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Topic

Nutrient Physiology of Siliceous Phytoplankton Under Warming and Acidification in Arctic and Subtropical Oceans

Department of Biology

Date & location

• Wednesday, December 18, 2024
• 10:00 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Diana Varela, Department of Biology, University of Victoria (Supervisor)
• Dr. Rana El-Sabaawi, Department of Biology, UVic (Member)
• Dr. Roberta Hamme, School of Earth and Ocean Sciences, UVic (Outside Member)
• Dr. Paul Covert, Institute of Ocean Sciences, Fisheries & Oceans Canada (Outside Member)

External Examiner

• Dr. Jeffrey Krause, Senior Marine Scientist III, Dauphin Island Sea Lab

Chair of Oral Examination

• Dr. Jon Husson, School of Earth and Ocean Sciences, UVic

Abstract

Steadily rising atmospheric CO₂ concentrations have impacted marine ecosystems by increasing the temperature, stratification, and acidity of surface waters of the world’s ocean. In the sunlit upper ocean, phytoplankton affect elemental cycling and contribute to nutrient export to deeper waters by incorporating nutrients into biomass and supporting higher tropic levels. One unique group of phytoplankton, diatoms, are characterized by their typically larger size and heavy silica frustules, and they make considerable contributions to global phytoplankton productivity. Diatoms are expected to be impacted by oceanic change in various ways, but the degree of this effect is still uncertain. The overall objective of this thesis is to improve our understanding of how marine diatom physiology, specifically the utilization of silicon (Si), and the contribution by diatoms to the cycles of carbon (C) and nitrogen (N) are affected by climate-induced increases in temperature and acidification. I investigated the impact of mesoscale physical processes on diatom contributions to utilization rates of C (ρC) and nitrate (ρNO₃) in the Sargasso Sea in the North Atlantic subtropical gyre, an ecosystem impacted by increased stratification due to ocean warming. Results show that diatoms played a minor role in nutrient utilization and biomass during the lowest-productivity time of year, but they increased and dominated nutrient utilization rates in the deeper euphotic zone of the Sargasso Sea when nutrient concentrations were enhanced by eddy-driven upwelling. In the contrasting environment of the Bering and Chukchi Seas, I investigated the effects of a warming ocean on diatom physiology and elemental composition as part of an on-going oceanographic time-series in the Pacific Arctic Region (PAR). I found significant trends in ocean temperature and sea ice breakup dates for different regions of the PAR, and evidence for declining diatom biomass in one area of the northern Bering Sea. The response of diatom assemblages in the PAR to a sustained warming period and marine heatwave (MHW) in 2019 otherwise varied substantially latitudinally from the north Bering to the Chukchi Sea. Estimates of diatom contributions to ρC and ρNO₃ in the PAR were improved compared to previous studies, and show with greater certainty that diatoms are responsible for the vast majority of nutrient utilization. I found anomalously low C:N values across the PAR during the 2019 MHW, and show that higher Si:C and Si:N are associated with colder temperatures across the entire 2006 – 2022 period. Ocean acidification experiments were conducted with a model diatom species and two natural phytoplankton assemblages to assess the effects of decreased pH on nutrient physiology. Overall, diatom Si utilization and physiology in laboratory and field culture experiments were mostly unaffected by pH. However, I found that the cell size of a model species of diatom, Thalassiosira rotula, decreased under OA, and that high levels of OA may negatively affect diatom biomass in the PAR. In subtropical phytoplankton assemblages, OA had no meaningful impacts on different groups of siliceous phytoplankton. This dissertation provides valuable insights into how siliceous phytoplankton, particularly diatoms, interact with marine cycles of Si, C, and N across cold and warm water marine ecosystems. It also deepens our understanding of how these dynamic systems respond to oceanic changes, and sets the stage for future research on the evolving impacts of climate-driven physical processes.