The importance of marine biological processes for carbon storage and biogeochemistry

Changes in the climate have direct implications for marine ecosystems, and as a result, incur feedbacks on ocean biogeochemistry. The response of the marine ecosystem to climate-driven perturbations must be accounted for if ocean biogeochemical dynamics and the ocean's role in the climate are to be understood. This dissertation uses a global ocean model to further our understanding of how marine biological processes affect ocean biogeochemistry, with particular focus on the carbon inventory. To begin, the role of marine biological processes for setting the climate of the Last Glacial Maximum (LGM; 21,000 years ago) is addressed. Despite many years of research, the combination of physical and biochemical processes that drove carbon into the ocean during the LGM, causing a reduction of approximately 95 ppmv of atmospheric CO\(_2\), are not well defined. The model is used to explore this problem. Marine biological changes are found to contribute the bulk of carbon storage (55 ppm) and also reconcile other biogeochemical properties under glacial conditions. These changes involved increased production due to iron fertilisation, reduced calcifer biomass and increased transfer of organics to depth, all consistent with prior theories. The importance of biological processes motivated the introduction of more sophisticated routines to the ocean biogeochemical model. They included variable functions for nutrient limitation, remineralisation and stoichiometry. Their integration is important for simulating marine biogeochemical cycles. The new, dynamic ecosystem model produced realistic features of marine communities and consistently improved biogeochemical fields. It also buffered biogeochemical properties against physical changes. Differences in carbon content between six unique global ocean states are reduced by 50 % when dynamic biological functioning is included. In the final experimental chapter of this thesis, a fully prognostic marine nitrogen cycle alongside the ecosystem developments of the previous chapter, are used to explore how changes in the nitrogen cycle affect the ocean's carbon content. Nitrogen fixation is found to be strongly related to the biological carbon store, in fact linearly predicting carbon storage. The strong relationship between nitrogen fixation and the biological carbon store is due to the unique ability of nitrogen fixers to simultaneously drive ecosystems towards more complete consumption of phosphorus and elevate carbon to phosphorus ratios, and therefore increase the effciency of the biological carbon pump. Moreover, the increase in nitrogen fixation is supported by nitrate depletion and iron addition to Southern Ocean intermediate waters that occur as a result of aeolian iron fertilisation. This thesis provides several lines of evidence that biological processes are of primary importance for setting ocean biogeochemical properties and carbon storage on millennial timescales. The multifarious biogeochemical roles of marine communities therefore must be invoked to understand the glacial cycles of pCO\(_2\). We complete the thesis with a discussion that readdresses the glacial drawdown of atmospheric CO\(_2\) in the context of the findings, and list directions for future research.