Photosynthesizing phytoplankton serve as primary producers in the upper ocean, where they take up atmospheric carbon dioxide and incorporate it into their biomass. In a process known as carbon export, some of this biomass is ultimately transported to the deep sea. New research by Henson et al. examines the factors that drive variations in export efficiency—the fraction of organic carbon produced by primary productivity that is eventually exported.
To conduct their investigation, the researchers combined and analyzed relationships among several global data sets collected since the mid-1980s. These included measurements or estimates of carbon export, phytoplankton community structure and primary productivity, zooplankton and bacterial abundance, and water column structure and nutrient availability.
Previous research has shown that typically, only a small fraction of organic carbon from primary production is exported to deeper waters. However, the new analysis revealed the existence of rare, high–export efficiency events. These events appear to occur mainly when macrozooplankton and bacterial populations are low. For instance, at the beginning of a springtime phytoplankton bloom, growth of zooplankton may lag behind growth of the phytoplankton they feed on. Instead of being eaten, a larger proportion of phytoplankton cells and the carbon they contain may sink, boosting export efficiency.
These rare occurrences of high carbon export efficiency result in a global inverse relationship between primary productivity and export efficiency. This relationship poses a potential problem for empirical models of carbon export that rely on satellite data and that typically assume a positive relationship between the two variables. In some cases, these models may be underestimating carbon export.
The new analysis highlights the importance of the entire upper ocean ecosystem, including phytoplankton, zooplankton, and bacteria, in determining export efficiency and suggests that different factors drive export efficiency in different regions of the world. The authors note that incorporating region-specific information into computational models could improve the models’ ability to accurately simulate carbon export. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB006158, 2019)
—Sarah Stanley, Freelance Writer