How do algal blooms cause fish to die?

Basically this type of fish kill is caused by nature not being able to contend with mans input. Usually caused by soil or fertiliser run off from farms, excess nutrients get into the water ways/lakes etc. Then nature tries to use up these excess nutrients in the only way it can and that is by the growth of Algae. Algae are tiny plants that will use up the chemicals N.P.& K. and the many other trace elements that have become suddenly available in the water. When this happens it is called an "algae bloom". Then, when all the excess nutrients have been used up by the algae, there is nothing for the it to use up (eat?) so it dies. Now we have a different problem. The dead algae decays. This decaying process is speeded up by aerobic (oxygen using) bacteria which create a lack of oxygen in the water. At this stage both the bacteria and the fish become short of oxygen so they die off (suffocate) too. This creates another load of decaying matter and another excess of nutrients N.P.K. etc etc etc. This has started a continuing circle of algae bloom and decay which continues on taking nature a considerable time to clean up, settle down and get back to normal.