Pesticides enter the sea from agricultural runoff, rivers and by airborne transfer (particularly DDT). In estuaries the quantities of pesticide in the water tend to vary seasonally according to local agricultural practice. Around the British Isles there are often two peaks, one in early summer following springtime dressings on crops and orchards, and a second peak in late autumn following the use of pesticides on autumn-sown wheat.

When it was first made, DDT was heralded as a wonder chemical capable of killing malarial mosquitoes and many other pests, with consequent saving of human life. It was only much later that the environmental consequences both on land and in the sea were realized. Chlorinated hydrocarbons such as DDT and PCBs have become widely dispersed throughout the oceans, even to the Arctic and Antarctic.

The main problem is that they are only very slowly degraded and are readily available for biomagnification in food chains. Phytoplankton take up the DDT because the large hydrocarbon molecules are not very soluble in water but are very soluble in fats such as the oils in diatom cells. Zooplankton such as copepods eat the phytoplankton with its burden of DDT and do not break down or excrete it. Small fish eating the copepods gain yet more DDT, and so on right up to mammals, birds and the human population. The effects of DDT pollution are varied. Some marine organisms, especially crustacea, are extremely sensitive to both or-ganochlorine and organophosphorus compounds and are killed by very small concentrations. In others, reproductive success is affected. Dramatic reproductive failures in fish-eating seabirds such as pelicans, cormorants, terns and fish eagles and in land raptors such as ospreys in the early 1970s, were finally attributed to a thinning of their egg shells and consequent breakage, caused by DDT. Some animals, including seagulls, have a high resistance to DDT and were not affected.

The use of DDT was banned outright in many countries in 1972 and restrictions placed on its use in others. However, this and other persistent pesticides are still used in some Third World countries.

A recently identified problem concerns the use of anti-fouling paints containing the organotin, tri-butyl-tin (TBT). The use of paints containing TBT has been banned since 1987 in the UK for boats under 25 metres long and for aquaculture equipment. TBT is still in use for larger boats and can also enter the marine environment when ships' hulls are stripped and re-painted. TBT causes various deformities in molluscs and had such a bad effect on oyster farming that the UK industry practically collapsed. In dogwhelks (Nucella lapillus), it causes a condition known as imposex, where females develop male sexual characteristics and breeding is impaired. Dogwhelks live for 5 to 6 years and spend all their lives on the same stretch of shore. They are therefore very good indicators of TBT pollution and were used as such when the case for banning TBT was being investigated.

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