Ocean gyres are vast, rotating systems of ocean currents formed by global wind patterns and the planet’s rotation. Among the five major subtropical gyres, the South Atlantic Gyre is a significant basin-wide feature. This immense vortex dictates oceanic conditions across a huge expanse of the southern ocean.
Defining the Gyre’s Boundaries and Currents
The South Atlantic Gyre dominates the ocean basin between the eastern coast of South America and the western coast of Africa. Its counter-clockwise rotation is governed by four distinct ocean currents. This rotational pattern is a consequence of the Coriolis effect in the Southern Hemisphere, which deflects moving water to the left, and the influence of continental landmasses. The system is driven by the interplay of the southeast trade winds and prevailing westerlies.
The gyre’s western boundary is the Brazil Current, a warm current that flows southward along the Brazilian coastline, transporting equatorial waters toward the pole. Upon reaching southern latitudes, it meets the eastward-flowing South Atlantic Current. This cold, broad current forms the southern edge of the gyre, running parallel to the Antarctic Circumpolar Current.
On the eastern side of the Atlantic, the system is completed by the Benguela Current, which brings cold water northward along the coast of Africa. This current warms and turns westward, becoming the South Equatorial Current, which flows back towards South America. This circuit defines the gyre and serves as a mechanism for redistributing heat, influencing weather patterns in both South America and Africa.
A Marine Biological Desert
The center of the South Atlantic Gyre is often described as a marine biological desert. The rotational forces that drive the currents cause surface water to converge and sink in the middle of the gyre. This process, known as downwelling, pushes surface waters downward, trapping nutrients deep below the sunlit upper layers. The result is an oligotrophic, or nutrient-poor, environment in the gyre’s core.
This scarcity of nutrients like nitrates and phosphates limits the growth of phytoplankton, the microscopic plants at the base of most marine food webs. Consequently, the central gyre cannot support large populations of fish, marine mammals, or seabirds. The clear, deep blue water in this region is a visible indicator of its low biological productivity, as the lack of phytoplankton means there is little to scatter sunlight.
Despite its reputation as a desert, the gyre is not devoid of life. It is home to a specialized community of microorganisms adapted to low-nutrient conditions, including vast populations of cyanobacteria like Prochlorococcus and Synechococcus. These tiny photosynthetic bacteria are efficient at absorbing scarce nutrients and are the primary producers, forming the foundation of a microbial-based food web.
Accumulation of Marine Debris
The circular motion of the gyre’s currents makes it an effective trap for floating materials. Debris from coastal areas, shipping lanes, and rivers is drawn into the rotating system and guided toward the calm center. This process leads to the concentration of marine debris, particularly persistent materials like plastics.
A significant accumulation of this debris, often called a garbage patch, has formed within the gyre. While a substantial feature, it is considered less dense and is not as extensively studied as the Great Pacific Garbage Patch. The exact size and density fluctuate based on wind and current patterns.
This accumulation of plastic poses a threat to the marine life that inhabits the gyre. As plastic items are exposed to sunlight and wave action, they break down into smaller fragments known as microplastics. These particles can be ingested by microscopic organisms at the base of the food web, introducing harmful chemicals. Animals may also mistake plastic pieces for food, leading to internal injury, toxicity, and starvation.