Aquatic Pollution and Coastal Management

Unit 4 : Pesticide pollution

4.13.The Effect of the Pollutant
Different organisms show varying degrees of sensitivity to toxins. Even within a species, sensitivity to a particular toxin may depend on age, sex, food availability, reproductive condition and genetic make-up. Young larval forms tend to be much more sensitive than the adults of the same species.

Organisms may ingest or absorb more than one pollutant at a time. Two pollutants may interact to produce a toxic effect which is greater than the combined effect of the two pollutants simply added together i.e. one of the pollutants may increase the mortality caused by the other. This is called synergism. High concentrations of lead, zinc and mercury are each capable of slightly reducing the growth rate of aquatic protozoans, but when acting together the overall effect is much higher.

Organochlorines
Organochlorines are hydrophobic (water-hating) and show low solubility in water, but are readily soluble in fat (lipophilic). Consequently, they will often accumulate in the fatty (adipose) tissue of an organism. In the past, organochlorines have been widely used as pesticides. They have two important characteristics:

• They are chemically stable and remain active in the environment for many years.
• They are fat-soluble and readily concentrate in adipose tissue.

Whilst there, even high concentrations may cause little if any harm. However, in times of food shortage, fat reserves may be metabolised, releasing the pesticide into the blood of the organism. This may be fatal.

Polychlorinated biphenyls (PCBs)
PCBs were used in the manufacture of paint, plastics and adhesives. PCBs are chemically stable, hydrophobic and lipid soluble, hence they can rapidly bioaccumulate. There is strong evidence to suggest that PCBs are at least partly to blame for the seriously declining numbers of the European Otter and for the decline in some populations of seals. PCBs interfere with the hormonal control of oestrous and ovulation, seriously reducing reproductive ability and weaken the immune system of marine mammals, predisposing them to illness. In humans, PCBs may increase the risk of anaemia, oedema and susceptibility to infectious diseases.

PCBs enter the environment by leakage from landfill sites, incineration of waste and from sewage effluent. Eventually, large quantities of the PCBs will end up in the sea and high levels may accumulate in sediment on the sea bed. Organisms which burrow in or feed on such sediment may accumulate very high concentrations.