Contrasting the primary production and carbon export in the different environments of the Southern Ocean

Abstract

Since the main driving forces of CO2 uptake is photosynthesis by phytoplankton and the vertical export of organic matter to the deep ocean determines the residence time of the carbon fixed by phytoplankton, the Southern Ocean ecosystem dynamics and biogeochemistry must be accurately understood today. Although iron concentration and solar irradiance are the most important factors controlling primary production of the Southern Ocean, studies on the quantification of the limitations of these two factors in the various environments of the Southern Ocean are still lacking due to difficulty in controlling experimental conditions. Moreover, decoupling between surface primary production and particulate organic carbon (POC) flux to the ocean interior has not been accurately explained. Applying a 1-dimensional modeling approach, 1) we quantified and compared the limitation factors of iron and irradiance for primary production in different environments of the Southern Ocean, and further 2) showed that the difference on particulate flux in each region can be due to physiological/biogeochemical differences in dominant phytoplankton species in the upper layer. The model was adopted to three regions in the Southern Ocean: the polar front, the marginal ice zone, and polynya in the inner-shelf area. The simulation results showed that iron plays the most significant role in determining the magnitude of summer primary production and community structure. Efficiency of POC export is large in the continental shelf of high iron concentration, high primary production, and high Phaeocystis biomass. On the other hand, biogenic silica export occurred most efficiently in the polar front region where diatoms were dominant and primary production was extremely low. Given the importance of residence time of organic carbon in the ocean, needs for a detailed understanding of vertical mixing, bacterial activity, and dynamics of macronutrient such as silicon were raised as well.