South African
Marine Ecosystems
Two oceans meet at the southern tip of Africa. By the end of this topic you will be able to place our two basins, three coastal regions and five biogeographic zones on a map, and explain the currents that built them.
Why is the water at Cape Town's beaches freezing cold, but at Durban it's warm enough to swim in all year round?
Two completely different ocean currents are responsible. The cold Benguela Current moves northward up the west coast, out of the Southern Ocean. Meanwhile the warm Agulhas Current rushes tropical water down the east coast from Mozambique.
They meet near Cape Agulhas — and that is why South Africa has three completely different coastal worlds in one country. The two coastlines do not just differ in water temperature either: they get very different weather, because the ocean beneath the air is the thing heating or cooling it.
Two Basins, Three Regions, Five Zones
South Africa is bordered by two ocean basins
The Atlantic Ocean Basin covers our west coast, as far as Cape Agulhas. The Indian Ocean Basin covers everything east from there, all the way up to Mozambique. The dividing line was a geographical decision — somebody picked the southernmost tip of Africa because it was a convenient landmark — but it turned out to be a real ecological boundary too, because each basin is dominated by a different current and very different weather.
Those two currents set the water temperature, and water temperature sorts the coast into three coastal regions. Each region is then subdivided by geography and by which species actually live there, giving five inshore biogeographic zones (BGZs). The whole of this topic hangs off that one sentence, so it is worth learning which zone belongs to which region — and which region belongs to which basin:
| Ocean basin | Coastal region | Biogeographic zones |
|---|---|---|
| Atlantic (west, to Cape Agulhas) | Cool Temperate CAPS: Cold Temperate | Namaqua · South Western Cape |
| Indian (east of Cape Agulhas) | Warm Temperate | Agulhas |
| Subtropical | Natal · Delagoa |
Two basins · three regions · five zones. Notice the Atlantic basin holds one region and two zones, while the Indian basin holds two regions and three zones.
Placing the Names
This is the single most examinable thing in the block: you must be able to place the basins, the regions and the zones on a map of South Africa, and name the landmark at each boundary. Tap any coloured ribbon to read that zone.
Cool Temperate
- Namaqua BGZ
- South Western Cape BGZ
Warm Temperate
- Agulhas BGZ
Subtropical
- Natal BGZ
- Delagoa BGZ
Four landmarks do all the work
Every zone boundary in this block is one of four places: Cape Columbine (Namaqua ends), Cape Point (South Western Cape ends), East London (Agulhas ends, Natal begins) and St Lucia (Natal ends, Delagoa begins). Learn those four and you can rebuild the whole map from memory. Cape Agulhas is the fifth name to know, but it divides the basins, not the zones — a distinction examiners like.
The Two Currents, and Where They Meet
The regions above exist because of these two currents. Tap the numbered points to follow the Agulhas from Mozambique to the place where it turns back on itself.
Why the Agulhas Bank matters more than it looks
The Agulhas Bank is just a shallow bit of seafloor off our south coast — but it is wide, and a current that fast cannot cross it without being pushed. That single obstruction deflects the Agulhas south-west, forces the retroflection, and creates every eddy in this diagram. It is also the reason the Warm Temperate region gets an "influx of colder water from the west along the shallower coastal shelf". You will meet the Bank again in Topic 3 as a piece of continental shelf.
Local connection — Algoa Bay sits on a seam
Gqeberha is near the eastern end of the Warm Temperate region, and Algoa Bay is one of the places where the "influx of colder water from the west along the shallower coastal shelf" actually arrives. That is why our water temperature jumps around so much through the year, and why you can find species here that belong to the region on either side of us.
It also makes Algoa Bay an unusually good place to do this topic rather than read it. A rock pool at Cape Recife and a rock pool at Sodwana are in two different coastal regions, and the difference is visible without any equipment at all.
Three Coastal Regions, Compared
Cool Temperate
- Driven by the cold Benguela Current
- Frequent upwelling of cold, nutrient-rich water
- Nutrients drive phytoplankton blooms, coastal algae and kelp forests
- Which in turn feed large populations of consumers
- 2 zones: Namaqua (to Cape Columbine) + SW Cape (to Cape Point)
Warm Temperate
- Edges fluctuate seasonally as temperatures rise and fall
- Often gets an influx of colder water from the west along the shallow coastal shelf
- Some species overlap with both neighbours
- But numerous endemic species: Red Stumpnose, Knysna Seahorse, Leopard Catshark, Basket Star
- 1 zone: Agulhas BGZ
Subtropical
- Agulhas water is clear and nutrient-poor
- Onshore winds cause downwelling, which keeps it that way
- Low-growing coralline algae, but very high species diversity
- High animal diversity, smaller population sizes
- 2 zones: Natal (to St Lucia) + Delagoa
A wording difference worth knowing
Your booklet and this course say Cool Temperate. The CAPS document says Cold Temperate for the same region. They are the same thing — use whichever the question uses, and do not lose a mark second-guessing yourself.
Upwelling vs Downwelling
These two processes explain why our west and east coasts look so different ecologically. Same country, opposite worlds — and they are opposite because the wind blows a different way.
↑ Upwelling
Strong offshore air flow pushes surface sea water out, away from the coast. Cold, deep, nutrient-rich water is drawn upwards against the coastline to replace it. Result: phytoplankton blooms, coastal algae, kelp forests — and the large consumer populations they feed.
↓ Downwelling
Prevailing onshore winds push warm surface water against the shoreline and force it downward, displacing the deeper, colder, potentially nutrient-rich water. Nothing brings those nutrients up. Result: clear blue water, very little plankton — and, in the far north, coral.
The Subtropical Puzzle
Here is the contradiction this region is built on: the water is nutrient-poor, yet it holds the most diverse ecosystems on our coast. Working out how is most of the marks in this section.
The seaweeds go low and go hard
Algae here are low-growing — moss-like, carpeting most rocks rather than standing up like kelp. Many are coralline: their cells incorporate calcium carbonate (CaCO₃), which makes them hard and unpalatable. That is a defence: a grazer trying to eat one is essentially chewing rock.
Despite the small size, species diversity is very high.
Seaweed gives way to coral
Move north and the rocks change hands. Seaweeds on rocks give way to coral in northern Natal and the Delagoa BGZ. In the southern part of the region, rougher ocean conditions and silt flowing out of the many estuaries discourage coral from forming at all — so the switch is not random, it tracks water clarity and wave energy.
Encrusting coralline algae build the reef
Encrusting coralline algae are the pink or red seaweeds that lay down increasingly thick layers of calcium-rich material on hard surfaces like rock and coral. Further north, new coral polyps and these algae build on top of multiple layers of older coral — which is how a reef becomes a reef rather than a thin coating.
The answer to the puzzle: a partnership
If subtropical water is so nutrient-poor, how do coral reefs flourish? The coral polyps — the tiny animals that build the reef — have a partner. Single-celled algae called zooxanthellae live inside the polyps' tissues, photosynthesising there. This is a symbiosis: the algae get shelter and raw materials, the coral gets food.
Multiply one polyp by the enormous collective surface area of a reef, and you get one of the most productive ecosystems in the ocean built in water that should not support one. Zooxanthellae are not limited to hard corals either — they also partner with soft corals, jellies and clams.
| What CAPS says | What it means on the reef |
|---|---|
| Animal species diversity is high, but population sizes are smaller | Many different species, few of each — the opposite of the west coast, where a handful of species occur in enormous numbers |
| Coastal fish exhibit tropical characteristics | Generally smaller and very colourful, and usually territorial |
| Larger territorial reef fish, sharks and larger pelagic fish visit periodically | It would be inaccurate to say all reef fish are small. Big eels hold territory on the reef, and sharks and open-ocean pelagic fish come in to hunt. Most residents are carnivores, so everything here is adapted either to compete or to avoid being an easy meal |
Beyond the Five
The five zones on the map are the inshore biogeographic zones. Further BGZs are identified in deeper water, out beyond the continental fringe to the edge of the South African Economic Exclusion Zone — the band of ocean within which the South African government's permission is required to harvest resources.
You are not examined on the offshore zones — CAPS says so explicitly. Know only that they exist, and that the five you are examined on are the inshore ones.
In the Lab — the CAPS Investigations
CAPS sets two investigations for this block, and both are research tasks rather than experiments. The practical sheet adds a third — the map work — because placing the names is the outcome this block is actually assessed on.
- 🟢 desk Place the names. The map above with every label stripped off, and a table to write them back into: two basins, three regions, five zones, four boundary landmarks.
- 🟢 desk 🌊 shore Investigate endemic species along the coast. The four CAPS names — Red Stumpnose, Knysna Seahorse, Leopard Catshark, Basket Star — plus what makes the Warm Temperate region a nursery for endemics in the first place.
- 🟢 desk Investigate endangered species along the coast. Including a real population graph from Algoa Bay that you will plot and interpret.
→ Open the Topic 2 practical sheet — printable, with a supplied data set for every task.
Why CAPS asks you to investigate both
Endemic and endangered are different ideas, and the second investigation only makes sense once you have done the first. Endemic is about where a species lives; endangered is about how close it is to disappearing. A species can be one, both, or neither.
When a species is both, conservation stops being a local matter. The African penguin breeds only in southern Africa, and its national population has fallen by roughly 93% since the 1950s — so if it goes here, it is gone everywhere. That collapse is not distant history either: St Croix Island in Algoa Bay held the largest African penguin colony in the world within the last decade, and lost almost all of it. The Knysna Seahorse is the same argument in miniature — three estuaries, and no fourth anywhere on Earth.
You will plot the penguin data yourself in the practical.
Key Vocabulary
Tap each card to flip it. The seven marked 📕 are the definitions your booklet flags as examinable; the rest are terms CAPS uses in the key concepts for this block.
An area based on geographical features and distinctive local biodiversity. South Africa has 5 inshore BGZs.
The upward movement of deep ocean water along a coastline, because surface water is being driven offshore.
A downward movement of water along a coastline, due to prevailing onshore winds pushing surface water up against the coast.
Relating to coral. Coralline algae incorporate CaCO₃ into their cells, making them hard and unpalatable. They are low-growing and moss-like, yet very diverse.
Pink or red seaweeds which form increasingly thick layers of calcium-rich material on hard surfaces like rock or coral reefs.
Backward swirls in a current, due to friction with a bank, a coastline, or a current running in a different direction.
A region along the South African coastline within which the permission of the South African government is required for the harvesting of resources.
The wide, shallow shelf off the south coast that deflects the Agulhas Current south-west — the cause of the retroflection and every eddy in it.
Found naturally nowhere else on Earth. The Knysna Seahorse is endemic to three South African estuaries — lose them and the species is gone.
At high risk of extinction in the wild. Different from endemic: endemic is about where a species lives, endangered is about how close it is to disappearing. A species can be either, both, or neither.
Single-celled algae living inside coral tissue. They photosynthesise there and feed the coral — which is how reefs are productive in nutrient-poor water.
The individual animal that builds coral. A reef is millions of polyps plus the calcium carbonate skeletons of all the generations underneath them.
A close long-term relationship between two different species. Coral and zooxanthellae is the example CAPS wants here, and both partners benefit.
Quick Check
Question 1
Well done!
Move on to Topic 3 when you're ready.