Abstract
AbstractThe ocean contributes to regulating atmospheric CO2 levels, partly via variability in the fraction of primary production (PP) which is exported out of the surface layer (i.e., the e ratio). Southern Ocean studies have found that contrary to global‐scale analyses, an inverse relationship exists between e ratio and PP. This relationship remains unexplained, with potential hypotheses being (i) large export of dissolved organic carbon (DOC) in high PP areas, (ii) strong surface microbial recycling in high PP regions, and/or (iii) grazing‐mediated export that varies inversely with PP. We find that the export of DOC has a limited influence in setting the negative e ratio/PP relationship. However, we observed that at sites with low PP and high e ratios, zooplankton‐mediated export is large and surface microbial abundance low suggesting that both are important drivers of the magnitude of the e ratio in the Southern Ocean.
Highlights
Over the last decades, algorithms have been developed to predict surface ocean carbon export, often based on satellite-derived estimates of primary production (PP) and sea surface temperature (SST)
We find that the export of dissolved organic carbon (DOC) has a limited influence in setting the negative e ratio/PP relationship
An inverse relationship between surface ocean PP and export efficiency (e ratio, defined as the ratio between PP and particulate organic carbon (POC) flux exported from the upper ocean [Buesseler, 1998]) has been observed in the Southern Ocean (SO) [Cavan et al, 2015; Laurenceau-Cornec et al, 2015; Maiti et al, 2013; Morris et al, 2007; Savoye et al, 2008]
Summary
Algorithms have been developed to predict surface ocean carbon export, often based on satellite-derived estimates of primary production (PP) and sea surface temperature (SST). Zooplankton grazing, surface ocean bacterial recycling, and enhanced downward export flux of DOC are plausible drivers [Cavan et al, 2015; Hansell et al, 2009; Laurenceau-Cornec et al, 2015; Maiti et al, 2013]. We further explore the processes that may explain the inverse relationship between PP and e ratio by comparing our estimates of e ratio with the zooplankton fecal pellet flux [Cavan et al, 2015], the downward export of DOC, and surfaceintegrated bacterial and heterotrophic flagellate abundance, used as simple indicators of surface bacterial recycling rates
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