7. Explain what causes and effects of eutrophication and algal bloom and how the inadequate treatment of sewage does may lead to the deterioration of water quality and microbial hazard.

Eutrophication is the enrichment of water bodies with inorganic nutrients, typically nitrates and phosphates. It results in algal bloom where water bodies become densely populated with phytoplankton. Algal bloom not only causes unpleasant smells in water, but also threatens aquatic life.

The causes of eutrophication

Eutrophication is mainly caused by human activities. With heavy application of fertilizers on agricultural lands, the excessive fertilizers are easily washed away by rain water into rivers or lakes. The manipulation of soil in ploughing, road building and reclamation may also speed up the leaching of inorganic nutrients into water bodies. The wastes of humans and livestock are rich in nitrogen-containing compounds. They will be broken down into ammonia and then nitrate. Further, the phosphate –rich detergents and washing powder in sewage are also the food sources of phytoplankton. All these are responsible for the phenomenon of eutrophication.

The cause of algal bloom

Algal bloom is caused by a combination of various factors, including sunlight intensity, availability of nutrients, salinity, water temperature and water flow. In winter, the temperature and light intensity are so low that the photosynthetic organisms consume only a very small amount of nutrients in water, leading to the accumulation of nutrients at the bottom of water bodies. When spring comes, the increase in temperature and light intensity, together with the eutrophicated water, cause a rapid growth of phytoplankton, i.e. algal bloom..

The effects of algal bloom

Although the majority of algal bloom is basically harmless to human, it has a great impact on aquatic life. The most significant effect is the depletion of oxygen in water as a result of the following:

1. Although the large amount of floating phytoplankton produces a great quantity of oxygen during daytime, the amount of dissolved oxygen will be greatly reduced during cellular respiration at night and on cloudy days.

2. The thick mat of floating phytoplankton diminishes the amount of light penetrating into the water. This affects the photosynthesis of submerged plants. As a result, the oxygen content in water is further reduced.

3. The rapid growth of phytoplankton is usually followed by a massive and sudden death of algae due to the rapid exhaustion of nutrients. Decomposition of the large amount of dead phytoplankton by microorganisms consumes a large quantity of dissolved oxygen in water.

The level of dissolved oxygen may fall to such a level that most aquatic life, especially fish, will die of suffocation. The changes in fish communities disturb the feeding relationship between organisms in food chains. This may seriously affect the aquatic ecosystem.

If there is inadequate treatment of sewage before disposal to aquatic bodies, various physical, chemical and biological changes of water are resulted. The discharge of sewage may introduce a high level of organic nutrients and bacterial population into water bodies. The decomposition of organic wastes by microorganisms leads to an increase in the BOD value. This dissolved oxygen in the water bodies will decrease dramatically. In addition, Ammonia present in the sewage is converted to nitrates by the action of nitrifying bacteria. Nitrates are important nutrients for the growth of phytoplankton in water. Although the population of phytoplankton initially decreases due to the reduction of light penetration in water in the presence of sewage, the great increase in nitrate contents will then lead to an increase in its population to a considerable level.