Atlantic‐origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific‐origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic‐origin cold saline water into the halocline boundary between Pacific and Atlantic‐origin waters in 2017 lifted Pacific‐origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer.

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; P_IIIIP₂₅, P_BIP₂₅, and P_DIP₂₅, 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. P_IIIIP₂₅-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. P_BIP₂₅-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, P_DIP₂₅ 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, P_BIP₂₅ 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.

Satellite observations and modeling data have suggested a significant increase in net primary production in the Arctic Ocean over the last decade due to retreating sea ice and the development of light availability caused by Arctic warming. Subsequently, under‐ice blooms (UIBs) are being recognized as an important phenomenon from the traditional perspective. However, the role of sea‐ice algae in UIBs is still unknown due to the limited availability of continuous observations. We analyzed data on primary producer‐derived lipid biomarkers from sinking particles collected over 1 year using time‐series sediment traps on the East Siberian Sea and Chukchi Sea slopes. Based on the seasonal changes in sympagic organic carbon derived from the data of the ice proxy (IP₂₅) flux and pelagic biomarkers, such as highly branched isoprenoid trienes, epi‐brassicasterol and dinosterol, a UIB was identified in summer 2018 on the East Siberian Sea slope. Compared to the nutrient distribution on the Chukchi Sea slope, the UIB on the East Siberian Sea slope might have been triggered by the nutrient supply. The estimated flux‐weighted mean sympagic organic carbon value measured during the UIB period (May−August) was 1.04 mg m⁻² d⁻¹ on the East Siberian Sea slope, approximately five times greater than recorded that on the Chukchi Sea slope (0.23 mg m⁻² d⁻¹) during the same period. Our findings suggest that the importance of sea‐ice algae as primary producers has increased as the UIB phenomenon has become more important in the Arctic Ocean and that sea‐ice environments face changes due to Arctic warming.

Theory predicts that sympatric predators compete for food under conditions of limited resources. Competition would occur even within the same species, between neighboring populations, because of overlapping foraging habits. Thus, neighboring populations of the same species are hypothesized to face strong competition. To test the hypothesis that intra-specific competition is more intense than inter-specific competition owing to a lack of niche partitioning, we estimated the foraging area and diving depths of two colonial seabird species at two neighboring colonies. Using GPS and time-depth recorders, we tracked foraging space use of sympatric breeding Chinstrap and Gentoo penguins at Ardley Island (AI) and Narębski Point (NP) at King George Island, Antarctica. GPS tracks showed that there was a larger overlap in the foraging areas between the two species than within each species. In dive parameters, Gentoo penguins performed deeper and longer dives than Chinstrap penguins at the same colonies. At the colony level, Gentoo penguins from NP undertook deeper and longer dives than those at AI, whereas Chinstrap penguins did not show such intra-specific differences in dives. Stable isotope analysis of δ¹³C and δ¹⁵N isotopes in blood demonstrated both inter- and intra-specific differences. Both species of penguin at AI exhibited higher δ¹³C and δ¹⁵N values than those at NP, and in both locations, Gentoo penguins had higher δ¹³C and lower δ¹⁵N values than Chinstrap penguins. Isotopic niches showed that there were lower inter-specific overlaps than intra-specific overlaps. This suggests that, despite the low intra-specific spatial overlap, diets of conspecifics from different colonies remained more similar, resulting in the higher isotopic niche overlaps. Collectively, our results support the hypothesis that intra-specific competition is higher than inter-specific competition, leading to spatial segregation of the neighboring populations of the same species.

Reconstructions of ocean primary productivity (PP) help to explain past and present biogeochemical cycles and climate changes in the oceans. We document PP variations over the last 50 kyr in a currently oligotrophic subtropical region, the Gulf of Cadiz. Data combine refined results from previous investigations on dinocyst assemblages, alkenones, and stable isotopes (¹⁸O, ¹³C) in planktonic (Globigerina bulloides) and endobenthic (Uvigerina mediterranea) foraminifera from cores MD04‐2805 CQ and MD99‐2339, with new isotopic measurements on epibenthic (Cibicides pachyderma‐Cibicidoides wuellerstorfi) foraminifera and dinocyst‐based estimates of PP using the new n = 1,968 modern database. We constrain PP variations and export production by integrating qualitative information from bioindicators with dinocyst‐based quantitative reconstructions such as PP and seasonal sea surface temperature and information about remineralization from the benthic Δδ¹³C (difference between epibenthic and endobenthic foraminiferal δ¹³C signatures). This study also includes new information on alkenone‐based SST and total organic carbon which provides insights into the relationship between past regional hydrological activity and PP regime change. We show that PP, carbon export, and remineralization were generally high in the NE subtropical Atlantic Ocean during the last glacial period and that the Last Glacial Maximum (LGM) had lower Δδ¹³C than the Heinrich Stadials with sustained high PP, likely allowing enhanced carbon sequestration. We link these PP periods to the dynamics of upwelling, active almost year‐round during sadials, but restricted to spring‐summer during interstadials and LGM, like today. During interstadials, nutrient advection through freshwater inputs during autumn‐winter needs also to be considered to fully understand PP regimes.