We examined temporal variation in nudibranch assemblages within intertidal mudflats (INT) and fouling communities (FC) on floating jetties, over a 56 mo (August 2020-March 2025) period in northern Norway. Nudibranchs were identified and counted approximately monthly at 2 INT and 4 FC sites along shallow transects, recording a total of 47 species. At INT, 28 species were recorded, with a mean monthly species richness ranging from 2.2 to 5.2 species, while FC had 42 species (6.0-12.2 species). Species richness was highest in autumn-winter in both habitats. Overall nudibranch abundance peaked in autumn at both habitats, largely due to mass occurrences of spawning ephemeral species. Multivariate analyses revealed a clear significant difference in assemblage structure between habitats, with season being a key factor in variability, particularly at FC sites. Species abundance patterns fell broadly into 2 main phenology groups: Group A, consisting of taxa with a defined peak in abundance in autumn-winter (INT: 6 taxa, FC: 11 taxa) and Group B, peaking in summer (INT: 0, FC: 6). Remaining species were assigned to Group C, an ad hoc residual category comprising taxa with ambiguous patterns, or species found in densities too low to allow for reliable pattern detection (INT: 23, FC: 27). This study provides insights into the composition and seasonal dynamics of sub-Arctic nudibranch assemblages for the first time, highlighting notable habitat differences and strong seasonal variability. These findings underscore the importance of long-term monitoring in marine biodiversity studies, especially in regions with poorly understood taxa.

Spatio-temporal dynamics of sediment oxygen consumption and its contributions from different benthic faunal groups and their activities are not yet well resolved, especially in the Arctic. Here, we assess total sediment oxygen demand (SOD) by means of incubation techniques in four geomorphological settings of the northwestern Barents Sea during four times of the year. Additionally, we estimate metazoan respiration (i.e. macro- and meiofauna) using allometric relationships and its contributions to SOD, and the bioturbation potential of macrofaunal communities. Seasonally, SOD rates were higher in August-December than in March-May, being particularly high in August at the Atlantic shelf and the margin of the Nansen Basin. Spatially, SOD rates tended to decrease from south to north, especially in early spring. Estimated metazoan respiration remained stable throughout the year and contributed usually >50% to SOD rates, especially at shelf and slope stations. However, its relative contributions to SOD decreased in August, indicating that most of the fluctuations in SOD rates were potentially attributed to non-metazoan respiration. In the adjacent deep Nansen Basin, unexpectedly high summer SOD rates may result from down-slope transport of organic matter from the shelf, sustaining elevated microbial activity in otherwise poor-metazoan sediments. On the Atlantic-influenced shelf, intense bioturbation from dominant macrofaunal taxa, such as Spiochaetopterus typicus, likely amplified microbial oxygen demand. Our findings suggest, with limitations by the methods used, that changes in total organic carbon in sediment have an interaction effect with non-metazoan processes, shaping the seasonal fluctuations in sediment remineralization in the northern Barents Sea. On the other hand, spatial patterns in SOD rates are highly affected by the bioturbating activities of macrofaunal assemblages, with highest influence south of the polar front, most likely shaping non-metazoan processes there as well.

Climate change is playing a major role in the current global biodiversity crisis. However, despite climate change being most pronounced in the Arctic, its impacts on biodiversity in this region remains largely unknown. Here, we combined three decades of abundance data from various animal groups (from zooplankton to megafauna) and regions in the European Arctic and Greenland to assess recent changes in biodiversity within Arctic coastal communities. Our results support the "borealization" hypothesis in all regions and provide evidence that marine ecosystems in the North Atlantic Arctic are shifting toward a boreal (i.e. cold temperate) state. Arctic endemic species are generally declining in abundance, while boreal species are increasing. These changes in abundance are associated with an average increase in biodiversity (e.g. species richness), although there are important variations among animal groups. This increase might be transient and the long-term implications of the ongoing changes in Arctic coastal biodiversity on ecosystem functioning and services remain uncertain.

Increasing ocean temperatures are altering phenological synchrony across the marine food web, especially at high latitudes. These shifts increase the probability of trophic mismatch events between the phytoplankton bloom, Calanus nauplii zooplankton, and planktivorous cod larvae, during which other potential prey with different phenology like meroplankton may become more prominent. To assess the potential of cirripede nauplii as a buffering prey source, we conducted a 20-day feeding experiment rearing first-feeding Atlantic cod larvae under conditions reflecting the spring-to-summer transition in Lofoten-Vesterålen, a major cod spawning area. Larvae were assigned to one of three diet treatments: a cirripede nauplii-only diet using Balanus crenatus, a copepod nauplii-only diet using Acartia tonsa, and a mixed diet, to evaluate the effect on larval growth, condition, and survival. Cod larvae reared on the cirripede-only diet showed the highest survival and growth rates compared to larvae reared on the copepod-only. Additional preference experiments tested whether cod larvae developed prey preferences related to their rearing diet. The experiment showed that cod larvae from the copepod-only treatment preferred B. crenatus nauplii over A. tonsa nauplii when exposed to both prey types. Overall, our results indicate that cirripede nauplii represent a suitable first-feed prey for cod larvae. Combined with their synchronized release and high abundances in the spring zooplankton community, this further highlights their potential as a buffering prey source during tropic mismatch events. We suggest that cirripede-rich prey fields may partially offset reduced availability of suitable copepod prey during the critical first-feeding period of North Atlantic cod larvae.

Meroplankton are frequently overlooked within the zooplankton community, yet they provide an important link between benthic and pelagic ecosystems. Long-term studies of holoplankton have revealed connections between climate and community dynamics, yet such studies are rare for meroplankton. We monitored the spring meroplankton community of a sub-Arctic fjord over 11 years (March-May 2014-2024), identifying 36 taxa across six phyla. Spring abundance peaked in April, averaging 5899 ind/m2, with the highest overall yearly abundance in 2018 and followed by a 30-fold decline across all taxonomic groups in 2019. The typical spring meroplankton succession began with a dominance of cirripedes around mid-March that shifted to high abundances of spionid larvae during April. However, two periods deviated from this succession pattern: From 2016 to 2018, when ophiopluteus larvae succeeded cirripeds, and from 2019 onwards, when cirripeds dominated before almost disappearing in 2023-2024. We do not report statistically significant relationships between climate indices (North Atlantic Oscillation or Arctic Oscillation) and meroplankton abundance. Instead, our results suggest that meroplankton respond to environmental variability related to deep water advection into the fjord's basin and bottom temperature. Abrupt changes in the meroplankton community composition and succession might have strong implications for benthic recruitment and the pelagic food web.

Oceanic fronts are often characterized by high primary productivity and increased localized levels of species diversity and biomass; this is due in part to the mixing of distinct water masses as well as the aggregation of organisms by ocean currents. How these interactions between the physical and chemical parameters of the Barents Sea Polar Front impact pelagic ecology are poorly understood, particularly across seasons. By combining findings from continuous acoustic and hydrographic recordings with discrete sampling, we measured how hydrographic changes associated with the Barents Sea Polar Front modify the spatial distribution and biomass of fish and zooplankton across three seasons (spring, winter, and summer). We found the influence of the Polar Front on the ecology of pelagic organisms to be highly dependent on season, with spring having the strongest correlation between hydrographic parameters and species distribution. The Front's influence on zooplankton and fishes differed considerably, as zooplankton and fish biomass often peaked at opposing ends of our survey transects, and in spring resulted in a zooplankton refuge from grazing fishes. The importance of seasonal sea ice was evident, and both the spring ice melt and new winter ice were strongly correlated with species distribution and in spring also biodiversity. Abiotic variables, and in particular temperature, were found to be the strongest predictors of pelagic fish and macrozooplankton biomass in all seasons, although in winter a greater influence by biotic variables was observed. While oceanic fronts are traditionally considered hotspots of biodiversity, the strong seasonal changes in the structure of the Barents Sea Polar Front create everchanging conditions where high biodiversity and pelagic biomass do not persist year-round. Our findings highlight the importance of considering seasonality in management decisions, and the need for increased in-situ winter data collection.

Monitoring biodiversity patterns and their changes in Arctic coastal ecosystems is critical under ongoing climate change. However, common current approaches require high effort and expertise and this in turn limits the spatial and temporal scale of these monitoring efforts. Here, we investigated both the fish and the marine invertebrate communities across Svalbard using a multi-marker environmental DNA metabarcoding approach. We collected and analysed marine water, sediment and zooplankton filtered from marine water from sites influenced by the warm West Spitsbergen Current and the cold East Spitsbergen Current. Following metabarcoding amplification using mitochondrial COI, 12S, 16S and nuclear 18S markers and high-throughput sequencing, we retrieved an extensive overview of Svalbard marine biodiversity. Water, sediment and especially zooplankton samples collected across Svalbard revealed spatial differences in community composition, with significantly distinct assemblages in the northwest and southeast of Svalbard. We identified potential bioindicator species for use in rapid assessment of impacts of marine temperature increase and confirmed observed patterns of ongoing shifts in community structure as a response to changes in dominant water masses. Overall, our findings show that species composition depending on fine-scale climate variation of Arctic waters can be effectively studied and monitored using environmental DNA. These insights can help us understand current and evolving climate-driven changes.

The Barents Sea is a hotspot for ongoing Arctic climate change, manifested in a rapid warming of the ocean and the atmosphere and a strong decline of the winter sea-ice cover. These changes in the physical environment have large consequences for marine ecosystems, including commercial fish populations. In a warmer future climate, both physical and ecological changes are expected to intensify. Here, we provide a first comprehensive overview of future climate change projections for the Barents Sea, and the associated physical, biogeochemical, and ecological consequences based on climate models and end-to-end ecosystem models. We also discuss potential future changes in human activities and their impacts, including changes in shipping activity and contaminants. We analyze results for two time horizons—the near-future (2040–2050) and the far-future (2090–2100)—and for two different emission scenarios: one with moderate future greenhouse gas emissions (SSP2-4.5) and one high-emission scenario (SSP5-8.5). The projections show that the future Barents Sea will be warmer, less ice-covered, more acidic, and more productive, with fish populations and spawning sites moving northward. There are small differences in multi-model mean physical and biogeochemical projections between the two emission scenarios by 2050, while large scenario differences emerge toward the end of the century. The implications of these results are far-reaching, including identifying the sensitivity of ecosystem change to future emissions, informing regional management strategies, and potentially identifying needs for adaptation to changes already likely to occur.

The projected transition of the central Arctic Ocean (CAO) into a warmer, seasonally ice-free ocean requires more knowledge of this environment to predict changes in the structure and dynamics of its ecosystems. We aimed to compare the state and underlying processes of Nansen Basin and Amundsen Basin ecosystems observed in August–September 2021 and assess impacts of Atlantic Water inflow and fresher Transpolar Drift waters, respectively, on these ecosystems. The basins differed in features of sea ice, hydrography, and chemical and biological compositions. The near-slope open water in western Nansen Basin showed a clear fingerprint of warm, saline Atlantic Water, with larger vertical turbulent fluxes facilitating nutrient transport across the pycnocline and supporting larger standing stocks of bacteria, protists, and zooplankton. Pelagic primary production and microbial and faunal stocks decreased northward and into Amundsen Basin, likely due to lower nutrient concentrations, stronger stratification, and reduced light through the more continuous and thicker ice and snow cover in Amundsen Basin, possibly also impacted by seasonally declining light levels. Transpolar Drift signals included lower salinity, stronger stratification, and higher silicate concentrations in Amundsen Basin surface waters. Similarities to earlier observations included the increase in small-sized algae from Nansen Basin into Amundsen Basin and overall low faunal abundances in the CAO, suggesting that overarching patterns remained unchanged over past decades. Examples of species range extensions and notable taxon absences relative to earlier studies, however, could be due to borealization and changes in sea-ice conditions, respectively. Higher density ecosystem sampling and consistent time series are recommended to confirm such conclusions. The distinct basin differences call for a regional approach to future management of the CAO. We especially caution against using the area of strong Atlantic Water inflow in southern Nansen Basin as representative of the entire basin, let alone Amundsen Basin or the CAO.

Addressing global challenges such as climate change requires large-scale collective actions, but such actions are hindered by the complexity and scale of the problem and the uncertainty in the long-term benefit of short-term actions (Jagers et al., 2019). In addition to climate change, socio-ecological systems face the cumulative pressures associated with resource needs, technology development, industrial expansion, and area conflicts. In marine systems, this has been called “the blue acceleration” (Jouffray et al., 2020) and is referred to as “socio-ecological pressures” in this paper. These socio-ecological pressures reduce our ability to reach the UN Sustainable Development Goals and meet the challenges of the UN Ocean Decade, and require integrating knowledge within a shared conceptual framework. For example, achieving sustainable growth must integrate ecological, socioeconomic, and governance perspectives on a larger scale by considering ecological impacts, ecosystem carrying capacities, economic trade-offs, social acceptability, and policy realities. This requires capacity development whereby actors unite to bridge disciplinary boundaries to meet challenges of complex systems.

We analyzed stable carbon and nitrogen isotope values (δ13C and δ15N, respectively) for pan-Arctic coastal primary producers and consumers to detect large-scale regional trends both temporally and spatially. To facilitate comparison, we grouped coastal habitats into fjords, lagoons, shelves, and straits as four "coastscapes". We gathered over 12,000 rows of data collected over 24 years (between 1999 and 2022) from 34 different field campaigns across the coastal Arctic (63 to 81°N and 177°W to 33°E). Our goal was to examine the isotopic patterns in pelagic and sediment particulate organic matter (pPOM and sPOM, respectively) and four consumer groups (deposit feeders, opportunists/scavengers, predators, and suspension feeders) among the four coastscapes. We found that despite the enormous spatial range of data, both pPOM and sPOM became 2.1‰ and 2.2‰ more 13C-depleted per decade, respectively, with parallel decreases in the δ13C values in consumers. The significant decrease is likely attributed to the increased contributions of 13C-depleted terrestrial organic matter across the Arctic coasts from freshwater inputs and coastal erosion in concert with diminishing sea ice that supports sympagic microalgae. Across all Arctic coastscapes, consumer groups exhibited overlapping isotopic composition, notably with wide δ13C ranges that indicated assimilation of multiple organic matter sources, including terrestrial organic matter, organic matter derived from marine phytoplankton and sea ice algae, macroalgae, and potentially benthic microalgae or degraded organic matter. This consistent pattern across coastscapes provides evidence of the trophic plasticity possessed by Arctic consumers, how coastal food webs respond to climate warming, and the signature of terrestrialization imprinted on the pan-Arctic coastal isoscape.

Arctic coastal biodiversity faces increasing threats from anthropogenic activities and climate change. However, the effects on biodiversity are still poorly understood, hindering actions aimed at mitigating the impacts at a pan-Arctic scale. We present the results of a horizon scan that provides a road map to address knowledge gaps on the influence of anthropogenic activities, from increased shipping and harvesting to consequences of climate change including increasing temperatures, cryosphere loss, and freshwater runoff. Predictions on ecological change, species range expansions, and anthropogenic impacts on Arctic coasts are hampered by the lack of biodiversity data and scarcity of biological long-term monitoring programs. Filling these knowledge gaps will require coordinated international efforts and standardized experiments across the diverse ecosystems characterizing the Arctic.

EcoQS assessment of the marine intertidal zone based on its fauna is challenging because the assemblages have a low diversity and consist of stress tolerant species. The new approach we propose is to pool foraminiferal diversity (effective number of species exp(H'bc)) across the whole intertidal zone including the salt marsh and tidal flat. In seven fjordheads studied in northern Fennoscandia, polycyclic aromatic hydrocarbon (PAH) concentrations indicated low levels of pollution (EcoQSPAH Excellent to Moderate). Jadammina or Balticammina dominated the salt marsh, Elphidium albiumbilicatum, Elphidium williamsoni, Elphidium clavatum, and Buccella frigida occurred in the tidal flat. Ovammina opaca thrived in both belts. While foraminiferal test abnormalities are often proposed to measure pollution impacts, we did not detect any correlation with PAHs. EcoQS based on foraminiferal diversity (EcoQSforam Excellent to Good) matched EcoQS based on PAHs suggesting that pooled foraminiferal diversity reliably measures intertidal EcoQS.

AbstractThe impacts of climate change on Arctic marine systems are noticeable within the scientific “lifetime” of most researchers and the iconic image of a polar bear struggling to stay on top of a melting ice floe captures many of the dominant themes of Arctic marine ecosystem change. But has our focus on open‐ocean systems and parameters that are more easily modeled and sensed remotely neglected an element that is responding more dramatically and with broader implications for Arctic ecosystems? We argue that a complementary set of changes to the open ocean is occurring along Arctic coasts, amplified by the interaction with changes on land and in the sea. We observe an increased number of ecosystem drivers with larger implications for the ecological and human communities they touch than are quantifiable in the open Arctic Ocean. Substantial knowledge gaps exist that must be filled to support adaptation and sustainability of socioecological systems along Arctic coasts.

On Arctic shelves, benthic food-webs are tightly linked to overlying primary production. In the seasonal ice zone, sympagic (ice-associated) primary production can be a major source of carbon for the benthos on productive inflow shelves. However, the role of sympagic organic matter is less well-understood in food webs of heavily ice-covered, less- productive outflow shelves, such as the northeast Greenland shelf. Highly branched isoprenoid biomarkers (HBIs) were used to track the relative distribution of sympagic and pelagic organic matter in the water column, sediments, and benthic fauna of the northeast Greenland shelf and fjords. Low pelagic HBI presence throughout the study area indicated a generally low production by pelagic diatoms (at the time of sampling). This was reflected in the benthos, as ~90% of their assimilated carbon was estimated to come from sympagic sources, indicating a benthic food-web highly reliant on sympagic production. This reliance was higher in coastal areas than on the open shelf, where the potentially higher pelagic productivity and shallower water on banks likely increased contributions of pelagic organic matter. As declining ice cover and reduced production of fast-sinking ice algae projected for Arctic shelves will likely result in weaker coupling between ice algae and the benthos, with possible consequences for future benthic-community structure and function.

Monitoring programs that integrate both structural and functional ecosystem components play integral roles in ecosystem management and conservation planning. In the early 1990’s, the marine ecosystem of the waters surrounding Newfoundland and Labrador (NL) underwent a regime shift. Several demersal and pelagic fish stocks collapsed simultaneously, and this had significant ecological and socioeconomic consequences. As this regime shift impacted numerous commercial and non-commercial species, assessments based on individual species would be insufficient. We explored a variety of metrics that capture different facets of diversity across multiple species to provide a more robust ecosystem assessment. These were species richness, evenness, community-weighted means of maximum body length and trophic level (i.e., the mean maximum body size or trophic level of the species present in a community), and functional dispersion (FDis). The objectives of this study were 1) to assess trends in community structure of the NL demersal community during the post-collapse period (1995-2018), 2) explore how the various community-level metrics differ or are redundant, and 3) investigate how these metrics are associated with important covariates. Several metrics were redundant and displayed strongly positive, temporal trends being consistent with expectations for a recovery encompassing the entire demersal community. In particular, unweighted community-weighted means of body length and trophic level displayed nearly equal temporal patterns, showing increasing trends throughout the study period which were most prominent in the northern study area and within a limited depth range at the upper shelf break. Corresponding biomass-weighted metrics were also correlated with each other but only showed similar increasing tendencies after the first decade. In contrast, species richness did not show any temporal increase. Evenness and biomass-weighted FDis showed similar temporal patterns, decreasing during the first decade followed by strong increases during subsequent years, patterns that were directly linked to variation in Northern shrimp and Atlantic cod biomass. This study demonstrates how a variety of community metrics can provide insight into different aspects of the post-collapse recovery of the demersal community and help us better understand the complexity of the changes the ecosystem is undergoing.

Please provide a 100 word synopsis of the article, which will be used to summarize the work when presented online. Coastal systems are increasingly challenged by anthropogenic pressures. Both social and ecological systems are vulnerable to climate change and other anthropogenic impacts. Adaptive measures are important to increase the resilience of the coastal social-ecological system. This chapter provides an overview of the adaptive measures from six selected country sites in the EuroAsian continent, with the focus on monitoring, sustainable fisheries, nature-based solutions, marine spatial monitoring and awareness-raising. However, the adaptive measures that focus on coastal waters alone are not sufficient, and it is important to combine the coastal adaptive measures with the management of land/terrestrial areas.

Food-web structure determines the cycling pathways and fate of new production in marine ecosystems. Herbivorous zooplankton populations are usually seasonally coupled with pelagic primary producers. Synchrony of phytoplankton blooms with reproduction, recruitment and seasonal ascent of their main grazers ensures efficient transfer of organic carbon to higher trophic levels, including commercially harvested species, especially in high-latitude systems. Changes in light, nutrient, and sea-ice dynamics due to accelerating climate change in the Arctic, however, create large uncertainties in how these systems will function in the future. To address such knowledge gaps, we surveyed the pelagic ecosystem of the Barents Sea Polar Front in May of two consecutive years (2021 and 2022) to investigate the pelagic food-web from primary producers to planktivorous fish. In both years we observed unprecedentedly high phytoplankton chlorophyll a values in open as well as ice-covered waters, much of which was invisible to satellite remote sensing. We also measured very low densities of grazing zooplankton across a wide area and extending for at least one month. This extreme mismatch resulted in low feeding by capelin, and further suggests a high potential for vertical export of carbon to the benthos rather than efficient assimilation into the pelagic food web. As the Arctic continues to warm and is characterized by thinner and more mobile sea ice, we may expect higher variability in phytoplankton bloom phenology and more frequent mismatches with grazer life-histories. This could have significant impacts on ecosystem functioning by re-directing the flow of energy through the system towards seafloor rather than to the production of commercially valuable pelagic marine resources.

Benthic (seafloor) remineralization of organic material determines the fate of carbon in the ocean and its sequestration. Bottom water temperature and labile carbon supply to the seafloor are expected to increase in a warming Arctic and correspondingly, benthic remineralization rates. We provide some of the first experimental data on the response of sediment oxygen demand (SOD), an established proxy for benthic remineralization, to increased temperature and/or food supply across a range of Arctic conditions and regimes. Each factor significantly increased SOD rates (with different degrees of variability); however the largest increases were seen with both factors combined (50% to ten-fold increases), consistently across the four seasons and the spatial gradient covering shelf to deep basin included in our study. This ability of the Arctic benthos to process increased pulses of carbon suggests that increased sedimented carbon under warming conditions is likely to be utilized and processed, not accumulated, impacting carbon storage and decreasing the Arctic’s role as a global carbon sink.

Arctic marine ecosystems are becoming more boreal due to climate change. Predictions of ecosystem change focus mainly on Arctic inflow or interior shelves, with few comprehensive studies on Arctic outflow regions. During September–October 2017, soft-bottom communities were sampled and benthic ecosystem processes were quantified at 12 stations on the Northeast Greenland shelf (outflow shelf) and compared to the last regional ecosystem study, conducted in 1992 and 1993. The benthic habitat was characterized in terms of sediment granulometry, pigment concentrations, and porewater chemistry (dissolved inorganic carbon, nutrients). Total abundance and biomass of macrobenthos and meiobenthos, bacterial abundance, porewater dissolved inorganic carbon and ammonium concentrations were higher on the outer shelf compared to locations adjacent to the Nioghalvfjerdsfjorden glacier at 79°N and the inner shelf stations (e.g., macrofauna: 1,964–2,952 vs. 18–1,381 individuals m−2). These results suggest higher benthic production in the outer parts of the NEG shelf. This difference was also pronounced in macrobenthic and meiobenthic community structure, which was driven mainly by food availability (pigments with 1.3–4.3 vs. 0.3–0.9 µg g−1 sediment, higher total organic carbon content and bacterial abundance). Compared to the early 1990s, warmer bottom water temperatures, increased number of sea-ice-free days and lower sediment pigment concentrations in 2017 were accompanied by decreased polychaete and increased nematode abundance and diversity, and a different community structure of nematode genera. The present study confirms previous reports of strong pelagic-benthic coupling on the NEG shelf, but highlights a possible weakening since the early 1990s, with a potential shift in importance from macrofauna to meiofauna in the benthic community. Increasing inflow of Atlantic water and decreasing volume transport and thickness of sea ice through the Fram Strait, probably affecting the Northeast Water Polynya, may be responsible, suggesting ecosystem-wide consequences of continued changes in sea-ice patterns on Arctic shelves.