Dominating diatoms: investigating the coupling between biogenic silica dynamics, primary production and nitrate uptake in a highly productive coastal fjord

Diatoms, microscopic algae with siliceous cell walls, account for up to 40% of the annual marine biological carbon fixation and for a significant portion of the export of carbon from the surface to the deep ocean. Diatoms are the largest consumers of dissolved Si (Si(OH)₄) in the oceans and, through the photosynthetic process, couple the marine cycles of silicon, carbon, and nitrogen (Si, C, and N). However, current knowledge of the marine Si cycle and the processes affecting its relationship to other marine biogeochemical cycles is limited. To better understand how diatoms link the marine cycles of Si, C and N, we evaluated surface biogenic silica (bSiO₂) dynamics and the contribution of diatoms to primary production and nitrate uptake in Saanich Inlet, a highly productive coastal fjord on the west coast of British Columbia, Canada. We examined the monthly euphotic zone distributions of dissolved nutrient, chlorophyll a, and bSiO₂ concentrations, and determined bSiO₂ production rates and ¹³C and nitrate (¹⁵NO₃) uptake rates over a two-year period. Our results indicate that diatoms tend to account for most of the primary production and nitrate uptake in Saanich Inlet, though there are occasional periods of high productivity, and in some cases high diatom biomass, that are not linked to high bSiO₂ production. Our results also show that these periods can result in changes to the Si:C and Si:N ratios of suspended particles, which are otherwise relatively stable and close to the expected ratios for nutrient replete diatoms throughout the year. These high primary production/low bSiO₂ production periods appear to be connected to changes in the nutrient supply, which in turn leads to changes in phytoplankton community composition. Such changes to the status of the phytoplankton community, if sustained, could have significant implications for the strength of the biological carbon pump.