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Newsletter and Technical Publications
Lakes and Reservoirs vol. 3
Water Quality: The Impact of Eutrophication
Why Is Eutrophication Such a Serious Pollution Problem?
Eutrophication is one of the most widespread
environmental problems of inland waters, and is their unnatural enrichment with
two plant nutrients, phosphorus and nitrogen.
One important result of lake and reservoir enrichment is increased growth of
microscopic floating plants, algae, and the formation of dense mats of larger
floating plants such as water hyacinths (Photos 1 and 2) and Nile cabbage.
Growth results from the process of
photosynthesis which is how the plants generate
organic compounds and biomass through the uptake of nutrients (nitrogen,
phosphorus and others) from the soil and water. In the process light acts as the
energy source and carbon dioxide dissolved in water as the carbon source. As a
result of the photosynthetic process oxygen is also produced.
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| Photo 1: Algal bloom in a lake. |
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Photo 2: Overgrowth of floating aquatic plants.
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When the plants die they decompose due to bacterial and fungi activity; in
the process oxygen is consumed and the nutrients are released together with
carbon dioxide and energy. In many lakes and reservoirs in the world plants
growing in the surface during spring and summer will die during autumn and sink
to the bottom where they decompose.
During spring and summer, lakes and reservoirs are often supersaturated with
oxygen due to the amount of plants. The oxygen surplus is released to the
atmosphere and no longer available to decompose organic matter. This causes
oxygen depletion or
anoxia in the deeper layers of lakes,
particularly in autumn. Oxygen depletion is therefore caused by the shifts in
time and space between photosynthesis and decomposition. In tropical areas the
same process takes place, but seasonally speaking it is not as representative as
in temperate areas because temperature and daylight duration is very similar
throughout the year.
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| Fig. 1: Thermocline and the relationship between temperature/oxygen and
depth in lakes within temerpate regions. |
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| Photo 3: Black sediment from the bottom of a lake. |
At certain times, lakes may form a thermocline some metres below the surface
(Fig. 1). In the
thermocline the temperature declines several
degrees over a few metres and divides the lake into two zones an upper warmer
one (
epilimnion),
and a lower colder one (
hypolimnion).
Lakes in temperate regions are lakes with a depth of about five to 10 meters or
more and typically form a thermocline during the summer time, therefore they
stratify. Shallow tropical lakes can also stratify, but stratification can be
broken down by strong winds.
A thermocline prevents the upper and lower layers of the lake from mixing.
The result is a change in vertical oxygen concentrations, as shown in Fig. 1,
where concentration is high in the upper layer or epilimnion and very low in the
lower layer or hypolimnion (the low oxygen concentration may degrade water
quality downstream of the lake or reservoir, particularly downstream of
reservoirs with short retention times, as mentioned in Volume 1, p.15).
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| Photo 4: Fish mortality due to lack of oxygen in an Indonesian lake. |
Oxygen depletion often leads to complete deoxygenation or anoxia in the deep
layers of the lake or reservoirs also because oxygen poorly dissolves in water.
In shallow lakes and where plant production is high, deoxygenation of the
sediment and water occur frequently too (black
sediment, Photo 3). Such conditions kill fish and invertebrates (Photo 4).
Moreover, ammonia and hydrogen sulfide originated from bacterial activity can be
released from sediments under conditions of anoxia, and their concentrations can
rise to levels which adversely affect plants and animals as they act as
poisonous gases (also hydroelectrical power facilities in reservoirs often
suffer because of the corrosive power of hydrogen sulfide). Phosphorus and
ammonia may also be released into the water, further enriching it with
nutrients.
Some particular type of algae, which grow in highly nutrient enriched lakes
and reservoirs (blue-green algae or cyanobacteria, Photos 5 and 6 and so-called
dinoflagelates which produces red tide, Photos 7 and 8), release in the water
very powerful toxins which are poisonous at very low concentrations. Some of the
toxins produce negative effects on the liver of life stock at minimal
concentrations but they can lead to the death of cattle and other animals even
to humans when ingested in drinking water at higher concentrations. Although one
way to treat and disinfect surface waters where these algae grow and/or to
prevent high concentration of organic matter is to use chlorine, unfortunately
this leads to the formation of compounds which may produce or induce cancer -a
serious threat to the safety of drinking water supplies. |
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| Photos 5 & 6: Macro- and micro-scopic viwes of Microcystis aeruginosa. |
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Photos 7 & 8: Macro- and microscopic views of Uroglena americana, a culprit
of red tide.
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High concentrations of nitrogen in the form of nitrate in water can also
cause public health problems. They can inhibit the ability of infants to
incorporate oxygen into their blood and so result in a condition called the blue
baby syndrome or methemoglobinemia. For this to occur, nitrate levels must be
above 10mg per liter in drinking water. The condition can be life-threatening.
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| Photo 9: Eutrophied waters (left down) in Barra Bonita reservoir, São
Paolo, Brazil. |
One of the main problems occurring as a result of algal blooms or other
aquatic plants (disproportionate growth, Photo 9) is the reduction in
transparency in the water which reduces the recreational value of lakes,
particularly for swimming and boating. Water hyacinth and Nile cabbage can cover
large areas near the shore and can float into open water spreading at times over
the entire surface. These mats can block light to submerged plants and produce
large quantities of dead organic matter that can lead to low oxygen
concentrations and the emission of unpleasant gases such as methane and hydrogen
sulfide due to its decomposition or decay. Masses of these plants can restrict
access for fishing or recreational uses of lakes and reservoirs and can block
irrigation and navigation channels.
Shifts in the abundance of, and significant reduction in diversity of species
(biodiversity) of aquatic organisms within a lake or reservoir may also be
caused by eutrophication (Fig. 2). This results from the changes in the water
and food quality together with decreased oxygen concentration which often alter
the composition of the fish fauna from more to less desirable species.
Nevertheless, yields of certain species of fish tend to increase as
eutrophication increases since there is more food available. However, oxygen
depletion and high ammonia concentrations under hypereutrophic conditions can
lead to decreases in fish yields as eutrophication rises.
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| Fig. 2: Relationship between number of species and volume of chlorophyll
a. |
In the following Table 1 the general effects of eutrophication in the aquatic
environment are presented.
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