Mario Uchimiya

Localized accumulation and a shelf‐basin gradient of particles in the Chukchi Sea and Canada Basin, western Arctic

Transparent exopolymer particles (TEP), particulate organic carbon (POC), and particles (size range: 5.2–119 μm) as determined by laser in situ scattering and transmissometry (LISST) were measured in the water column from the Chukchi Sea to the Canada Basin in the western Arctic Ocean, during the late summer of 2012. In general, the percentages of TEP-carbon to POC were high (the mean values for the shelf and slope-basin regions were 135.4 ± 58.0% (± standard deviation, n = 36) and 187.6 ± 73.3% (n = 58), respectively), relative to the corresponding values reported for other oceanic regions, suggesting that TEP play an important role in regulating particle dynamics. A hotspot (extremely high concentration) of particles, accompanied by high prokaryote abundance and production, was observed near the seafloor (depth 50 m) of the shelf region. Localized accumulation of particles was also found in the thin layer near the pycnocline (depth 10–30 m) and on the slope. Over a broader spatial scale, particle concentration gradients were identified from the shelf to the basin in the upper water column (<50 m). The particle-size distribution analysis indicated that relatively small particles were dominant in the shelf region compared to the slope-basin region. These results suggest that particles containing large amounts of TEP are produced in the shelf region and are potentially delivered to the slope-basin region along the pycnocline, which might support productivity and material cycles in the nutrient-depleted basin region of the western Arctic Ocean.

Coupled Response of Bacterial Production to a Wind-Induced Fall Phytoplankton Bloom and Sediment Resuspension in the Chukchi Sea Shelf, Western Arctic Ocean

Heterotrophic bacterial abundance and production, dissolved free amino acid (DFAA) and dissolved combined amino acid (DCAA) concentrations, and other microbial parameters were determined for seawater samples collected at a fixed station (maximum water depth, 56 m) deployed on the Chukchi Sea Shelf, in the western Arctic Ocean, during a 16-day period in September 2013. During the investigation period, the sampling station experienced strong winds and a subsequent phytoplankton bloom, which was thought to be triggered by enhanced vertical mixing and upward nutrient fluxes. In this study, we investigated whether bacterial and dissolved amino acid parameters changed in response to these physical and biogeochemical events. Bacterial abundance and production in the upper layer increased with increasing chlorophyll a concentration, despite a concomitant decrease in seawater temperature from 3.2°C to 1.5°C. The percentage of bacteria with high nucleic acid content during the bloom was significantly higher than that during the prebloom period. The ratio of the depth-integrated (0–20 m) bacterial production to primary production differed little between the prebloom and bloom period, with an overall average value of 0.14 ± 0.03 (± standard deviation, n = 8). DFAA and DCAA concentrations varied over a limited range throughout the investigation, indicating that the supply and consumption of labile dissolved amino acids were balanced. These results indicate that there was a tightly coupled, large flow of organic carbon from primary producers to heterotrophic bacteria during the fall bloom. Our data also revealed that bacterial production and abundance were high in the bottom nepheloid (low transmittance) layer during strong wind events, which was associated with sediment resuspension due to turbulence near the seafloor. The impacts of fall wind events, which are predicted to become more prominent with the extension of the ice-free period, on bacterial processes and the dynamics of organic matter in the Chukchi Sea Shelf could have far-reaching influences on biogeochemical cycles and ecosystem dynamics in broader regions of the Arctic Ocean.

Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.

Horizontal and vertical distribution of polycystine radiolarians in the western Arctic Ocean during the late summers of 2013 and 2015

A drastic sea-ice reduction has been observed in the Pacific sector of the Arctic Ocean during the last few decades. However, it is still poorly understood how the future reduction in sea-ice cover will impact the microzooplankton communities within the Arctic marine ecosystems. To elucidate the relationship between hydrographic conditions and the horizontal and vertical distribution of polycystine radiolarians, we analyzed 84 plankton tow samples from 51 stations in the western Arctic Ocean. Radiolarians were commonly observed in the continental slope area and the deeper basin area but were scarce or absent on the continental shelf area during the late summers of 2013 and 2015. The horizontal distribution of radiolarians during this time interval was primarily related to the horizontal distribution of low salinity waters. Radiolarian abundances increased at the stations where the seasonally mixed layer, which was formed by sea-ice melt and river runoff, was observed. This result suggests that freshwater inputs would affect the distribution of radiolarians in the western Arctic, presumably via the modification of their food sources. Vertical distribution of radiolarian species was controlled by temperature and salinity characteristics of each water mass, but their abundance decreased in water masses with low dissolved oxygen.