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The metals are deposited by gas exchange at the sea surface , by fallout
of particles (dry deposition) , or are scavenged from the air column by
precipitation ( wet deposition).
Rivers alo make a major contribution of metals to the sea. And other
quantities of metals are added to the sea by direct discharges of
industrial and other wastes by pipelines , and by dumping sewage sludge
and industrial wastes at sea.
Mercury :
Mercury is the only contaminant introduced by humans into the sea.
Natural inputs of mercury to the sea are from the weathering mecrury
bearing rocksand degassing of the earth’ s crust , particularly through
volcanic activity .
Following the discovery , in the early 1960s,of the dangers to human
health of mercury in the marine environment , there has been a steady
reduction of man-made inputs , partly through the imposition of strict
controls on discharges of wastes containing mercury , and partly
through elimination of the use of mercury and mercurial compounds.
Bivalve molluscs take up mercury from the surrounding water very
quickly.
Some fish species notably Tuna , swordfish
and marlin
naturally
contain high concentration of mercury . Concentration of 1 ppm in the
muscle are common , and may be as high as 4.9 ppm.
Seabirds
naturally acquire high concentrations of mercury without
detrimental effect . High concentrations of mercury are found in the
liver , where it may be demethylated and selenium also accumulates in
the tissues , wher it gives protection against mercury toxicity.
The toxicity of mercury to humans has been known for centuries.
The
World Health Organisation
recommended a maximum tolerable
consumption of mercury in food of 0.9 mg of methyl mercury or 0.3
mg of total mercury per week.