Industrially-derived heavy metals are increasingly responsible for contamination of coastal and estuarine waters. All stages of metal production are sources of contamination, the main contributors being acidic mine drainage waters and smelting works. Other major sources are industrial water discharges, sewage sludge, the atmosphere, shipyard paints and electricity power stations (Bryan, 1984). The most contaminated sites are the rivers and estuaries that directly receive the industrial outfalls. It is a common misconception that metal wastes are simply washed out to sea and dispersed. Estuaries are in fact efficient traps of heavy metals. The scrubbing processes of precipitation, chelation and adsorption onto particulate materials ensure that only small amounts of metals escape to the open sea (Turekian, 1977). The accumulation of heavy metals in estuaries raises the question; at what levels do metals have a detrimental effect on the biota?