Answer. Pesticides are chemicals that are used either directly to kill or control the growth of pests. They are used in agricultural fields. When they run-off from these fields enter into pounds, streams or rivers, the salts containing phosphate ions causes the rapid growth of algae. When algae dies, bacteria consumes oxygen of the water for decomposition of algae. As & result, oxygen, depletes in the water that affects other organisms living in the water. Thus, pesticides cause water pollution.