide the water needed by the human population, agriculture, and industry. Locally, these increase the
salinity of the sea water and so alter the local pattern of
currents. On a global scale, however, the major impact is the interception of fresh water for irrigation schemes before it reaches the ocean. For this reason, some seas such as the Aral Sea have completely dried up. It is estimated that around 50% of all river discharges are now intercepted and used for irrigation. This is best illustrated by the building of the High Aswan Dam in Egypt. This single project decreased the freshwater inflow into the eastern Mediterranean by over half. In the short term, this resulted in the collapse of the delta fishery and substantial decline in the productivity of the offshore waters. In 2004, over 30 years after the completion of the dam, the currents in the eastern Mediterranean are changing and it is only a matter of time before the western Mediterranean will show signs of being affected as well. The environmental fallout cannot be predicted, but is likely to be deleterious to both marine communities and fisheries, and perhaps even to the tourists on the beaches.
M. V. Angel
