Over use of fertilizers have destroyed local lakes and rivers. Eutrophication is the term given to killing of life in a lake as a result of excessive growth of algae due to an overabundance of nutrients. Nitrates are very soluble and tend to be washed away by rain into local lakes. As the nitrates build up in the water algal and plant growth increases. Algae grows at the surface of the lake and prevents light from reaching plants at the bottom of the lake. As plants die, the production of oxygen in the lake is reduced. To compound the already difficult situation of low oxygen production, bacteria decomposing the dead plant matter, at the bottom of the lake, use oxygen in the process.

With the absence of oxygen all life undergoing aerobic respiration dies. Anaerobic organisms take over the decomposition of plant and animal matter. Instead of carbon dioxide and water being the products of decomposition anaerobic decomposition produces methane(CH₄) and hydrogen sulfide(H₂S). This creates a horrible stench as hydrogen sulfide smells like rotten egg. Methane make the body of water a fire hazard

1) Explain how farming can damage local river systems or lakes.

2) The fact that fertilisers are water soluble causes the farmers to periodically apply more of it. Why?

3) What happens to the fertiliser when it rains?

4) Lakes close to farmland are particularly vulnerable to eutrphication. Explain why.

5) How does an excess supply of nutrients in a large body of water cause the complete demise of a health ecosystem in that body of water?