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A multidisciplinary survey was carried out in the Pacific Arctic and sub-Arctic regions of the North Pacific Ocean on the Korean icebreaking research vessel Araon. During this survey, ichthyoplankton fishes in the Pacific Arctic and sub-Arctic region ranged from the Bering Sea to the northern Chukchi Shelf in summer. The most dominant species was Gadus chalcogrammus, followed by Pleuronectes quadrituberculatus and Boreogadus saida. Gadus chalcogrammus and P. quadrituberculatus were particularly abundant near the Bering Sea and Bering Strait, whereas B. saida was dominant in the Chukchi Sea. Hierarchical cluster analysis revealed four distinct ichthyoplankton communities in Pacific Arctic and sub-Arctic regions based on geographical regions. However, Eleginus gracilis, which was previously known to be seen between latitudes 66.5°N and 69.5°N, was found above 70°N, suggesting that its distribution extends further north. Furthermore, we noticed that Benthosema glaciale, which is usually found in the Atlantic sector of Arctic Ocean, was observed in the northern Chukchi Sea. In addition to these unusual species distributions, several species that are mainly observed in coastal areas are observed in the Chukchi Sea region. The observed influx of various uncommon fish species into the Chukchi Sea can be attributed to multiple factors, including freshwater inflow from the East Siberian Sea and the intrusion of warm Atlantic and Pacific waters, which are strongly affected by global warming. Consequently, it is imperative to conduct rigorous monitoring of the Pacific Arctic region, with a particular focus on the Chukchi Sea, to better understand the implications of global warming.

In the Arctic Ocean, the northern Chukchi and East Siberian Seas (NCESS) are vulnerable to climate change due to warming, sea ice melting, and surface freshening. To investigate how local physical forcing affects phytoplankton biomass and communities in this area, the multi-year (2015-2020) distributions of summer (August) chlorophyll-a concentrations and size structures using data collected by the Korea-Arctic Ocean Observing System (K-AOOS) program were investigated. The environmental characteristics and phytoplankton communities in the study area showed east-west regional differences. It is characterized by warm fresh waters and strong stratification to the east and cold saline waters and relatively high nutrient and sea ice concentrations to the west. Despite the differences between the east and west regions, patterns of the inter-annual variation in phytoplankton biomass and communities were similar across the entire study area, implying environmental controllers regulating phytoplankton in the NCESS. Inter-annually, higher sea surface salinity and weaker water column stratification were observed in 2017-2019 than in 2015 and 2020, implying the possibility of a potential modulation by the Arctic Oscillation. The shallower nitracline depth and higher surface nutrient concentrations since 2017 compared to the period from 2015-2016, indicate improved nutrient availability due to Atlantic water intrusion. However, average insolation has been relatively low since 2017, with the exception of August 2018. August mean phytoplankton biomass was highest in 2018 in the study area (average 83.7 mgm-2) and was dominated by large-sized phytoplankton. The low phytoplankton biomass in 2017 (23.9 mgm-2) and 2019 (62.4 mgm-2), despite similar characteristics of nutrient concentration to 2018, is likely due to lower average daily insolation in both years (206 μEm-2d-1 and 184 μEm-2d-1 in 2017 and 2019, respectively) compared to the 2018 average (271 μEm-2d-1). These results suggest that increased intrusion through upwelling of Atlantic water, along with nutrient loading, could be a pivotal driving factor contributing to this enhanced production in the NCESS, particularly under conditions where light levels are sufficient for phytoplankton growth and bloom development.

Increase in river discharge and seasonal primary production and decline in sea ice coverage in the Arctic Ocean in summer can significantly affect the distribution and composition of dissolved organic matter (DOM). This study aimed to enhance the current available knowledge about the impacts of environmental changes on the characteristics of DOM in the rapidly changing Arctic Ocean. Seawater samples were collected from the western Arctic Ocean during the summer of 2019 and analyzed for fluorescent DOM (FDOM), dissolved organic carbon (DOC), and stable oxygen isotope (δ18O) content in conjunction with biophysical properties. We identified two humic-like (C1 and C2) and one protein-like (C3) components using fluorescence excitation-emission matrix coupled with parallel factor (EEM–PARAFAC) analysis. Remarkably high intensities of humic-like FDOM were found in the upper halocline layer (32 < salinity < 33.5 psu, at depths between 50–200 m) with high inorganic nutrient concentrations and low N* values, indicating that the humic-like FDOM was supplied from the shelf sediment. Furthermore, shoaling of the upper halocline layer brought high levels of humic-like FDOM to the euphotic zone, resulting in an increased probability of photodegradation of humic-like FDOM due to exposure to solar radiation in the surface layer. Tryptophan-like FDOM was positively correlated with river water fraction (friver) and riverine DOC but not with chlorophyll-a (Chl-a) and heterotrophic bacterial abundance, indicating river discharge as a potential additional source of tryptophan-like FDOM. The correlation coefficients between tryptophan-like FDOM and river water parameters (friver and riverine DOC) differed across the Chukchi Sea, Chukchi Borderland, and East Siberian Sea, implying that the influence of river discharge on tryptophan-like FDOM is region-dependent. An increase in river discharge in future might lead to a greater supply of tryptophan-like FDOM, impacting the dynamics of DOM cycling in the western Arctic Ocean.

The Arctic Ocean environment is drastically changing because of global warming. Although warming-induced processes, such as the decrease in sea-ice extent and freshening of the surface layer, have the potential to alter primary production, the changes that will likely occur in their production and their mechanisms are still poorly understood. To assess the potential changes in net community production, which is a measure of biological carbon pump, in response to climate change, we observed the O2/Ar at the surface of the northern Chukchi Sea in the summers of 2017 and 2020. The net community production (NCP) estimates that we derived from O2/Ar measurements were largely in the range of 1 – 11 mmol O2 m-2 d-1 in the northern Chukchi and Beaufort Seas, close to the lower bounds of the values in the global oceans. The average NCP of 1.5 ± 1.7 mmol O2 m-2 d-1 in 2020 was substantially lower than 7.1 ± 7.4 mmol O2 m-2 d-1 in 2017, with the most pronounced decrease occurring in the ice-free region of the northern Chukchi Sea; the NCP of the ice-free region in 2020 was only 12% of that in 2017. The decrease in NCP in 2020 was accompanied by a lower salinity of > 2, which resulted in shallower mixed layer depths and stronger stratification. We speculated that the anomalously low pressure near the east Russian coast and the lack of strong winds contributed to the strong stratification in 2020. With a continuing decrease in the extent of sea ice, the northern Chukchi Sea will likely experience earlier phytoplankton blooms and nitrate exhaustion. Unless winds blow strong enough to break the stratification, the biological pump in late summer is likely to remain weak.

Vertical distributions of phytoplankton in the Arctic Ocean are characterized by a very narrow subsurface chlorophyll maximum (SCM) layer formed every summer after the sea ice retreats. Despite the prevalence of this narrow SCM layer, phytoplankton photosynthetic response to climate change remains to be elucidated. Here, we examined the photophysiological properties of phytoplankton in the SCM layer in the northern Chukchi Sea during the summers of 2015–2018. There was a significant difference in the SCM depth between the northwestern and northeastern Chukchi Sea determined by the distribution of Pacific Summer Water (PSW) around the SCM layer (34 ± 14 m vs. 49 ± 10 m, respectively). The maximum quantum yield of photochemistry in photosystem II (Fv/Fm) in the SCM phytoplankton was high (Fv/Fm ≥ 0.54) and similar in both regions until 2016; however, since then, Fv/Fm in the northeastern Chukchi Sea has decreased by approximately 10%. This decrease was accompanied by a marked decrease in the fraction of microplankton, which are known to be susceptible to nutrient limitation. This result suggests a reduction in nitrogen availability in the SCM layer in the northeastern Chukchi Sea. Meanwhile, the maximum electron transfer rate (ETRmax) did not have a significant relationship with the nitrogen availability and phytoplankton community size structure in the SCM layer; however the improved light conditions (with an approximately two-fold increase in the relative ratio of surface PAR reaching the SCM layer) increased ETRmax by up to 30% in the SCM phytoplankton in the northwestern Chukchi Sea. Therefore, these results provide a better understanding of how changes in nitrogen and light availability could affect phytoplankton photosynthesis and primary production in the Arctic Ocean.

The Nordic Seas have been widely implicated by deep water formation as a part of Atlantic Meridional Overturning Circulation. This study explores the spatiotemporal variations in the near-surface salinity over the Nordic Seas associated with surface freshening factors by using monthly TOPAZ4 reanalysis data from 1991 to 2019. We first show that reliability of TOPAZ4 data compared to the salinity products of other reanalysis data, satellite data, and in-situ measurements in the Nordic Seas. The salinity variability was larger in the Greenland Sea (GS) than in the Norwegian Sea (NS) on both time scales of seasonal and interannual. The seasonal change of GS salinity was coincident with the seasonality of sea ice extent. The longer-time variations are decomposed by empirical orthogonal function (EOF) analysis. The GS salinity is mainly affected by current advection (29%) and sea ice extent (11%). The interannual response of salinity to the sea ice extent over the GS differs by season. NS salinity variability responds to the strength of the Subpolar Gyre associated with a large-scale atmospheric system that caused the freshening event in the mid-1990s. The propagation of the northward Atlantic Water core is observed over the period of about 3 years from the Faroe Shetland Channel to the Fram Strait at a speed of 2.6-6.5° year-1. Other freshening factors such as sea ice export from the Arctic, freshwater flux at the Fram Strait, and net precipitation are also discussed. For the past three decades, the continuous trend appeared only in the sea ice extent, which might be a signal of climate changes over high latitude. However, there was no significant trend other than the periodic change in a few years to the decadal time scale in the salinity of GS and NS. As preconditioning for deep convection, near-surface salinity within Greenland Sea Gyre was influenced by salinity fluctuation in both GS and NS.

Limited information is available regarding the phytoplankton communities in the Arctic Ocean, especially in the Chukchi Sea. We conducted research cruises in the Arctic Ocean in the late summers of 2015, 2016, 2018, and 2019, and used 18S ribosomal DNA metabarcoding to examine the dynamic changes in core phytoplankton. Environmental factors were divided into three Groups: “surface layers in low latitude”, “subsurface chlorophyll maximum layers in low latitude”, and “high latitude.” The phytoplankton community was divided into two Groups. One Group, “the phytoplankton community in lower latitudes affected by the Pacific Influx “, comprised Dinophyta (56.33%), Bacillariophyta (22.28%), and Chlorophyta (20.77%), while the other Group, “the phytoplankton community in higher latitudes affected by the Arctic Sea “, comprised Dinophyta (57.51%), Bacillariophyta (10.71%), and Chlorophyta (27.57%). The common phytoplankton taxa in Group of lower latitudes included 33 operational taxonomic units (OTUs) (99.04%), while the other Group included nine OTUs (98.80%). In these groups, OTUs #005 (Heterocapsa rotundata), #001 (Micromonas pusilla), and #003 (Chaetoceros gelidus) were core OTUs (>5%). These core OTUs were significantly different between the two Groups; OTU #001 and #003 were distributed at high relative abundance and rapidly increased at higher latitudes. OTU #005 showed a relatively high abundance at lower latitudes. Analysis of the indicator species revealed that 34 OTUs in lower latitudes and only one OTU (#003) in higher latitudes were observed to have a strong value of >0.8. Our results indicate that core phytoplankton taxa and community structures by latitudinal differences are highly dependent on different water masses and show their dynamic ecological responses to extreme environmental conditions.

Time-series sediment traps were deployed on the Chukchi Sea and East Siberian Sea slopes from August 2017 to August 2018 with the aim of elucidating the temporal and spatial variations in particle fluxes and identifying the main processes affecting those variations. Particle fluxes showed a typical seasonal pattern, with high values in summer and low values in other seasons, and a large inter-annual variation was observed only on the East Siberian Sea slope, where particle fluxes were one order of magnitude higher in early August 2018 than in late August 2017. This large inter-annual variation in particle flux resulted from the episodic intrusion of nutrient-enriched shelf water in the East Siberian Sea, which enhanced biological production at the surface and particle fluxes. The Chukchi Sea slope was influenced by the inflow of Anadyr Water, with high salinity and high nutrient concentrations, which had little annual variability. Therefore, particle flux showed little inter-annual variation on the Chukchi Sea slope. Under-ice phytoplankton blooms were observed in both the Chukchi Sea and East Siberian Sea slopes, and increases in particulate organic carbon (POC) flux and the C:N ratio under the sea ice were related to transparent exopolymer (TEP) production by ice algae. On the East Siberian Sea slope, particle fluxes increased slightly from 115 to 335 m, indicating lateral transport of suspended particulate matter; POC and lithogenic particles may be laterally transported to the slope as nutrient-rich shelf waters flowed from the East Siberian Sea to the Makarov Basin. Annual POC fluxes were 2.3 and 2.0 g C m–2 year–1 at 115 and 335 m, respectively, on the East Siberian Sea slope and was 2.1 g C m–2 year–1 at 325 m on the Chukchi Sea slope. Annual POC fluxes were higher on the Chukchi Sea and East Siberian Sea slopes than in Arctic basins, lower than on Arctic shelves, and generally similar to those on western Arctic slopes.

The Bering Strait is the only gateway to the Chukchi Sea from the Pacific Ocean and is a major route of Pacific water inflow. We recently investigated the occurrence of Pacific copepod species along with the warming of the Chukchi Sea and sought to identify the cause of the mass occurrence these copepods through an analysis of the water masses flowing through the Bering Strait. Zooplankton and Conductivity-Temperature-Depth (CTD) data collection was conducted in the Chukchi Sea and Bering Strait from 2014 to 2016. In addition, mooring systems installed in the Bering Strait were analyzed to obtain water temperature and salinity data during summer to understand the properties of the water masses. In 2015, a high abundance of Pacific copepod species (Eucalanus bungii, Metridia pacifica, and Neocalanus spp.) was observed in Bering Summer Water (BSW), which was relatively warm compared to measurements obtained from 2014 to 2016. As further confirmation, our results were consistent with 2007, 2009, and 2012 data, which showed that the abundance of Pacific copepod species was proportional to the temperature of the BSW entering the Chukchi Sea. In conclusion, we reconfirmed that Pacific copepod species are entering the Chukchi Sea along with BSW, and we newly discovered that their high abundance coincided with the relatively warm BSW, instead of other water masses. These findings suggest that the inflow of the high-temperature BSW (>3°C) plays an important role in the mass occurrence of Pacific copepod species in the southern Chukchi Sea.