There has been a steady increase in the quantity and diversity of discharges that reach aquatic environment, both organic and inorganic, either synthesized or mobilized by man as a result of high population growth and development activities. This has led to the appearance of several types of pollution. Inland and coastal waters have been the most affected, since most major cities are located near water bodies. Monitoring programs and research for metals in the environment have become widely established because of concerns over accumulation and toxic effects, particularly in aquatic organisms. Quantifying metal concentrations in water and sediments may have limited value in this respect, particularly in circumstances in which it is difficult to define biologically relevant fractions. Therefore, the analysis of aquatic organisms have been used increasingly as a direct measure of the abundance and availability of metals and micro-pollutants in the environment and has led to the adoption of the bio-indicator (bio-monitor, sentinel organism) concept 50 years ago. All heavy metals (trace elements) are potentially toxic, even the essential ones if accumulated above (or below) levels needed by the organism. Temperature and salinity are the two major factors to be considered when considering abundance, availability, bioaccumulation and excretion of these metals in the aquatic environment. Almost all developed countries are found in the temperate region while the developing ones are located or/and found in the tropics where these factors are at the extreme. Hot spots for heavy metal pollution are found both in the developed and developing world.
Key words: Heavy metals, aquatic environment, bivalves, bio-indicator/monitors, seasonal variation, accumulation.