Iron in the Southern Ocean: A model study of iron sources and their impact on the phytoplankton

Abstract

In the iron limited Southern Ocean, the iron sources have a large impact on the spatial distribution and magnitude of the phytoplankton blooms, and thereby also on the net primary and export production (NPP and EP). In this thesis, the Southern Ocean input of iron from the sediments and from vertical supply to the mixed layer is investigated in a new Ocean General Circlation Biogeochemical Model (OGCBM). Focus is especially on the model s representation of the iron sources, how they affect the biological production in the model and what can be learned from this about the iron sources in nature. The first part of the thesis documents the global performance of the Regulated Ecosystem Model (REcoM2) coupled to the Finite Element Sea-ice Ocean Model (FESOM). It shows that the model does a reasonable job reproducing the large scale surface patterns of the biogeochemical fields when compared to observations and other non-eddy resolving OGCBMs. The spatial fields of chlorophyll a and NPP are especially good. Further analysis of the Southern Ocean shows that the total net primary and export production here fits well with previous estimates. The second part of the thesis assesses the impact of the Ocean General Circulation Model (OGCM) on the vertical iron supply to the mixed layer of the Southern Ocean. Two similar model runs were carried out with REcoM2 coupled to two different ocean models; FESOM and MITgcm. The study shows that the modeled vertical iron fluxes, and thereby the NPP and EP, are strongly dependent on the ocean model. This is partly due to differences in the mixed layer dynamics, but is also affected by other mechanisms, such as the strength of the meridional overturning circulation. The study highlights the importance of considering the skill of the OGCM forcing the biogeochemistry. This is important for contemporary runs, but especially when predicting future changes to the biological pump using large scale OGCBMs. The rate of iron release from the sediments in nature is not well constrained, leading to large variations in its strength between biogeochemical models. The third part of the thesis focuses on the implications of changes in the modeled strength of the sediment source, and accompanying higher scavenging rates, on the phytoplankton composition and the opal export. The study shows, that while variations in the strength of the benthic iron flux can be counteracted by an appropriate scavenging rate, thereby producing very similar values for the total NPP in the Southern Ocean, it has a large impact on the spatial distribution of the NPP, on the diatoms and on the opal export. In order to optimize the sedimentary iron input to large scale OGCBMs, it is thus important to look beyond the modeled NPP and assess the biological production by diatoms, as well as the vertical export of opal, especially for the Southern Ocean. The general discussion focusses on the relative importance of the different iron sources in the Southern Ocean, how the different sources may change in the future and what factors modelers need consider when predicting these changes. Lastly further directions for work regarding the iron sources in the Southern Ocean as well as the new FESOM-REcoM2 model are described.