What causes inverse relationships between primary production and export efficiency in the Southern Ocean?

In the ocean, the proportion of the primary production (PP) exported as particulate organic carbon (POC) below the surface (export efficiency or e-ratio) is critical for the marine carbon cycle. Several studies have found inverse relationships between the e-ratio and PP in cold, high latitude regions. This suggests that cold high productivity regimes may not export as much POC as predicted by used empirical export flux algorithms. So far, causes of the occurrence of low export in high PP cold regions remains elusive. Here, we investigate the PP and POC export in the Scotia Sea, Southern Ocean during summer 2013. A cruise surveyed included high and low PP waters yielding a wide range of e-ratio. We find that the export of DOC has a limited influence on the e-ratio/PP correlations. However, we observed that at low PP and high e-ratio sites, zooplankton mediated export was larger and surface microbial abundance was lower than in high PP, low e-ratio stations suggesting that both are important processes driving the magnitude of the e-ratio in cold waters regions. This highlights the need for integrating more ecosystem related processes to mechanistic models of POC export in cold water regions such as the Southern Ocean.