First trait-based characterization of Arctic ice meiofauna taxa

Trait-based approaches connect the traits of species to ecosystem functions to estimate the functional diversity of communities and how they may respond to environmental change. For the first time, we compiled a traits matrix across 11 traits for 28 species of Arctic ice meiofauna, including Copepoda (Subclass), Nematoda (Phylum), Acoela (Order), Rotifera (Phylum), and Cnidaria (Phylum). Over 50 years of pan-Arctic literature were manually reviewed, and trait categories were assigned to enable future trait–function connections within the threatened ice-associated ecosystem. Approximately two-thirds of the traits data were found at the genus or species level, ranging from 44% for Nematoda to 100% for Cnidaria. Ice meiofauna were shown to possess advantageous adaptations to the brine channel network within sea ice, including a majority with small body widths < 200 μm, high body flexibility, and high temperature and salinity tolerance. Diets were found to be diverse outside of the algal bloom season, with most organisms transitioning to ciliate-, omnivore-, or detritus-based diets. Eight species of the studied taxa have only been recorded within sea ice, while the rest are found in a mixture of sympagic–pelagic–benthic habitats. Twelve of the ice meiofauna species have been found with all life stages present in sea ice. Body width, temperature tolerance, and salinity tolerance were identified as traits with the largest research gaps and suffered from low-resolution taxonomic data. Overall, the compiled data show the degree to which ice meiofauna are adapted to spending all or portions of their lives within the ice

What we do in the dark: Prevalence of omnivorous feeding activity in Arctic zooplankton during polar night

During the productive polar day, zooplankton and sea-ice amphipods fulfill a critical role in energy transfer from primary producers to higher trophic-level species in Arctic marine ecosystems. Recent polar night studies on zooplankton and sea-ice amphipods suggest higher levels of biological activity than previously assumed. However, it is unknown if these invertebrates maintain polar night activity on stored lipids, opportunistic feeding, or a combination of both. To assess how zooplankton (copepods, amphipods, and krill) and sea-ice amphipods support themselves on seasonally varying resources, we studied their lipid classes, fatty acid compositions, and compound-specific stable isotopes of trophic biomarker fatty acids during polar day (June/July) and polar night (January). Lipid storage and fatty acid results confirm previously described dietary sources in all species during polar day. We found evidence of polar night feeding in all species, including shifts from herbivory to omnivory. Sympagic-, pelagic-, and Calanus spp.-derived carbon sources supported zooplankton and sea-ice amphipods in both seasons. We provide a first indication of polar night feeding of sea-ice amphipods in the pelagic realm

Light and freshwater discharge drive the biogeochemistry and microbial ecology in a sub-Arctic fjord over the Polar night

The polar night has recently received increased attention as a surprisingly active biological season. Yet, polar night microbial ecology is a vastly understudied field. To identify the physical and biogeochemical parameters driving microbial activity over the dark season, we studied a sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on the impact of mixing, terrestrial organic matter input and light on microbial ecosystem dynamics. Our study highlights strong differences in the key drivers between spring, autumn, and winter. The spring bloom started in March in a fully mixed water column, opposing the traditional critical depth hypothesis. Incident solar radiation was the key driver maximum Chlorophyll was reached in April. The onset of the autumn phytoplankton bloom was controlled by vertical mixing, causing nutrient upwelling and dilution of zooplankton grazers, which had their highest biomass during this time. According to the dilution-recoupling hypothesis grazer dilution reduced grazing stress and allowed the fall bloom formation. Mixing at that time was initiated by strong winds and reduced stratification as a consequence of freezing temperatures and lower freshwater runoff. During the light-limited polar night, the primary production was extremely low but bacteria continued growing on decaying algae, their exudates and also allochthonous organic matter. A melting event in January could have increased input of organic matter from land, supporting a mid-winter bacterial bloom. In conclusion, polar night biogeochemistry and microbial ecology was not only driven by light availability, but strongly affected by variability in reshwater discharge and allochthonous carbon input. With climate change freshwater discharge will increase in the Arctic, which will likely increase importance of the dynamics described in this study

Light and freshwater discharge drive the biogeochemistry and microbial ecology in a sub-Arctic fjord over the Polar night

The polar night has recently received increased attention as a surprisingly active biological season. Yet, polar night microbial ecology is a vastly understudied field. To identify the physical and biogeochemical parameters driving microbial activity over the dark season, we studied a sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on the impact of mixing, terrestrial organic matter input and light on microbial ecosystem dynamics. Our study highlights strong differences in the key drivers between spring, autumn, and winter. The spring bloom started in March in a fully mixed water column, opposing the traditional critical depth hypothesis. Incident solar radiation was the key driver maximum Chlorophyll was reached in April. The onset of the autumn phytoplankton bloom was controlled by vertical mixing, causing nutrient upwelling and dilution of zooplankton grazers, which had their highest biomass during this time. According to the dilution-recoupling hypothesis grazer dilution reduced grazing stress and allowed the fall bloom formation. Mixing at that time was initiated by strong winds and reduced stratification as a consequence of freezing temperatures and lower freshwater runoff. During the light-limited polar night, the primary production was extremely low but bacteria continued growing on decaying algae, their exudates and also allochthonous organic matter. A melting event in January could have increased input of organic matter from land, supporting a mid-winter bacterial bloom. In conclusion, polar night biogeochemistry and microbial ecology was not only driven by light availability, but strongly affected by variability in reshwater discharge and allochthonous carbon input. With climate change freshwater discharge will increase in the Arctic, which will likely increase importance of the dynamics described in this study

Overview of the MOSAiC expedition: Physical oceanography

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.publishedVersio

Photophysiological responses of bottom sea-ice algae to fjord dynamics and rapid freshening

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Earlier sea-ice melt extends the oligotrophic summer period in the Barents Sea with low algal biomass and associated low vertical flux

The decrease in Arctic sea-ice extent and thickness as a result of global warming will impact the timing, duration, magnitude and composition of phytoplankton production with cascading effects on Arctic marine food-webs and biogeochemical cycles. Here, we elucidate the environmental drivers shaping the composition, abundance, biomass, trophic state and vertical flux of protists (unicellular eukaryotes), including phytoplankton, in the Barents Sea in late August 2018 and 2019. The two years were characterized by contrasting sea-ice conditions. In August 2018, the sea-ice edge had retreated well beyond the shelf break into the Nansen Basin (>82°N), while in 2019, extensive areas of the northwestern Barents Sea shelf (>79°N) were still ice-covered. These contrasting sea-ice conditions resulted in marked interannual differences in the pelagic protist community structure in this area. In August 2018, the protist community was in a post-bloom stage of seasonal succession characterized by oligotrophic surface waters and dominance of small-sized phytoplankton and heterotrophic protists (predominantly flagellates and ciliates) at most stations. In 2019, a higher contribution of autotrophs and large-celled phytoplankton, particularly diatoms, to total protist biomass compared to 2018 was reflected in higher chlorophyll a concentrations and suggested that the protist community was still in a late bloom stage at some stations. It is noteworthy that particularly diatoms contributed a considerably higher proportion to the protist biomass at the ice-covered stations in both years compared to the open-water stations. This pattern was also evident in the higher vertical protist biomass flux in 2019, dominated by dinoflagellates and diatoms, compared to 2018. Our results suggest that the predicted transition toward an ice-free Barents Sea will lengthen the oligotrophic summer period with low algal biomass and associated low vertical flux.publishedVersio

Still Arctic? — The changing Barents Sea

The Barents Sea is one of the Polar regions where current climate and ecosystem change is most pronounced. Here we review the current state of knowledge of the physical, chemical and biological systems in the Barents Sea. Physical conditions in this area are characterized by large seasonal contrasts between partial sea-ice cover in winter and spring versus predominantly open water in summer and autumn. Observations over recent decades show that surface air and ocean temperatures have increased, sea-ice extent has decreased, ocean stratification has weakened, and water chemistry and ecosystem components have changed, the latter in a direction often described as “Atlantification” or “borealisation,” with a less “Arctic” appearance. Temporal and spatial changes in the Barents Sea have a wider relevance, both in the context of large-scale climatic (air, water mass and sea-ice) transport processes and in comparison to other Arctic regions. These observed changes also have socioeconomic consequences, including for fisheries and other human activities. While several of the ongoing changes are monitored and quantified, observation and knowledge gaps remain, especially for winter months when field observations and sample collections are still sparse. Knowledge of the interplay of physical and biogeochemical drivers and ecosystem responses, including complex feedback processes, needs further development.Still Arctic? — The changing Barents SeapublishedVersio

Temporal and spatial variability of sympagic metazoans in a high-Arctic fjord, Svalbard

Svalbard is one of the fastest warming regions in the Arctic including massive loss in fjord sea ice both in terms of area coverage, ice thickness and duration. Sea ice is a habitat for a wide variety of microscopic flora and fauna, and we know little about the impact of accelerated loss of sea ice on this unique sea ice community. Here, we present the first study on the seasonal progression and spatial distribution of the sympagic meiofauna community, in a Svalbard fjord. Further, the meiofauna community in sea ice versus the water column below were compared to investigate the link between the two habitats. In total, we found 12 taxa associated with the sea ice and 15 taxa in the water column below with 11 taxa occurring in both habitats. However, a Canonical-analysis (CA) showed that despite similarities in taxa the two mediums were distinctly different (potentially) due to the low abundance of ice nematodes and polychaete juveniles, in pelagic samples. Temporally, ice meiofauna abundances ranged from 9.7 to 25.3 x 103 ind m-2 from beginning of March to end of April, following the seasonal build-up of ice algal biomass from 0.02 to 15.99 mg Chl a m-2 during the same time span. For the transect stations, the lowest ice meiofauna abundance was recorded at the outermost station (VMF2) with 1.6 x 103 ind m-2 and the highest abundance at the mid-station MS with 25.3 x 103 ind m-2. Our results indicate that fjord ice harbors most ice algae and sympagic meiofauna in its lower 10-cm with highest values in the lowermost 2-cm, at the sea ice water interface. Sympagic meiofauna communities were mostly dominated by nematodes or polychaete juveniles. We observed the phenology of ice nematodes through the maturation of females and hatching of juveniles from eggs. Polychaete larvae developed (quickly) into juveniles and grew morphological features indicative of readiness for settlement. Thus, we propose, that as with other parts of the Arctic, sea ice in Svalbard fjords plays an important role in the life cycle of ice nematodes and for accelerating the growth of polychaete larvae. Loss of coastal sea ice may therefore negatively impact coastal biodiversity and affect recruitment for some benthic infauna in Svalbard