Because the water in some currents is more biologically productive than in other currents, we can use productivity measurements to investigate the location and shape of the currents. Black areas are parts of the ocean where insufficient data were collected. Light changes seasonally, particularly at high latitudes. High nutrient levels are often observed along the edges of oceans, where upwelling currents bring nutrient-rich water to the surface. Low nutrient levels are often observed in the centers of the large oceanic gyres.
Winds can also generate vertical water motions by a process called upwelling.
Upwelling currents bring deeper, colder water to the surface. Whereas surface waters are usually depleted of nutrients such as phosphates and nitrates that are critical to plant growth, deeper waters have high concentrations of these nutrients. Upwelling replenishes the surface layers with the nutritional components necessary for biological productivity. Regions of upwelling are among the richest biological areas of the world. Satellites orbiting the earth can measure chlorophyll pigment and water temperature.