Drivers of Carbon Export Efficiency in the Global Ocean.

Stephanie Henson,Fred Le Moigne,Sarah Giering

doi:10.1029/2018gb006158

Stephanie Henson, Fred Le Moigne + Show 1 more

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https://doi.org/10.1029/2018gb006158

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Abstract

The export of organic carbon from the surface ocean forms the basis of the biological carbon pump, an important planetary carbon flux. Typically, only a small fraction of primary productivity (PP) is exported (quantified as the export efficiency: export/PP). Here we assemble a global data synthesis to reveal that very high export efficiency occasionally occurs. These events drive an apparent inverse relationship between PP and export efficiency, which is opposite to that typically used in empirical or mechanistic models. At the global scale, we find that low PP, high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low. This implies that a decoupling between PP and upper ocean remineralization processes can result in a large fraction of PP being exported, likely as intact cells or phytoplankton‐based aggregates. As the proportion of PP being exported declines, macrozooplankton and bacterial abundances rise. High export efficiency, high PP regimes also occur infrequently, possibly associated with nonbiologically mediated export of particles. A similar analysis at a biome scale reveals that the factors affecting export efficiency may be different at regional and global scales. Our results imply that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting export efficiency. Further, the existence of low PP, high export efficiency regimes imply that biogeochemical models that parameterize export efficiency as increasing with PP may underestimate export flux during decoupled periods, such as at the start of the spring bloom.

Highlights

The ocean's biological carbon pump is a major factor in determining the air‐sea partitioning of CO2 (Kwon et al, 2009); without it, atmospheric CO2 concentration would be ~200 ppm higher than currently (Parekh et al, 2006)
We find that low primary productivity (PP), high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low
This latter is the functional form encapsulated in many global export efficiency algorithms (e.g., Laws et al, 2000; Laws et al, 2011) and implies that export flux increases at a faster rate than PP

Summary

Introduction

The ocean's biological carbon pump is a major factor in determining the air‐sea partitioning of CO2 (Kwon et al, 2009); without it, atmospheric CO2 concentration would be ~200 ppm higher than currently (Parekh et al, 2006). The export efficiency is affected by multiple factors such as phytoplankton community structure (which may affect formation of aggregates or sinking by ballasting; Buesseler, 1998; Boyd & Newton, 1999; Francois et al, 2002), zooplankton (which package phytoplankton into fast‐sinking fecal pellets or may transfer carbon directly to depth via diel vertical migration; Cavan et al, 2015; Dagg et al, 2014; Steinberg et al, 2002), and bacterial remineralization (Belcher et al, 2016; Buchan et al, 2014; Buesseler et al, 2007; Le Moigne et al, 2016) Each of these factors varies seasonally and regionally, and the export efficiency is unlikely to be constant either spatially or over the course of a year, as has been demonstrated in both observational (Buesseler, 1998; Henson et al, 2012) and modelling (Henson et al, 2015; Wassmann, 1998) studies. Zooplankton grazing and other upper ocean remineralization processes increase, establishing recycling pathways and leading

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