UC San Diego

The cycling of iron in the ocean is intimately tied to whole ecosystem processes and other biogeochemical cycles. While ocean basin surveys of iron concentration have revealed important sources to the ocean, questions remain regarding the controls on supply of this critical micronutrient to productive planktonic ecosystems. Using a combination of oceanographic-scale observations and mechanistic experiments, work in this thesis seeks to advance our understanding of controls of new and regenerated iron supply to coastal polar and temperate ecosystems and microbially-mediated cycling in such marine environments.

In Chapter 2 we comprehensively characterized the cryospheric inputs of iron to Andvord Bay, a cold glaciomarine fjord located on the west Antarctic Peninsula. Iron content and speciation were measured to characterize inter-seasonal changes in the water column, a subglacial meltwater plume and glacial ice, fjord sediments, including a first assessment of iron-binding organic ligands in a highly productive fjord environment. We utilized multi-elemental analyses to understand the cycling of iron sources. Results from a high-resolution numerical model revealed episodic atmospheric forcing events are important for supplying subglacial meltwater to the surface, and for the export of surface meltwater and associated iron away from the coast. In Chapter 3 we characterize iron supply and cycling in a coastal upwelling filament, a mesoscale feature which commonly occurs during times of intensified along-shore winds in eastern boundary current systems, such as the southern California Current. We used a Lagrangian framework to capture the biogeochemical evolution of a filament, including: the development of iron-limitation of the phytoplankton community, assessment of in situ geochemical proxies of iron stress, and impacts of diminishing iron flux on phytoplankton allometric ratios, namely the biogenic silica-to-organic carbon content of diatoms. We have determined filaments are not likely significant sources of dissolved iron to the open ocean, yet their importance for advecting particulate iron remains to be tested. Coincident with the development of iron-limitation of the phytoplankton community we observed greater biogenic silica-to-organic carbon ratios and enhanced export efficiency to depth. In Chapter 4, within microcosm a field experiment, we examined the production of organic iron-binding ligands during remineralization – an important process supplying iron regenerated from phytoplankton blooms. As a part of this work we developed an improved data processing methods for multiple analytical windows electrochemical iron ligand titration data, enabling more robust detection of distinct pools of iron-binding ligands produced by heterotrophic bacteria and in diverse oceanic waters.

Together, this work examines the processes involved in the flux of new iron to the ocean, and internal transformations affecting its fate within the water column. We hope this work results in an improved understanding of the controls on modern iron supply and ties to the marine biogeochemical cycles of macronutrients.