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Dryad

Acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron

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Mar 08, 2023 version files 8.20 GB

Abstract

Dissolved iron (dFe) plays an important role in regulating marine biological productivity. In high-nutrient, low-chlorophyll regions (> 33% of the global ocean) iron is the primary growth-limiting nutrient, and elsewhere iron can regulate nitrogen fixation and growth by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas, or cellular Fe:C ratios. This ratio can vary by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in sparse field observations. The Community Earth System Model ocean component was modified to simulate dynamic, group-specific, phytoplankton iron quotas (Fe:C) that vary as a function of ambient iron concentration. The simulated Fe:C ratios match the spatial trends in the observed  Fe:C ratios. Acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply still has first-order impacts on global carbon and nitrogen fluxes, and on nutrient limitation spatial patterns. Our results suggest pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to future variations in atmospheric deposition.