Article: Omand, Melissa et al. (2015) “Eddy-driven subduction exports particulate organic carbon from the spring bloom” Science 10.1126
Phytoplankton represent one of the few ways the environment can pull carbon dioxide out of the atmosphere and store it somewhat permanently. These photosynthetic microorganisms, which mostly live near the ocean surface, take in CO2 from the atmosphere in order to grow. Being heavier than water, phytoplankton tend to sink, and if they sink deep enough, that carbon can stay down in the depths of the ocean for tens of thousands of years. This process, termed the ‘biological pump’, keeps the atmospheric CO2 concentration appreciably lower than it otherwise would be [Fig. 1].
Figure 1: Diatoms are a type of large phytoplankton that often dominate blooms in many parts of the ocean. Their size and hard silicate shells cause them to sink quicker than other plankton. Image courtesy UC Berkeley.
The biological pump is generally thought to be driven by this process of plankton sinking from the surface, where there’s enough light for them to grow, into deeper waters where they eventually serve as the energy source for deep ocean communities. Phytoplankton, however, are only slightly denser than the water they live in, and very small, so they sink very slowly compared to the total depth of the ocean. Many are too small and buoyant to sink at all. Moreover, this picture ignores the fact that the ocean moves!
Well, The Ocean Does Move – So What?
Recent work by scientists from Woods Hole Oceanographic Institution, the University of Washington, and the University of Maine, suggests that much of this carbon export to the deep ocean may actually be due to mixing by ocean eddies (oceanic whirlpools). This represents a paradigm shift in our understanding of how carbon is removed from the atmosphere; the turbulence of the ocean may play a major and active climate role by pushing carbon-rich water into the deep.
A crucial time to investigate the role between ocean turbulence and sinking, carbon-rich phytoplankton is during a “bloom.” Large phytoplankton blooms are generally triggered in springtime, similar to the flowering of terrestrial plants. In spring, more light is available, and weaker winds allow the surface layer of the ocean to shallow, bringing nutrients up into well-lit regions and stimulating plankton growth.
The investigators took measurements during one of these blooms in the North Atlantic using both gliders (underwater autonomous vehicles) and ‘Lagrangian floats’ which passively drift with water below the surface [Fig. 2]. The gliders and floats collected all sorts of physical, chemical, and biological data from which the authors could determine details about the carbon content of the water and how it had gotten there. They compared the data they collected with high-resolution computer simulations of ocean stirring along fronts, which included a model of phytoplankton.
Figure 2: a) a Seaglider, which flies through the water and collects data. Image courtesy the Alfred Wegner Institute. b) a Lagrangian float, which follows the motion of a parcel of seawater. Image courtesy U. Washington.
How Does This Work?
Just like in weather patterns, ocean fronts occur at the boundaries of different water masses. Where a heavier (either colder or saltier) body of water meets a lighter (warmer or fresher) one, this results in a horizontal gradient, or change, in density. These fronts are often the sites of photosynthetic activity as well as stronger and more turbulent ocean activity. Eddies that spin along such fronts will swap lighter water from one region with heavier water from the other, and in the process will cause filamentary tongues of the heavier or intermediate density water to slide underneath the lighter water [Fig. 3]. This sinking of heavier filaments also pushes deeper, nutrient-rich water upwards, stimulating further plankton growth. Thus, this horizontal mixing process pushes carbon-rich surface waters downwards, in exchange for nutrient-rich, carbon-depleted waters. This process ultimately results in injections of carbon into the deep ocean.
Figure 3: A computer simulation of an eddy along a front. The front starts very smooth – with warm/light water (red) on the right side and cold/heavy water (blue) on the left – then turbulence sets in and eddies form. As a result, some denser surface water (white and light blue) is sucked underneath the warmer surface water. If the surface water being pulled down by the eddy contains carbon-rich plankton communities, this will effectively remove carbon from the atmosphere. Image courtesy Dr. John Taylor, U. Cambridge.
This mixing process was found both in the observational data and in the model simulations. By combining multiple approaches, the researchers provide a compelling picture of the importance of ocean eddies for the biological pump. They found that in regions with strong currents & fronts, like the waters off the coast of East Asia, the Eastern US, and around Antarctica, subduction by eddies during spring blooms may be responsible for half of the sinking carbon. Beyond changing our ideas about what contributes to the ocean’s uptake of atmospheric carbon, these results may lead to new estimates of the deep ocean’s ability to uptake carbon. A better understanding where, how, & how fast carbon can go into the ocean’s remote depths can lead to better predictions of the global impacts of anthropogenic carbon emissions.