This study investigates spatial and temporal patterns in zooplankton abundance and community composition in the Fram Strait between 2011 and 2023. The region is shaped by contrasting hydrographic regimes: warm, saline Atlantic Water transported northward by the West Spitsbergen Current (WSC) in the east, and cold, fresh Polar Water carried southward by the East Greenland Current (EGC) in the west. While the WSC has been extensively studied, the EGC remains less well understood. Yet, monitoring both regimes is essential to detect changes related to ongoing Atlantification. As part of the LTER observatory HAUSGARTEN, zooplankton were sampled at nine sites across Fram Strait using vertical multinet hauls (150 mu m mesh, 1500-1000-500-200-50-0 m depth). Preserved samples were scanned and analysed using the web-based platform EcoTaxa. In total 86 taxa were identified, with Copepoda, particularly Oithona and Calanus spp., dominating the communities. Abundances in the EGC were relatively stable and consistently lower than in the WSC. In the WSC, zooplankton abundances varied considerably and occasionally reached extremely high values that might be linked to mesoscale hydrographic features. We applied Generalized Additive Models to our data, that revealed significantly declining abundances over time in the WSC while they increased in the EGC. Shannon diversity and Pielou's evenness, however, had increased slightly in both regions in the last three years. Climate change is likely to affect the two regimes differently, leading to diverging ecosystem responses. Thus, our findings underscore the importance of sustained long-term monitoring to fully understand zooplankton dynamics in the entire Fram Strait.

Embedded in the interdisciplinary design of pelagic and benthic long-term observations at the Arctic long-term ecological research site HAUSGARTEN in Fram Strait (78 degrees 36,46 ' N-79 degrees 56,63 ' N 10 degrees 50,91 ' E-5 degrees 22,24 ' W; 250-5500 m), environmental DNA (eDNA)-based biodiversity studies span multiple habitats and trophic levels. These studies provide valuable insights into marine ecosystem functionality and carbon fluxes, including seasonal and interannual dynamics. Since 2009, eDNA studies of pelagic eukaryotic microbial communities have regularly been conducted. Recently, these studies have been expanded to cover the biodiversity of bacteria, archaea and metazoa such as zooplankton (including gelatinous taxa) and nekton (cephalopods and fish) alongside with microbial eukaryotes. The eDNA surveys have been coupled with measurements of physical and chemical properties, such as sea-ice coverage and water masses. By applying these approaches across spatial and trophic scales in a nested approach, we characterize the dynamics in biodiversity, its role in ecological processes, with a focus on current climate change. This is achieved by considering the complexity and interconnectivity of organisms, integrating information from different disciplines across scales and habitats, and focusing on functional processes including species composition. The resulting data improve our predictive capabilities for conservation and sustainable marine management in a rapidly changing Arctic. Here, we review the design of the first interdisciplinary Arctic eDNA long-term observatory and the results of more than ten years of method development and observations.

Taxonomic discussions often permeate the broader scientific community slowly, yet they may hold more relevance than typically assumed. In many zooplankton groups, identification issues arise from cryptic species complexes, increasingly revealed by molecular approaches, and from groups with high morphological similarity. These challenges can lead to substantial uncertainties in species-level identification, questioning whether the expected species are truly covered and whether those sharing names across ecosystems are indeed distinct entities. This review provides a condensed overview on identification challenges of key species in the ICES zooplankton time series from the North Atlantic and adjacent seas. Examples are given across all relevant groups, including copepods, gelatinous plankton, and meroplanktonic larvae. The high prevalence of challenging species complexes underscores the need to further explore the implications of an accurate species assignment for understanding what defines a species’ role in an ecosystem. This review highlights the dynamic nature of taxonomy, with species being split and cryptic species eventually becoming morphologically distinguishable. It provides examples showing that relying solely on molecular methods without deep taxonomic expertise poses significant risks. It also aims to serve as a starting point for delving deeper into the taxonomy of the ICES zooplankton time series.

Oxygen Minimum Zone (OMZ) expansion is a major challenge to marine ecosystems and associated zooplankton. Calanoid copepods include lineages that tolerate hypoxia and exhibit functional traits such as diel vertical migrations to, or dormancy within, hypoxic mesopelagic zones. However, the evolutionary origins and molecular drivers of these traits remain unclear. Herein, we integrate a time-calibrated phylotranscriptomic tree of 50 copepod species with ancestral trait reconstruction, gene family copy number variation, and palaeoceanographic data to infer the evolutionary timing and ecological drivers of hypoxia adaptation. Our results support that post-embryonic dormancy originated in calanoid ancestors, accompanied by widespread gene expansions primarily involving hypoxia-response pathways as well as lipid and amino acid metabolism. Mesopelagic colonisation by calanoid lineages likely occurred during the Ordovician deep-sea oxygenation event. This was followed, during the Carboniferous deep-sea deoxygenation, by a secondary habitat shift toward shallower waters and embryonic dormancy, and gene contractions in the superfamily Diaptomoidea. We further analysed the hypoxia-induced transcriptomic response of Eucalanus hyalinus from the Benguela upwelling OMZ, and identified a coordinated response involving extracellular matrix remodelling, amino acid recycling for anaerobic energy and antioxidant production as well as triglycerides to wax ester conversion. Gene family expansions upstream (proteolysis, transport) and downstream (antioxidant biosynthesis) of core metabolic pathways suggest purifying selection on dosage-sensitive nodes. Together, these results link palaeoclimate change to lineage-specific genome evolution patterns supporting copepod adaptation to oxygen limitation.

Seasonal vertical migration of large lipid-rich copepods is often described as a mass descent of animals when primary production ceases, with important implications for mesopelagic food webs and global carbon sequestration. This view ignores the existence of surface-resident individuals, but here we show that non-migrants can form a substantial part of the populations of polar migrant species. In the Central Arctic Ocean, the biomass-dominant Calanus hyperboreus was evenly distributed throughout the water column from November 2019 to March 2020, with ~20% of subadults and adult females remaining in the upper 200 m and ~41% migrating to 1000-2000 m. These vertical positions aligned with differences in the copepods' cholesterol content, which can enhance the tissue density at higher temperatures. Gonad development and the vertical distribution of their offspring indicate that both non-migrant and migrant females contribute to the population recruitment. We reinterpret copepod seasonal migration as a bet-hedging strategy that balances nutritional benefits near the surface with survival benefits at depth, and thereby contributes to the species' resilience under climatic change.

Due to the high sensitivity of zooplankton to environmental fluctuations, monitoring their taxonomic composition, abundance and biomass is of high priority to identify changes in the ecosystem. Recent advances in imaging and molecular technologies promise to greatly accelerate the processing of samples to determine both the diversity and quantity of the zooplankton community. In our study, we analyzed the diversity and quantity of an epipelagic Arctic zooplankton community using multi-marker metabarcoding and imaging analysis (ZooScan). We identified a total of 11 phyla and 58 species in the northern Barents Sea and the Nansen Basin. Metabarcoding identified more taxa than image analysis, while imaging provided quantitative information on abundance and biomass. Multivariate analyses revealed overall the same significant environmental drivers (temperature and percentage of Polar Surface Water in the sampling depth layer) explaining the similarity and spatial distribution of the zooplankton community. For all approaches, similar spatial patterns of the zooplankton community were found. Abundance, biovolume and biomass decreased with increasing latitude within the analyzed regions. Based on this study, we recommend ZooScan image analysis in combination with COI metabarcoding for future monitoring of Arctic zooplankton diversity and quantification to ensure the detection of changes in both aspects of these communities.

The use of trait-based approaches and trait data in zooplankton ecology is rapidly growing to better understand and predict the patterns of zooplankton distributions and their role in aquatic ecosystems and biogeochemical cycles. Although the number of zooplankton trait-based studies and available trait datasets is increasing, several challenges remain for the findability, accessibility, interoperability, and reusability (FAIR) in trait-based approaches that, if unaddressed, may stifle progress in this research area. Here, we review recent applications of trait-based approaches in zooplankton research and summarize the currently available trait data resources. To realize the potential of trait-based approaches to resolve ecological roles of zooplankton, datasets and approaches must adhere to FAIR principles. We provide recommendations and pathways forward to ensure FAIRness while highlighting the importance of collaborative efforts. These practical and easily implementable strategies will enhance the FAIRness of trait data, ultimately advancing zooplankton ecological research and connecting these findings to aquatic ecosystem functioning.

The international and interdisciplinary sea-ice drift expedition "The Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) was conducted from October 2019 to September 2020. The aim of MOSAiC was to study the interconnected physical, chemical, and biological characteristics and processes from the atmosphere to the deep sea of the central Arctic system. The ecosystem team addressed current knowledge gaps and explored unknown biological properties over a complete seasonal cycle focusing on three major research areas: biodiversity, biogeochemical cycles, and linkages to the environment. In addition to the measurements of core properties along a complete seasonal cycle, dedicated projects covered specific processes and habitats, or organisms on higher taxonomic or temporal resolution in specific time windows. A wide range of sampling instruments and approaches, including sea-ice coring, lead sampling with pumps, rosette-based water sampling, plankton nets, remotely operated vehicles, and acoustic buoys, was applied to address the science objectives. Further, a broad range of process-related measurements to address, for example, productivity patterns, seasonal migrations, and diversity shifts, were made both in situ and onboard RV Polarstern. . This article provides a detailed overview of the sampling approaches used to address the three main science objectives. It highlights the core sampling program and provides examples of habitat- or process-specific sampling. The initial results presented include high biological activities in wintertime and the discovery of biological hotspots in underexplored habitats. The unique interconnectivity of the coordinated sampling efforts also revealed insights into cross-disciplinary interactions like the impact of biota on Arctic cloud formation. This overview further presents both lessons learned from conducting such a demanding field campaign and an outlook on spin-off projects to be conducted over the next years.

Polar cod (Boreogadus saida) is an endemic key species of the Arctic Ocean ecosystem. The ecology of this forage fish is well studied in Arctic shelf habitats where a large part of its population lives. However, knowledge about its ecology in the central Arctic Ocean (CAO), including its use of the sea-ice habitat, is hitherto very limited. To increase this knowledge, samples were collected at the under-ice surface during several expeditions to the CAO between 2012 and 2020, including the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The diet of immature B. saida and the taxonomic composition of their potential prey were analysed, showing that both sympagic and pelagic species were important prey items. Stomach contents included expected prey such as copepods and amphipods. Surprisingly, more rarely observed prey such as appendicularians, chaetognaths, and euphausiids were also found to be important. Comparisons of the fish stomach contents with prey distribution data suggests opportunistic feeding. However, relative prey density and catchability are important factors that determine which type of prey is ingested. Prey that ensures limited energy expenditure on hunting and feeding is often found in the stomach contents even though it is not the dominant species present in the environment. To investigate the importance of prey quality and quantity for the growth of B. saida in this area, we measured energy content of dominant prey species and used a bioenergetic model to quantify the effect of variations in diet on growth rate potential. The modeling results suggest that diet variability was largely explained by stomach fullness and, to a lesser degree, the energetic content of the prey. Our results suggest that under climate change, immature B. saida may be at least equally sensitive to a loss in the number of efficiently hunted prey than to a reduction in the prey's energy content. Consequences for the growth and survival of B. saida will not depend on prey presence alone, but also on prey catchability, digestibility, and energy content.

In marine ecosystems, most physiological, ecological, or physical processes are size dependent. These include metabolic rates, the uptake of carbon and other nutrients, swimming and sinking velocities, and trophic interactions, which eventually determine the stocks of commercial species, as well as biogeochemical cycles and carbon sequestration. As such, broad-scale observations of plankton size distribution are important indicators of the general functioning and state of pelagic ecosystems under anthropogenic pressures. Here, we present the first global datasets of the Pelagic Size Structure database (PSSdb), generated from plankton imaging devices. This release includes the bulk particle normalized biovolume size spectrum (NBSS) and the bulk particle size distribution (PSD), along with their related parameters (slope, intercept, and R2) measured within the epipelagic layer (0–200 m) by three imaging sensors: the Imaging FlowCytobot (IFCB), the Underwater Vision Profiler (UVP), and benchtop scanners. Collectively, these instruments effectively image organisms and detrital material in the 7–10 000 µm size range. A total of 92 472 IFCB samples, 3068 UVP profiles, and 2411 scans passed our quality control and were standardized to produce consistent instrument-specific size spectra averaged to 1° × 1° latitude and longitude and by year and month. Our instrument-specific datasets span most major ocean basins, except for the IFCB datasets we have ingested, which were exclusively collected in northern latitudes, and cover decadal time periods (2013–2022 for IFCB, 2008–2021 for UVP, and 1996–2022 for scanners), allowing for a further assessment of the pelagic size spectrum in space and time. The datasets that constitute PSSdb's first release are available at https://doi.org/10.5281/zenodo.11050013 (Dugenne et al., 2024b). In addition, future updates to these data products can be accessed at https://doi.org/10.5281/zenodo.7998799.

As Arctic sea ice deteriorates, more light enters the ocean, causing largely unknown effects on the ecosystem. Using an autonomous biophysical observatory, we recorded zooplankton vertical distribution under Arctic sea ice from dusk to dawn of the polar night. Here we show that zooplankton ascend into the under-ice habitat during autumn twilight, following an isolume of 2.4 × 10 −4 W m −2 . We applied this trigger isolume to CMIP6 model outputs accounting for incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice habitat could be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, and carbon and nutrient fluxes. These findings highlight the importance of biological processes during the twilight periods for predicting change in high-latitude ecosystems.

Zooplankton are major consumers of phytoplankton primary production in marine ecosystems. As such, they represent a critical link for energy and matter transfer between phytoplankton and bacterioplankton to higher trophic levels and play an important role in global biogeochemical cycles. In this Review, we discuss key responses of zooplankton to ocean warming, including shifts in phenology, range, and body size, and assess the implications to the biological carbon pump and interactions with higher trophic levels. Our synthesis highlights key knowledge gaps and geographic gaps in monitoring coverage that need to be urgently addressed. We also discuss an integrated sampling approach that combines traditional and novel techniques to improve zooplankton observation for the benefit of monitoring zooplankton populations and modelling future scenarios under global changes.

Abstract As Arctic sea ice deteriorates, more light enters the Arctic Ocean, causing largely unknown effects on the ecosystem. A novel autonomous bio-physical observatory provided the first record of zooplankton vertical distribution under sea ice drifting across the Arctic Ocean from dusk to dawn of the polar night. Its measurements revealed that zooplankton ascend into the under-ice layer during autumn twilight, following an isolume of 5.5 10-4 W m-2. We applied this trigger isolume to IPCC models enabled to incorporate incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice layer will be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, carbon- and nutrient fluxes. These findings highlight the importance of processes in the twilight periods for predicting change in high-latitude ecosystems.

Comau Fjord is a stratified Chilean Patagonian Fjord characterized by a shallow brackish surface layer and a >400 m layer of aragonite-depleted subsurface waters. Despite the energetic burden of low aragonite saturation levels to calcification, Comau Fjord harbours dense populations of cold-water corals (CWC). While this paradox has been attributed to a rich supply of zooplankton, supporting abundance and biomass data are so far lacking. In this study, we investigated the seasonal and diel changes of the zooplankton community over the entire water column. We used a Nansen net (100 µm mesh) to take stratified vertical hauls between the surface and the bottom (0-50-100-200-300-400-450 m). Samples were scanned with a ZooScan, and abundance, biovolume and biomass were determined for 41 taxa identified on the web-based platform EcoTaxa 2.0. Zooplankton biomass was the highest in summer (209 g dry mass m−2) and the lowest in winter (61 g dry mass m−2). Abundance, however, peaked in spring, suggesting a close correspondence between reproduction and phytoplankton spring blooms (Chl a max. 50.86 mg m−3, 3 m depth). Overall, copepods were the most important group of the total zooplankton community, both in abundance (64–81%) and biovolume (20–70%) followed by mysids and chaetognaths (in terms of biovolume and biomass), and nauplii and Appendicularia (in terms of abundance). Throughout the year, diel changes in the vertical distribution of biomass were found with a daytime maximum in the 100–200 m depth layer and a nighttime maximum in surface waters (0–50 m), associated with the diel vertical migration of the calanoid copepod family Metridinidae. Diel differences in integrated zooplankton abundance, biovolume and biomass were probably due to a high zooplankton patchiness driven by biological processes (e.g., diel vertical migration or predation avoidance), and oceanographic processes (estuarine circulation, tidal mixing or water column stratification). Those factors are considered to be the main drivers of the zooplankton vertical distribution in Comau Fjord.

The pelagic ecosystem of the Arctic Ocean is threatened by severe changes such as the reduction in sea‐ice coverage and increased inflow of warmer Atlantic water. The latter is already altering the zooplankton community, highlighting the need for monitoring studies. It is therefore essential to accelerate the taxonomic identification to speed up sample analysis, and to expand the analysis to biomass and size assessments, providing data for modeling efforts. Our case study in Fram Strait illustrates that image‐based analyses with the ZooScan provide abundance data and taxonomic resolutions that are comparable to microscopic analyses and are suitable for zooplankton monitoring purposes in the Arctic. We also show that image analysis allows to differentiate developmental stages of the key species Calanus spp. and Metridia longa and, thus, to study their population dynamics. Our results emphasize that older preserved samples can be successfully reanalyzed with ZooScan. To explore the applicability of image parameters for calculating total mesozooplankton and Calanus spp. biomasses, we used (1) conversion factors (CFs) translating wet mass to dry mass (DM), and (2) length–mass (LM) relationships. For Calanus spp., the calculated biomass values yielded similar results as direct DM measurements. Total mesozooplankton biomass ranged between 1.6 and 15 (LM) or 2.4 and 21 (CF) g DM m −2 , respectively, which corresponds to previous studies in Fram Strait. Ultimately, a normalized biomass size spectra analysis provides 1 st insights into the mesozooplankton size structure at different depths, revealing steep slopes in the linear fit in communities influenced by Atlantic water inflow.

Characterization of species diversity of zooplankton is key to understanding, assessing, and predicting the function and future of pelagic ecosystems throughout the global ocean. The marine zooplankton assemblage, including only metazoans, is highly diverse and taxonomically complex, with an estimated ~28,000 species of 41 major taxonomic groups. This review provides a comprehensive summary of DNA sequences for the barcode region of mitochondrial cytochrome oxidase I (COI) for identified specimens. The foundation of this summary is the MetaZooGene Barcode Atlas and Database (MZGdb), a new open-access data and metadata portal that is linked to NCBI GenBank and BOLD data repositories. The MZGdb provides enhanced quality control and tools for assembling COI reference sequence databases that are specific to selected taxonomic groups and/or ocean regions, with associated metadata (e.g., collection georeferencing, verification of species identification, molecular protocols), and tools for statistical analysis, mapping, and visualization. To date, over 150,000 COI sequences for ~ 5600 described species of marine metazoan plankton (including holo- and meroplankton) are available via the MZGdb portal. This review uses the MZGdb as a resource for summaries of COI barcode data and metadata for important taxonomic groups of marine zooplankton and selected regions, including the North Atlantic, Arctic, North Pacific, and Southern Oceans. The MZGdb is designed to provide a foundation for analysis of species diversity of marine zooplankton based on DNA barcoding and metabarcoding for assessment of marine ecosystems and rapid detection of the impacts of climate change.

Two new species of Paracalanidae, Giesbrecht, 1893 , have been described. Paracalanus brasiliensis sp. nov. and Bestiolina brasiliensis sp. nov. were registered in four estuaries on the central coast of Brazil. Paracalanus brasiliensis sp. nov. females differ from their congeners mainly with regard to body size, the structure of swimming legs 1–4, absence of bristles in the coxopodites, spinules between the spines in the third segment of the exopodite, and the shape of the seminal receptacles. The uniformity of the number of spinules and their location on the anterior face of the second exopodite of legs 2–3 and the absence of spinules on the endopodite of legs 3–4 differentiate Bestiolina brasiliensis sp. nov. females from other Bestiolina Andronov, 1991 , species. In the males of both species, the main diagnostic features (swimming leg seta, spine formula, and ornamentation) are generally observed in females with a few additional characteristics. Genetic divergence analyses based on partial mitochondrial COI (mtCOI) sequences revealed no genetic divergence between Paracalanus brasiliensis sp. nov. and Paracalanus sp. E. sensu Cornils and Held ( 2014 ), demonstrating that they are mutually conspecific. mtCOI sequence data from Bestiolina brasiliensis sp. nov. identified a clade with high bootstrap support that separated the specimens in this study from other Bestiolina species. The present report provides the first morphological description of females and males of both Paracalanus brasiliensis sp. nov. and Bestiolina brasiliensis sp. nov. and presents molecular evidence for species specificity. Matters regarding the validity of these species are also discussed.

Raw and supplementary data and detailed statistical results for the publication "The carbon and nitrogen budget of Desmophyllum dianthus – a voracious cold-water coral thriving in an acidified Patagonian fjord"

Zooplankton community structure is often characterized by using traits as a function of environmental conditions. However, trait-based knowledge on Southern Ocean mesozooplankton is limited, particularly regarding size and elemental composition. Nine stations around the northern Antarctic Peninsula were sampled during austral autumn to investigate the spatial variability in mesozooplankton taxonomic composition, size structure and stoichiometry in relation to environmental predictors, but also to the abundance of Antarctic krill and salps. The mesozooplankton communities around the South Shetland Islands were dominated by small copepods, mainly Oithonidae and Oncaeidae, while stations along the frontal zones and the Weddell Sea revealed a higher proportion of larger organisms. Spatial differences in taxonomic composition and size structure were significantly altered by salp abundance, with stronger impact on small-sized copepods. Furthermore, taxonomic composition was significantly related to temperature and total carbon but not chlorophyll a, indicating reduced relevance of phytoplankton derived food during autumn. Bulk mesozooplankton stoichiometry, however, showed no significant relation to environmental conditions, mesozooplankton size structure or dominant taxa. Our results indicate that aside from bottom-up related drivers, top-down effects of salps may lead to mesozooplankton communities that are more dominated by larger size classes with potential consequences for trophic interactions and nutrient fluxes.

With rapid sea-ice decline, ocean warming and increasing Atlantic inflow, the ecosystem of the Central Arctic Ocean (CAO) is experiencing an unprecedented, potentially disruptive transformation. While this transformation is affecting the biodiversity of marine communities and the ecosystem functions they fulfil, major knowledge gaps about the distribution of pelagic macrofauna (zooplankton and fish) complicate the assessment of the impact of this transformation on biodiversity and marine resources. The largest blind spot remains in the central Arctic Basin, which has been difficult to sample with large sampling gear such as fishing nets due to a year-round ice coverage. However, in the face of increasing human activities and international efforts to implement marine management in the CAO, it becomes important to monitor pelagic fauna in this remote area. One possibility to enable a better sampling of pelagic macrofauna is to use sea-ice thethered autonomous observatories. Within the British/German project EcoLight, we developed a new autonomous sea-ice observatory comprising an ASL Acoustic Zooplankton and Fish Profiler (AZFP). The device has 4 frequencies targeting different size classes of animals. It samples automatically at predefined intervals and transmits the data to a server in Europe via Iridium. It is possible to change the sampling parameters via a remote connection at any time. The AZFP buoy was deployed in the CAO in September 2020, shortly before the end of the MOSAiC expedition. Since then, the buoy has been recording the vertical zooplankton distribution in the water column under the ice. First data show a light-induced change of the vertical distribution of scatterers, transitioning from deep distribution during the polar day, through a short period of diel vertical migration during the twilight period, to a constant presence of scatterers in the surface layer in the polar night. Furthermore, AZFP data suggest an enhancement of zooplankton between the upper pycnocline and ~50 m depth during in an eddie transition. The data collected by the EcoLight AZFP buoy constitute the first hydroacoustic record of zooplankton distribution near the North Pole sampled with a fully autonomous system in the absence of disturbing light sources. They demonstrate the feasibility of year-round automated monitoring of macrofauna in the CAO in relation to environmental properties. Similar autonomous devices may serve as key elements in the future monitoring of biological resources in the CAO and other inaccessible areas.