Marine Snow and the Sea of Milk


In the great nautical adventure tale, "20,000 Leagues Under the Sea," narrator Pierre Aronnax observes one evening that the Nautilus is "sailing in a sea of milk." The professor is correct in attributing the white color to marine life but wrong in thinking it comes from tiny animals.

Once again, the truth is stranger than fiction: not only is the "milk sea" phenomenon actually caused by microscopic plants, but it has huge implications for global climate.

These microscopic plants are called coccolithophores, and they are highly abundant throughout the global ocean. They build shells from chalk by removing dissolved inorganic carbon from seawater.

Bigelow Laboratory researchers recently developed a new algorithm that uses satellite images to estimate the amount of coccolithophores, and therefore carbon, in the surface layers of the ocean. Bigelow Laboratory scientists Catherine Mitchell and Barney Balch worked with coauthor Chuanmin Hu from the College of Marine Science at University of South Florida, and published their results in the Journal of Geophysical Research.

"Coccolithophores shells are highly reflective, so much of the sunlight that hits them is scattered," said Mitchell, lead author of the study. "Because they're white, this makes the water a milky blue color. We can detect that color change by measuring the light reflected back out of the ocean."

That process begins in outer space, where earth-observing satellites detect the light scattering off the coccolithophores' shells. Measuring this light with the new method can tell scientists how much calcium carbonate is suspended in surface waters, much of which sinks when the phytoplankton die and "fall like snowflakes to the seafloor," according to Mitchell.

"Carbon from coccolithophores plays a major role in the global carbon cycle, ultimately exporting carbon from the surface of the ocean to the sea floor," Mitchell said. "While they're falling, the shells are also dissolving and that carbon is eventually recirculated into the deep sea."

Seventy-five percent of carbon deposited on the seafloor comes from calcium carbonate. The ability to quickly and accurately quantify gigatons of carbon in the surface waters will teach scientists about major ocean chemistry patterns and how they may shift in response to a changing climate.

The next step is to expand this methodology to other ocean color satellites. Mitchell wants it to be useful to those working with data from the United States' VIIRS, MODIS-Aqua and MODIS-Terra satellites, the recently-launched JPSS-1 weather satellite, as well as the European family of "Sentinel" satellites.

"Just as satellites revolutionized our ability to predict weather from space, ocean color satellites allow us to visualize the vast ocean garden of microscopic plants that covers three-quarters of the Earth," said Balch. "These are the very phytoplankton that serve as the bottom of the marine food web on which all life in the ocean depends."

Eventually, the authors would like the new algorithm to translate to the new PACE (Plant, Aerosol, Cloud, and ocean Ecosystem) satellite that the United States plans to launch in 2022, which is designed to observe the world's oceans with unprecedented power and resolution, enabling more detailed study of ocean-atmosphere interactions as well as ocean ecology.

"The new NASA PACE mission will revolutionize ocean color remote sensing, allowing us to view the Earth's ocean in a whole new way," said Balch.

However, in the president's proposed 2018 budget, the PACE mission is de-funded. As Congress debates and finalizes the budget, the future of this NASA Earth observing satellite mission, and others like it, remains in flux.

"These missions take decades of effort by thousands of people to conceive, build and launch. We mustn't jeopardize this important ocean scientific mission, especially given the tremendous scientific advances that will result once it is on orbit," said Balch. "Losing the ability to look at ocean change over large spatial scales, and over decadal time scales, would be a great loss to Earth science."

The above image shows bands of different types of phytoplankton blooming along the Patagonian Shelf, north of the Falkland Islands. The coccolithophore blooms are the bright milky blue strips that run through the center of the image and around the islands.