Bio essay 7 Chan Yuk Ying 6S (22)

Explain what causes and effects of eutrophication and algal bloom. How dose the inadequate treatment of sewage may lead to the deterioration of water quality and microbial hazards.

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 blooms not only cause unpleasant smells in water, but also threaten aquatic life.

Eutrophication is mainly caused by human activities. With heavy applications of fertilizers on agricultural lands, the excessive fertilizers are easily washed away by rainwater 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.

Algal bloom is the main cause of eutrophication. It 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.

Although the majority of algal bloom is basically harmless to human, it has a great impact on aquatic life.

The phytoplankton produces a great quantity of oxygen during daytime but the amount of dissolved oxygen will be reduced during cellular respiration at night and on cloudy days. In addition, the thick mat of floating phytoplankton reduces the amount of light penetrating into the water. The oxygen content in water is further reduced. The rapid growth of the phytoplankton is usually followed by a massive and sudden death of algal due to the rapid exhaustion of nutrients.

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. The 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 initially deceases due to the reduction of light penetration in water in the presence if sewage, the great increase in nitrate contents will then lead to an increase in its population to a considerable level.