Many heavy metals such as arsenic, lead, cadmium, mercury and copper are naturally present in seawater at very low concentrations reflecting their low solubility (see Table 4.3). Various organisms need some of these in very small amounts, for normal metabolism. However, increased concentrations resulting from pollution may be harmful both to marine organisms and to humans.
The concentration of heavy metals in the water may be raised locally by discharges from many industrial processes, and in sediments they may become very high. Sewage sludge dumping provides a significant input. Metals may also be released into the water from sediments disturbed by dredging, or by changes in pH or redox potential.
Shellfish such as oysters, mussels and clams bioaccumulate metals but do not seem themselves to be affected. In contrast, most fish and crustaceans excrete any metals they take in with their food. The exceptions are mercury and cadmium. Levels of these metals in top predatory fish such as tuna may exceed levels considered safe for human consumption. High levels have also been found in dead killer whales washed ashore and may have contributed to their deaths.
Mercury is present in the effluents from several industries, for example those involved in the manufacture of chlorine, acetaldehyde, caustic soda and paper. It is contained in many agricultural fungicides and some is released by the burning of fossil fuels. Mercury poisoning was the cause of the most serious human catastrophe yet arising from marine pollution, an outbreak in 1953 at Minamata in Japan of what was at first thought to be a mysterious disease. Later the problem was traced to the consumption of local-caught fish and shellfish contaminated by mercuric wastes dumped into the bay by a nearby chemical factory. Two hundred and thirty people subsequently died and a thousand others suffered permanent cerebral and nerve damage from this source. A second outbreak killing five people and affecting thirty others occurred in Japan in 1965 near the mouth of the river Agano from a similar cause. These tragic events illustrate the folly of assuming that discharges into coastal waters are safely diluted and dispersed.
Metallic mercury is virtually insoluble in water and very little is absorbed into tissues except in those who expose themselves to long and continuous contact, such as dentists. In water, metallic mercury is acted upon by micro-organisms which gradually convert it into a variety of organic compounds, notably to methyl-mercury compounds which are soluble and highly toxic, and readily absorbed and concentrated by organisms.
Arsenic is present in some detergents and was widely used in pesticides and herbicides until at least the 1960s. In seawater it exists mainly as arsenate but a proportion becomes converted to the more highly toxic arsenite. Arsenic compounds are readily concentrated in the tissues of certain marine fish.
Lead is often present in the effluents from mine workings in metalliferous areas, and becomes concentrated in the tissues of some marine species. Other metals which may also be present in marine sediments near river mouths carrying mine-washings include cadmium, chromium, nickel, copper and zinc, all of which have been found in high concentrations in various worms and molluscs from these areas (Clark, 1986). Lead also enters the sea in appreciable quantities from the air due to atmospheric lead pollution from the exhaust fumes of internal combustion engines.
Many marine organisms concentrate heavy metals and it appears that this increases their tolerance to even greater concentrations. Certain metals, such as copper, are essential for normal enzyme activity but may become enzyme inhibitors at high concentrations. Except for the Minamata tragedy there is not much evidence of human detriment from metal contamination of marine foods.
Some fatalities have been reported where chromium and cadmium contamination of shellfish were implicated. There is also the danger that several metals may act additively or synergistically.
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