Leaching is the process by which nutrients are removed from the soil. Rain water will dissolve any soluble nutrients, such as nitrates, and carry them deep into the soil and eventually beyond the reach of plant roots. The leached nitrates find their way into watercourses. Here they may have a harmful effect on humans as the water becomes polluted. High nitrate levels can prevent efficient oxygen transport and can cause stomach cancer.
Eutrophication is the process by which nutrients build up in bodies of water. It is a natural process that occurs mostly in freshwater lakes.
1. In most lakes and rivers there is naturally very little nitrate and so nitrate is a limiting factor for plant and algal growth
2. As the nitrate concentration increases as a result of leaching, it ceases to be a limiting factor for the growth of plants and algae and both grown exponentially
3. As algae growth occurs mostly at the surface, the upper layers of water become densely populated with algae
4. This dense surface layer of algae absorbs light and prevents it from penetrating to lower depths
5. Light then becomes the limiting factor for the growth of plants and algae at lower depths and so they eventually die
6. The lack of dead plants and algae is no longer a limiting factor for the growth of saprobiotic algae and so these too grow exponentially using the dead organisms as food
7. The saprobiotic bacteria require oxygen for their respiration, creating an increased demand for oxygen
8. The concentration of oxygen in the water is reduced and nitrates are released from the decaying organisms
9. Oxygen then becomes the limiting factor for the population of aerobic conditions These organisms ultimately die as the oxygen is used up altogether
10. Without the aerobic organisms there is less competition for the anaerobic organisms whose populations now rise exponentially
11. The anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes, such as hydrogen sulphide which makes the water putrid