Definition • Oxygen levels begin to drop. •

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Definition in general:

The needless amount of nutrients in a lake or other body /source
of water, that is exposed to the water regularly due to run-off from the land
causes a massed growth of plant life.


Definition in terms of biology:

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The progress by which a body of water becomes enhanced in
diffused nutrients such as phosphates, that encourages the growth of aquatic
plant life usually resulting in the deficiency of dissolved oxygen.


How eutrophication occurs:


Overabundance of nutrients enters the source of water.

Nutrients encourage plant growth, especially algae.

Algal bloom occurs.

Algae succumb and are decomposed by bacteria.

Decomposition of algae increases organic oxygen demand.

Oxygen levels begin to drop.

Macro-invertebrates, fishes and other aquatic life forms die.






Major Contributors:

The major influencing factors on water eutrophication include
hydrodynamics, nutrient enrichment, environmental factors such as carbon
dioxide, salinity, temperature, element balance, etc., and biodiversity and


Hydrodynamics affect the water mainly by the drift of winds and
waves, which move the sediment in the water body away from a certain area. When
the flow velocity is swift, it is difficult for eutrophication to occur even if
the concentration of nutrients are high enough to trigger it, because some
algae could be washed away downstream by the flow before their growth has
reached its highest point. Then the necessity for growth is destroyed and will
not result in eutrophication. However, in slow-flowing water bodies like,
reservoirs, lakes, bays, estuaries, inland seas, the flow of velocity is slow
and the water body changes slowly. This condition slows down the spread of the
nutrients and provokes the accumulation of the nutrients especially nitrogen
and phosphorus, which offers the main nutrients for the rapid reproduction of



All actions in the whole drainage area of a lake or reservoir
are related either indirectly or directly in the water quality of these waters.
A lake or reservoir may, however, be spontaneously atrophied when situated in a
fruitful area with common nutrient enriched soils. Drainage water from
agricultural land also contains nitrogen and phosphorus. It usually has much
more nitrogen because phosphorus is usually bound to soil components. Excessive
use of fertilizers results in significant percentages of nutrients particularly
nitrogen, in agricultural runoff. If eroded soil reaches the lake, both
phosphorus and the nitrogen in the soil contribute to eutrophication. Erosion
is often caused by deforestation that also is caused from unwise planning and
management of the resource.


and salinity are the two major contributors that urge alga bloom. Alga bloom
always occurs at salinity between 23% and 28% and temperature between 23 °C and
28 °C. The variation of temperature and salinity also affects algal bloom, and
the ideal conditions for algal bloom is the rise in temperature and rapid
decrease of salinity than ever in a short period. Carbon dioxide level is one
of general factor controlling water eutrophication. Cyanophytes are more
capable of adapting to low levels of carbon dioxide and become more buoyant at
low levels of carbon dioxide and high pH.


activity is the inducement factor to alga bloom. It can increase high levels of
alga bloom breeding. Nutrient-enhanced microbial production of organic elements,
or eutrophication, is easily accompanied by changed microbial community
structure and purpose.
The amount of microbial biomass is directly related to the content of organic
matter and the volume of plankton in eutrophicated water. The decomposition of
organic matter by bacteria actions may create nutrients and organic substances,
which may encourage the algal bloom, break out.







The most noticeable effect of development of eutrophication is
the production of dense blooms of toxic, foul-smelling phytoplankton that decrease
the clarity of water and ruin the water quality. Algal blooms reduces light
penetration, lessen growth and causing die-offs of plants in certain zones
while lowering the success of predators that use light to pursue and get their
prey. Furthermore, high quota of photosynthesis associated with eutrophication
can deplete dissolved inorganic carbon and increase pH to intense levels during
the day. Elevated pH can in turn ‘blind’ organisms that rely on perception of
dissolved chemical cues for their survival by impairing their chemosensory
abilities. When these dense algal blooms eventually decease, microbial
decomposition rapidly depletes dissolved oxygen, creating a hypoxic or anoxic
‘dead zone’ lacking sufficient oxygen to support the organisms found in the
waters. Dead zones are found in many freshwater lakes including the Laurentian
Great Lakes (e.g., central basin of Lake Erie; Arend et al. 2011). Lastly, such
hypoxic events are particularly common in marine coastal environments surrounding
large, nutrient-rich rivers.












Detectors Used and Info of Effected Areas:

Inglett and Reddy reported evidences to
support the use of stable C (delta C-13) and N (delta N-15) isotopic ratios as
indicators for eutrophication and shifts between N and P limitation. Lin et al.
in 2006 compared the stable isotopes from liquefy nutrients and plants and
water column nutrient parameters and integration of multiple proxies in a
sediment core from Meiliang Bay of Taihu Lake, and discovered the differences
between aquatic plant species and trophic status between East Taihu Bay and
Meiliang Bay are indicated by their variations in delta C-13 and delta N-15 of
aquatic plants and delta N-15 of NH4 +-N. A significant influence of external
nutrient inputs on water quality of Meiliang Bay is reflected in temporal
changes in delta N-15 of NH4 +-N and hydro-environmental parameters. The
synchronous change between delta C-13 and delta N-15 values of sediment organic
matter (OM) has been attributed to elevated primary production at the beginning
of eutrophication between 1950 and 1990, and then recent inverse correlation
between them has been caused by the uptake of N-15-enriched inorganic nitrogen
by phytoplankton grown under eutrophication and subsequent OM decomposition and
denitrification in surface sediments, proving that the lake has suffered from developing
eutrophication since 1990.









Measures taken:

Water resource managers routinely employ a
variety of strategies to minimize the effects of cultural eutrophication,


(1) Diversion of excess nutrients 

(2) Altering nutrient ratios 

(3) Physical mixing 

(4) Shading water bodies with opaque liners or
water-based stains

(5) Application of potent algaecides and


Sadly, these strategies have not been able to
improve the situation and are considered to be impractical, especially for
large, complex ecosystems. Water quality may be improved by decreasing nitrogen
and phosphorus inputs into aquatic channels, and there are several well-known
examples where bottom-up control of nutrients has greatly improved water
clarity. However, nutrient reduction can be difficult and expensive to control,
especially in agricultural areas where the algal nutrients come from nonpoint
sources. The use of algaecides, such as copper sulfate, is also effective at
reducing HABs temporally. It may pose risks to humans, livestock, and wildlife,
in addition to harming a variety of non-target aquatic organisms. Another
alternative to improve water quality in nutrient-rich lakes is bio manipulation – the alteration of a
food web to restore ecosystem health. The basic premise is that secondary
consumers (planktivorous fishes) are removed either through the addition of
tertiary consumers (piscivorous fishes) or harvesting, which allows the
dominance of large-bodied, generalist grazers (e.g., Daphnia) to control






Categories: Management


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