Aquatic Pollution and Coastal Management

Unit 4 : Pesticide pollution

4.14.1.Toxicity of Heavy Metals
The toxicity of metal is extremely dependent on a wide range of chemical, biological and environmental factors, which in some cases change the rate of absorption of metals and in other, change the physiological processes, with the result the effect of metal may further increase.
The concentration of heavy metals in sea water is so low that they can easily be increased by local contamination to a level which organisms have never been previously encountered. These low levels are presumed to be the optimum concentration, since no large animal ever suffer from the deficiency of essential metals. However, higher concentrations of these essential heavy metals posses problems by blocking the enzyme systems. Consequently heavy metal whether essential or not become toxic to organisms.
For the purpose of discussion, the toxicity can be described in two distinct categories, such as lethal toxicity and sub-lethal toxicity.

Lethal Toxicity
There are sporadic evidences of mass mortality of the fish in the areas where tale mining of zinc was discharged. However, there are greater evidences of accumulation of metals in all invertebrates. Among the various invertebrates molluscs are notorious for their ability to accumulate a very high concentrations of metals, although specific abilities vary widely. The consumption of such contaminated shellfish and fin fish has resulted in the recognition of deadly diseases such as Minamata, Itai-Itai etc., which will be discussed later in this chapter.

Sub-lethal Toxicity
Sub-lethal toxicity is the exposure of organisms to lower concentration of pollutants (metals), which are found to produce a variety of observable effects without causing death to the organisms. The heavy metals in their sublethal concentrations cause damage in three ways.

• Enzyme inhibition resulting from the formation of stable complexes with active groups on protein molecule.
• Disrupting respiratory surface
• After accumulation, it may damage the internal organ