In places with large concentrations of people, animals and natural fresh water is usually not enough, especially if it is used to collect sewage and transport them away from populated areas. If dirt gets into the soil is not much soil organisms recycle them, re-using nutrients, and in the neighboring streams seeps have clean water. But if you get sewage directly into the water, they rot, and their oxidation consumes oxygen. Created so called biochemical oxygen demand. The higher the demand, the less oxygen remains in the water for living organisms, especially fish and seaweed. Sometimes because of lack of oxygen dies every living thing. Water become biologically dead – there are only anaerobic bacteria, and they thrive without oxygen and in the course of his life isolated hydrogen sulphide – a toxic gas with a specific smell of rotten eggs. And without that lifeless water becomes putrid smell and becomes quite unfit for humans and animals. The same thing could happen with an excess of water in substances such as nitrates and phosphates, they fall into the water from the agricultural fertilizers in the fields or from contaminated waste water detergents. These nutrients stimulate the growth of algae, the algae begin to consume much oxygen, and when it becomes insufficient, they die. Under natural conditions, the lake silted up before and disappear, there are about 20 thousand. years. Excess nutrients accelerates the aging process, or , and reduces the life of the lake, making it to the Besides unattractive.