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Amid the alarming atmospheric and oceanic warming rates taking place in the Arctic, western fjords around the Svalbard archipelago are experiencing an increased frequency of warm water intrusions in recent decades, causing ecological shifts in their ecosystems. However, hardly anything is known about their potential impacts on the until recently considered stable and colder northern fjords. We analyzed macrobenthic fauna from four locations in Rijpfjorden (a high-Arctic fjord in the north of Svalbard) along its axis, sampled intermittently in the years 2003, 2007, 2010, 2013 and 2017. After a strong seafloor warm water temperature anomaly (SfWWTA) in 2006, the abundance of individuals and species richness dropped significantly across the entire fjord in 2007, together with diversity declines at the outer parts (reflected in Shannon index drops) and increases in beta diversity between inner and outer parts of the fjord. After a period of three years with stable water temperatures and higher sea-ice cover, communities recovered through recolonization processes by 2010, leading to homogenization in community composition across the fjord and less beta diversity. For the last two periods (2010-2013 and 2013-2017), beta diversity between the inner and outer parts gradually increased again, and both the inner and outer sites started to re-assemble in different directions. A few taxa began to dominate the fjord from 2010 onwards at the outer parts, translating into evenness and diversity drops. The inner basin, however, although experiencing strong shifts in abundances, was partially protected by a fjordic sill from impacts of these temperature anomalies and remained comparatively more stable regarding community diversity after the disturbance event. Our results indicate that although shifts in abundances were behind important spatio-temporal community fluctuations, beta diversity variations were also driven by the occurrence-based macrofauna data, suggesting an important role of rare taxa. This is the first multidecadal time series of soft-bottom macrobenthic communities for a high-Arctic fjord, indicating that potential periodic marine heatwaves might drive shifts in community structure, either through direct effects from thermal stress on the communities or through changes in environmental regimes led by temperature fluctuations (i.e. sea ice cover and glacial runoff, which could lead to shifts in primary production and food supply to the benthos). Although high-Arctic macrobenthic communities might be resilient to some extent, sustained warm water anomalies could lead to permanent changes in cold-water fjordic benthic systems., Competing Interests: Declaration of competing interest The authors state that they have no competing financial interests or personal relationships that could influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Pelagic-benthic coupling describes the connection between surface-water production and seafloor habitats via energy, nutrient and mass exchange. Massive ice loss and warming in the poorly studied Arctic Chukchi Borderland are hypothesized to affect this coupling. The strength of pelagic-benthic coupling was compared between 2 years varying in climate settings, 2005 and 2016, based on δ 13 C and δ 15 N stable isotopes of food-web end-members and pelagic and deep-sea benthic consumers. Considerably higher isotopic niche overlap and generally shorter isotopic distance were found between pelagic and benthic food web components in 2005 than in 2016, suggesting weaker coupling in the latter, low-ice year. δ 15 N values indicated more refractory food consumed by benthos in 2016 and fresher food reaching the seafloor in 2005. Higher δ 13 C values of zooplankton indirectly suggested a higher contribution of ice algae in 2005 than 2016. The difference in pelagic-benthic coupling between these years is consistent with higher energy retention within the pelagic system, perhaps due to strong stratification in the Amerasian Basin in the recent decade. Weaker coupling to the benthos can be expected to continue with ice loss in the study area, perhaps reducing benthic biomass and remineralization capacity; monitoring of the area is needed to confirm this prediction., (© 2023. The Author(s).)

The iconic picture of Arctic marine ecosystems shows an intense pulse of biological productivity around the spring bloom that is sustained while fresh organic matter (OM) is available, after which ecosystem activity declines to basal levels in autumn and winter. We investigated seasonality in benthic biogeochemical cycling at three stations in a high Arctic fjord that has recently lost much of its seasonal ice-cover. Unlike observations from other Arctic locations, we find little seasonality in sediment community respiration and bioturbation rates, although different sediment reworking modes varied through the year. Nutrient fluxes did vary, suggesting that, although OM was processed at similar rates, seasonality in its quality led to spring/summer peaks in inorganic nitrogen and silicate fluxes. These patterns correspond to published information on seasonality in vertical flux at the stations. Largely ice-free Kongsfjorden has a considerable detrital pool in soft sediments which sustain benthic communities over the year. Sources of this include macroalgae and terrestrial runoff. Climate change leading to less ice cover, higher light availability and expanded benthic habitat may lead to more detrital carbon in the system, dampening the quantitative importance of seasonal pulses of phytodetritus to seafloor communities in some areas of the Arctic. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

The current understanding of Arctic ecosystems is deeply rooted in the classical view of a bottom-up controlled system with strong physical forcing and seasonality in primary-production regimes. Consequently, the Arctic polar night is commonly disregarded as a time of year when biological activities are reduced to a minimum due to a reduced food supply. Here, based upon a multidisciplinary ecosystem-scale study from the polar night at 79°N, we present an entirely different view. Instead of an ecosystem that has entered a resting state, we document a system with high activity levels and biological interactions across most trophic levels. In some habitats, biological diversity and presence of juvenile stages were elevated in winter months compared to the more productive and sunlit periods. Ultimately, our results suggest a different perspective regarding ecosystem function that will be of importance for future environmental management and decision making, especially at a time when Arctic regions are experiencing accelerated environmental change [1]., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The recently completed European Census of Marine Life, conducted within the framework of the global Census of Marine Life programme (2000-2010), markedly enhanced our understanding of marine biodiversity in European Seas, its importance within ecological systems, and the implications for human use. Here we undertake a synthesis of present knowledge of biodiversity in European Seas and identify remaining challenges that prevent sustainable management of marine biodiversity in one of the most exploited continents of the globe. Our analysis demonstrates that changes in faunal standing stock with depth depends on the size of the fauna, with macrofaunal abundance only declining with increasing water depth below 1000 m, whilst there was no obvious decrease in meiofauna with increasing depth. Species richness was highly variable for both deep water macro- and meio- fauna along latitudinal and longitudinal gradients. Nematode biodiversity decreased from the Atlantic into the Mediterranean whilst latitudinal related biodiversity patterns were similar for both faunal groups investigated, suggesting that the same environmental drivers were influencing the fauna. While climate change and habitat degradation are the most frequently implicated stressors affecting biodiversity throughout European Seas, quantitative understanding, both at individual and cumulative/synergistic level, of their influences are often lacking. Full identification and quantification of species, in even a single marine habitat, remains a distant goal, as we lack integrated data-sets to quantify these. While the importance of safeguarding marine biodiversity is recognised by policy makers, the lack of advanced understanding of species diversity and of a full survey of any single habitat raises huge challenges in quantifying change, and facilitating/prioritising habitat/ecosystem protection. Our study highlights a pressing requirement for more complete biodiversity surveys to be undertaken within contrasting habitats, together with investigations in biodiversity-ecosystem functioning links and identification of separate and synergistic/cumulative human-induced impacts on biodiversity.

Climate warming can trigger abrupt ecosystem changes in the Arctic. Despite the considerable interest in characterizing and understanding the ecological impact of rapid climate warming in the Arctic, few long time series exist that allow addressing these research goals. During a 30-y period (1980-2010) of gradually increasing seawater temperature and decreasing sea ice cover in Svalbard, we document rapid and extensive structural changes in the rocky-bottom communities of two Arctic fjords. The most striking component of the benthic reorganization was an abrupt fivefold increase in macroalgal cover in 1995 in Kongsfjord and an eightfold increase in 2000 in Smeerenburgfjord. Simultaneous changes in the abundance of benthic invertebrates suggest that the macroalgae played a key structuring role in these communities. The abrupt, substantial, and persistent nature of the changes observed is indicative of a climate-driven ecological regime shift. The ecological processes thought to drive the observed regime shifts are likely to promote the borealization of these Arctic marine communities in the coming years.