Graduation Year

2023

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

Timothy Conway, Ph.D.

Committee Member

Bethany D. Jenkins, Ph.D.

Committee Member

Peter L. Morton, Ph.D.

Keywords

dissolved and particulate fractions, iron, nutrient limitation

Abstract

The trace metals iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) are essential to primary production in the oceans. They act as micronutrients, used in intracellular enzymes for macronutrient acquisition, light harvesting, and mediation of reactive oxygen species. In some cases, they can also substitute for each other when a primary trace metal micronutrient is depleted. Characterizing the interplay amongst trace metals, macronutrients, and phytoplankton growth is vital for understanding the biogeochemical underpinnings of primary production, and for modelers who develop biological pump frameworks for global carbon cycling.

In Chapter two, I investigated the timing and magnitude of dissolved (<0.4 µm) Fe, Mn, Co, Ni, Cu, Zn, and Cd concentration changes throughout phytoplankton growth in control versus Fe and/or vitamin B12-amended shipboard incubations of macronutrient-replete austral spring seawater of the Southern Ocean. I observed no evidence for Fe-vitamin B12 co-limitation, but Mn appeared to be a secondary limiting nutrient. An evident lack of bacterial Mn oxidation in the Southern Ocean may have facilitated the efficient Fe recycling observed offshore. Overall uptake of dissolved metal (M): phosphate (P) was closely related to initial dissolved M:P concentrations for Fe, Mn, Co (when no B12 was added), Ni, and Cd, and for Cu inshore. Results also indicated substitution of dissolved Co and Cd for Zn as Zn bioavailability plateaued in the incubations. Amongst this suite of trace metals, Ni uptake was the weakest and the most delayed in onset.

In Chapter three, I monitored dissolved (<0.4 µm) and leachable particulate (0.4-5 µm, >5 µm) Ni to assess uptake from seawater into different size classes of plankton, a novel approach. I did this for a series of Fe-, Cu-, and macronutrient-amended incubation experiments designed to induce nutrient stress or relieve nutrient limitation in late summer at Ocean Station Papa in the Fe-limited subarctic North Pacific. All Fe-amended treatments exhibited more phytoplankton growth than treatments without added Fe, consistent with Fe limitation of the study region. Dissolved Ni uptake into the >5 µm leachable particulate fraction, which was associated with diatoms, was delayed relative to chlorophyll increases between the two time points studied, 6 and 8 days. The unique combination of macronutrient and Fe treatments used in the experiments allowed an assessment of Ni uptake as a function of nitrate concentrations, revealing the onset of dissolved Ni uptake coincident with depletion of nitrate below ~5 µM. My results suggest that dissolved Ni uptake by large diatoms is a function of a threshold in nitrate availability, which offers a new hypothesis for the incomplete depletion of dissolved Ni in the surface ocean.

The Piney Point wastewater emergency response offered the chance to monitor dissolved and particulate trace metal concentrations in situ during consecutive phytoplankton blooms in Tampa Bay. This estuarine system experienced exceptionally large inputs of nitrogen and P when fertilizer mining waste was discharged into Tampa Bay following a leak in the plastic liner of a holding pond in early April 2021. A large diatom bloom immediately followed the macronutrient inputs, and then a Karenia brevis bloom was advected in from offshore. Archived field samples collected before the spill provided a baseline for assessing trace metal contributions from the spill. Phosphate and nitrate appeared to support the diatom bloom near Port Manatee, but leveled off to baseline concentrations by June. Following the spill in April, dissolved and total dissolvable Mn and Ni concentrations were highest spatially near Port Manatee and then returned to baseline concentrations by June. Cadmium in April was 60-80% dissolved in upper Middle Tampa Bay, yet 60-100% particulate in association with the diatom bloom near Port Manatee. However, when Karenia brevis blooms were present, Cd concentrations were ~60% dissolved, suggesting different Cd quotas across these phytoplankton groups. The results of my dissertation work provide novel insights into factors governing trace metal uptake by phytoplankton communities in nearshore and offshore environments with varying macronutrient supply.

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