Understanding ocean iron dynamics and impacts on marine ecosystems

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Pham, Anh Le-Duy
Ito, Takamitsu
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Fe is one of the most important nutrients for phytoplankton growth in the ocean, making it a crucial element in the regulation of the ocean carbon balance and biogeochemical cycles. Atmospheric deposition of dissolved Fe (dFe) to the ocean has increased over the last decades partly due to human activities, which can significantly alter marine ecosystems. Thus, a comprehensive understanding of how the ocean Fe cycling operates and how it will respond to human perturbations is urgently needed. In this work, I first significantly improve the Fe parameterization in a global ocean biogeochemistry model, constrained by new high-quality ocean Fe datasets. Then, I identify key mechanisms that control the ocean Fe cycle in various ocean basins and examine the responses of marine phytoplankton to an increasing Fe deposition through a suite of model simulations. These simulations are performed in an ocean biogeochemistry and an ecosystem models, which incorporate the newly improved Fe scheme. The refinement of model Fe parameterization and its evaluation are undertaken in chapters two to four. In these chapters, I show that my newly developed Fe scheme displays a remarkable improvement in reproducing observations over the old scheme. Through a suite of model simulations, I reveal the crucial role of the concurrent release of dFe and ligands from sinking organic particles in forming and maintaining the subsurface dFe maxima observed in many ocean transects. Moreover, the inclusion of spatially varying ligand classes with different binding strengths in the model is important to explain the strong vertical dFe gradient observed in the upper ocean. I also identify the relative roles of different external dFe sources in different ocean basins. While atmospheric deposition is an important source of dFe in the Atlantic and Indian Oceans, sedimentary and hydrothermal dFe inputs are more important in the Pacific Ocean. The relative contributions of external sources and ocean interior processes on regulating the upper ocean dFe pattern are explored in chapter five. This task is done by analyzing the dFe budget and the dFe distribution field simulated in different ocean Fe models, using an unsupervised classification technique. The results show that the upper ocean dFe patterns are largely controlled by interior ocean processes and that without an appropriate representation of these processes, Fe models cannot reproduce observations, even with a correct magnitude of the external fluxes. In chapter six, I explore the impact of an increasing dFe atmospheric deposition on the Indian Ocean phytoplankton and carbon balance by using an ocean ecosystem model, which incorporates the newly improved Fe scheme. I found that while diatom growth and export organic carbon flux are enhanced south of 40 degree S, they decrease in some regions in the northern Indian Ocean, compensated by increases in coccolithophores growth and carbonate carbon export. These changes lead to a decrease in the carbon dioxide uptake over the Indian Ocean.
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