7: Explain what causes and effect of eutrophication and algal bloom. How does 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 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. It induces algal bloom where the water becomes densely populated with phytoplankton. In addition, the inorganic nutrients, especially nitrates and phosphates are the nutrients most commonly limiting primary productivity in aquatic ecosystems. An increase in 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.

An algal bloom is caused by a combination of factors that encourage algal growth which include sunlight, nutrients, trace elements, salinity, water temperature and water flow. The continuation of any algal bloom is therefore dependent on the changes of these factors. 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. Moreover, 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. Besides, 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. Consequently, the level of dissolved oxygen ay 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.