South Africa's Ocean Basins: Atlantic and Indian

• South Africa is bordered by two ocean basins to the south, east, and west.

Ocean Basins

• The Atlantic Ocean Basin is off the west coast of South Africa, down to Cape Agulhas.
• The Indian Ocean Basin is along the south-eastern and eastern coastline, east from Cape Agulhas.
• The distinction between the two is based on geography.
• Major currents and associated weather reinforce distinction.
• These basins contain three coastal regions based on water temperatures.
• These can be further divided into five inshore biogeographic zones (BGZs).
• BGZs are formed by geographical features and local biodiversity.

Atlantic Ocean Basin

• Stretches from Cape Agulhas to Namibia on the west coast.
• The cold Benguela Current moves northward from the Southern Ocean towards the Equator.
• Contains the Cool Temperate coastal region and two biogeographic zones.

Indian Ocean Basin

• Stretches from Mozambique to Cape Agulhas on the east coast.
• The warm Agulhas Current flows southward, and is among the strongest globally
• It moves at speeds of up to 13 km/h.
• The Agulhas Current flows from Mozambique along the KZN and Eastern Cape coastlines.
• It is deflected south-west by the Agulhas Bank, then turns south in swirls or eddies.
• The largest eddy joins the Indian Ocean circulation.
• Smaller eddies recirculate into the Agulhas Current.
• Some eddies break off and move into the Atlantic Ocean basin as pockets of warm water.
• Contains the Warm Temperate region along the southern coast, the Subtropical region along the east coast, and three biogeographic zones.

Cool Temperate Region

• Located on the west coast, with average temperatures from 9 °C to 15 °C.
• Divided into two biogeographic zones based on biodiversity.
• The Namaqua BGZ stretches from Namibia to Cape Columbine.
• The South Western Cape BGZ stretches from Cape Columbine to Cape Point.
• Experiences frequent upwelling of cold, nutrient-rich water due to strong offshore winds.
• Prevailing winds blow surface water away from the coast, replaced by cold, nutrient-rich water from the Benguela Current.
• Nutrient-rich water promotes phytoplankton blooms and the growth of seaweeds and kelp forests.
• Drives food webs for large consumer populations.

Warm Temperate Region

• Located on the South Coast, with average temperatures from 16 °C to 20 °C.
• Contains the Agulhas BGZ, extending from Cape Point to just north of East London,
• Experiences influxes of colder water from the west along the shallow coastal shelf.
• A degree of species overlap with bordering regions.
• Contains endemic species such as Red Stumpnose, Knysna Seahorse, Leopard Catshark, and Basket Star.

Subtropical Region

• Stretches from East London to Mozambique, with average temperatures from 18 °C to 27 °C.
• Subdivided into two biogeographic zones.
• The Natal BGZ stretches from East London up to St Lucia.
• The Delagoa BGZ is further to the north.
• Clear, nutrient-poor water due to the Agulhas Current.
• Onshore winds cause downwelling, displacing colder, nutrient-rich water, and maintaining nutrient-poor conditions.
• Seaweed species diversity is high, with low-growing, moss-like, coralline species.
• Coralline seaweeds incorporate CaCO3, making them unpalatable.
• Rock surfaces are covered with these seaweeds, especially in the southern parts of the region where conditions discourage coral formation.
• In the northern parts of the Natal and Delagoa BGZs, coral colonies become more common.
• Lower seawater nutrient levels result in negligible primary production from seaweeds.
• Corals form a thin layer, blanketing reefs in shallower water.
• Further north, corals form actual reefs, with new coral polyps and encrusting coralline algae.
• Non-coralline seaweeds and eel grass are limited to sheltered intertidal zones.
• Primary production is almost entirely from the symbiotic interactions between coral polyps and zooxanthellae.
• Large surface area of coral polyps and zooxanthellae populations enables a productive ecosystem.
• Zooxanthellae interact with hard corals, soft corals, jellies, clams, and other tropical organisms.
• Animal species diversity is high, but populations are smaller.
• Fish on coral reefs are smaller, colourful, and territorial.
• Most consumers are carnivorous with adaptations to compete and protect themselves.

Other BGZs

• Identified in deeper water, beyond the continental fringe, towards the end of the South African Economic Exclusion Zone.

Key Terms

• Biogeographic Zones (BGZs): Areas with geographical features and local biodiversity.
• Coralline: Relating to coral.
• Downwelling: Downward movement of water along a coastline due to onshore winds.
• Eddies: Backward swirls in a current.
• Encrusting Coralline Algae: Pink or red seaweeds forming thick layers of calcium-rich material.
• South African Economic Exclusion Zone: A region requiring South African government permission for resource harvesting.
• Upwelling: Upward movement of deep ocean water along a coastline, due to offshore driven surface water.