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The River Basin  Introduction

GeographyClimate and WeatherHydrologyWater QualityEcology & Biodiversity

  Ecology  Aquatic Ecology  Building Blocks  Aquatic Habitats

  Life in Aquatic EcosystemsFactors Affecting Aquatic Ecosystems

  Environmental Flows  Wetlands  Biodiversity

Watersheds  References

 

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You are here: Home>The River Basin >Ecology & Biodiversity >Aquatic Ecology >Factors Affecting Aquatic Ecosystems

Factors Affecting Aquatic Ecosystems  

DamsThe building of dams can negatively affect ecosystems. It can result in the inundation of habitats of limited distribution. If the water flow is interrupted during the construction or filling of a dam, a biodiversity hotspot such as the Kunene River mouth could fall temporarily dry with potentially devastating long- term effects for its fauna. Dams would also obstruct the movement of fish species which could result in the loss of certain species, some of which may be endemic, for example the Kneria Maydelli in the Kunene River basin. On the other hand dams can create new lake habitats and can have other positive effects such an improved fishery potential or regulated flows.

Abstraction of waterThe 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 in Angola. Water volumes (supply/ debit versus demand/ extraction) need to be carefully managed to avoid negative effects on the ecosystems.

Water is abstracted from the Kunene River to supply Northern Namibia.Source: GTZ 2006( click to enlarge )

Degradation of riparian vegetationIn 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 speciesA 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

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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, inter- basin transfers, hydro- electrical flow releases, irrigation and mining abstraction

Modified flow regime or hydrology

Alien species Pioneer alien species out- compete indigenous species for space, nutrients and sunlight

Degradation of riparian and in- stream vegetation

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.

Loss of indigenous trees and gallery forest in the riparian belt because of reduced floods (moisture), reduced seed dispersal, more 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