Graduation Year

2025

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Kristen N. Buck, Ph.D.

Co-Major Professor

Robert H. Byrne, Ph.D.

Committee Member

Ana Aguilar-Islas, Ph.D.

Committee Member

Mya Breitbart, Ph.D.

Committee Member

Brad Rosenheim, Ph.D.

Keywords

Distributions, HR-ICP-MS, Limiting nutrients, Ocean, Phytoplankton, Size fractions

Abstract

Phytoplankton in the marine environment require the trace metals manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) to catalyze cellular processes driving photosynthesis, including macronutrient (nitrogen, phosphorus, and carbon) acquisition, light harvesting, and protection from oxidative stress. The high biological demand for these elements, coupled with their low concentrations in the ocean, can limit primary production and shape community composition in marine ecosystems, highlighting the need to understand the drivers behind trace metal distributions and their interplay with phytoplankton growth across biogeochemical regimes. This dissertation characterizes the spatial and temporal distributions of dissolved (<0.2 µm) and leachable particulate (>0.4 µm) trace metals in surface waters of the West Florida Shelf, as well as the temporal evolution of dissolved trace metals during the decline of the North Atlantic Spring Bloom.

On the West Florida Shelf, turbidity was observed to be an important driver of Mn, Fe, and Pb, which were present primarily in the leachable particulate size fraction, whereas Co, Ni, Cu, Zn, and Cd were mostly present in the dissolved size fraction, and concentrations increased primarily as a function of decreasing salinity, with distinct riverine endmembers contributing to spatio-temporal variability across the shelf. Samples with high cell counts of the dinoflagellate Karenia brevis were associated with distinct size fractionation of Co and Cd, suggestive of substantial biological uptake of these elements during red tide events. At the Porcupine Abyssal Plain-Sustained Observatory station, temporal changes in dissolved trace metals were characterized during the decline of the North Atlantic Spring Bloom in a Lagrangian field study conducted as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign. Both field and experimental studies were coupled to provide insights into trace metal uptake across different temporal scales of phytoplankton growth in the North Atlantic. Persistently low Fe concentrations and low Fe to nitrate concentration ratios in the upper water column were consistent with Fe-limiting conditions for large phytoplankton, which was also observed in the shipboard bioassay experiments indicating serial Fe-silicic acid limitation during the decline of the bloom. Characterization of a broader suite of trace metals revealed that dissolved Zn concentrations were also depleted in the upper water column, and depletion of Zn appeared to be followed by enhanced drawdown of Cd and/or Co. This trend was also observed in both shorter (2-3 day) and longer (6-8 day) incubation experiments, consistent with known substitution of these elements in cellular enzymes. Dissolved Al was observed to closely follow silicic acid, suggesting that diatom growth and remineralization were important drivers of Al concentrations. Overall, results from this dissertation improve our understanding of trace metal cycling on the subtropical West Florida Shelf characterized by low inorganic macronutrients and the high-latitude North Atlantic Ocean with high inorganic macronutrients.

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