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.
Inorganic nutrients, especially
nitrates and phosphates, are the nutrients most commonly limiting primary
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 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 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 the 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 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
microorganism 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.
The problems of inadequate treatment of sewage
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 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.