Kathy Poon 6S 28

7.Explain what causes and effects of eutrophication and algal bloom. How does the

Inadequate treatment of sewage may lead to the deterioration of water quality and

Microbial hazards.

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

If the pollution is not so serious, the oxygen content, ammonia content, nitrate concentration, BOD, and the population of bacteria and phytoplankton will return to their normal level as water flows downstream by self-purification. However, if the discharge of sewage is continuing and persistent, microorganisms will use up almost all the dissolved oxygen for the decomposition of organic wastes. The lack of oxygen will thus induce the incomplete decomposition of organic wastes. The lack of oxygen will thus induce the incomplete decomposition process by anaerobia bacteria may produce gases with unpleasant smells such as ammonia and hydrogen sulphide. The depletion of oxygen also leads to the depth of most aquatic organisms due to suffocation. The accumulation of their dead bodies further brings anaerobic decomposition that makes the condition even worse. The combination of low oxygen content, anaerobic decomposition and poisonous gas production makes water septic and dangerous to health. It also renders the water unsuitable for drinking and other purposes. Therefore, sewage treatment is important in controlling water pollution.

Inorganic nutrients, especially nitrates and phosphates, are the nutrients most commonly limiting productivity in aquatic ecosystems. An increase in the amount of these nutrients can significantly enhance the growth of phytoplankton. However, an excessive amount of these nutrients may lead to dramatic proliferation of phytoplankton.

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.

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 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.

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 that, although the large amount of floating phytoplankton produce 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. 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. The rapid growth of phytoplankton is usually followed by a massive a 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 relationships between organisms in food chains. This may seriously agent the aquatic ecosystem. Red tide is a kind of algal bloom, which gives the seawater colors such as pink, reddish brown. The majority of red tides are basically harmless. Only a few may kill fish, contaminate seafood and cause allergic eyes and skin irritation when people swim in such polluted water. Red tides occur quite frequent in Hong Kong, About 20-30 incidents on average in a year. Fortunately, the harmful effects associated with the algal bloom are not very serious and its impact of a large scale on organisms is rarely reported.