Graduation Year

2023

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Graduate School

Major Professor

Kristen N. Buck, Ph.D.

Co-Major Professor

Brad Rosenheim, Ph.D.

Committee Member

Mya Breitbart, Ph.D.

Keywords

bioavailability, biogeochemistry, organic ligands, trace metals

Abstract

Nickel (Ni) is an important micronutrient for phytoplankton and bacteria that serves as a required co-factor in several metalloenzymes. Despite these known biological uses, total dissolved Ni concentrations remain elevated in global surface waters, in contrast to the surface depletion commonly observed for macronutrients and other nutrient-type trace elements. A prevailing hypothesis for the muted depletion of dissolved Ni concentrations in surface waters is that dissolved Ni in seawater is not in a bioavailable form. The chemical lability of Ni in seawater provides insight into Ni speciation and bioavailability, but few measurements have been made in the open ocean to date. Here we present the first depth profiles of measured labile Ni concentrations in the upper ocean water column (to 1,000 m). Measurements were performed at eight stations across three distinct biogeochemical regimes in the Northeast Pacific Ocean, namely coastal upwelling in the Northeast Pacific, the subtropical gyre, and iron-limited waters of the subarctic Pacific at Ocean Station Papa. Labile Ni concentrations were generally nutrient-type in the profiles, with lowest concentrations associated with fluorescence maxima, including near complete depletion in the subsurface fluorescence maximum of the oligotrophic gyre, and increasing labile Ni observed with macronutrient regeneration and silica frustule dissolution at depth. Deviations from a classic nutrient-type profile were evident in a surface maximum of labile dissolved Ni and an apparent deficit in the deepest samples, which may reflect distinct sources and sinks of this chemical form of dissolved Ni. Samples in this study were accidentally buffered to pH 9.0 instead of the calibration pH of 8.4, which may have resulted in an overestimation of labile Ni throughout the dataset, necessitating further research into the sensitivity of this operationally-defined method to pH changes.

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