Anna Olivia Melville

Topic

The role of siliceous Rhizaria in the silicon cycle in the Eastern Subarctic North Pacific Ocean

Department of Biology

Date & location

• Wednesday, April 10, 2024
• 8:30 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Diana Varela, Department of Biology, University of Victoria (Supervisor)
• Dr. John Dower, Department of Biology, UVic (Member)
• Dr. Akash Sastri, Department of Biology, UVic (Member)

External Examiner

• Dr. Fabrice Not, Station Biologique de Roscoff, Sorbonne Université

Chair of Oral Examination

• Dr. Laurence Coogan, School of Earth and Ocean Sciences, UVic

Abstract

Siliceous Rhizaria are zooplankton protists found in all ocean basins from the surface to the deep aphotic zone. Like diatoms, a group of phytoplankton with silica (SiO2) cell walls, siliceous Rhizaria utilize silicon (Si) to form their skeletons. While diatom production is known to strongly control the Si cycle, an important biogeochemical process that modulates global long-term stability, the role of Rhizaria on Si cycling is poorly understood. Therefore, the Si cycle is not fully constrained. Furthermore, long-term shifts in community composition and seasonal changes to Si uptake rate (ρSi) in siliceous Rhizaria have never been studied. To address these knowledge gaps, field work was conducted in the Eastern Subarctic North Pacific (ESNP) Ocean and Pacific Arctic Region (PAR) between 2010-2023. Stations were sampled with bongo net tows which were vertically hauled from 250 m and 1200 m to the ocean surface. In this thesis, chapter 2 presents a historical time series (2010-2020) of siliceous Rhizaria community composition in the ESNP. The results showed that there were no decadal trends in species abundance, richness, or composition. Rhizaria abundance data were incorporated into a published dataset to update the estimated Si production by Rhizaria to a total of 0.4 – 5.5 Tmol Si year-1 in polar waters (>40°N/S). Chapter 3 compares Rhizaria community composition in the ESNP (March, July, and August 2022) and the PAR (July 2022). In the ESNP, wintertime conditions promoted the mixotrophic taxa of Rhizaria, while summer favoured phagotrophic Rhizaria. The sampling locations in the PAR displayed an absence of siliceous Rhizaria during this study. Chapter 4 presents the results from diatom and Rhizaria ρSi experiments conducted in August 2022 and May 2023 along the Line P transect in the ESNP. Overall, the contribution of siliceous Rhizaria to total daily ρSi was less than 0.12% and their contribution to total SiO₂ biomass was less than 4% confirming that diatoms are the main drivers of Si cycling in the ESNP. However, it can be estimated that up to 38% of SiO₂ produced by Rhizaria could be buried along Line P suggesting Rhizaria contribution to Si flux might be important. This work provides a novel contribution to our understanding of the ecology of Rhizaria in the ESNP and PAR. Furthermore, the thesis provides new data which can be used to constrain the global Si cycle and predict how the Si cycle might shift with climate change.