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We assessed the applicability of the nutrient diol index (NDI) as a proxy for sea surface nutrients by analyzing sediments from a box core (ES14-BC03) collected in the southwestern continental slope of the East Sea, or the Japan Sea (hereafter the East Sea). The estimated sedimentation rate based on the 210Pb chronology was 0.15 cm yr−1 over the last 240 yrs. The NDI-derived phosphate (0.69 μmol L−1) and nitrate (8.63 μmol L−1) concentrations for the core-top sediment were within the average phosphate (0.26 ± 0.08 μmol L−1) and nitrate (3.38 ± 2.32 μmol L−1) concentration ranges for the last ∼20 years, accounting for both estimation error and variation in observational data. The distributions of the NDI-diols varied slightly through time, grouping into two distinct clusters (Cluster 1 and Cluster 2) in both principle component analysis (PCA) and hierarchical clustering of principal components (HCPC). Cluster 2 represented a time period with higher relative abundances of the C28 and C30 1,14-diols, resulting in higher NDI-derived nutrient concentrations than those of other periods. Interestingly, during this period, the U 37 K ' -derived sea surface temperatures (SSTs) also decreased. These results indicate that higher surface nutrient conditions and colder SSTs occurred between 1884 CE and 1911 CE, which might be associated with stronger upwelling intensity at the study site. This study is the first application of the NDI to a down-core, demonstrating that the NDI can be a useful proxy that provides helpful information about past sea surface nutrient conditions.

The concentration of n-alkanes (C17–C35) and sterols in marine particulate matter were investigated to trace the origin of organic carbon in Kongsfjorden in early spring (April). The spatial distributions of environmental factors (seawater temperature, salinity, density, turbidity, chlorophyll a (chl. a) and particulate organic carbon (POC) concentrations) and the cell density of phytoplankton differed between the inner and outer fjord regions. In addition, brassicasterol, diatom biomarker, showed a high concentration in the outer fjord and positive correlations with the chl. a and POC concentrations in the water column. In contrast, some sterols originating from terrestrial organic matter (OM), such as stigmasterol and campesterol, showed relatively higher concentrations in the inner fjord than in the outer fjord. Based on the distance-based redundancy analysis (db-RDA) result, the distributions of organic compounds are predominantly controlled by the water density and the POC and chl. a concentrations, and these distributions allowed us to divide the inner and outer fjord regions. However, the hierarchical clustering of principal components (HCPC) results obtained based on principal component analysis (PCA) using lipid biomarkers (C17–C35 alkanes and sterols) and environmental factors indicated that the clusters were distinguished by surface (0 m) and subsurface (>4 m) seawater samples rather than by any regional division. Notably, the concentration of relatively short-chain alkanes (average chain length (ACL): 24.6 ± 3.7) without a carbon preference for odd numbers (carbon preference index (CPI): 0.97 ± 0.11) in the sea surface layer was significantly higher than that of subsurface seawater (ACL: 31.1 ± 0.5 and CPI: 1.06 ± 0.03) in the early spring. This suggests the potential of these compounds as indicators for tidewater glacier-derived OM and freshwater input by snow melt into the fjord system. Hence, these results demonstrate that the distributions of lipid biomarkers in the water column possibly provide important information for a comprehensive understanding of the origin and transport of OM in an Arctic fjord.

Analysis of the biochemical composition (carbohydrates, CHO, proteins, PRT, lipids, LIP) of particulate organic matter (POM, mainly phytoplankton) is used to assess trophic states, and the quantity of food material is generally assessed to determine bioavailability, however, bioavailability is reduced or changed by enzymatic hydrolysis. Here, we investigated the current trophic state and bioavailability of phytoplankton in the Chukchi Sea (including the Chukchi Borderland) during the summer of 2017. Based on a cluster analysis, our 12 stations were divided into three groups: the southern, middle, and northern parts of the Chukchi Sea. A principal component analysis (PCA) revealed that relatively nutrient-rich and high-temperature waters in the southern part of the Chukchi Sea enhanced the microphytoplankton biomass, while picophytoplankton were linked to a high contribution of meltwater derived from sea ice melting in the northern part of the sea. The total PRT accounted for 41.8% (&plusmn, 7.5%) of the POM in the southern part of the sea, and this contribution was higher than those in the middle (26.5 &plusmn, 7.5%) and northern (26.5 &plusmn, 10.6%) parts, whereas the CHO accounted for more than half of the total POM in the northern parts. As determined by enzymatic hydrolysis, LIP were more rapidly mineralized in the southern part of the Chukchi Sea, whereas CHO were largely used as source of energy for higher trophic levels in the northern part of the Chukchi Sea. Specifically, the bioavailable fraction of POM in the northern part of the Chukchi Sea was higher than it was in the other parts. The findings indicate that increasing meltwater and a low nutrient supply lead to smaller cell sizes of phytoplankton and their taxa (flagellate and green algae) with more CHO and a negative effect on the total concentration of POM. However, in terms of bioavailability (food utilization), which determines the rate at which digested food is used by consumers, potentially available food could have positive effects on ecosystem functioning.

Drastic sea ice retreats in the Antarctic Peninsula, and the consequent environmental changes have brought about the consequences of biological adaptation and food competition. The isotopic niche is reflected by the ecological position and functional role of a species, which can be altered depending on these environmental changes. We assessed the isotopic niche and trophic positions (TP) of 3 seabirds on the Barton Peninsula, King George Island, Antarctica, to understand ecological interaction among the species. The average TP of Antarctic krill (Euphausia superba) (hereafter “krill”) (2.6 ± 0.1) was estimated by using the compiled published data of the compound-specific nitrogen isotope ratio of glutamic acid and phenylalanine in the Southern Ocean, and TP values of the seabirds based on that of krill to be reliable according to ecological knowledge. Our results on the overlap of the isotopic niches of seabirds suggests potential diet competition among the consumers, such as the brown skua (Stercorarius antarcticus) and south polar skua (Stercorarius maccormicki), whereas the distinct and broad isotopic niche width of kelp gulls (Larus dominicanus) suggesting that their adaptation through generalization was related to their extended habitat and various food sources. Our research indicates that isotopic niche of seabirds reflects their survival strategy for food competition. Consequently, these seabirds can be easily influenced by alternative food sources, including terrestrial and human-derived sources, by rapid environmental changes, indicating that they are valuable as key environmental species and require long-term monitoring in the Antarctic Peninsula.

We investigated the source, composition and reactivity of sedimentary organic carbon (OC) in a river-dominated continental marginal sea, the eastern Yellow Sea. A multi-proxy approach was applied to 9 riverbank sediments and 69 marine surface sediments, combining bulk and molecular organic parameters. The riverbank sediments (n=9) have on average low C/N ratio (4.8±0.5) and enriched δ13CTOC values (–21.5±0.6%) while the BIT index is on average 0.27. The sedimentary OC in the marine surface sediments appears to have a predominantly marine origin (on average C/N ratio=7.0±0.6 and δ13CTOC=–21.9±0.5%, n=69) with minor contribution of continental (i.e. soil- and lake/river-derived) OC (on average BIT index=0.00±0.01, n=69). However, the Δ14C values were depleted (on average –227±53%, n=8). Accordingly, our results highlight that fossil OC, potentially derived from erosion of sedimentary bedrocks in the catchment areas and/or human activities is being contributed to the sedimentary OC pool in the eastern Yellow Sea. More work is needed to better constrain the source, composition, and age of the organic material supplied to the eastern Yellow Sea, given the lack of biogeochemical data from the Korean rivers.

In this study, we aimed to reconstruct spring (April–June) sea ice changes in the western Arctic Ocean over recent centuries (ca. the last 250 years) by measuring biomarker distributions in a multicore (ARA01B-03MUC) retrieved from the Chukchi Shelf region and to evaluate outcomes against known or modelled estimates of sea ice conditions. Specifically, we analyzed for the Arctic sea ice proxy IP25 and assessed the suitability of a further highly branched isoprenoid (HBI) lipid (HBI III), epi-brassicasterol, and dinosterol as complementary biomarkers for use with the so-called phytoplankton marker-IP25 index (PIP25; PIIIIP25, PBIP25, and PDIP25, respectively). The presence of IP25 throughout core ARA01B-03MUC confirms the occurrence of seasonal sea ice at the study site over recent centuries. From a semi-quantitative perspective, all three PIP25 indices gave different trends, with some dependence on the balance factor c, a term used in the calculation of the PIP25 index. PIIIIP25-derived spring sea ice concentration (SpSIC) estimates using a c value of 0.63, determined previously from analysis of Barents Sea surface sediments, were likely most reliable, since SpSIC values were high throughout the record (SpSIC > 78%), consistent with the modern context for the Chukchi Sea and the mean SpSIC record of the 41 CMIP5 climate models over recent centuries. PBIP25-based SpSIC estimates were also high (SpSIC 108%−127%), albeit somewhat over-estimated, when using a c value of 0.023 obtained from a pan-Arctic distribution of surface sediments. In contrast, PDIP25 values using a pan-Arctic c value of 0.11, and PIP25 data based on the mean biomarker concentrations from ARA01B-03MUC, largely underestimated sea ice conditions (SpSIC as low as 13%), and exhibited poor agreement with instrumental records or model outputs. On the other hand, PBIP25 values using a c factor based on mean IP25 and epi-brassicasterol concentrations exhibited a decline towards the core top, which resembled recent decreasing changes in summer sea ice conditions for the Chukchi Sea; however, further work is needed to test the broader spatial generality of this observation.