ecosystem. Changing the annual water curve (dam regulation for example) can have similar
effects bymodifyingwater flow dynamics and disturbing sediment movement. Reducing or
blocking a river’s continuity by creating a sill or dam can also impact the living community of
aquatic ecosystems.
Numerous examples exist of dysfunction due to abstraction or diversions caused by human
activity: the drying-up of the Aral Sea followingmassivewithdrawals to irrigate cotton crops is
a symptomatic case that has been the centre of muchmedia attention. However, the reduction
in the number of wetlands around theworld illustrates the extent of this type of dysfunction,
which can affect much smaller areas than the Aral Sea.
Watercourse diversions andmany hydromorphological alterations that modify thewater curve
(channelization andmeander cut-off) also come under this category, since the natural flows of
the low-flow and high-flow channels are affected.
2.2.4 Chemical and physico-chemical: Organic pollution, eutrophication and
micropollutants
Many human activities cause organic pollution. In addition to the discharge of domestic
wastewater (untreated wastewater) or industrial wastewater that has not been treated
adequately (e.g. from food and wood industries), agriculture can be a significant source of
pollution, via livestock farming including silage production or manure / sludge spreading on
land. This input of organicmatter into the aquatic environment can usemassive amounts of
oxygen, with the potential to kill fish and disturb the aquatic ecosystem.
Depending on its content, organic pollution can also add high levels of nutrients to the
aquatic ecosystem. The excessive use of fertiliser can have a similar effect via erosion or
leaching.
Excess nutrients trigger the phenomenon of eutrophication. The exaggerated growth of
aquatic plants with access to a significant mass of nutrients (phosphates and nitrates)
results in a considerable reduction of dissolved oxygen and environmental asphyxia. This
phenomenon can be natural in closed environments, like ponds, where plants produce
nitrogen and phosphate compounds when they die and sink. However, the phenomenon of
eutrophication through inputs of human origin creates disequilibrium in the ecosystem that
is visible as “algal bloom”, and causes the death of fish and putrefaction.
Human activities can also dischargemicropollutants. Micropollutants are chemical
substances found in low quantity that have a polluting effect on the aquatic environment.
Threemain groups are generally identified: organic, metallic and organometallic. They are
generated by a number of human activities linked to extracting, transforming and using raw
materials. It also can be generated through their end-of-life breakdown (waste). Depending
on their physico-chemical and toxic characteristics, they have a varying impact on all or part
of the ecosystem. People can then be exposed through environmental factors (especially air
or skin contact), through food, or at work.
Other parameters can also have an impact on the functioning of ecosystems. For instance,
discharges of hot water (e.g. from industry and nuclear power stations), when regular,
modify the ecosystem by changing the structure of the biocenosis.
2FUNCTIONSANDBENEFITSOFAQUATICECOSYSTEMS
THEHANDBOOK FORMANAGEMENT
ANDRESTORATIONOF AQUATIC ECOSYSTEMS INRIVERAND LAKE BASINS
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