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Polar research is an interdisciplinary and multi-faceted field of research ranging from history to geology and geophysics to social sciences and education. Thus, several different universities and institutions within Germany participate in polar research. The seminar series POLARSTUNDE, organized by the German Society for Polar Research (Deutsche Gesellschaft für Polarforschung) and the German National Committee of the Association of Polar Early Career Scientists (APECS Germany) regularly features different topics of German polar research. Although initially a "pandemic solution", the seminar series has established itself as a valuable and highly successful part of the German polar research landscape. The seminar series was held in German and was aimed at both scientists and the general public. This article addresses the first season of POLARSTUNDE and provides (1) comprehensive summaries of the talks and (2) insight into the planning and execution from an organizational point of view.

Petrography of recrystallised ikaite from Ocean Drilling Program material has been presented previously from Nankai Trough and Congo (ex-Zaire) deep-sea fan. This paper expands on the Nankai Trough ikaite observations, drawing on evidence from Laptev Sea, South Georgia, Okhotsk Sea, and coastal lagoon Point Barrow. However, even though many ikaite and glendonite sites occur at high latitudes, it cannot be that ikaite forms exclusively in polar environments, as demonstrated by the occurrences in the low latitude low temperature deep sea sediments offshore Gulf of Guinea (Angola Congo) and mid-latitude deep-sea trenches offshore Japan. Recrystallised ikaite occurs as mm large, zoned calcite crystals in all samples, along with secondary phases of calcite. Our data set is unique in that the origin, storage, and recrystallisation process of natural formed ikaite is recorded in detail and confirms that glendonite petrographic characteristics are a consequence of the structure and chemistry of recrystallising ikaite and not the physical or geochemical environment. The transformation of man-made ikaite to calcite as recorded in laboratory studies, is a process very similar to the one we have observed for natural ikaite. Most significant is that there is variation in the order of the calcite types within a single sample, leading to the conclusion that the variation is a consequence of impurities and geochemical variability in the ikaite, not the external environment. Morphological observations reveal similarities in ikaite and glendonite, this and the similarity in internal textures in glendonite and recrystallised ikaite confirms that glendonite may be used as an indicator of past presence of ikaite.

The Arctic is warming two times faster than the global mean, and a phenomenon known as the ‘Atlantification of the Arctic’ via the Fram Strait is having growing influences on both biological and physical processes in the region. Changes to community composition and function are already underway and these environmental changes will continue to rapidly alter Arctic ecosystems. Greater understanding of how this impacting local marine biodiversity is crucial for formulating accurate predictions of future Arctic marine ecosystems and management decisions. Gelatinous zooplankton (GZP) is a highly diverse group of taxa, including cnidarians, ctenophores and tunicates. Very little is currently known about GZP ecology, particularly in the Arctic Ocean, and even less about how they are being impacted by climate-related changes. Not only are they often actively left out of zooplankton surveys, but GZP are notoriously difficult to catch in good condition because their fragile bodies are easily destroyed by traditional sampling methods. These challenges contribute to GZP taxa regularly being underestimated in biodiversity, distribution and abundance, which has led to a lack of reliable and comprehensive baseline data available, especially in the Arctic. The aim of this study is to apply non-invasive environmental DNA (eDNA) metabarcoding methods to investigate GZP biodiversity and distribution across the Fram Strait. COI mitochondrial and 18s rRNA amplicons from water and sediment samples will be sequenced with Next Generation Sequencing (NGS) and validated with depth-stratified net-catch data. Net-caught specimens will be barcoded where possible, to complement existing reference databases. Oceanographic data will then be incorporated in order to form a holistic baseline dataset of GZP biodiversity across the region. Such data would be a valuable contribution to future research into the deviations in Arctic GZP biodiversity, community composition and distribution over time, as a

Gelatinous zooplankton (GZP) comprising ctenophores, cnidarians and tunicates, gained more interest in recent years. During favorable conditions GZP is known to rapidly increase in biomass and several dominant species are able to exploit the zooplankton standing stock. It is assumed that GZP will increase in biomass with the ongoing warming and Atlantification of the Arctic Ocean. Hence, it is crucial to gain insights into the role of gelatinous zooplankton in the current and future Arctic marine food web . This study targets both the benthic and pelagic components of the Arctic marine food web . First, we aim to determine the role of so-called “jellyfalls”, or jelly carcasses on the seafloor, as a food source for the benthic community. To do so, we investigate the stomach content of benthic scavenging amphipods in Kongsfjorden sampled during Polar night with DNA metabarcoding. By doing so,. Second, we look at the role of jellyfish as predators in the pelagic system. We focus on a widely distributed boreal-Arctic hydrozoan species, Aglantha digitale, which is also assumed to be a climate-change winner in an “Atlantified” Arctic. Its feeding ecology is scarcely studied, in particular in the Arctic region? This jellyfish is believed to feed on a wide range of zooplankton including copepods. Due to its high abundances in the Arctic regions, both in summer and in winter, it is crucial to investigate spatio-temporal patterns in its diet. Hence, we use DNA metabarcoding of Aglantha’s stomach content to reveal its prey composition in different seasons and regions in order to better predict its future role in an ice-free and Atlantified Arctic.

Rapid warming in the Arctic is drastically impacting marine ecosystems, affecting pelagic communities and food web structure. Themisto amphipods are dominant in the Arctic zooplankton community and represent a key link between secondary producers and higher trophic levels. In the Arctic seas, two coexisting species are found in high biomass: Themisto libellula, considered a true Arctic species and Themisto abyssorum, a sub-Arctic, boreal species. Many aspects of the ecology and genetic structure of these two species are not well studied, despite their importance in the food web. First, we tested both species for levels of genetic diversity and then assessed their diet spectrum with molecular methods, with a regional focus on the Greenland shelf, Fram Strait and Svalbard. Spatial genetic structure was evaluated using the mitochondrial cytochrome c oxidase subunit 1 gene (COI). Our results revealed strikingly different levels of genetic diversity: low levels in T. libellula contrasted with higher diversity in T. abyssorum. No spatial genetic structure was found, and both species exhibited high levels of connectivity and evidence of historic demographic expansion. The observed low genetic diversity, in combination with cold adaptations, could cause T. libellula to be more susceptible to the warming Arctic. In contrast, high diversity likely increases adaptive potential in T. abyssorum. In order to comprehensively characterize the prey spectrum of both Themisto species, we also applied DNA metabarcoding, also using COI, on gut contents. Both species showed a regional variation in prey spectrum. T. abyssorum’s diet showed a clear dominance of reads identified as chaetognath species, whereas T. libellula had a broader prey spectrum, including ice-associated taxa such as polar cod. Calanoid copepods did not appear as important as prey as assumed from previous (morphological) studies. Several previously overlooked jellyfish taxa were found in the stomachs of both species. T

The Southern Ocean, in particular the southwest Atlantic sector, is experiencing rapid environmental changes. A long-term trend of density changes of key pelagic species has been noted over the last decades: Antarctic krill populations are declining whilst salps are on the rise and shifting their distribution poleward. A similar poleward expansion is anticipated for a third key player, the hyperiid amphipod crustacean Themisto gaudichaudii, leading to an increasing overlap of the distributions of these three species. Due to major knowledge gaps in the ecology, and genetic connectivity of T. gaudichaudii, the likelihood of this shift and its consequences for the pelagic food web structure remain largely unexplored. In this context, Themisto’s genetic and trophic connectivity as well as thermal response were investigated with state-of-the-art molecular methods. Phylogeographic analyses showed genetic homogeneity between localities in the Southern Ocean and Atlantic waters combined with high degree of phenotypic plasticity enabling different lineages to thrive in regions further south. Diet analyses using DNA metabarcoding were applied to characterize regional variation in diet. These analyses showed a diet predominantly composed of krill, in particular in the Antarctic Peninsula region, showing that Themisto’s poleward range expansion can further impact the already declining krill stocks. It also unexpectedly revealed ctenophores to be an important prey, despite their reputation as “trophic dead-end”. Transcriptome analyses were used to study the thermal response of Themisto individuals from different geographic populations that were experimentally exposed to heat and cold treatments. The analysis of differentially expressed genes showed that genetic lineages differ in thermal tolerances. It also revealed a wide range of molecular mechanisms in Themisto amphipods to cope with thermal stress. These findings contribute to better predict the impact of climate-driven rang

Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures. Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation a

The Laptev Sea and the East Siberian Sea are remote areas of the Arctic region where detailed data on phytoplankton composition and spatial distribution remain limited. In the context of the ongoing environmental changes (increasing warming and ice melting) and prospective exploration activities (oil and gas production) on the Arctic shelves, understanding of the seasonal and interannual phytoplankton community dynamics is of critical importance. Our study provides new specifying data on species composition of phytoplankton over the vast area of the Laptev Sea shelf and the East Siberian Sea shelf. We found that the outer shelf of the Laptev and East Siberian seas was characterized by typical late spring diatom species (Chaetoceros furcellatus, Chaetoceros diadema, Chaetoceros debilis, Chaetoceros constrictus). On the inner shelf of the Laptev Sea, which is strongly affected by the Lena River water masses, the phytoplankton were characterized by the transition from the summer to an autumn stage of development. Local algal assemblages were composed by mixo- and heterotrophic dinoflagellates (Dinophysis and Protoperidinium genera) together with marine and brackish water-marine diatoms (Thalassiosira hyperborea, Thalassiosira baltica, Thalassiosira gravida, Thalassiosira nordenskioeldii) accompanied by sporadically occurring freshwater riverine planktonic diatom species (Aulacoseira granulata, Aulacoseira italica, Asterionella formosa). These variations in species composition over the Laptev Sea shelf were attributed to differences in the hydrography, marine chemical conditions, and the sea-ice regime.

The lifetime and thickness of Arctic sea ice have markedly decreased in the recent past. This affects Arctic marine ecosystems and the biological pump, given that sea ice acts as platform and transport medium of marine and atmospheric nutrients. At the same time sea ice reduces light penetration to the Arctic Ocean and restricts ocean/atmosphere exchange. In order to understand the ongoing changes and their implications, reconstructions of source regions and drift trajectories of Arctic sea ice are imperative. Automated ice tracking approaches based on satellite-derived sea-ice motion products (e.g. ICETrack) currently perform well in dense ice fields, but provide limited information at the ice edge or in poorly ice-covered areas. Radiogenic neodymium (Nd) isotopes (εNd) have the potential to serve as a chemical tracer of sea-ice provenance and thus may provide information beyond what can be expected from satellite-based assessments. This potential results from pronounced εNd differences between the distinct marine and riverine sources, which feed the surface waters of the different sea-ice formation regions. We present the first dissolved (< 0.45 µm) Nd isotope and concentration data obtained from optically clean Arctic first- and multi-year sea ice (ice cores) collected from different ice floes across the Fram Strait during the RV POLARSTERN cruise PS85 in 2014. Our data confirm the preservation of the seawater εNdsignatures in sea ice despite low Nd concentrations (on average ~ 6 pmol/kg) resulting from efficient brine rejection. The large range in εNd signatures (~ -10 to -30) mirrors that of surface waters in various parts of the Arctic Ocean, indicating that differences between ice floes but also between various sections in an individual ice core reflect the origin and evolution of the sea ice over time. Most ice cores have εNd signatures of around -10, suggesting that the sea ice was formed in well-mixed waters in the central Arctic Ocean and transported dire

Arctic summer and winter sea-ice extent is continuously declining as a result of climate change, affecting the hydrography and biogeochemical cycles on the seasonally ice-free Eurasian Shelves. The prolongation of the open-water season causes higher sediment resuspension and coastal erosion due to larger wind fetch and wave heights. This impacts the optical properties of the water column and hence biological productivity in this region. During “Transarktika-2019” leg 1 in late winter 2019, a comprehensive dataset of and optical data was collected throughout the central and northern Barents Sea. Combining suspended particulate matter concentrations obtained from water samples and optical data revealed a pronounced bottom nepheloid layer on the Barents Sea shelf even under ice-covered conditions. Moreover, the data indicate that the Franz Viktoria Trough could be a major pathway for sediment transport into the Eurasian Basin. Therefore, to link changes in sediment distribution and its impact on the ecosystem under a warming climate, further studies of sediment dynamics are required, particularly during winter.

A coordinated effort involving trailblazing science — and icebreaking ships — from many nations is needed to fill gaps in our understanding of the Arctic Ocean and how it’s changing.