Summertime Biogenic Silica Production and Silicon Limitation in the Pacific Arctic Region From 2006 to 2016

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AbstractWe present a decade of dissolved and particulate silica concentrations within five biological “hot spots” in the Pacific Arctic Region (PAR) and the first measurements of both biogenic silica production rates (ρSi) and the kinetics of silicon utilization from a period of four years at the same sites. The “hot spots” were located within the Bering and Chukchi Seas and identified as part of the Distributed Biological Observatory (DBO). Across all hot spots, the highest concentrations of silicic acid (Si(OH)4) and biogenic silica were found near the bottom of the euphotic zone and often correlated with increased ρSi. For the entire region, the average ρSi was 19 mmol m−2 day−1 and siliceous microplankton (i.e., diatoms) contributed an average of 62% to primary productivity and 82% to nitrate utilization. Irradiance and [Si(OH)4] had separate and interactive effects on ρSi. Irradiance modulated both the magnitude of ρSi and the response of diatoms to changes in Si(OH)4. Availability of Si(OH)4 limited ρSi in all hot spots in at least one of the four years. Kinetic experiments conducted in all hot spots demonstrated that the half‐saturation constant (Ks) for ρSi was 4–8 times higher than ever reported in the literature. In the southeastern Chukchi Sea, an east to west gradient in [bSiO2] and ρSi may have been driven by differences in the availability of NO3− rather than Si(OH)4. Despite strong interannual variability, we suggest that phytoplankton phenology responds to short‐term climatic changes, which can have far‐reaching effects on Arctic regions influenced by the Pacific‐origin waters flowing through the PAR.