This thesis presents improvements to a method for measuring the production of biogenic silica (bSiO2) by diatoms, a group of microscopic algae with siliceous cell walls (frustules) that dominate the marine cycling of silicon (Si) and account for a significant proportion of global marine primary productivity. Using the fluorescent dye PDMPO, diatom bSiO2 can be labeled as it is produced and then quantified using fluorometry to determine community-wide bSiO2 production. A distinct advantage of PDMPO over more traditional tracers of bSiO2 production is that the combination of measurements of PDMPO by fluorometry and by fluorescence microscopy allows for the quantification of cell (and thus taxa) specific bSiO2 production within a mixed community. However, the robustness of PDMPO as a quantitative tracer of diatom bSiO2 production has not been sufficiently investigated. To address this, experiments were conducted both in the lab, and at two field locations where diatoms are known to be abundant, namely the continental shelf off the west coast of Vancouver Island, and Saanich Inlet, a highly productive fjord located on southern Vancouver Island. Laboratory culture experiments demonstrated that concentrations of PDMPO >500 nmol L-1 reduced growth rate in the diatom Thalassiosira pseudonana, and affected the Si:PDMPO ratio of incorporation. The relationship between SiO2 and PDMPO incorporation was significantly affected by diatom species, though this effect was small (8%) when cells were lysed. From these experiments, a Si:PDMPO incorporation ratio of 4200 ± 380:1 was determined, which predicted 30% more bSiO2 production for PDMPO incorporation than previous studies, and better agreed with bSiO2 production rates determined using established methods in Saanich Inlet. However, bSiO2 production rates were over-estimated by the PDMPO method when rates were less than 1 µmol L-1 d-1. In a few cases, this occurred when dinoflagellates were numerically dominant, but for the majority of samples, dinoflagellates were low in abundance, and over-estimation by PDMPO may be related to low dissolved Si(OH)4 concentration. Protocols for quantifying PDMPO fluorescence by microscopy were optimized by using a low numerical aperture microscope objective. Additionally, measurements of fluorescence intensity were calibrated using a fluorescent microscope slide as a standard, which served to correct for unevenness of illumination across the field of view. With these protocol modifications, quantification of PDMPO by microscopy agreed with PDMPO measured by fluorometry. When PDMPO was measured by microscopy in the field, the contribution of diatom taxa to PDMPO fluorescence differed from their contribution to cell numbers. In many cases this was due to large diatom taxa producing more bSiO2 per cell than smaller taxa. However, much of the difference between cell numbers and PDMPO fluorescence was not explained by differences in cell size. This suggests that the diatom taxa had different specific bSiO2 production rates, which could be estimated using PDMPO. This thesis highlights the strength of the PDMPO tracer for understanding diatom community dynamics. The use of PDMPO should allow the relationship between diatom community composition, growth and productivity to be better illuminated in the oceans. / Graduate / 0416 / jelong@uvic.ca

Marine diatoms are the dominant primary producers in coastal and shelf regions, and contribute to about 20% of the annual photosynthesis on Earth. Diatoms also exert a major control on the marine silicon (Si) cycle through the formation of biogenic silica (bSiO2). Continental shelves account for half of the total marine area in the Arctic, yet our knowledge of the cycling of Si for this critically climate-impacted region is limited. The overall objective of this thesis was to improve our understanding of marine bSiO2 dynamics and Si cycling in marine Arctic and Subarctic ecosystems using novel techniques. Phytoplankton and nutrient observations, including dissolved and particulate silica concentrations, are presented from a period of ten years within five biological ‘hotspots’ in the Bering and Chukchi Seas. The first measurements of bSiO2 production and dissolution rates are also presented from a period of four years at the same sites. Results from this work show that (i) although interannual variability is high, diatoms are responsible for most of the high primary productivity in the Bering and Chukchi Seas, (ii) bSiO2 is primarily re-dissolved within the euphotic zone rather than exported, and (iii) phytoplankton phenology and marine Si cycling are affected by short-term climatic changes in this region. We also present the first measurements of bSiO2 production rates along a transect from the Canadian Arctic Archipelago (CAA), through Baffin Bay and into the Labrador Sea. We show that diatoms are both abundant and productive throughout these regions in summer, despite widespread Si limitation in the low-nutrient surface waters. Finally, we also investigated the natural variations in the Si isotopic composition of silicic acid (30Si(OH)4). On a transect through the Bering and Chukchi Seas, Canada Basin and CAA, and finally to Baffin Bay and the Labrador Sea, we found that δ30Si(OH)4 signals reflect water mass composition, the dissolution of bSiO2 throughout the water column, and the biological utilization of Si in surface waters. Ultimately, this work provides insight into the processes controlling marine Si cycling within the Arctic and its links to the global marine Si cycle and other biogeochemical cycles. / Graduate / 2020-03-13