Lillian Liem 6S 26

Date: 19-01-2004

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.

Eutrophication

Eutrophication is the enrichment of water bodies with inorganic nutrients, typically nitrates and phosphates. It is mainly caused by human activities. With heavy applications 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.

Also, it results in algal bloom where water bodies become densely populated with phytoplankton. Algal bloom not only causes unpleasant smells in water, but also threaten aquatic life.

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

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. 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. Furthermore, 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.

A well-known water polluted area is characterized by low dissolved oxygen and high Escherichia coli levels. The poor water quality is mainly due to the lack of proper treatment of the sewage that enters the harbours, streams and rivers, etc. The inadequate treatment lead to suffocation of fishes, disturb the food chain of marine ecosystem thus affect the aquatics ecosystem.