Where Two Oceans Shape Life: The Temperate Southern Africa Marine Realm
- Shanti
- Jul 24
- 13 min read
How cold upwelling, warm currents and remote islands support rich marine life—and why this productive ocean region needs careful stewardship.
Shanti Plants
The Temperate Southern Africa marine realm covers a remarkable group of coastal and continental-shelf environments around southern Africa, together with the distant Amsterdam–Saint-Paul island region in the southern Indian Ocean.
It includes cold, nutrient-rich waters influenced by the Benguela Current along Namibia and western South Africa, as well as warmer waters shaped by the Agulhas Current along South Africa’s southern and eastern coasts. Far to the east, Amsterdam and Saint-Paul islands support another distinctive temperate marine community.
The realm is part of the Marine Ecoregions of the World system, which divides coastal and shelf waters into broad biological regions. It is important to understand that a marine realm is a scientific mapping unit, not one uniform ecosystem. Conditions, species, threats and laws differ greatly between Namibia, South Africa and the remote French islands of Amsterdam and Saint-Paul (Spalding et al., 2007).
What connects these places is the strong influence of ocean currents. The cold Benguela system brings nutrient-rich deep water towards the surface, while the warm Agulhas Current carries tropical and subtropical water southwards. Together, these forces help create kelp forests, rocky reefs, sandy seabeds, open-water food webs and important breeding and feeding areas for marine wildlife.
This productivity supports fish, seabirds, marine mammals, sharks, coastal livelihoods. However, it also faces growing pressure from commercial fishing, pollution, coastal development, habitat disturbance and other uses of the sea. These pressures do not affect every part of the realm in the same way, making long-term cultivation and local management protection.
In This Article |
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What Is Happening?
An ocean current acts a little like a moving river within the sea. It carries heat, salt, nutrients, oxygen and tiny drifting organisms from one area to another.
On the west coast, winds help push surface water away from the shore. Cold water then rises from deeper layers to replace it. This process is called upwelling. The deeper water carries nutrients that feed microscopic algae called phytoplankton.
Phytoplankton form the base of much of the marine food web. They feed small animals and fish, which in turn support larger fish, penguins, seals, whales and sharks. This is one reason the Benguela system is among the world’s most productive marine environments.
Along the southern and eastern coast, the Agulhas Current carries warmer water southwards. The current interacts with the continental shelf, winds, bays and underwater landscape. This creates warm reefs, local upwelling areas and transition zones where cold-water and warm-water species can occur relatively close to one another.
The result is not a simple line where the Atlantic Ocean ends and the Indian Ocean begins. Instead, it is a shifting mixture of currents, eddies, temperatures and habitats.
Why it's Happening
The Temperate Southern Africa marine realm is not undergoing one simple change. Different sections are warming, cooling or experiencing altered upwelling, currents, oxygen levels and species distributions at different times.
Long-term observations and research have found changes in the abundance and location of some marine organisms within the Benguela and South African systems. Fishing has also changed food webs and population sizes, while pollution, mining, coastal modification and climate change add further pressure. Researchers caution that natural seasonal and decade-to-decade variation can make it difficult to attribute every observed change to one cause (Jarre et al., 2015; Moloney et al., 2013).
The Benguela system
The Benguela Current’s upwelling supports very high biological productivity. However, the strength, timing and location of upwelling can vary. Changes in wind, temperature, oxygen and nutrient movement can alter phytoplankton growth and the distribution of fish.
This matters because many predators depend on small schooling fish such as sardines and anchovies. When these fish move, decline or become less predictable, the effects can spread through the food web.
The Benguela Current Convention identifies overexploitation, pollution, coastal degradation and climate change as growing environmental and social pressures across the wider Benguela Current Large Marine Ecosystem.
The Agulhas system
The Agulhas Current is warm, fast and highly dynamic. It can influence coastal temperature, local upwelling and the movement of eggs and larvae.
Modelling for the Agulhas Bank indicates that warming and changes in current strength or position could affect important nursery and feeding areas. For example, stronger or differently positioned currents may carry some squid larvae away from suitable shelf habitats. These projections are useful for planning, but they remain modelled outcomes rather than certain predictions for every year or location (Asdar et al., 2022).
Climate effects can also appear contradictory. A warming climate does not mean every local event becomes warmer. Research published in 2024 linked an intense cold upwelling event in the Agulhas system to deaths across 81 marine species. The study found evidence that some cold upwelling events had become more frequent or intense, illustrating how changing currents can expose warm-water animals to sudden cold conditions at the edges of their ranges (Lubitz et al., 2024).
Remote islands
Amsterdam and Saint-Paul islands remain comparatively isolated, but isolation does not remove all threats. Their ecosystems may be affected by global climate change, fishing interactions, introduced species and disease.
Their remoteness also makes research and management difficult. Some habitats and deep-water communities remain less well documented than accessible coastal areas. The surrounding French Austral Lands and Seas reserve protects important seabird, marine mammal and kelp habitats, while scientific monitoring helps detect environmental change.
Why it matters
Plants and algae
Kelp and other seaweeds are algae rather than flowering plants, but they perform many plant-like ecological roles. They use sunlight, create habitat and feed marine food webs.
Kelp forests provide shelter and nursery habitat for fish, shellfish and other animals. Phytoplankton support almost every higher level of the offshore food web. Seagrasses and saltmarsh plants, where present, trap sediment, provide nursery areas and help store carbon.
Changes in temperature, water quality, grazing pressure, storms or nutrient supply can change where these foundation species grow.
Soil, seabeds and coastal sediments
The sea does not contain soil in exactly the same way as a garden, but seabed sediment performs many similar ecological functions.
Sand, mud, gravel and rocky seabeds provide habitat for worms, shellfish, corals, sponges, fish and microorganisms. These organisms recycle nutrients, filter water and provide food for larger animals.
Dredging, mining, anchors, bottom-contact fishing gear and coastal construction can disturb these habitats. Polluted runoff can also carry nutrients, chemicals and sediment from land into estuaries and coastal waters. SANBI identifies mining, development, pollution and unsustainable harvesting as important pressures at South Africa’s land–sea boundary.
Water
Water temperature, oxygen, salinity, acidity and nutrient levels influence which species can survive.
Upwelling normally brings valuable nutrients towards the surface, but deeper water can also contain less oxygen. Under certain conditions, low-oxygen water may place stress on fish and seabed animals.
Pollution from plastics, sewage, industrial discharge, oil, fertilisers and other land-based sources can further reduce water and habitat quality. The exact importance of each pollutant differs between cities, ports, estuaries, fishing grounds and remote islands.
Wildlife
Marine wildlife depends on connected food webs.
Small fish feed larger fish, penguins, seals and seabirds. Kelp forests shelter juvenile animals. Healthy seabeds provide feeding areas for fish and invertebrates. When one part of the system changes, predators may need to travel further or switch prey.
South Africa’s 2025 National Biodiversity Assessment reported that 36% of assessed marine taxa were threatened, while also noting important gaps in knowledge. This figure applies to assessed South African taxa, not automatically to every species throughout the entire Temperate Southern Africa realm.
Food
The Benguela and Agulhas systems support fisheries for species including hake, sardine, anchovy, horse mackerel, squid, rock lobster and abalone.
These fisheries provide food, employment and export income. However, a productive sea does not provide an unlimited supply. Fish populations depend on successful breeding, suitable nursery grounds, food availability and responsible harvest levels.
Good management must account for both fishing pressure and environmental change. A catch level that was sustainable under previous conditions may need to be reviewed when species distributions or recruitment change.
People
Coastal communities depend on marine ecosystems in many ways:
commercial and small-scale fishing;
tourism and recreation;
cultural practices and local knowledge;
seafood processing and transport;
education and research;
protection from waves, erosion and coastal hazards.
Environmental decline can therefore become a food, employment, health and equity issue—not only a wildlife issue.
Community resilience
A resilient community is better able to prepare for change, respond to disruption and continue meeting essential needs.
Healthy ecosystems contribute to resilience by providing diverse livelihoods, food sources, tourism opportunities and natural coastal protection. Communities are also stronger when they have access to reliable information, participate in decisions and are not dependent on a single vulnerable fishery.
Main Causes
Causes supported by direct evidence
Climate change and ocean variability
Rising greenhouse-gas concentrations affect ocean heat, currents, acidity, oxygen and sea level. Around southern Africa, these global effects interact with strong natural variability in the Benguela and Agulhas systems.
Research shows that the result will not be uniform warming everywhere. Changes may include altered upwelling, shifting current boundaries, marine heat, extreme cold events and movement of species into or away from particular areas.
Unsustainable harvesting
Fishing can reduce target species and may also affect predators, prey and seabed habitats. Accidental capture, known as bycatch, can harm seabirds, turtles, sharks and other non-target animals.
The scale of the effect depends on the fishery, gear, location, compliance and condition of the stock. Fisheries should therefore be assessed individually rather than described as equally harmful.
Pollution
Pollution can enter the ocean through rivers, stormwater drains, sewage systems, ships, industries and coastal activities.
Plastic waste may entangle wildlife or break into smaller pieces. Excess nutrients can contribute to poor water quality, while oil and chemical pollution may cause direct harm to organisms and habitats.
Habitat degradation
Coastal development, mining, dredging, damaging fishing gear and poorly planned infrastructure can remove or disturb habitat.
Loss of estuaries, kelp forests, reefs, dunes, wetlands and nursery grounds may reduce the capacity of species to recover from other pressures.
Contributing factors that vary by location
The following factors can increase risk, but their importance is not the same throughout the realm:
invasive plants, animals or microorganisms;
wildlife disease;
harmful algal blooms;
unusually low oxygen;
marine mining and energy development;
shipping, noise and collision risks;
changes in predators or prey;
limited monitoring and enforcement;
poverty or unequal access to alternative livelihoods;
gaps in cross-border management.
These factors should be assessed with local data. Their presence in one province or island should not be assumed elsewhere.
What the Evidence Shows
Ocean currents create the broad biological pattern | The Benguela and Agulhas currents help explain why western and eastern southern Africa support different marine communities. Cold-water kelp systems dominate much of the west, while warmer-water communities become more prominent towards the east. Transition zones contain mixtures of species and can be especially diverse (Griffiths et al., 2010; Spalding et al., 2007). |
Upwelling supports productivity | The rise of cold, nutrient-rich water supports phytoplankton growth. This primary production feeds zooplankton, fish and larger predators. High productivity, however, does not guarantee that every species will remain abundant. The timing and location of nutrients, prey, breeding and migration must also match. |
Species and food webs can change | Long-term studies have documented changes in marine species’ abundance and distribution. In some cases, fish have shifted southwards or eastwards. Researchers have also reported possible changes in food-web relationships. However, causes can be difficult to separate because fishing, climate variability, pollution and other pressures may occur together (Jarre et al., 2015; Moloney et al., 2013). |
Future effects will differ across the realm | Climate models project changes in temperature, currents and ecological conditions, but local results depend on winds, depth, seafloor shape and the behaviour of each species. This means management should not rely on one prediction for the entire realm. Monitoring and plans must be regional and regularly updated. |
Management can reduce avoidable pressure | Climate change cannot be solved by marine protected areas alone, but protected habitats, science-based harvest limits, bycatch reduction, pollution controls and restoration can improve the ability of ecosystems to cope with change. Regional cooperation is particularly important because currents, fish, seabirds and pollution cross national boundaries. The Benguela Current Convention provides a formal structure for cooperation between Angola, Namibia and South Africa. |
What You Can Do at Home
Personal actions will not replace government and industry responsibility. They can still reduce local pressure, improve awareness and support broader change.
Home gardens
Use fertilisers only when needed and follow the label rate. Excess nutrients can travel through drains and waterways.
Keep soil covered with mulch or vegetation to reduce erosion.
Choose low-toxicity pest controls and prevent chemicals entering stormwater drains.
Plant suitable local species that support biodiversity and stabilise soil.
Pick up loose plastic, line, cable ties and packaging before they can reach waterways.
Ask your seafood supplier about origin, fishing method and current sustainability guidance.
Balconies
Use drip trays so potting mix and fertiliser do not wash into drains.
Secure lightweight pots, labels and plastic items during storms.
Avoid releasing aquarium water, plants, fish, snails or other organisms into drains or waterways.
Reduce single-use plastic where practical.
Join online citizen-science projects or support a recognised marine conservation group.
Schools
Schools can:
audit litter and stormwater around the campus;
study local catchments and trace where drains lead;
organise safe clean-ups with adult supervision;
compare marine food webs from the Benguela and Agulhas systems;
investigate sustainable seafood and fisheries management;
contribute observations to approved citizen-science programs;
invite a local scientist, Traditional Custodian, ranger or community group to speak.
Community gardens
Community gardens can reduce land-to-sea pollution by:
composting correctly;
preventing loose soil and fertiliser runoff;
installing rain gardens or vegetated drainage areas where suitable;
avoiding overwatering;
safely storing chemicals;
teaching visitors that rivers, estuaries and oceans are connected;
holding a litter-free event or catchment education day.
Local groups
Local groups can:
adopt a beach, creek or stormwater site for regular monitoring;
record litter types rather than only collecting rubbish;
support marine protected areas and responsible fisheries;
report injured or entangled wildlife to the appropriate authority;
advocate for effective sewage, stormwater and waste systems;
participate in public consultation on coastal development;
support community-led fisheries and habitat-restoration projects;
share locally verified information without exposing sensitive wildlife locations.
What Communities Can Do
Build shared understanding
Community education should explain both the value of marine resources and the limits to their use.
Useful activities include public talks, school materials, fisher knowledge exchanges, coastal signs and simple guides to local marine habitats. Information should be available in relevant local languages wherever possible.
Support monitoring
Long-term monitoring helps distinguish a lasting change from an unusual season.
Communities can assist through:
beach and wildlife surveys;
water-quality observations;
catch and effort records;
reporting tagged or stranded animals;
kelp, seagrass or reef monitoring;
photographic records taken from repeat locations;
documenting local ecological knowledge with permission.
Community observations should support—not replace—trained scientific assessment.
Restore connected habitats
Restoration may include dunes, wetlands, estuaries, saltmarshes, seagrass or kelp, depending on the site.
A restoration project should begin with:
confirming the original cause of damage;
checking land ownership and legal permissions;
selecting locally appropriate species and methods;
preventing new damage;
setting measurable outcomes;
monitoring whether the habitat and wildlife recover.
Planting without solving pollution, erosion or physical disturbance may waste resources.
Strengthen fisheries resilience
Fisher communities, scientists and managers can work together to:
improve catch information;
protect nursery and breeding habitats;
reduce bycatch;
adjust fishing times or areas when evidence supports it;
diversify livelihoods where appropriate;
improve safety and cold-chain infrastructure;
include small-scale fishing communities in decision-making.
Coordinate across borders
The Benguela system crosses national boundaries. Fish stocks, currents, pollution and wildlife do not stop at political borders.
Shared monitoring, compatible rules and transparent scientific information can help prevent one country’s actions from undermining another’s conservation work.
Conservation and Safety Notice |
Marine wildlife, fishing, collecting and access laws vary between South Africa, Namibia, France and individual protected areas. Before fishing, gathering seaweed, collecting shells, entering restricted islands, launching a boat, flying a drone or undertaking research:
Amsterdam and Saint-Paul form part of a strictly managed nature reserve and World Heritage property. Human activity is regulated, and some areas are prohibited or require formal permission. Introduced organisms can cause severe damage on isolated islands, making biosecurity particularly important. For stranded, entangled, injured or unusually behaving wildlife, keep people and pets away and contact the relevant local wildlife or marine authority. Do not attempt a rescue unless directed by trained responders. |
Key Takeaways |
The Temperate Southern Africa marine realm is shaped by some of the world’s most powerful and biologically important ocean processes. Cold Benguela upwelling supports productive food webs, while the warm Agulhas Current creates contrasting habitats and complex transition zones. This richness supports wildlife, food, employment, research and community wellbeing, but it is not unlimited or equally secure everywhere. Climate change, fishing, pollution and habitat disturbance can combine in different ways across the realm. Protecting it requires reliable monitoring, responsible use, strong local action and cooperation across communities and national borders. |
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References & Further Reading
Asdar, S., Jacobs, Z. L., Popova, E., Noyon, M., Sauer, W. H. H., & Roberts, M. J. (2022). Projected climate change impacts on the ecosystems of the Agulhas Bank, South Africa. Deep Sea Research Part II: Topical Studies in Oceanography, 200, 105092. doi:10.1016/j.dsr2.2022.105092.
Benguela Current Convention. (2026). BCLME IV: Mainstreaming a climate-resilient blue economy in the Benguela Current Large Marine Ecosystem.
Griffiths, C. L., Robinson, T. B., Lange, L., & Mead, A. (2010). Marine biodiversity in South Africa: An evaluation of current states of knowledge. PLOS ONE, 5(8), e12008. doi:10.1371/journal.pone.0012008.
Jarre, A., Hutchings, L., Kirkman, S. P., Kreiner, A., Tchipalanga, P. C. M., Kainge, P. I., Uanivi, U., van der Plas, A. K., Blamey, L. K., Coetzee, J. C., Lamont, T., Samaai, T., Verheye, H. M., Yemane, D. G., Axelsen, B. E., Ostrowski, M., Stenevik, E. K., & Loeng, H. (2015). Synthesis: Climate effects on biodiversity, abundance and distribution of marine organisms in the Benguela. Fisheries Oceanography, 24(S1), 122–149. doi:10.1111/fog.12086.
Lubitz, N., Daly, R., Smoothey, A. F., Vianello, P., Roberts, M. J., Schoeman, D. S., Sheaves, M., Cowley, P. D., Dagorn, L., Forget, F. G., Soria, M., Peddemors, V. M., Filmalter, J. D., Butcher, P. A., Brett, G., & Barnett, A. (2024). Climate change-driven cooling can kill marine megafauna at their distributional limits. Nature Climate Change, 14, 526–535. doi:10.1038/s41558-024-01966-8.
Moloney, C. L., Fennessy, S. T., Gibbons, M. J., Roychoudhury, A., Shillington, F. A., von der Heyden, B. P., & Watermeyer, K. (2013). Reviewing evidence of marine ecosystem change off South Africa. African Journal of Marine Science, 35(3), 427–448. doi:10.2989/1814232X.2013.836135.
South African National Biodiversity Institute. (2026). South Africa has exceptional marine biodiversity—what is its status?
Spalding, M. D., Fox, H. E., Allen, G. R., Davidson, N., Ferdaña, Z. A., Finlayson, M., Halpern, B. S., Jorge, M. A., Lombana, A., Lourie, S. A., Martin, K. D., McManus, E., Molnar, J., Recchia, C. A., & Robertson, J. (2007). Marine ecoregions of the world: A bioregionalization of coastal and shelf areas. BioScience, 57(7), 573–583. doi:10.1641/B570707.
UNESCO World Heritage Centre. (2019). French Austral Lands and Seas.
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