Drivers of Carbon Export Efficiency in the Global Ocean

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.

Phytoplankton Growth and Productivity in the Western North Atlantic: Observations of Regional Variability From the NAAMES Field Campaigns

“…In the comparably unstable conditions of the high latitudes, phytoplankton that possess an ability to grow rapidly in highly changeable conditions tend to prosper, leading to a more "classic" food chain (e.g., diatom-copepod-fish) often typical of upwelling and other productive regions (Schmoker et al, 2013). The larger phytoplankton cells that thrive in such conditions allow for more rapid sedimentation of particulate matter and increased export efficiency (Henson et al, 2019). Although only pigmentbased composition of phytoplankton was considered in the study presented here, Imaging FlowCytobot data show both seasonal and regional differences in community composition.…”

Review article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?

“…3e). Patterns in Si are more variable (Cauwet and Sidorov, 1996;Emmerton et al, 2008;Hessen et al, 2010). Dissolved Si concentration at low salinity is higher in rivers than in glacier discharge ( Fig.…”

“…3c and f). River water is, relatively, a much more important source of NO 3 (Cauwet and Sidorov, 1996;Emmerton et al, 2008;Hessen et al, 2010), and in river estuaries this nutrient can show a sharp decline with increasing salinity due to both mixing and biological uptake ( Fig. 3e).…”

“…Linear regression details are shown in Table S1 in the Supplement. (d) Si, (e) NO 3 and (f) PO 4 distributions in surface waters of three major Arctic river estuaries: the Lena, Mackenzie and Yenisey (Cauwet and Sidorov, 1996;Emmerton et al, 2008;Hessen et al, 2010). Note the different yand x-axis scales.…”

“…3d). Where evident, this release of dissolved Si typically occurs at low salinities (Cauwet and Sidorov, 1996;Emmerton et al, 2008;Hessen et al, 2010), with the behaviour of Si being more conservative at higher salinities and in estuaries where pronounced drawdown by diatoms is not evident (e.g. Brown et al, 2010).…”