Marine Pollution, Oil Spills and Ocean Acidification

Marine pollution is the entry of harmful substances or energy into the sea, directly or through rivers, the air and the coast, in quantities the ocean cannot absorb without damage. Most of it begins on land, not at sea: sewage, farm runoff, industrial effluent and plastic carried down by rivers account for the greater part, while shipping, offshore drilling and dumping account for the rest. For exams the chapter divides into three blocks, each with its own vocabulary: the kinds of pollution, oil spills and their clean-up, and the chemistry of ocean acidification.

Kinds of marine pollution

Nutrient pollution is the enrichment of coastal water with nitrogen and phosphorus from fertiliser, detergent and sewage. It causes eutrophication: algae multiply into blooms, the bloom dies, bacteria decompose it and use up the dissolved oxygen, and the water becomes hypoxic. Where oxygen falls too low for fish and bottom life, the area is called a dead zone; the mouths of heavily farmed river basins are the classic sites. Harmful algal blooms also colour the water, giving the names red tide and red water.

Plastic is the most visible pollutant. Particles smaller than five millimetres are called microplastics; they come from the breakdown of larger plastic, from synthetic fibres in laundry water and from microbeads once used in cosmetics. Marine animals swallow them or become entangled in nets and rings, and the rotating currents of an ocean gyre gather floating debris into patches, the best known being in the North Pacific. Heavy metals bioaccumulate and biomagnify up the food chain: methylmercury discharged into Minamata Bay in Japan caused Minamata disease, a crippling nervous disorder first recognised in 1956, and cadmium caused the bone disease known as itai-itai. Other categories examiners list are thermal pollution from power-plant cooling water, radioactive waste, sewage and pathogens, antifouling chemicals, and underwater noise from shipping and seismic surveys.

Oil spills and how they are cleaned

Oil forms a film on the surface that cuts off light and gas exchange, coats the feathers of seabirds so they lose buoyancy and insulation, smothers mangroves and corals, and tars beaches. Clean-up uses containment booms to stop the slick spreading, skimmers to lift oil off the water, sorbents to soak it up, chemical dispersants to break the slick into droplets, controlled in-situ burning, and bioremediation with oil-degrading bacteria, for which the Indian product Oilzapper, developed by The Energy and Resources Institute, is the standard Indian example.

Ocean acidification

The ocean absorbs a large share of the carbon dioxide added to the air. Dissolved carbon dioxide reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions, and the extra hydrogen ions lower the pH. Average surface pH has fallen from about 8.2 before industrialisation to about 8.1, a small-looking change that means roughly a thirty per cent rise in hydrogen ion concentration, because pH is a logarithmic scale. The hydrogen ions also combine with carbonate ions, leaving less carbonate for the animals that build shells and skeletons of calcium carbonate: corals, molluscs, sea urchins and the planktonic pteropods near the base of the food web. Where water becomes undersaturated in aragonite, the form of calcium carbonate corals use, existing skeletons begin to dissolve. Note the distinction examiners test: acidification is caused by carbon dioxide and lowers pH, while coral bleaching is caused mainly by warm water, which makes the coral expel its symbiotic algae, the zooxanthellae, and turn white. The ocean is still alkaline, so acidification means a shift towards acidity, not acid water.

Conventions and Indian law

MARPOL, the International Convention for the Prevention of Pollution from Ships of 1973, modified by the 1978 Protocol, is the main instrument of the International Maritime Organization, with annexes on oil, noxious liquids, packaged harmful substances, sewage, garbage and air emissions. The London Convention of 1972 restricts dumping of wastes at sea. The United Nations Convention on the Law of the Sea of 1982 defines maritime zones, including the exclusive economic zone up to two hundred nautical miles, and obliges states to protect the marine environment. India applies the Water (Prevention and Control of Pollution) Act of 1974, the Environment (Protection) Act of 1986, the Coastal Regulation Zone notification issued under it and the Merchant Shipping Act of 1958. Oil spill response is run under the National Oil Spill Disaster Contingency Plan, for which the Indian Coast Guard is the central coordinating agency. Sustainable Development Goal 14, Life Below Water, is the global target students should be able to name.