Phytoplankton bloom

In the northern hemisphere, the sea surface erupts every spring in a massive phytoplankton bloom. Like plants, these floating unicellular organisms use photosynthesis to convert light into energy, consuming carbon dioxide and releasing oxygen in the process. When the phytoplankton dies or is eaten by zooplankton, the carbon-rich fragments sink deeper into the ocean, where they are eaten by other organisms or buried in the sediments. This process is the key to the “biological carbon pump”, an important part of the global carbon cycle. While scientists have long known that the ocean plays an essential role in capturing carbon from the atmosphere, a new study by the Woods Hole Oceanographic Institution (WHOI) shows that the efficiency of the ocean’s “biological carbon pump” has been drastically underestimated, with implications for future climate assessments.

According to a paper published in the Proceedings of the National Academy of Sciences on 6 April, WHOI geochemist Ken Büsseler and his colleagues showed that the depth of the sunlit area where photosynthesis takes place varies considerably across the ocean. This is important because the ability of the phytoplankton to absorb carbon depends on the amount of sunlight that is able to penetrate the upper layer of the ocean. Taking into account the depth of the euphotic or sunlit zone, the authors found that about twice as much carbon sinks into the ocean each year as previously estimated. This paper draws on previous studies of the carbon pump, including the authors’ own studies. “When you look at the same data in a new way, you get a completely different view of the ocean’s role in processing carbon and thus its role in regulating the climate,” says Büsseler.

Biological Carbon Pump

Through photosynthesis, the phytoplankton floating on the sea surface absorbs carbon dioxide from the atmosphere. The depth of this sunlit layer influences the efficiency of the ocean’s “biological carbon pump” or its ability to absorb carbon.

“With the new metrics, we will be able to refine the models to say not only what the ocean looks like today, but also what it will look like in the future,” he adds. “Is the amount of carbon sinking in the ocean going up or down? This figure affects the climate of the world we live in”.In the paper, Büsseler and his co-authors ask their colleagues in oceanography to consider their data in relation to the actual boundary of the euphotic zone.

“If we want to call something a euphotic zone, we have to define it,” he says. “So we insist on a more formal definition so we can compare locations.

Instead of taking measurements at fixed depths, the authors used chlorophyll sensors – which indicate the presence of phytoplankton – to quickly determine the depth of the sunlit region. They also propose to use the signature of a naturally occurring thorium isotope to estimate the rate of descent of carbon particles.

The WHOI project Ocean Twilight Zone

Buesseler is one of the main researchers in the WHOI project Ocean Twilight Zone, which focuses on the little-understood but enormously important region of the mid-ocean. In a commentary published in Nature on 31 March, Buesseler and his colleagues call on the international marine research community to intensify its investigations of the twilight zone during the upcoming United Nations Ocean Decade (2021-2030). A better understanding of the twilight zone ecosystem and its role in regulating the climate, the authors say, will lead to a global policy to protect the area from exploitation.