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The River Basin
Factors Affecting Aquatic Ecosystems
Introduction
Geography
Climate and Weather
Hydrology
Water Quality
Ecology & Biodiversity
Ecology
Dams
Aquatic Ecology
Building Blocks
The building of dams can negatively affect ecosystems. It can result in the inundation
Aquatic Habitats
of habitats of limited distribution. If the water flow is interrupted during the
Life in Aquatic
construction or filling of a dam, a biodiversity hotspot such as the Kunene River
Ecosystems
Factors Affecting Aquatic mouth could fall temporarily dry with potentially devastating long- term effects for its Explore the sub- basins of the
fauna. Dams would also obstruct the movement of fish species which could result in Kunene River
Ecosystems
the loss of certain species, some of which may be endemic, for example the Kneria
Environmental Flows
Maydelli in the Kunene River basin. On the other hand dams can create new lake
Wetlands
habitats and can have other positive effects such an improved fishery potential or
Biodiversity
regulated flows.
Watersheds
References
Factors Affecting Aquatic Ecosystems Abstraction of water
The abstraction of water for water supply to settlements and for irrigation can also
have a negative impact on the ecosystems with effects similar to those described
above. The high demand of water for the arid parts of the Lower Kunene especially in
Namibia needs to be balanced with the demands in the upper sections of the watershed Video Interviews about the
integrated and transboundary
in Angola. Water volumes (supply/ debit versus demand/ extraction) need to be
management of the Kunene
carefully managed to avoid negative effects on the ecosystems.
River basin
Explore the interactions of living
organisms in aquatic
environments
Water is abstracted from the Kunene River to supply Northern Namibia.
Source: GTZ 2006
( click to enlarge )
Degradation of riparian vegetation
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In general population densities are low to very low especially in the Lower Kunene
basin. However, human activities in agriculture and pastoralism lead to the
degradation of the riparian vegetation. This can have negative effects such as the
reduction or fragmentation of habitats for wildlife in the floodplains of the Middle and
Upper Kunene watershed, where population pressure is highest.
Alien species
A significant threat to aquatic ecosystems in southern Africa (and elsewhere) lies in
the invasion of alien species. The most prominent example for problems with invasive
species is the Nile Perch in Lake Victoria. After being introduced to the lake this
species has out- competed most of the indigenous fish population and robbed the lake
Examine how the hydrologic
cycle moves water through and
around the earth
A significant threat to aquatic ecosystems in southern Africa (and elsewhere) lies in
the invasion of alien species. The most prominent example for problems with invasive
species is the Nile Perch in Lake Victoria. After being introduced to the lake this
species has out- competed most of the indigenous fish population and robbed the lake
of its formerly high fish diversity.
Another well known invader is the water hyacinth (Eichhornia crassipes ) which has
spread in most African fresh water habitats leading to clogged waterways,
eutrophication and problems for hydropower schemes resulting in economical and
ecological losses. The water hyacinth is for the moment absent from the Lower
Kunene.
Compared to other regions, Namibia has been relatively spared from invasive species.
The integrity of forest ecosystems is relatively unscathed by the impact of evasive
aliens with the exception of ephemeral river forests with severe infestation of
Mesquite (Prosopis sp.) (Kohli et al 2008).
Factors Affecting Aquatic Ecosystems
Factor
Impact
Dams, interbasin transfers,
hydro- electrical
flow releases, Modified flow regime or hydrology
irrigation and
mining
abstraction
Alien species
Pioneer alien species out- compete indigenous species for space,
nutrients and sunlight
Floating aquatic plants increase with reduced flow
Changes to the shape of the wetted perimeter of the river channel,
with lower water levels causing banks to dry out, temporary
exposure of unprotected banks and bank collapse
Enhanced benefit to pioneer reeds, such as the Common Reed (
Phragmites australis ), under reduced flow, with increased
distribution and patch size, thereby accumulating sediments,
blocking channels and resulting in large disturbances when washed
out during large floods. These often form reed mats that cause
blockages downstream and exacerbate the effect of floods.
Degradation of
riparian and inLoss of indigenous trees and gallery forest in the riparian belt
stream
because of reduced floods (moisture), reduced seed dispersal, more
vegetation
frequent hot fires because of increase in reed beds and less cooling
effect as previously moist riverbanks are drier
Increased agricultural encroachment into the riparian belt because of
reduced flooding and waterlogged soils
Invasion by alien vegetation, notably Mesquite ( Prosopis spp.) ,
exacerbated by a loss of indigenous vegetation and disturbance (e.g.,
through fires and agricultural activities)
Changes in species composition and abundance as a result of
fertilizers and salts draining into the river
Source: UNDP- GEF (2008)
For more information refer to section Threats to Biodiversity .
Next: Environmental Flows