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Nutrient and phytoplankton pigment data were collected during summer cruises in 2008–2014 to examine how the Beaufort Gyre (BG) affected regional differences in nutrient stocks and the phytoplankton community. The results showed a gradual decrease in nutrient concentrations and increasing depth of the chlorophyll maxima toward the center of the gyre. A horizontal map of nitrate at a depth of 50 m indicated nitrogen deficiency in the gyre compared with that in the surroundings. Biogeochemical model simulations suggested that an increased freshwater convergence, driven by the strengthened BG and sea ice retreat, was primarily responsible for the decrease in nutrient stocks. In the BG zone, the contribution of diatoms to the phytoplankton community decreased, accompanied by the increased contribution of small flagellates. The current state of the BG under anticyclonic winds negatively affected the diatom biomass and biological carbon pump. The possible rotation of wind regimes from anticyclonic to cyclonic might relieve the nitrogen deficiency for phytoplankton growth in the western Arctic Ocean. [ABSTRACT FROM AUTHOR]

Spatial variability of ammonium concentrations along repeat transects were examined on the Chukchi shelf during 2012–2018. Two distinct near-bottom high ammonium pools (>1 μmol/kg) near 67.5°N and 72.5°N of the transects were identified in all years. The accumulation of ammonium in the regions is driven primarily by a combination of biogeochemical processes (e.g., dynamic bacterial remineralization of organic matter) and physical controls (e.g., strong density-contrast barrier limits upward mixing of ammonium). The ammonium pool on the shelf may became larger in the expectation of the stronger bacterial remineralization following elevate primary production, and may have potential impact on the structure and productivity of ecosystem on the Chukchi shelf. • High ammonium concentrations were measured in near-bottom water of the Chukchi shelf during summer of 2012–2018. • The accumulation of ammonium may be caused by a combination of intense remineralization and inhibited vertical diffusion. • The ammonium pool on the shelf may became larger with elevated primary production and enhanced bacterial remineralization. [ABSTRACT FROM AUTHOR]

The nutrient-rich Pacific Ocean seawater that flows through the Bering Strait into the Chukchi Sea is generally considered to be the most important source of nutrients to the Arctic euphotic zone. The inflow is characterized by nitrogen deficit and low nitrate/phosphate (N/P) ratios; this is ascribed to sedimentary denitrification on the Chukchi shelf by preoccupant opinions. However, the Chukchi Sea also has high primary production, which raises the question of whether the biological pump may also significantly modulate nutrient properties of the throughflow. Here, we show that nitrate concentrations of the Pacific inflow gradually decrease northward in association with notable biological utilization. The phytoplankton N/P uptake ratio was 8.8±2.27, higher than the N/P ratio of Pacific inflow water (5–6). This uptake ratio, in combination with efficient vertical nitrogen export, serves to preferentially remove nitrogen (relative to phosphorus) from upper waters, thereby further intensifying the Arctic nitrogen deficit. Accordingly, as large as about 111.7×109 mol N yr−1 of nitrate was extra consumed, according to the real N/P uptake ratio rather than the ratio of the Pacific inflow, which may be as great as half the nitrogen loss ascribed to sedimentary denitrification. Our findings suggest that besides sedimentary denitrification, biological disproportionate utilization of nutrients in the Chukchi Sea upper water is another important contributor to the nitrogen limitation and excess phosphorus in the upper Arctic Ocean. In the rapid Arctic change era, the predicted reinforced biological carbon pump could further impact the nutrient dynamics and biogeochemical process of the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Siliceous planktons are valuable indicators of the environmental conditions in both modern and past marine settings. However, in contrast to diatoms and radiolarians, other siliceous micro- and nanoplankton in the Arctic Ocean have been rarely explored. In this study, silicoflagellates, endoskeletal dinoflagellate Actiniscus pentasterias and chrysophyte cysts were investigated in one-year mooring sediment trap material (from August 2008 to September 2009) collected in the Northwind Ridge, western Arctic Ocean. The silicoflagellate assemblage was dominated by Stephanocha speculum , accounting for > 71% of the total silicoflagellate composition. While S. speculum was overwhelmingly abundant in summer, S. medianoctisol and S. octonari a were more frequent during winter. The export fluxes of endoskeletal dinoflagellate A. pentasterias did not show clear seasonal pattern except for a peak value in September 2009. We suggest that high flux of A. pentasterias reflects nutrient rich environment at sea ice edge rather than cold under sea ice conditions. High fluxes of chrysophyte cysts were recorded in summer 2009 peaking in late July and early August, ~10 times higher than in summer 2008. Peak of chrysophyte cysts and sea ice melting occurred simultaneously. In addition, observed encystment might be triggered by the population density, hence indicating a favorable environment for phytoplankton blooming. These siliceous micro- and nanoplankton provides information on the modern Arctic Ocean environment but requires further investigations to consolidate knowledge for robust use in paleoceanography. • First report on seasonal fluxes of chrysophyte cysts in the western Arctic. • Productive sea ice edge is indicated by Actiniscus pentasterias and chrysophyte cysts. • Silicoflagellate Stephanocha speculum reflects influence of subarctic Pacific water. [ABSTRACT FROM AUTHOR]