“It doesn’t take a lot of technology to see that the ocean is blue. And when it comes to the blueness of the ocean, it doesn’t get much more blue than where I am.”

That’s how Joaquín Chaves-Cedeño, an ocean scientist from NASA’s Goddard Space Flight Center, described the waters of the South Pacific in September 2017. During an expedition onboard the R/V Nathaniel B. Palmer, researchers sailed eastward along a route just south of the South Pacific Gyre—the largest of the five major oceanic gyres, which form part the global system of ocean circulation. The gyre, on average, holds the clearest, bluest waters of any ocean basin.

Sea water is often colored by phytoplankton—tiny plant-like organisms that form the center of the marine food web and play a key role in the global carbon cycle. Individual phytoplankton are tiny, but when they bloom by the billions, the high concentrations of chlorophyll and other light-catching pigments change the way the seas reflect light.

The difference in abundance is apparent in this map, assembled from data acquired in September 2017 by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite. Greens and yellows represent areas with the highest chlorophyll concentrations; blues have the lowest.

Notice how phytoplankton tend to thrive near coastlines and along the equator. In general, these areas have ample nutrients and sunlight for phytoplankton growth. When coastal or trade winds move surface waters, nutrient-rich waters rise from the depths to the surface (upwelling). In contrast, the bluest waters occur where the circulation has stunted the supply of nutrients and prevented life from thriving.

“With water flowing towards the center [of the South Pacific gyre] from all directions, literally piling up and bulging the surface of the ocean—albeit, by just a few centimeters across thousands of miles—gravity pushes down on this pile of water,” Chaves-Cedeño wrote. “This relentless downward push puts a lock on life...Layers of denser water trap the nitrogen and phosphorus-rich water at depth, keeping it too far down, where not enough light can reach it to spark the engine of photosynthesis that allows plants to grow.”

Since the late 1970s, NASA has been measuring ocean color from satellites as a way to quantify the abundance of phytoplankton. But satellites do not measure it directly. Instead, they detect the wavelength and intensity of light reflecting off of the ocean surface to their sensors. Researchers then translate those signals into meaningful measurements of plant life.

“To properly calibrate a satellite sensor and validate its data products, we must obtain field measurements of the highest possible quality,” Chaves-Cedeño wrote. “The waters of the South Pacific Gyre are an ideal location for gathering validation quality data, perhaps one the most desirable to do so, because there are few complicating factors and sources of uncertainty that blur the connection we want to establish between the color of the water and phytoplankton life abundance.”

NASA Earth Observatory image by Joshua Stevens, using VIIRS data from the Suomi National Polar-orbiting Partnership. Story by Kathryn Hansen, based on text by Joaquín Chaves-Cedeño.