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MODULE 2
THE TWO OCEANS OF SOUTHERN AFRICA
TABLE OF CONTENTS
1. THE TWO OCEANS OF SOUTHERN AFRICA AND THEIR DIVERSITY
2
1.1
Southern Africa’s currents
4
1.2
Differences between the three main coastal regions
4
1.3
Impacts of ocean currents
7
2. RED TIDE
8
3. TIDES
10
4. ROCKY SHORES
12
5. REFERENCES
14
Key Concepts
Underline /highlight the answers to these questions as you read
What is Biodiversity?
a) What is the difference between the two oceans off South Africa’s Coast
b) Names of the two oceans, currents and temperatures
c)
How this affects animal and plant variety in the oceans
d) How this affects the climate along the coast and vegetation on land
e) What is Upwelling?
f)
How does it happen?
g) What effect does it have?
h) What is red tide? Describe the process.
Tides:
i)
What causes tides?
j)
What is spring tide?
k) When do we experience spring tide?
l)
When do we experience neap tide?
m) How the tides affects the distribution of animals and plants along the shore.
n) Names of the different rocky shore zones and main organism found in each zone.
(placed in correct position on figure)
Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
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1. The two oceans of Southern Africa and their diversity
Biodiversity describes the different variety of living animals or plants in an area, including the
number of different species, the genetic wealth within each species, the interrelationships
between the species, and the natural areas where they occur.
South Africa has the third-highest level of biodiversity of plants and animals in the world. Brazil is
ranked first and Indonesia second highest. South Africa, and particularly the W. Cape, is called a
‘biodiversity hotspot’. South Africa is the only country to contain an entire floral region (The Cape
Floristic Region)!
The Table Mountain National Park for example has more plant species within its 22 000 hectares
than the whole of the British Isles or New Zealand. Some 18 000 species of vascular plant (plants
with vessels for bearing sap) occur within South Africa's boundaries, of which 80% occur nowhere
else. The table below illustrates that South Africa’s biodiversity is not restricted to plants.
Number of species found in South Africa
Plants
23 420
Marine Invertebrates
8 859
Marine Algae
800
Marine Fishes
2 200
Birds
694
Mammals
258
Reptiles
363
Amphibians
111
Freshwater Fishes
220
Table 1: Biodiversity in South Africa.
Percentage of Earth’s
Species
9%
8%
10%
15%
7%
6%
5%
2%
2.6%
Our incredible biodiversity is due to our unique physical features. Most of the country is situated
on a high-lying plateau and the coast is flanked by two very different oceans (fig. 1).
The oceans surrounding South Africa can be divided into three coastal regions. They are:
a) The Atlantic Ocean off the West Coast, with cold sea temperatures ranging from 9 –150 C.
b) The Indian Ocean off the East Coast, with warm sea temperatures ranging from 18 – 270 C.
c) a) and b) meet and mix along the South Coast with sea temperatures ranging from 160 – 200C.
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Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
Figure 1. The Two Oceans of southern Africa showing the major currents
Quick review
1. Which two oceans are found off South Africa’s coast
a) Pacific
b) Indian
c) Atlantic
d) Southern
2. The temperature of the water off the South Coast is:a) 9-15 °C
b) 18-27 °C
c) 16-20 °C
3. The three main coastal regions differ in terms of temperature, nutrients, plant life and
wind direction. True or False?
4. Which two currents mostly influence SA’s coast
a) Benguela
b) Agulhas
c) Mozambique
d) Humboldt
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1.1 Southern Africa’s currents
The southern African coastline is influenced by two
major currents –the benguela and agulhas currents.
THE BENGUELA CURRENT
This current flows slowly northwards from the
Southern Ocean along the West Coast carrying colder
water. A process called upwelling further enhances
the cooling effect of the Benguela current.
UPWELLING:
 Upwelling occurs when there is an offshore
wind.
 Off the west coast the South Easterly winds
blow the surface water away from the coast.
 This surface water is replaced by water from
deep below which is icy cold. (See figure to
Figure 2. An illustration of the upwelling process.
the right).
 The water close to the ocean floor is full of nutrients from the dead animals and plants
that have sunk to the sea floor.
 When this nutrient rich water reaches the sunlit surface it provides a food source for
phytoplankton and the kelp forests.
 Heat from the sun results in rapid growth of marine algae and phytoplankton.
Ironically, sea temperatures off the West Coast are colder in summer than they are in winter. This
is because the South Easterly wind that causes upwelling occurs in summer.
THE AGULHAS CURRENT
This fast moving powerful current flows southwards down the East Coast from the Madagascar
region bringing warm water from the tropics. The main part of the Agulhas current is deflected
away from the coast close to East London but streams of warm water continue to flow along the
south coast to False Bay. The Agulhas Current carries relatively low concentrations of nutrients, so
that the productivity of the water off the East coast is not very high.
Quick review
1. True or False? Upwelling is caused by the following process:a) SE wind blows the surface water away from the W coast
b) Cold Water containing nutrients is brought to the surface
2. The Agulhas current contains the highest amount of upwelling experienced on earth. True or False?
1.2 Differences between the three main coastal regions
The differences in temperature and nutrients results in changes in plant and animal life as one
moves around the coast.
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Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
A. WEST COAST: Vast numbers, but few species

One of the most nutrient rich coastlines: Nutrients brought to the surface allow
microscopic floating plants called phytoplankton. Tiny animals known collectively as
zooplankton eat this phytoplankton, and both types of plankton become a rich source of
food for plankton-feeding fish such as pilchards and anchovies.

These small fish in turn are consumed by shoaling predatory fish such as snoek and tuna.
Pelagic (swimming) species such as pilchard and anchovy, and benthic (bottom-dwelling)
species such as hake and kingklip occur in enormous numbers and sustain major fishing
industries. The west coast supports a relatively small number of fish species, but their
biomass ('meat' per unit volume) is high.

Enormous kelp forests dominate the inshore region on the west coast. Kelp is a very fast
growing sea plant that supports many animals in the region. Perlemoen and Argenville’s
limpets feed on the kelp fronds by trapping them under their shells. Mussels, sponges and
redbait sieve the liquidized kelp that is broken up by the waves. The West Coast rock
lobster feeds on the mussels.

There are relatively few species of invertebrates in the region but it supports enormous
numbers. The richest rock lobster and perlemoen grounds lie on the west and southwest
coasts. On rocky shores, limpets occur at densities unmatched anywhere else in the world.

Seals, penguins and other sea birds such as cormorants and gannets are common on the
west coast, particularly on offshore islands where they breed undisturbed by predators.

Human activities: many people make a living by harvesting kelp, fish and shellfish from the
west coast.

Phytoplankton Blooms: the west coast of South Africa is also known for its
phytoplankton blooms which can result in rock lobster walk outs (see red tides)
B: EAST COAST: Marine life is diverse and colourful
The water is warm, clear and has low levels of nutrients. Marine life on the east coast consists of a
large diversity of species, most of which occur in relatively low numbers. The competition for
food is strong and there is a high proportion of predators. Many species are brightly coloured and
have remarkable adaptations to allow them to survive in a competitive environment where there
are many predators.
Examples of some of the adaptations are
listed below:
1. Bright colours - Fish and other
animals use distinctive markings
and colours like flags on the reef.
This helps them to identify friend
from foe. Many fish are territorial
and will chase away fish of the same
species. The females and juveniles
are often differently patterned so
that the male will allow its mate and
offspring to feed in its territory but
will chase off foreign males (e.g
Emperor Angelfish). Sometimes
Fig. 3 Emperor Angelfish: Adult and juvenile.
Which is which?
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colours advertise danger such as venomous spines or poisonous flesh. While in the
case of cleaner wrasses and shrimps the colours advertise useful cleaning services that
these animals perform by removing dead tissue and parasites from larger fish. Some
fish have special bony or spiny armour for protection.
2. Venomous and poisonous animals are more common in warmer water than in colder
regions. These include the deadly
stonefish, devil fire fish, pufferfish,
certain sea urchins, cone shells, fire
corals and fire worms.
3. Algal cultivation - Corals, colonial
anemones (zoanthids) and giant
clams
house
tiny
algae
(zooxanthellae) in their body walls
to provide extra food for
themselves. The algae manufacture
food by photosynthesis and gain
fertilizer from theirs host’s waste
Fig. 4 Devil fire fish or lionfish
material which helps to produce
calcium carbonate (limestone) to build coral skeletons.
4. Coral Reefs are formed by the rock-like accumulation of calcareous (calciumcontaining) exoskeletons of coral animals. Built up layer upon layer by living corals
growing on top of the skeletons of past generations of coral, coral reefs grow upwards
at rates of 1 to 100 cm per year. Coral reefs are tropical, extending to about 30° north
and south of the equator and forming only where surface waters are never cooler than
16° C.
5. Clown fish sheltering among anemones. The anemone protects the fish and the fish, in
turn, protects the anemone by warding off predators, removing damaged tissue and by
preventing silt from settling on the anemone.
The Greatest shoal on earth
Part of the pilchard (sardine) stock
moves slowly eastwards from the south
in autumn and appear to mass off at
Port Elizabeth before starting their
migration along the east coast. Their
movement east past East London
heralds the start of the annual ‘sardine
run’ into Kwa-Zulu Natal waters. They
are followed by hungry predators such
as elf, yellowtail, garrick (leervis),
geelbek and kob. Sharks and dolphins
also follow the sardine run. In southern
Kwa-Zulu Natal the cold current is
pushed closer to the shore and
enormous quantities of sardines are
sometimes forced into the shallows and
even onto the beaches. Sea birds and
humans join in on the feast.
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Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
Sea plants found along the east coast are generally iridescent, small and extremely varied. There
is no kelp to break force of the waves so the algae have to be tough and flexible. Many contain
calcium or chemicals to deter grazers.
Human activities: Holidaymakers enjoy swimming and snorkelling in the warm waters of the East
coast. Line fishing is popular because of the wide variety of fishes.
C. SOUTH COAST: The intermediate region
This is an intermediate region with a wide range of sea plants, including kelp as far as Stilbaai, on
the south east coast.
 Many fish migrate through this area, including silvery schooling species such as pilchards
(sardines), maasbanker and mackerel. A number of fish species undertake seasonal
migrations to the East coast in winter, after spending their summer on the South coast.
Examples are the white musselcracker, the Poenskop (also known as the black
musselcracker) and the red steenbras.
 Several species of whales visit the south coast of South Africa in winter. Southern Right
whales can be seen along the South Coast during June to November, where they mate and
give birth. They return to the icy Antarctic waters to feed on krill during summer.
1.3 Impacts of ocean currents
The ocean currents that flow along the coast of Southern Africa have profound effects on the
climate, rainfall and the resulting vegetation on land.
Effect of the Benguela current on the West Coast





The cool waters of the west coast provide little moisture for the air moving onshore
resulting in very dry conditions on land along the coast.
In summer conditions are particularly dry because the south easter blows offshore and
the land is left hot and dry. At night fog banks develop over the cold sea and bring some
moist relief to the coastal vegetation, but inland there are desert or semi-desert
conditions.
In winter there are north westerly winds bringing winter rain to the region but even
then the amount of rain is small over most of the West Coast.
A shortage of fresh water is one of the most critical problems facing this region.
The fynbos vegetation of the Western Cape is adapted to hot dry summers and cold wet
winters. Most of the plants have small curled or hairy leaves to reduce water loss and
many of them spend the hot dry summer either as underground bulbs or seeds. The
broken nature of the terrain and the harsh climate has led to an amazing array of
different species which burst into bloom in spring!
EAST COAST


A warm sea allows abundant evaporation and onshore winds bring more rain to the
region.
This rain results in lush sub-tropical vegetation. However, in the rain shadow behind the
mountain ranges lies the dry Karoo region.
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SOUTH COAST

The south coast is an intermediate region where both winter and summer rainfall
occurs. Generally there is good rainfall on the coastal belt, especially adjacent to the
mountain ranges.
Quick Review
EAST COAST
WEST COAST
Population
Precipitation
Nutrient levels
Sea plants
Appearance of
the fish
Vegetation along
the coast
Fisheries
2. RED TIDE
Red tide refers to discolorations caused by dense concentrations of phytoplankton (tiny
microscopic plants). The three types of red tide organisms are dinoflagellates, diatoms and ciliates.
The term red tide is misleading, because discolouration of the water may be red, brown, orange,
purple or yellow.
Red tides usually occur along the west and south coasts of the Cape in late summer and autumn.
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
Upwelling of the cold, nutrient rich water brings with it the dinoflagellate cysts, which have
been lying dormant in the sediment of the sea floor.

This nutrient rich water combined with ideal conditions of temperature, salinity and light
trigger the dinoflagellates to develop and grow. The rapid increase in their numbers
(sometimes millions of cells per litre of water) results in ‘blooms’ of phytoplankton.

Wind and currents concentrate the phytoplankton blooms in certain areas leading to the
formation of red tides. In addition dinoflagellates have the ability to swim to the surface
using two whip-like appendages called flagella.

Most red tides are useful and add to plankton production, but some produce harmful
results. Harmful algal blooms are most commonly reported in the media. Harmful algal
blooms are natural and seasonal phenomena and they occur widely off the west coast.

Certain types of red tide cause huge mortalities of abalone, white mussels and black
mussels.

Mussels are filterfeeders and filter the ocean of phytoplankton which could include red
tide organisms. The mussels itself is not harmed but the phytoplankton is stored and
Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
concentrated in the mussel - so that they become poisonous if eaten by humans. Mussels
can remain contaminated for about four months.

Most of the mass mortalities of fish and rock lobster, however, are due to oxygen
depletion, either because their gills become clogged or because the oxygen in seawater is
used up when the red tide organisms die and rot.

During the decay process aerobic bacteria soon deplete (use) the oxygen in the water and
are replaced by anaerobic bacteria.

Anaerobes obtain their energy by converting sulphur into toxic hydrogen sulphide gas. This
gas smells of rotten eggs, turns the water black and poisons the remaining organisms. This
is known as a black tide event.

A "black tide" event in St Helena Bay killed 95% of all marine life in 1978. Rock lobsters are
particularly susceptible to oxygen depletion on the ocean floor. Their only refuge is in the
surf zone, where wave action generates oxygen-rich water, but they are easily left stranded
when the tide retreats.
Figure 5. If calm weather follows a bloom the plankton may become trapped in a bay and use up all
the nutrients and consequently die or sink to the bottom and form dormant cysts.
Before eating shellfish contact the Red Tide Hotline: 021 434 4457
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Quick Review
Circle the letter(s) of the most accurate answer(s) to the questions.
Note: in some cases more than one answer may be correct.
1. Red tide depends on
[a] the phase of the moon
[b] the south easter blowing on the east coast
[c] the concentration of the phytoplankton in certain areas
[d] upwelling of the cold nutrient rich water
2. Red tide occurs in
[a] late autumn
[b] late spring
[c] at full moon
[d] late summer
3. One of the following conditions is absolutely essential for red tide to develop
[a] upwelling of nutrient rich water
[b] wind
[c] currents
[d] temperature, salinity and light
4. Most red tides are useful but some are harmful. The harmful red tide may either directly kill
large numbers of abalone, white and black mussels or kill them because it has
[a] used up all the oxygen
[b] become concentrated in the flesh of mussels for months because the mussels are
filter feeders
[c] used up all the carbon dioxide for the fish and lobsters
[d] died and rotted and so all the oxygen for the fish and lobsters has been used up
3. TIDES
The sea level rises and falls twice a day which results in a section of the shore being flooded or
exposed. This rising and falling sea level is called a tide. This is mainly caused by the gravitational
pull of the moon.
The sun also exerts a pull on the sea causing extra high and extra low tides when the earth,
sun and moon are in a straight line. These are called spring tides which happen when there is
either a new moon or a full moon.
Look at the tide table on the next page to plan your beach visit to the sea shore when it is spring
tide.
1. Check the date of the next or closest new moon and full moon.
10 Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
2. You may plan your visit to the beach either 2 days before or 2 days after new moon or full
moon.
3. Then check the HIGH tide table for the date you have chosen.
4. Low tide will be approximately 6 hours after high tide – so add 6 to the time given for high
tide.
5. In the Western Cape spring low is mainly during the morning.
Information on tides can be obtained from the newspaper, a diary which has the phases of the
moon included (see figure 6), internet www.saweather.co.za or www.sanho.co.za or purchase a
tide table book.
Refer to the phases of the moon.
1. On which date(s) in April 2013 would spring tide occur?
2. What would be the best time during the day to go and visit the rocky shore?
Figure 6. 2010 September Tide Table
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What does this mean for animals and plants living between the high and
low water marks?
The area between the high tide and low tide level is called the intertidal zone. Twice a day the
organisms in this zone (animals and plants) are underwater and uncovered by changes in tide
level (viz. high tide and low tide). One finds ecosystems of the intertidal zone along both rocky and
sandy shorelines.
Plants and animals that live on rocky shore intertidal zones have adapted their vital functions of
respiration, excretion and reproduction for two completely different environments:
 the aquatic environment when the tide is high
 the terrestrial environment when the tide is low.
The higher up they are on the shore, the more the intertidal organisms might:
1. dry out,
2. experience low temperatures in winter, or high temperatures in summer, and the
3. less food comes their way.
The lower they are on the shore, the
4. greater the competition in the space available.
At low tides,
5. birds move in to feed on them.
Intertidal =
between high and
low tide level
High tides
6. bring the predatory fishes.
4. ROCKY SHORES
Rocky shores range from headlands with vertical cliffs to wide, wave-cut platforms and jumbles of
boulders polished by the motion of the ocean. A great variety of plants and animals live on rocky
shores where they can attach to the firm substratum and shelter in nooks and crannies. Many
animals live totally submerged in pools and gullies while those on open rocks are exposed by the
rise and fall of the tide twice a day, which results in physical stresses such as wide temperature
fluctuations and water loss. Animals and plants occupy distinctive zones on the shore according to
their needs / limitations / levels of predation and competition.
12 Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
Rocky shore zonation
The marine organisms on rocky shores are well adapted to a variety of environmental conditions.
A close look at rocky shores at low tide reveals that the position of plants and animals within the
intertidal zone depends on the amount of exposure to air and wave action they are able to
withstand. For instance, those that dominate the lower shore are only out of water during low tide
and are able to tolerate short periods of exposure to the air but can tolerate strong wave action.
Biologists have described four distinct bands (zones) between the high and low tide levels that
typify South Africa's rocky shores.
Each zone is characterised by a well-defined group of plants and animals. The species that occur in
each zone vary according to their location around the coast.
The following are brief descriptions of the distinct zones that occur on rocky shores around South
Africa:
The Littorina zone is the highest and most barren zone on the shore. Small, air-breathing
littorinid snails, one species on the west and south coasts and three species on the east coast,
inhabit it. Purple laver (Porphyra capensis), a hardy sea plant able to withstand severe
desiccation, also occurs in this zone.
The upper balanoid (barnacle) zone is dominated by barnacles, winkles, limpets and a few
seaweeds able to survive the semi-dry conditions that prevail in this zone.
The lower balanoid (barnacle) zone on the west coast supports thick beds of fleshy seaweed.
On the east coast you will find slippery, green zoanthids and brightly coloured sponges, brown
mussels and coralline seaweeds.
The infratidal or subtidal zone is the lowest region on the shore and the richest in plant and
animal life especially red bait (sea squirts), anemones, sea urchins and starfish. Colourful,
branched seaweeds are found on the east coast, while large kelps flourish on the west coast.
Additional zones occur on particular coasts:
The oyster belt on the east coast is dominated by oysters.
The cochlear zone (between the infratidal and the lower Balanoid zones) on the south coast
supports dense bands of pear limpets at the low-tide mark.
The cochlear/argenville zone is a feature of the west coast. The zone takes its name from the
tall, Argenville's limpet, which occurs in a band, together with large numbers of pear limpets.
Black mussels are also found in this zone.
Rocky shore profile / outline
You can see that there are transverse and longitudinal lines in the rocky shore outline below.
These lines indicate different areas according to tides or indicator species. ‘Indicator species’
refers to those animals which are mostly found in that area.
Start filling in the names of the various areas or the names of the indicator species following the
order of the numbers (use the information on the previous page about the zones).
In the case of no. 1, 4, and 5 write the answer along the vertical line.
Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
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5_________________________ZONE
10 ____________________
6. ____________________________ zone
7 __________________
ZONE
3
________________zone
______________________
_______________ zone
Figure 7. Tidal zones found along the South African coast
REFERENCES
ATTWOOD, C. 2000. Coastcare Fact Sheets. Red Tide 1C.
BRANCH, G. and M. Branch. 1998. The Living Shores of Southern Africa. Cape Town: Struik.
14 Two Oceans Aquarium Vol. Manual Module 2 – THE TWO OCEANS OF SOUTHERN AFRICA
9. ___________________ tide
4:___________________________ZONE
8.___________ tide
1____________________________ zone
2 ________________ZONE