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.
Eutrophication
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.
The causes of
eutrophication
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 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.
The outbreak of 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.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.
The effects of 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:
-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.
-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 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.
The 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 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.