Zooplankton size composition and production just after drastic ice coverage changes in the northern Bering Sea assessed via ZooScan

Drastic environmental changes were noted in the northern Bering Sea in 2018. A reduction in sea ice affected several trophic levels within the ecosystem; this resulted in delayed phytoplankton blooms, the northward shifting of fish stocks, and a decrease in the number of seabirds. Changes in the community composition of zooplankton were reported in 2022, but changes in zooplankton interactions and production have not been reported to date. Therefore, this study examined predator-prey interaction, secondary production, and prey availability for fish to understand the effect of early sea ice melt. Zooplankton size data were estimated from the size spectra obtained using ZooScan based on samples collected in 2017 and 2018. A cluster analysis based on biovolume showed that the zooplankton community could be divided into three groups (Y2017N, Y2017S, Y2018). Y2017N, characterized by low abundance, biomass, and production, Y2017S, characterized by high biovolume and production, which contributed with Calanus spp., and Y2018, characterized by low biovolume but high production, contributed with small copepods, and Bivalvia. In 2017, the highest biovolume group was observed south of St. Lawrence Island, and it was dominated by Calanus spp. and Chaetognatha. Normalized size spectra of this group showed the highest secondary production with present predator-prey interactions, suggesting that the area provides high prey availability for fish larvae and juveniles. In contrast, small copepods and bivalve larvae were dominant in this area in 2018, which contain less carbons and energy, suggesting the prevalence of low-nutrient foods in this year in relation to early sea ice melt

Predation of Juvenile Japanese Sea Cucumber Apostichopus japonicus by Kelp Crab Pugettia ferox

The predation of commercially important Japanese sea cucumber Apostichopus japonicus by the kelp crab Pugettia ferox, widely distributed in coastal northeast Asia, was examined in field sampling and laboratory experiments. The ossicles of A. japonicus were detected from the stomach contents of a natural population of P. ferox at the rate of 32.4% of 68 individuals collected within artificial intermediate sea cucumber reefs for releasing hatchery-produced juveniles in December 2018. In the following laboratory experiments, a high mortality rate (7.7 ± 2.4 individuals day–1) of juvenile A. japonicus (15.35 ± 2.47 mm) was observed despite the different sizes and sex of P. ferox tested. It was also confirmed that a maximum of five sea cucumbers was killed and cut into small pieces within the first 2 h. Smaller and younger P. ferox individuals (adolescent) between carapace widths of 14.2–17.8 mm actively decorated themselves using pieces of chopped sea cucumber after feeding. Attached pieces of sea cucumber were observed to be fully eaten within a week, suggesting a possible strategy by P. ferox of short-term food storage as well as mimicry. This study demonstrates considerable evidence that predation mortality by sufficiently mobile P. ferox on commercially important A. japonicus can be significant, causing high mortality at the early life stage in the natural environment, especially in areas releasing hatchery-produced juveniles. It is also worth noting that the utilization of freshly chopped sea cucumbers as decoration material and food storage is a unique and novel ecological trait of P. ferox

Effects of Early Sea-Ice Reduction on Zooplankton and Copepod Population Structure in the Northern Bering Sea During the Summers of 2017 and 2018

A remarkable early sea-ice reduction event was observed in the northern Bering Sea during 2018. In turn, this unusual hydrographic phenomenon affected several marine trophic levels, resulting in delayed phytoplankton blooms, phytoplankton community changes, and a northward shift of fish stocks. However, the response of the zooplankton community remains uncharacterized. Therefore, our study sought to investigate the zooplankton community shifts in the northern Bering Sea during the summers of 2017 and 2018 and evaluate the effects of early sea-ice melt events on the zooplankton community, population structure of large copepods, and copepod production. Five zooplankton communities were identified based on cluster analysis. Further, annual changes in the zooplankton community were identified in the Chirikov Basin. In 2017, the zooplankton community included abundant Pacific copepods transported by the Anadyr water. In 2018, however, the zooplankton community was dominated by small copepods and younger stages of large copepods (Calanus glacialis/marshallae and Metridia pacifica), which was likely caused by reproduction delays resulting from the early sea-ice reduction event. These environmental abnormalities increased copepod production; however, this higher zooplankton productivity did not efficiently reach the higher trophic levels. Taken together, our findings demonstrated that zooplankton community structure and production are highly sensitive to the environmental changes associated with early sea-ice reduction (e.g., warm temperatures and food availability)

Zooplankton community structure and dominant copepod population structure on the southern Kerguelen Plateau during summer 2016

The Tenth Symposium on Polar Science/Ordinary sessions: [OB] Polar Biology, Wed. 4 Dec. / 3F Multipurpose conference room, National Institute of Polar Researc

Spin current generation from an epitaxial tungsten dioxide WO2_{2}

We report on efficient spin current generation at room temperature in rutile type WO$_{2}$ grown on Al$_{2}$O$_{3}$(0001) substrate. The optimal WO$_{2}$ film has (010)-oriented monoclinically distorted rutile structure with metallic conductivity due to 5$\it{d}$$^2$ electrons, as characterized by x-ray diffraction, electronic transport, and x-ray photoelectron spectroscopy. By conducting harmonic Hall measurement in Ni$_{81}$Fe$_{19}$/WO$_{2}$ bilayer, we estimate two symmetries of the spin-orbit torque (SOT), i.e., dampinglike (DL) and fieldlike ones to find that the former is larger than the latter. By comparison with the Ni$_{81}$Fe$_{19}$/W control sample, the observed DL SOT efficiency $\xi$$_{DL}$ of WO$_{2}$ (+0.174) is about two thirds of that of W (-0.281) in magnitude, with a striking difference in their signs. The magnitude of the $\xi$$_{DL}$ of WO$_{2}$ exhibits comparable value to those of widely reported Pt and Ta, and Ir oxide IrO$_{2}$. The positive sign of the $\xi$$_{DL}$ of WO$_{2}$ can be explained by the preceding theoretical study based on the 4$\it{d}$ oxides. These results highlight that the epitaxial WO$_{2}$ offers a great opportunity of rutile oxides with spintronic functionalities, leading to future spin-orbit torque-controlled devices.Comment: 14 pages, 4 figure

Characteristics of the summer decapod larvae community through Bering and Chukchi Seas

第6回極域科学シンポジウム分野横断セッション：[IA] 急変する北極気候システム及びその全球的な影響の総合的解明―GRENE北極気候変動研究事業研究成果報告2015―11月19日（木）　国立極地研究所１階交流アトリウ