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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-156
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-156
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  25 May 2020

25 May 2020

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This preprint is currently under review for the journal BG.

Variable phytoplankton size distributions reduce the sensitivity of global export flux to climate change

Shirley W. Leung1, Thomas Weber1,2, Jacob A. Cram1,3, and Curtis Deutsch1 Shirley W. Leung et al.
  • 1School of Oceanography, University of Washington, Seattle, 98195, USA
  • 2Department of Earth and Environmental Science, University of Rochester, Rochester, 14627, USA
  • 3Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, 21613, USA

Abstract. Earth System Models predict a 10–20 % decrease in ocean carbon export production by the end of the 21st century due to global climate change. This decline is caused by increased stratification of the upper ocean, resulting in reduced shallow subsurface nutrient concentrations and a slower supply of nutrients to the surface euphotic zone. These predictions, however, do not account for associated changes in sinking particle size and remineralization depth. Here we combine satellite-derived export and particle size maps with a simple 3-D global biogeochemical model to investigate how shifts in sinking particle size may buffer predicted changes in surface nutrient supply and therefore export production. We show that higher export rates are correlated with larger phytoplankton and sinking particles, especially in tropical and subtropical regions. Incorporation of these empirical relationships into a global model shows that as circulation slows, a decrease in export and associated shift toward smaller phytoplankton yields particles that sink more slowly and are thus remineralized shallower; this in turn leads to greater recycling of nutrients in the upper water column and faster nutrient recirculation into the euphotic zone, boosting productivity and export to counteract the initial circulation-driven decreases. This negative feedback mechanism (termed the particle size-remineralization feedback) slows export decline over the next century by ~14 % globally and by ~20 % in the tropical and subtropical oceans, where export decreases are currently predicted to be greatest. Thus, incorporating dynamic particle size-dependent remineralization depths into Earth System Models will result in more robust predictions of changes in biological pump strength in a warming climate.

Shirley W. Leung et al.

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Shirley W. Leung et al.

Shirley W. Leung et al.

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Latest update: 27 Sep 2020
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Short summary
A global model is constrained with empirical relationships to quantify how shifts in sinking particle sizes modulate particulate organic carbon export production changes in a warming ocean. Including the effect of dynamic particle sizes on remineralization reduces the magnitude of predicted 100-year changes in export production by ~14 %. Projections of future export could thus be improved by considering dynamic phytoplankton and particle size-dependent remineralization depths.
A global model is constrained with empirical relationships to quantify how shifts in sinking...
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