What is upwelling?
Upwelling occurs when winds push surface water away from the shore and deeper water rises to fill the gap.
During upwelling, wind-displaced surface waters are replaced by cold, nutrient-rich water that wells up from below. Figure modified by D. Reed from image by J. Wallace and S. Vogel, El Niño and Climate Prediction. Image courtesy of Sanctuary Quest 2002, NOAA/OER. Download image (jpg, 43 KB).
When the wind blows parallel to a coastline, surface waters are pushed offshore and water is drawn from below to replace the water that has been pushed away. The upward movement of this deep, colder water is called upwelling.
The deeper water that rises to the surface during upwelling is rich in nutrients. These nutrients “fertilize” surface waters, encouraging the growth of plant life, including phytoplankton. These phytoplankton serve as the ultimate energy base in the ocean for large animal populations higher in the food chain, providing food for fish, marine mammals, seabirds, and other critters.
Coastal upwelling ecosystems, such as along the west coast of the United States, are some of the most productive ecosystems in the world and support many of the world's most important fisheries. Although coastal upwelling regions account for only one percent of the ocean surface, they contribute roughly 50 percent of the world's fisheries landings.
Upwelling can also play an important role in the movement of marine animals. Most marine fish and invertebrates produce microscopic larvae which, depending on the species, may drift in the water for weeks or months as they develop. For adult marine creatures that live in shallow waters nearshore, upwelling that moves surface water offshore can potentially move drifting larvae long distances away from their natural habitat, thus reducing chances for survival.
In some ways, upwelling can be a mixed blessing to coastal ecosystems. It can infuse coastal waters with critical nutrients that fuel dramatic productivity, but it can also rob coastal ecosystems of offspring required to replenish coastal populations.