Toxic substances have always been present in the Earth’s waters. The presence is insured by natural solution processes although not necessarily in harmful concentrations.
Ammonia and hydrogen sulfide from the decay of organic matter are naturally occurring toxic substances. Other highly toxic organic substances produced by some plankton organisms and terrestrial plants are known to sometimes cause the death of aquatic animals.
Some waters normally contain heavy metals and other toxic minerals in concentrations harmful to life, but the activities of man have led to much larger problems of toxic substances in many waters.
We can hardly list the many kinds of industrial processes from which toxic wastes enter receiving waters. Many products of industry including pesticides, detergents and other chemicals find their ways into aquatic environments, sometimes with and sometimes without man’s intent.
There are waters that have become all but uninhabitable for aquatic organisms because of metals entering with drainage from mines.
Living in water as they do, aquatic organisms may be harmed by concentrations of toxic substances that have little effect on man. Toxic substances need not be present in concentrations that are acutely toxic to reduce the distribution and abundance of aquatic animals.
To do this, sufficiently high concentrations need only affect directly or indirectly the longevity, reproduction, growth or movement of aquatic organisms. Concern has arisen over the effect of organisms exposure to chronic and sub chronic concentrations of certain elements that industrial and other human activities are releasing into the aquatic environment.
Many elements are essential for plants or animals yet are toxic under certain conditions. To be toxic, an element must be biologically available to the plants or animals. That is it must exist in the form that can enter tissues of the organism either in solution or as a gas.
Total amounts of a toxic element in the environment are not an adequate estimation of the toxicity hazard unless it can be shown that the element exists in or is likely to assume an available form under the environmental conditions that it occurs.
Most of the available toxicity data are reported as median tolerance limits (TLM) which is the concentration that kills 50% of the test organisms within a specified time span, usually 96 hours or less. This system of reporting has been misapplied by some that erroneously inferred a TLM value is a safe value, whereas it is merely the level at which half the test organisms are killed. There is nothing safe about between TLM concentrations and concentrations that are low enough to permit reproduction and growth.
Substantial data on long term effects and safe levels are available for few toxicants. The effects of toxicants on reproduction are under study. This is a very important aspect of all long term toxicity tests.
For example, in chronic tests with six different toxicants, there were 3 toxicants with which certain concentrations permitted indefinite survival and normal appearance, but blocked spawning completely. Such evidence makes estimates of safe concentrations based on acute lethal test data alone very difficult and frequently erroneous.
Equally problematical is the lack of information on the sensitivity of various life stages of organisms. Many organisms are the most sensitive in the larval, nymph, molting or fry state. Others may be the most sensitive in the egg and sperm stage.