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hy does a quarter of the world's fish catch come from just five upwelling regions
Upwelling is a key phenomenon in oceanography, the wind-driven movement of denser, cooler and often nutrient-rich deep waters to the ocean surface to replace warmer and often nutrient-poor surface waters. These nutrient-rich upwelling waters promote the growth and reproduction of primary producers such as phytoplankton. According to the report, about 25% of the global marine fish catch comes from five upwelling regions, which account for only 5% of the world's ocean area.
The biomass and water temperature of upwelling areas allow these areas to be identified, as they have cooler sea surface temperatures and higher concentrations of chlorophyll a.
The mechanism of upwelling is driven by three main factors: wind, Coriolis effect and Ekman transport. Different types of upwelling may operate differently, but their general effects are the same. When the wind blows along the ocean surface, the surface water moves at an angle of approximately 90 degrees to the direction of the wind due to the Coriolis effect and Ekman transport.
Types of upwelling
Major upwelling in the ocean is usually associated with diverging currents that bring deeper, cooler, nutrient-rich waters to the surface. Common types of upwelling include coastal upwelling, large-scale wind-driven upwelling in the ocean interior, eddy-associated upwelling, topography-related upwelling, and widespread diffuse upwelling in the ocean interior.
Coastal upwelling
Coastal upwelling is the best-known type of upwelling and is closely associated with human activities because it supports some of the most productive fisheries in the world. This phenomenon occurs when the wind direction is parallel to the coast. When Ekman transport occurs away from the coast, surface water is carried away and with it comes deeper, colder, and denser water.
As coastal upwelling brings rich deep ocean waters to the surface, these nutrients are used by phytoplankton, which further drives the food chain and accounts for up to 50% of global ocean productivity.
Equatorial upwelling
In equatorial regions, upwelling is associated with the equatorial Intertropical Convergence Zone (ITCZ), which tends to move north or south. Although there is no Coriolis effect at the equator, upwelling still occurs to the north and south of the equator, making high concentrations of phytoplankton visible in the equatorial regions of the Pacific Ocean.
Antarctic upwelling
In the Antarctic Ocean, strong westerly winds drive the ocean water northward, which is effectively a coastal upwelling. This process draws in cold water from the deep ocean and has an impact on the climate and ecosystems in the Antarctic region.
Threats and human impacts
However, upwelling systems face many threats. One of them is overfishing. Since upwelling areas are productive and species-rich, this attracts a lot of commercial fishing. If one species is overfished, the rest of the organisms will be affected by a chain reaction and the entire ecosystem may be damaged.
For the upwelling system, every species in the balanced ecosystem is crucial. If one link is weakened, the impact will spread to the entire system.
Additionally, the El Niño phenomenon (ENSO) can affect productivity in these regions. In normal years, strong trade winds continue to drive upwelling, but during El Niño events, trade winds weaken, resulting in a reduction in global upwelling, which directly affects the productivity and stability of ecosystems. Does this mean that we should pay more attention to and protect these fragile ecosystems?
