Another harmful act caused by foreign substances entering the water is Biological Magnification. The run off from farms and lawn contain pesticides, these pesticides accumulate in "fatty tissue". As one organism eats another, the chemical becomes more concentrated. Thus organisms higher up on the food chain may take in more amount of toxins and die.

Sediment or soil particles carried to a streambed, lake, or ocean, can also be a pollutant when present in large enough amounts. Soil erosion is the typical reason for this and can be produced by many things. The removal of trees near waterways is a major contributor to erosion.

The Earth's topsoil can also find their way into streams by means of heavy rain runoff and floods in or near croplands, strip mines and roads. This high level of sediment and organic matter can damage a stream or lake by introducing too much nutrient matter, which can lead to eutrophication (nutrient rich and oxygen poor).

Not only do pollutants harm the organisms living in the water; they also harm land animals that live off the water. When oil is spilled in a body of water it spells bad news for birds; birds and oil don’t mix. Oil kills birds in many ways, ruining the water proofing of the bird for example. The structure of the feathers makes them waterproof. The separate strands, or barbs, in each feather are bound together by rows of tiny hooks, or barbules. Oil destroys the duck’s "raincoat" by clogging the tiny barbs and barbules. Cold water quickly soaks into the insulating down and reaches the skin. In cold climates, the temperature stresses on an oiled duck may be doubles to those of a clean one. The amount of oil is not important; even a small dab may be enough. The waterlogged duck burns up its fat reserves, losing at the same time its last layer of insulation against the cold. The bird can only save itself by spending even more energy, which it does not have to spare, in search of food. The oiled bird is caught in a vicious cycle. The bird’s first response is to clean their feathers. As the bird nibbles at its oily feathers, the bird also inhales and swallows toxic compounds that damage the liver, lungs, kidneys, intestines, and other internal organs. The poisoning is as lethal as the loss of waterproofing, though slower to take effect, so hypothermia is the actual cause of death.

Water pollution also harms us, as humans. We drink the contaminated water and become sick. Some organisms occur in nature and are only considered pollutants when found in drinking water. One such parasite is Cryptosporidium parvum, which caused more than 400,000 illnesses and 100 deaths in Milwaukee in 1993 when it contaminated the drinking water supply.

Water Pollution is the contamination of any body of water or water supply, such as rivers and streams, lakes, underground water, or oceans by substances harmful to plants and animals.

Algal Bloom is the rapid population growth of cyanobacteria and algae caused by the addition of nutrients to the water.

Cyanobacteria are one-celled organisms with no distinct nuclei. The cyanobacteria were formerly called the blue-green algae because they contain chlorophyll, which was thought to only occur in plants and algae. However, scientists eventually determined that these organisms more closely resembled bacteria than algae, and they were renamed; they are sometimes referred to as blue-green bacteria.

Algae are primitive, or simple, plant forms that come in many sizes and colours. Pond scum and seaweed are both examples of algae. Algae can be single-celled or contain many cells. Algae can be microscopic (diatoms) or as long as one hundred feet (seaweed). Even though they may appear colourless, red, brown, golden, or green, all algae contain the pigment chlorophyll, which is needed to photosynthesize, or make food and energy from sunlight.

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