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Freshwater Ecosystems
Table of Contents
Freshwater Ecosystems ........................................................................... 1
Western Cape Rivers ................................................................................ 2
How are Western Cape Rivers different to Eastern Cape and Southern
Cape rivers?................................................................................................ 4
Characteristics of a typical Western Cape river such as the Olifants
River. ....................................................................................................... 5
River Health is important for our health ................................................. 7
How do we affect our rivers? .................................................................. 9
Wetlands................................................................................................... 15
Glossary of terms................................................................................... 17
Key Concepts
Underline or highlight the concepts as you read through the notes.
1. Western Cape River system
 Characteristics of
 Plants and animals
 Threats to river ecosystem
2. Definition of wetlands and estuaries
 Importance of
3. Importance of frogs in the environment
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Western Cape Rivers
1.
Precious Water
Water is our most precious resource, without it the earth cannot continue to sustain life.
It might seem that there is an abundance of water on our planet –especially when viewed from outer
space - but the amount that humans can drink is very small.
Land
30%
Water
70%
Fig. 1. Percentage Land vs. Water on Earth
Fresh water
3%
Oceans
97%
Fig. 2. Percentage of Fresh Water vs. Sea Water
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Ninety seven Percent (97%) of the water on earth is found in our oceans with only 3% being
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freshwater. However, the bulk of this fresh water - 68% of all the freshwater or 2.2% of all the water on
earth – is found in the polar icecaps. This leaves us with only 0.25% in surface water, and 0.74 %, in
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groundwater, of all the water on earth that is ‘available’ as freshwater for humans to use .
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Davies B and Day J (1998) Vanishing Waters p 39
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Yet all we need do is go to a river, stream or lake nearest our homes or schools to realise that the
percentage surface water available for us to drink or use is even less than we first thought.
Surface Water
8%
Water in
Atmosphere
1%
Ground Water
23%
Ice
68%
Fig. 3. Distribution of Earth's Fresh Water
2.
South Africa a semi-desert region.
South Africa has very few rivers compared to many other parts of the world. This is due to the low rainfall
that we receive annually.
The average rainfall in South Africa is approximately 500mm per annum, while the average in Europe is
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approximately 1000mm per annum. Therefore particularly in South Africa water is our most precious –
and limited – natural resource. Our average rainfall is just over half of the world average which places
our country within a semi-desert area.
Furthermore rainfall and available water are very unevenly spread across the country 21% of the country
receives less than 200mm per annum. This situation is made worse by the fact that our average
evaporation rate per year is approximately four times more than our average rainfall per year. This
means that four times more water is lost through evaporation than is received as rainfall.
The map below shows the
different climatic regions in
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southern Africa.
The major rivers of the Western
Cape are the Berg, Olifants, Breede and Gouritz Rivers.
2
Strahler A N (1975) Physical Geography Fourth Edition.
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How are Western Cape Rivers different to Eastern Cape and Southern Cape
rivers?
Cape Rivers originate in a variety of areas. The various ecological conditions include different: 



gradients of slopes,
climates of areas,
soils and rock formations and
types of vegetation.
These varying conditions give rise to different types of rivers. For example types of rivers found include: 
perennial rivers or permanent rivers in the higher rainfall areas of the southern coastal
region and eastwards,

seasonal flowing rivers in the winter rainfall area of the southwestern Cape and

non-perennial rivers or periodic rivers in areas receiving sparse rain at any time of the year
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as are found in the dry regions of the Western Cape.
The table below compares the general features of the three ‘Cape’ water regions.
Table 1: General characteristics of the hydrological (water) regions
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Region
name
Flow rate
Peak time of
flow
Flow type
Flood
Characteristics
Western
cape
During winter
the flow is
moderate.
July/ August
Flow is strongly
seasonal
River doesn’t stay
in flood for long.
Type of
substrate
(bottom
surface)
sandy
Turbidity or
appearance of
water.
sandy
Tea- like/
Brown/ black in
colour.*
Tea- like
/brown/ black
in colour.*
Fast flowing
Rivers.
Southern
cape
Flow is not
dependent on
a season Aseasonal.
No distinctive
flow peak.
Flow and flood
frequency highly
unpredictable
Period of flood is
medium to long
Eastern
cape
Moderate to
high
November and
March
River is clear
and carries
cool, acid,
‘black’ waters.
muddy
It has more
places of slow
flow along river.
River stays in flood
for longer.
Warm and
turbid, with
neutral to
slightly alkaline
waters.
*Fynbos contain oils called tannins in its leaves to stop animals from eating it. When these leaves are shed
into rivers the tannins leach into the ground and water causing the rivers passing through fynbos areas to be
brown/ black in colour.
3
Scott H A and Hammond K C D (1984). Cape Conservation Series 5: Freshwater Fishes of the Cape. Cape
Department of Nature and Environmental Conservation, p2
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Eekhout S, King J M and Wackernagel A. (undated). Classification of South African Rivers, Volume 1. South African
Wetlands Conservation Programme, Department of Environmental Affairs and Tourism p.65 and 56
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Characteristics of a typical Western Cape river such as the Olifants River.
The conditions in and around a river changes as one travels from the mountain area where the river starts to
the river mouth where it flows into the ocean. To understand these changes and how they affect the type of
animals found in that section of a river it may be split into the following parts, the upper reach, middle reach
and lower reach.
Activity: Go to the Sappi River Meander Ecosystem. What differences do you see in
these regions in the exhibit? Make notes on the accompanying figure.
Fig. 5. A River Catchment .5
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The following table shows how conditions in a river change as you move from the upper reach to the
lower reach of the river.
Table 2: Shows the three parts of a river and the typical conditions found in each.
FACTORS
PART 1 UPPER
REACHES
PART 2 MIDDLE
REACHES
PART 3 LOWER
REACHES
Slope
Very steep
Less steep
Slight slope
Water speed
Very fast
Slower
Very slow
FACTORS
PART 1 UPPER
REACHES
PART 2 MIDDLE
REACHES
PART 3 LOWER REACHES
Abundant 02 (aq)
Very narrow
Less 02 (aq)
Wider
Little 02 (aq)
Very wide
Stones/rocks
Mainly stones
Sand/mud/silt
Oxygen
Width
Bottom surface
Soil erosion causes an increase in the suspended particles in the water. The turbidity of the water is
increased. This makes photosynthesis difficult because of no or little sunlight penetrating the water.
Particles, which sink to the bottom, form a 'woolly' layer on the river bed which has a harmful effect on
animal life
Vegetation
Plants are scarce
because of harsh action
of fast flowing water.
mosses/algae grow on
rocks, little/no plankton,
algae/plankton in water.
more photosynthesis.
plants along water's
edge
Lots of water plants /algae
/plankton
Bulrushes and reeds (such as
palmiet) line the riverbanks.
forest / canopy vegetation
on river/ stream bank.
Deforestation of riverbanks can have a negative effect on animal life, because it damages the habitat.
Sunlight
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Very little sunlight per
volume of water because
of shade from vegetation
on banks. Little
photosynthesis.
River is more exposed
therefore sunlight filters
through water.
Wide river allows more sunlight
to filter through water.
High rate of photosynthesis.
Water is warmer.
Juta (undated) Environmental Focus 3: Rivers as Ecosystems
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How is this achieved in the SAPPI river ecosystem?
Water used in the freshwater part of the ecosystem comes from the municipality, normal tap water.
Normal tap water is chlorinated and this may clog the gills of fish, so the water is passed through a
carbon filter to extract the chlorine.
The filtered water is introduced from the water fall area and over flows into the rest of the parts of
the river. No air stones are required as the waterfall provides all the Oxygen needed for the system
(exactly as is the case in a usual river system).
The temperature of the water is maintained at max of 210 C but may drop to 180 C during winter.
The fish in the upper reaches and middle reaches are indigenous to Western Cape river systems
and prefer more acidic pH of water. So to lower the pH ( to under 7) of the water Ferric Chloride is
added to the water.
The water is recirculated in this exhibit and topped up with 60 litres per hour.
River Health is important for our health
A healthy river may provide us with a number of direct and indirect services such as: 
transporting and providing clean fresh water

providing a suitable place for recreation, such as swimming and having picnics on the riverbank

providing water used for cattle and to irrigate crops. Along some areas of the river fish and reeds are
harvested

being part of our cultural and religious beliefs e.g. performing baptisms in the river pools

chemical cycling of detritus and nitrogen

Purifying Water: Rivers are able to absorb a certain amount of pollution. The river vegetation along
the banks of the river performs this function. The self-cleaning ability of rivers if protected can save
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thousands of rands that would otherwise have been spent on water purification plants.
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 Reducing soil erosion: Dense vegetation on riverbanks slows or prevents soil erosion and the loss
of topsoil.
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www.csir.co.za/rhp/state_of_rivers/state_of_umgeni
Juta (undated). Juta: Environmental Focus 4: River systems and their ability to recover
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Indicators of River Health
South Africa in an effort to better manage existing rivers has established the South African River Health
Programme (RHP). This programme focuses on selected animal groups that represent the larger
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ecosystem.
The RHP assessments are:1. Fish Assemblage Integrity Index (FAII)
Fish are good indicators of long term influences by humans on a section of a river and the condition of its
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habitat, because fish live for a longer period of time than invertebrates and are more mobile.
The absence of frogs and other amphibians from our wetland and river systems should serve as a
warning to us that the system is not healthy. Frogs respire through their skin and are susceptible to
chemical pollution in our freshwater systems. So if frogs are disappearing then soon we may
too…….(see more on Frogs on page 16)
2. South African Scoring System (SASS) is a scoring system that is based on the presence of
certain families of aquatic invertebrate present at a site.
Invertebrates need specific types of water habitats and water quality conditions for part of their life cycle.
Some of these invertebrates are unable to live in water of poor quality while others flourish in such
conditions. The presence or absence of these indicator invertebrates is a sign of changes in the overall
river condition. These are good indicators of the conditions of a section of river over a short-term for
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example a few months.
Table 3: The scoring system of mini - SASS
You can assist the river managers by studying rivers close to your home or school by using the mini
SASS scoring system. This system helps to identify which rivers are of concern, which rivers may need
further monitoring. You could download or print the sample sheets from the following website: www.riverhealth.co.za.
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How do we affect our rivers?
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All human activities within a catchment indirectly or directly drive changes in the health of a river . See
picture below.
a)
Alien vegetation is thirstier than fynbos
Trees such as Black wattle, Port Jackson and Sesbania
are alien to the Western Cape introduced from Australia to
stabilize the sand dunes.
They grow well in the riparian zone. These plants
disperse their seeds into the stream resulting in rapid
infestation of the riverbank downstream over a few years.
These tall alien trees are also much thirstier than fynbos
and need more water to grow.
Thus an area infested with alien plants will produce less
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runoff water and reduce river flow. These impacts on river
and wetland ecosystems, translating into less water in
dams for agricultural, industrial and domestic use.
Alien plants form a poor groundcover leaving a lot of soil
uncovered. The uncovered soil is easily eroded thus
rainwater runs off too quickly to soak into the soil. This
runoff of water also results in the erosion, collapse and
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flooding of riverbanks.
Fires in alien vegetation are more intense than fynbos fires. The flames in alien fires can reach up to
15m high.
The subsequent rains wash all the exposed soil into rivers or down the slope of mountains causing
mudslides as experienced in the Cape Peninsula 2 years ago. These mudslides cause rivers and dams
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to fill up with sand thus reducing the space to store water.
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Cape Nature Conservation and Department of Water Affairs and Forestry (1996) Mountain Catchment area pamphlet
Working for Water (undated). What is an invading alien plant? Working for Water programme leaflet.
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Solution: Clearing these mountain catchment areas of alien
vegetation can increase water yields by more than 30%.
Removing these trees, which ‘drink ‘ up the majority of the
water, is far more affordable and environmentally friendly than
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building new dams.
Disrupts ecosystems
Fishes and invertebrates in these systems exist mainly on a
detritus food web. Removal of natural fynbos vegetation can
seriously disrupt these food chains as these fish and
invertebrates rely on the leaves that fall into the rivers for
food. A change in the type of vegetation, for example alien
trees, can change the colour, amount of organic load of the
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water and the type of food source.
For example, fynbos sheds its leaves mostly in summer,
providing food during this period for the various animals and
microorganisms living in the river.
Alien trees, mostly from Europe, shed their leaves in a single pulse during autumn. The bulk of the food
is therefore not available at times when it is ‘expected’ by the animals. And they can’t cope with this
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overload of food source at one period.
The peak flow of Western Cape rivers is during winter- this means that all the leaves of alien trees are
washed down into the lower parts of the rivers and could cause a blockage of the river course and
flooding.
Aquatic alien plants such as Water hyacinth and Parrot feather are both South American natives
and are major economic pests.
Water hyacinth for example prevents recreational activities such as boating, skiing and angling by
blocking the river flow making it impossible for boats to pass through. Water hyacinth disrupts the water
flow in irrigation channels and it block sluices creating conditions for malarial mosquitoes and bilharzia
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carrying snails to breed.
Water Hyacinth
Parrot feather
Invasive alien vegetation blocks river courses and increases flooding while most indigenous river plants
are adapted to occasional flooding and are able to bend with the water. These dense mats reduce light
penetration into the water, reducing phytoplankton production and impacts on the rest of the ecosystem.
b)
Introducing alien fish
Alien fish are fish that have been introduced from a different area or country - a species that doesn’t
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belong naturally in an area.
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Gigher I G, Hammon K C D and Thorne S C (1980) The Distribution, conservation status and factors affecting the
survival of indigenous freshwater fishes in the Cape Province.
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Introducing alien fish into rivers is the second largest threat to the survival of many of the world’s
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10 000 freshwater fish species.
In the past indigenous fish were thought to be unsuitable for angling and eating. Therefore species such
as trout and black bass were imported from Europe and North America for recreational angling.
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Table 4: Examples of Indigenous fish species
and their ecological status
Name of fish
Picture
Status
Why is it
threatened
Habitat is
reduced.
Introduction of
predatory fish
and excessive
water abstraction
1. Clanwilliam
redfin
Vulnerable
2. Cape Kurper
Threatened
Habitat
destruction and
introduction of
alien fish species
e.g bass,
sharptoothed
catfish and
rainbow trout
Critically
Endangered
(50% probability
of extinction
within 5 years or 2
generations)
Dams block
breeding
migration of fish
4. Clanwilliam
sawfin
Endangered
Introduction of
bass, habitat
destruction and
dam obstructions
5. Clanwilliam
yellowfish
Vulnerable
(10% probability
of extinction in
100 years)
Construction of
dams,
abstraction of
water and
introduction of
bass
Found in southern
and south western
Cape rivers
3. Clanwilliam
sandfish
Live in clear pools in
Olifants river - only
found here.
May migrate upstream
to breed
Only found in slow
flowing regions of
Olifants river.
Migrate upstream to
spawn
Reduced water
flow and
introduction of
alien fish
For further information read the following:Envirofacts 22: Freshwater fishes
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Cambray J (2003). Science in Africa: Africa’s first On-line Magazine. Alien alert: the aliens are here! So are the
ecotourists.
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Skelton P (2001). Freshwater Fishes of Southern Africa
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Garrow C. and Marr S. (2012). Swimming on the Edge of Extinction
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Table 5: Examples of alien fish to Western Cape rivers include:(The names of the fish in bold are found in the lower reaches of the Sappi River Ecosystem)
Name of fish
Picture
1. Large mouth
black bass
2. Rainbow trout
Country /
continent it
originates from
Central and
Eastern
America
Threat to
ecosystem
North America
introduced via
England -sport
angling in 1897
Opportunistic
feeder preys on
insects, crabs,
fish and frogs.
3. Bluegill
sunfish
Mainly a fish
eater but will eat
anything including
crabs, frogs,
snakes and small
mammals
Declared noxious
because of its
potential to
overpopulate
rapidly
4. Carp
Europe
Breeds rapidly
and its bottom
feeding habits
make natural
waters very
muddy
5. Small mouth
black bass
Central and
Eastern
America
Preys heavily on
indigenous fish
Unfortunately these alien fish species are mostly carnivorous and indigenous fish soon fall prey to these
efficient predators, as the indigenous fish had developed in systems where such carnivores were
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absent.
These alien fish may have positive attributes. However in the absence of natural limiting factors such as
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diseases or predators they spread and displace indigenous fishes of that area.
With the ever-present threat of drought, water is transferred from catchments with good water supplies
and low water demand to those where demand for water is high and the supply is poor. This results in
mixing of fish species from these two different river systems. Such introductions result in indigenous fish
from one river system becoming ‘aliens’ in their new habitats.
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Did You Know!
In the SAPPI river system ecosystem the alien fish include Carp, large mouth black bass and
Sharptoothed catfish and are in the lower reaches. The alien freshwater fish are fed once daily with
khoi or flake fish food. The Sharptoothed catfish is fed twice a week with mullet.
The indigenous freshwater fish are in the upper and middle reaches of the exhibit. They are fed twice
daily with flake food, crickets and mealworms.
c)
Farming practices
Poor farming methods such as overgrazing and ploughing down a slope can cause excessive erosion of
the lands. Erosion of large farming areas may result in a build up of silt in rivers, which in turn affects the
spawning beds of fish and may lead to flood damage during heavy rains.
Some farmers enrich their lands with fertilisers – nutrients- in order to increase crop yields. A large
proportion of these nutrients is either washed off the soil into rivers or leaches through the soil into the
ground water and then into rivers. On reaching the rivers these nutrients are as effective in increasing
plant production as they are on land. When present in excess, they stimulate the development of blooms
of algae and floating pest plants. This build up of nutrients in a river is called eutrophication.
[(see page 9 a) Introducing alien plants]
d)
Water extraction
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Water is pumped out of rivers and used for irrigation, industrial processes and domestic purposes. This
is particularly devastating during the hot summer months of the Western Cape when farmers remove
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water from already depleted rivers.
This may lead to :
sections of the river drying out and many fish and other animals dying.


e)
an increase in salt concentration (salinity) of freshwaters.
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an increase in the effects of pollution by concentrating pollutants.
Damming of rivers
Dams restrict the movement of migrating fish ( page 11, Table 4 Indigenous fish) and break
populations into smaller units, which are then more susceptible to other threats such as pollution or
sedimentation.
The quantity and timing of flows in a river are critical to the maintenance of the ecosystem. Low flows
may influence the diversity of species or trigger spawning in fish, while flooding might flush out poor
water and trigger migration of fish and germination of seeds. Dams prevent a river from flowing and thus
change these environmental cues on which some organisms rely and change the riverine habitat from
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flowing water to still water.
If we use water wisely there may be no need to build more dams.
f)
Habitat damage
Where development results in habitat changes certain animals may not return to that part of the river
even after the disturbance has stopped.
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Share net (1999) Envirofacts 22: Freshwater fishes
Major N (2001). Notes from Two Oceans Aquarium Volunteers Freshwater fish Workshop
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Examples of these kinds of changes are draining of wetlands, canalisation of riverbeds, or removal of
vegetation on banks and margins. For a river to function naturally with a diverse animal and plant
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community, a number of different habitats are needed.
Canalisation
The purpose of canalisation was originally to prevent flooding of properties in low-lying parts of the
catchment and to route stormwater in the most direct (and rapid) way out of the area.
The bed and banks of a natural river is replaced with virtually smooth concrete on which no organic
material can be trapped and therefore few animals live in these canals. Such stretches do no more than
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channel water from source to sea or discharge point.
A canalized river
A river with Gabions
Solution:
A better way of managing or rehabilitating a riverbank is to use gabions (stone filled baskets). The gaps
facilitate plant and animal growth and therefore maintain a natural habitat.
Drilling one metre holes through the bottom of the canal reconnects the river with its sediment. This
allows for the accumulation of organic and inorganic material in the cores and some of the natural
functioning of the river and its organisms to redevelop. These functions include decomposition of
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decaying material, shelter for invertebrates during flooding and habitat for burrowing animals’.
i)
Pollution
Water pollution occurs when chemicals or nutrients enter water faster than natural processes can
remove it. Pollutants include: sewage, oil, silt, industrial chemicals, heavy metals, air-borne manmade chemicals dissolved in rain water, pesticides, fertilisers and herbicides leached from the
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land.
Water pollution comes from ‘point’ and ‘non-point’ sources. Point sources include pipe outlets and
other direct and observable sources of pollution. These sources may be easily identified and
controlled. Non-point sources include acid precipitation and the run-off of fertilisers from large
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farming areas. These pollution sources are less easy to identify and control.
Litter itself is seldom seriously damaging to water environments unless it is in such a large quantity
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to physically damage aquatic habitats. Litter such as plastic bags, paper, cooldrink tins etc. in rivers
are unsightly, especially because it takes years to decompose if at all.
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Shelembe E B (1995). Monitoring Visible Water Life.
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Wetlands
1. What are wetlands?
__________________________________________________________________________
2. The three important characteristics or features of wetlands are:____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Summary of importance of wetlands
1. _______________________________________________________________________________
2. ________________________________________________________________________________
3. ________________________________________________________________________________
4. ________________________________________________________________________________
5. ________________________________________________________________________________
6. ________________________________________________________________________________
7. ________________________________________________________________________________
8. ________________________________________________________________________________
9. ________________________________________________________________________________
Further reading : Envirofact sheet 2: Wetlands and Envirofact sheet 3: Estuaries
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Why are Frogs important?
1. Amphibians are the only group of animals able to truly live and exploit both aquatic and terrestrial
environments.
[The word amphibian is derived from two Greek words viz. amphi meaning double and bios
meaning life hence ‘living a double life’]
2. Amphibians are extremely interesting animals.
They are also extremely varied, showing a tremendous range of shapes, colour patterns, behaviour, different
vocalisations, breeding habits and habitats. They live in habitats that range from mountain tops to lowland
wetlands and extremely arid deserts to humid forests.
3. Amphibians form a very important part of the food web, especially in wetlands.
Tadpoles eat large quantities of algae, which makes them primary consumers or herbivores. The
tadpoles are in turn eaten by dragonfly larvae, fishes or waterbirds.
Conversely the adults are secondary consumers or carnivores eating large quantities of insects and
other invertebrates. This makes them useful animals for use as biological controls.
So would you notice if there were no more frogs?
Frogs in turn are food for fish, waterbirds, snakes, monitor lizards, young crocodiles and otters.
Interestingly the eggs are protected from predators by toxins and do not form part of the food chain.
4. Amphibians act as monitors of water quality and general environment quality/early ecological
warnings.
Frogs are sensitive to various pesticides (e.g. DDT used in malaria affected areas), fertilizers and
pollutants .If frogs have contact with any of these they die.
Severe decline in amphibian population world wide may be due to increased acid rain, global warming,
ozone depletion, pollution AND increase in urbanisation which has reduced natural habitats such as
wetlands.
Ozone depletion has resulted in an increase of ultraviolet light. An increase in UV light has caused the
chytrid fungus to grow on frogs’ skin. The frogs are now unable to breathe through their skins, suffocate
and die on a large scale. Instances of chytrid fungus have been discovered on frogs in Panama, South
America as well as in Australia.
5. Amphibians are walking pharmaceutical factories, the skins producing, amongst other things,
antibiotics and painkillers. [They do not have to be killed to extract these chemicals]
6. The amphibian the platanna or African clawed toad was used as a human pregnancy test from
1933 and was only abandoned in the 1960s.
[A female plattana would produce eggs if injected with a little urine from a pregnant woman].
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Glossary of terms
Alien species
A species which has been introduced from another area or country;
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a species that doesn’t belong naturally in an area.
Bilharzia
Bilharzia is a waterborne disease caused by parasitic flatworms. It is
spread when an infected person urinates in river water.
Animals which are used as an indicator of river health or pollution.
Bio indicators
Bio Pollution
Deforestation
Introducing animals or plants to an area where they are not normally
found.
Clearing of forests
Detritus
In ecological terms, the remains, often pieces, of plants and animals
Endemic
Eutrophication
Animals found nowhere else but in a specific area. This term is used
for species within a restricted range.
A build up (excess) of nutrients
Groundcover
Low-growing, spread plants that help to stop weeds growing.
Indigenous
Intercatchment link or
Interbasin transfer
One which occurs naturally in the area it is found, i.e. its natural
home range
The transfer of water from one catchment area to another by means
of tunnels and canals
Invertebrates
Animals which don’t have a backbone or internal skeleton
Non Perennial
Referring to a river which does not flow all year round.
Nutrients
Perennial
Elements needed for life processes e.g. nitrogen, phosphorous and
potassium are the most important nutrients.
Refers to a river which flows all year
Riparian
Refers to a river bank
River Catchment
It is all the land from the mountaintop to seashore, drained by a
single river and its tributaries.
This is a buildup of sand or mud in either rivers or dams.
Siltation
Turbidity
The immediate visual effect of a change in water is a change in
water clarity. In other words the degree of visibility in water
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