Michael Livingston

Topic

Transparent Exopolymer Particles and Phytoplankton Nutrient Physiology in the North Pacific and Arctic Oceans

Department of Biology

Date & location

• Monday, April 15, 2024
• 2:00 P.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Diana Varela, Department of Biology, University of Victoria (Supervisor)
• Dr. Rana El-Sabaawi, Department of Biology, UVic (Member)
• Dr. Jay Cullen, School of Earth and Ocean Sciences, UVic (Outside Member)
• Dr. Roberta Hamme, School of Earth and Ocean Sciences, UVic (Outside Member)

External Examiner

• Dr. Philip Boyd, Institute for Marine and Antarctic Studies, University of Tasmania

Chair of Oral Examination

• Dr. David Scoones, Department of Economics, UVic

Abstract

The export of organic carbon from the surface to the deep ocean is a key factor in regulating the level of carbon dioxide (CO2) in the atmosphere. The efficiency of this process, known as the biological carbon pump (BCP), is largely governed by biological influences in the upper layers of the water column, including the magnitude of primary production, as well as the sinking rate of particulate organic carbon. However, the efficiency of the BCP is still poorly understood in large parts of the ocean. A key factor affecting the BCP is the presence of transparent exopolymer particles (TEP) in the surface ocean. These particles originate from the exudation of organic exopolymeric substances and form sticky carbon (C) gels that facilitate the aggregation of organic matter. These particles are less dense than seawater and affect the sinking of particulate organic carbon from the surface to the deep ocean, and therefore play an important role in the ocean’s C cycle. The overall objective of this thesis was to quantify key biological factors that affect surface ocean C cycling and determine the effects of variation in environmental conditions on the strengths of these factors. Over a 4-year study, I examined the concentrations and rates of production of TEP, phytoplankton nutrient physiology, and phytoplankton size-fractionated contributions to nutrient cycling in the Eastern Subarctic North Pacific (ESNP), and Pacific and Central Arctic regions. Measurements of TEP and a variety of biological and environmental variables were made across all regions in the ESNP and Arctic, and total and size-fractionated nutrient uptake rates of C, nitrate (NO₃^-) and silicic acid (Si(OH)₄) were additionally measured across the ESNP. The concentrations of TEP were largely driven by the amount of phytoplankton biomass and productivity, but also by environmental variables such as temperature, mixed layer depth, and nutrient concentrations. By using a novel multivariate modelling approach, the concentration of C in the form of TEP (TEP-C) was constrained in the oceanic regions of the ESNP to a range between 5-15μg C L^-1 year-round. We further measured TEP concentrations relative to C uptake and export potential (i.e., new production), providing the first quantitative comparison among these variables across large spatiotemporal gradients in the ESNP. Results show that low productivity regimes are characterized by higher concentrations of TEP-C (and higher estimates of TEP-C turnover) relative to C uptake and new production, which suggests these regions may experience less efficient C export. This thesis presents the first field-based connection between excess C consumption and TEP concentrations. Size-fractionated measurements reveal that small-sized phytoplankton (< 5 μm) are responsible for most of the nutrient uptake and TEP production in the oceanic and less-productive regions of the ESNP. The contribution of the small size-class did not appear to be influenced by environmental variations. Overall, this work provides new perspectives on surface ocean C cycling in the ESNP by shedding light on the main drivers of TEP, predicting TEP-C concentrations, and by relating TEP-C to total primary productivity and new production.