Toxin is a term used to define a poisonous chemical substance that is produced within a living cell and organisms. Generally it is a natural phenomenon when organisms produce toxins as a form of excretion, mode of predation and protection against other organisms.
Toxins can be small molecules or proteins that can cause disease upon absorption or direct contact by the body tissues. Such contact produces interaction with the molecules of the body thus causing irritation, allergic reactions or cellular damage.
Toxin can contribute to water pollution when large amount of toxins are produced in a specific area of water. For example, botulinum toxin is produced by Clostridium botulinum that can be present in water samples which can cause food poisoning. Another example is the polluting the water by increased production of toxins of some algae. This phenomenon is called as algal bloom or red tide. Some of the algae produce harmful toxins that can affect the fishes living in the area.
Pesticides and herbicides are the chemicals used for killing the plant and animal pests. It is a general term that includes bactericides, fungicides, nematicides, insecticides and also the herbicides or weedicides. These enter a pond, lake, it is then taken as such by the plants of the pond, then reaches to zooplankton feeding on plants. Industrial effluents like Na, Cu, Cr, Cd, Hg, Pb, etc. are discharged from industries into water. Agriculture discharges are chemicals used as fertilisers and the pesticides used in disease control. Their discharges reach into the water bodies.