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A key challenge in understanding carbon cycling in the Southern Ocean is disentangling long-term responses from significant spatial and temporal variability in physical and biogeochemical parameters. As such, there is a critical need for regional long-term observations for the model validation and testing needed for a better mechanistic understanding of primary production drivers. We present a new macronutrient data product for the South Atlantic sector of the Southern Ocean, including depth profiles and underway surface measurements of nitrate, nitrite, ammonium, phosphate, silicic acid, temperature and salinity, collected from 1980 to 2009 and covering most months of the year (https://doi.org/10/h3qr). Using this data product, we explore the differences in shallow and deep-water nutrients around the island of South Georgia that are observed between years. We discuss both the biological and physical driving mechanisms behind this variability, which are interconnected with climate feedbacks. The new data product provides an unprecedented view of biogeochemical cycling in biologically productive regions of the Southern Ocean across a critical period in recent climate history, and illustrates the importance of building these scientifically valuable and FAIR (findability, accessibility, interoperability, and reusability) observational datasets.

Poleward range shifts are a global-scale response to warming, but these vary greatly among taxa and are hard to predict for individual species, localized regions or over shorter (years to decadal) timescales. Moving poleward might be easier in the Arctic than in the Southern Ocean, where evidence for range shifts is sparse and contradictory. Here, we compiled a database of larval Antarctic krill, Euphausia superba and, together with an adult database, it showed how their range shift is out of step with the pace of warming. During a 70-year period of rapid warming (1920s–1990s), distribution centres of both larvae and adults in the SW Atlantic sector remained fixed, despite warming by 0.5–1.0°C and losing sea ice. This was followed by a hiatus in surface warming and ice loss, yet during this period the distributions of krill life stages shifted greatly, by ~1000 km, to the south-west. Understanding the mechanism of such step changes is essential, since they herald system reorganizations that are hard to predict with current modelling approaches. We propose that the abrupt shift was driven by climatic controls acting on localized recruitment hotspots, superimposed on thermal niche conservatism. During the warming hiatus, the Southern Annular Mode index continued to become increasingly positive and, likely through reduced feeding success for larvae, this led to a precipitous decline in recruitment from the main reproduction hotspot along the southern Scotia Arc. This cut replenishment to the northern portion of the krill stock, as evidenced by declining density and swarm frequency. Concomitantly, a new, southern reproduction area developed after the 1990s, reinforcing the range shift despite the lack of surface warming. New spawning hotspots may provide the stepping stones needed for range shifts into polar regions, so planning of climate-ready marine protected areas should include these key areas of future habitat.

We present a database from substantial collections of macronutrient data made on 20 oceanographic cruises, primarily from around the island of South Georgia and the Scotia Sea. This sector of the Southern Ocean was studied comprehensively during the Discovery Investigations between ∼1920 and 1950 and resulted in the hugely influential Discovery Reports. Following this pioneering research period, there was a lull of several decades prior to the British Antarctic Survey (BAS) initiation of an offshore biological programme to study the ecology of the South Atlantic sector of the Southern Ocean. These studies began in the late 1970s and have continued until the present day. Between 1980 and 2009, the programme included macronutrient measurements as part of an integrated ecosystem analysis. In addition to South Georgia and the Scotia Sea, measurements were also made in the Bellingshausen Sea and the waters to the west of the Antarctic Peninsula. Data were collected during all months of the year with the exceptions of May and June and compiled into a database. Vertical profile samples were taken from water bottles, while data along transects were collected by monitoring the ship's non-toxic seawater supply. Nutrients measured were silicate (Si(OH)4–Si), phosphate (PO4–P), nitrate (NO3–N), ammonium (NH4–N), and nitrite (NO2–N). Our database includes nutrient data along with contemporaneous temperature and salinity data where available. Further background and supporting information is included for context. The profile and underway sample data have been deposited at the UK Polar Data Centre and are publicly available in NetCDF and CSV formats from https://doi.org/10.5285/4014370F-8EB2-492B-A5F3-6DC68BF12C1E (Whitehouse et al., 2022).

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

AimLanternfishes (Myctophidae) are one of the most species-rich families of mid-water fishes. They inhabit the mesopelagic zone, where physical barriers to dispersal and gene flow are permeable. Thus, modes of speciation that rely exclusively on geographical separation are potentially of less importance than those that rely more prominently on evolution of assortative mating through divergent habitat use and/or sexual signals, including visual signals from bioluminescent light organs. Here we used phylogenetic, ecological and morphological data to investigate the roles of geography, habitat use and lateral photophores in lanternfish speciation.LocationGlobal.Time periodData collected between 1950 and 2015.Major taxa studiedLanternfishes (Myctophidae).MethodsEcological niche models (ENMs) were developed for 167 species, enabling the community composition of 33 mesopelagic ecoregions to be determined. Sequence data for seven protein-coding regions from 175 species were used to reconstruct a phylogenetic tree of Myctophidae. Age-overlap correlation tests were conducted using this phylogeny with outputs from ENMs (pairwise geographical overlap and pairwise ecological niche overlap, n = 136), in addition to matrices of pairwise depth overlap (n = 158) and photophore pattern dissimilarity (n = 161).ResultsCommunities assembled according to nine broad climatic regions, and recently diverged species pairs possessed greater geographical and ecological niche overlap than more distantly related species, indicating that sympatric or parapatric speciation might be dominant modes of divergence. Differences in photophore patterns increased with the relative age of speciation events, suggesting that photophore patterns are largely constrained by phylogeny.Main conclusionsBased on this evidence, we suggest that large-scale oceanographic features structure the diversity of lanternfish communities and that speciation within this family of deep-water fishes might not have required geographical isolation.

Zooplankton form an integral component of epi- and mesopelagic ecosystems, and there is a need to better understand their role in ocean biogeochemistry. The export and remineralisation of particulate organic matter at depth plays an important role in controlling atmospheric CO2 concentrations. Pelagic mesozooplankton and micronekton communities may influence the fate of organic matter in a number of ways, including: the consumption of primary producers and export of this material as fast-sinking faecal pellets, and the active flux of carbon by animals undertaking diel vertical migration (DVM) into the mesopelagic. We present day and night vertical biomass profiles of mesozooplankton and micronekton communities in the upper 500 m during three visits to an ocean observatory station (P3) to the NW of South Georgia (Scotia Sea, South Atlantic) in austral spring, alongside estimates of their daily rates of ingestion and respiration throughout the water column. Day and night community biomass estimates were dominated by copepods >330 μm, including the lipid-rich species, Calanoides acutus and Rhincalanus gigas. We found little evidence of synchronised DVM, with only Metridia spp. and Salpa thompsoni showing patterns consistent with migratory behaviour. At depths below 250 m, estimated community carbon ingestion rates exceeded those of metabolic costs, supporting the understanding that food quality in the mesopelagic is relatively poor, and organisms have to consume a large amount of food in order to fulfil their nutritional requirements. By contrast, estimated community rates of ingestion and metabolic costs at shallower depths were approximately balanced, but only when we assumed that the animals were predominantly catabolising lipids (i.e. respiratory quotient = 0.7) and had relatively high absorption efficiencies. Our work demonstrates that it is possible to balance the metabolic budgets of mesopelagic animals to within observational uncertainties, but highlights the need for a better understanding of the physiology of lipid-storing animals and how it influences carbon budgeting in the pelagic.

We present a database from substantial collections of macronutrient data made on 20 oceanographic cruises, primarily from around the island of South Georgia and the Scotia Sea. This sector of the Southern Ocean was studied comprehensively during the Discovery Investigations between ~1920 and 1950 and resulted in the hugely influential Discovery Reports. Following this pioneering research period, there was a lull of several decades prior to the British Antarctic Survey’s (BAS) initiation of an offshore biological programme to study the ecology of the South Atlantic sector of the Southern Ocean. These studies began in the late 1970s and have continued until the present day. Between 1980–2009, the programme included macronutrient measurements as part of an integrated ecosystem analysis. In addition to South Georgia and the Scotia Sea, measurements were also made in the Bellingshausen Sea and the waters to the west of the Antarctic Peninsula. Data were collected during all months of the year with the exceptions of May and June and compiled into a database. Vertical profile samples were taken from water bottles while data along transects were collected through monitoring the ship’s non-toxic seawater supply. Nutrients measured were silicate, Si(OH)4-Si; phosphate, PO4-P; nitrate, NO3-N; ammonium NH4-N; and nitrite, NO2-N. Our database includes nutrient data along with contemporaneous temperature and salinity data where available. Further background and supporting information are included for context.

Species of the genus Euphausia dominate the euphausiid biomass of the Southern Ocean, the three largest being Euphausia superba, E. triacantha and E. crystallorophias. We measured a number of morphological features to identify differences between, and within, these species to obtain ecological insights. Interspecifically, the greatest difference was carapace size, with that of E. superba being by far the largest and most variable. This likely reflects its prolific spawning capacity compared with other euphausiid species. E. triacantha exhibited an extended sixth abdominal segment that could facilitate greater levels of thrust in the tail flip escape response. The pleopods, which provide propulsion in forward swimming, were more than 50% larger in E. superba, indicating a greater capacity for directional movement at high velocities. E. crystallorophias had eyes that were almost double the size of those in E. superba and E. triacantha, which may help retain visual resolution within its under-ice habitat. Intraspecifically, we found the above morphological features differed little between sexes and developmental stages in E. crystallorophias and E. triacantha, but differed significantly in E. superba. Compared to females and juveniles, male E. superba had significantly larger eyes and pleopods, whilst the carapace in males became shorter as a proportion of body length during growth. These features indicate a greater capacity for searching and swimming in males, which, we hypothesise, increases their ability to locate and fertilise females. This morphological specialisation in male E. superba is indicative of comparatively greater inter-male competition resulting from its tendency to form large, dense swarms.

Mesopelagic fish are an important component of marine ecosystems, and their contribution to marine biogeochemical cycles is becoming increasingly recognized. However, major uncertainties remain in the rates at which they remineralize organic matter. We present respiration rate estimates of mesopelagic fish from two oceanographically contrasting regions: the Scotia Sea and the Benguela Current. Respiration rates were estimated by measuring the enzyme activities of the electron transport system. Regression analysis of respiration with wet mass highlights regional and inter-specific differences. The mean respiration rates of all mesopelagic fish sampled were 593.6 and 354.9 µl O2 individual−1 h−1 in the Scotia Sea and Benguela Current, respectively. Global allometric models performed poorly in colder regions compared with our observations, underestimating respiratory flux in the Scotia Sea by 67–88%. This may reflect that most data used to fit such models are derived from temperate and subtropical regions. We recommend caution when applying globally derived allometric models to regional data, particularly in cold (

The changing Arctic environment is affecting zooplankton that support its abundant wildlife. We examined how these changes are influencing a key zooplankton species, Calanus finmarchicus, principally found in the North Atlantic but expatriated to the Arctic. Close to the ice-edge in the Fram Strait, we identified areas that, since the 1980s, are increasingly favourable to C. finmarchicus. Field-sampling revealed part of the population there to be capable of amassing enough reserves to overwinter. Early developmental stages were also present in early summer, suggesting successful local recruitment. This extension to suitable C. finmarchicus habitat is most likely facilitated by the long-term retreat of the ice-edge, allowing phytoplankton to bloom earlier and for longer and through higher temperatures increasing copepod developmental rates. The increased capacity for this species to complete its life-cycle and prosper in the Fram Strait can change community structure, with large consequences to regional food-webs.

Antarctic krill (Euphausia superba) are swarming, oceanic crustaceans, up to two inches long, and best known as prey for whales and penguins – but they have another important role. With their large size, high biomass and daily vertical migrations they transport and transform essential nutrients, stimulate primary productivity and influence the carbon sink. Antarctic krill are also fished by the Southern Ocean’s largest fishery. Yet how krill fishing impacts nutrient fertilisation and the carbon sink in the Southern Ocean is poorly understood. Our synthesis shows fishery management should consider the influential biogeochemical role of both adult and larval Antarctic krill., Swarms of crustaceans called krill dominate Antarctic ecosystems, yet their influence on biogeochemical cycles remains a mystery. Here Cavan and colleagues review the role of krill in the Southern Ocean, and the impact of the krill fishery on ocean fertilisation and the carbon sink.

Larval recruitment in Antarctic krill is known to be episodic and regional. We consider the importance of a range of recruitment factors using an environmentally driven model of larval development from spawning to post-larvae. Our model examines the timing of spawning, interaction with bathymetry, susceptibility to cold temperatures, temperature-driven development and the seasonal cycle of sea ice to identify those factors with the greatest impact. The model predicts that the seasonal location of sea ice is the main limiting factor for successful larval recruitment. Spawning in January leads to the greatest area of viable larval recruitment habitat. Dense sea ice cover, which we assume that adult krill do not spawn under, prevents spawning in large areas early in the breeding season (December). Nevertheless, later spawning in February, when sea ice is at a minimum, means there is often insufficient time for the larvae to reach a viable developmental stage before the sea ice advances. Meanwhile, although spawning is possible in more northerly areas throughout the breeding season, these are generally remote from winter sea ice, which is assumed to be necessary for larvae to overwinter. Interaction with bathymetry before hatching further limits suitable habitat. Over a 12 yr period, the model predicted larval re - cruitment from January spawning in all years in the Cooperation, Ross and Weddell Seas, with episodic larval recruitment in the Bransfield Strait in 9 of the 12 years. Additional understanding of the overwintering requirements of larvae, together with regional studies at higher spatial resolution, particularly in shelf regions, will better constrain the uncertainties in the model.

The biological carbon pump drives a flux of particulate organic carbon (POC) through the ocean and affects atmospheric levels of carbon dioxide. Short term, episodic flux events are hard to capture with current observational techniques and may thus be underrepresented in POC flux estimates. We model the potential hidden flux of POC originating from Antarctic krill, whose swarming behaviour could result in a major conduit of carbon to depth through their rapid exploitation of phytoplankton blooms and bulk egestion of rapidly sinking faecal pellets (FPs). Our model results suggest a seasonal krill FP export flux of 0.039 GT C across the Southern Ocean marginal ice zone, corresponding to 17–61% (mean 35%) of current satellite-derived export estimates for this zone. The magnitude of our conservatively estimated flux highlights the important role of large, swarming macrozooplankton in POC export and, the need to incorporate such processes more mechanistically to improve model projections., Particulate organic carbon (POC) fluxes in the oceans, particularly the Southern Ocean, remain poorly constrained. Here the authors modelled the potential underestimated flux of POC originating from Antarctic krill and discovered a seasonal krill faecal pellet export flux of 0.039 GT C yr-1 across the marginal ice zone (MIZ) of the Southern Ocean.

Antarctic krill are the dominant metazoan in the Southern Ocean in terms of biomass; however, their wide and patchy distribution means that estimates of their biomass are still uncertain. Most currently employed methods do not sample the upper surface layers, yet historical records indicate that large surface swarms can change the water colour. Ocean colour satellites are able to measure the surface ocean synoptically and should theoretically provide a means for detecting and measuring surface krill swarms. Before we can assess the feasibility of remote detection, more must be known about the reflectance spectra of krill. Here, we measure the reflectance spectral signature of Antarctic krill collected in situ from the Scotia Sea and compare it to that of in situ water. Using a spectroradiometer, we measure a strong absorption feature between 500 and 550 nm, which corresponds to the pigment astaxanthin, and high reflectance in the 600–700 nm range due to the krill's red colouration. We find that the spectra of seawater containing krill is significantly different from seawater only. We conclude that it is tractable to detect high-density swarms of krill remotely using platforms such as optical satellites and unmanned aerial vehicles, and further steps to carry out ground-truthing campaigns are now warranted.

The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS., SEA-DNA, SI

Funding Information: We acknowledge the financial support of the Portuguese Foundation for the Science and Technology (FCT / MCTES) through a PhD grant to Jose Seco (SRFH/PD/BD/113487) and CESAM (UIDP/50017/2020+UIDB/50017/2020), through national funds. The Institut Universitaire de France is acknowledged for its support to P. Bustamante as a Senior Member. This research was also within strategic program of MARE (MARE - UID/MAR/04292/2020). The work is a contribution to the Ecosystems component of the British Antarctic Survey Polar Science for Planet Earth Programme, funded by the Natural Environment Research Council, which is part of UK Research and Innovation. Funding Information: We thank the officers, crew and scientists aboard RSS James Clark Ross during cruises JR177 and JR16003 for their assistance in collecting samples. We also thank Giulia Pompeo for her help with the Hg analysis. We are grateful to G. Guillou from the “Plateforme Analyses Isotopiques” of LIENSs for his assistance during stable isotope analyses at the University of La Rochelle. We acknowledge the financial support of the Portuguese Foundation for the Science and Technology ( FCT / MCTES ) through a PhD grant to José Seco ( SRFH/PD/BD/113487 ) and CESAM ( UIDP/50017/2020+UIDB/50017/2020 ), through national funds. The Institut Universitaire de France is acknowledged for its support to P. Bustamante as a Senior Member. This research was also within strategic program of MARE (MARE - UID/MAR/04292/2020). The work is a contribution to the Ecosystems component of the British Antarctic Survey Polar Science for Planet Earth Programme, funded by the Natural Environment Research Council , which is part of UK Research and Innovation . Publisher Copyright: © 2021 Elsevier Ltd Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Biomagnification of mercury (Hg) in the Scotia Sea food web of the Southern Ocean was examined using the stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) as proxies for trophic level and feeding habitat, respectively. Total Hg and stable isotopes were measured in samples of particulate organic matter (POM), zooplankton, squid, myctophid fish, notothenioid fish and seabird tissues collected in two years (austral summers 2007/08 and 2016/17). Overall, there was extensive overlap in δ13C values across taxonomic groups suggesting similarities in habitats, with the exception of the seabirds, which showed some differences, possibly due to the type of tissue analysed (feathers instead of muscle). δ15N showed increasing enrichment across groups in the order POM to zooplankton to squid to myctophid fish to notothenioid fish to seabirds. There were significant differences in δ15N and δ13C values among species within taxonomic groups, reflecting inter-specific variation in diet. Hg concentrations increased with trophic level, with the lowest values in POM (0.0005 ± 0.0002 μg g−1 dw) and highest values in seabirds (3.88 ± 2.41 μg g−1 in chicks of brown skuas Stercorarius antarcticus). Hg concentrations tended to be lower in 2016/17 than in 2007/08 for mid-trophic level species (squid and fish), but the opposite was found for top predators (i.e. seabirds), which had higher levels in the 2016/17 samples. This may reflect an interannual shift in the Scotia Sea marine food web, caused by the reduced availability of a key prey species, Antarctic krill Euphausia superba. In 2016/17, seabirds would have been forced to feed on higher trophic-level prey, such as myctophids, that have higher Hg burdens. These results suggest that changes in the food web are likely to affect the pathway of mercury to Southern Ocean top predators. Changes in foodweb dynamics influence Hg bioaccumulation in top predators. Postprint

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22037-y

Diel vertical migration is a widespread behaviour amongst zooplankton, yet its effect on the rate at which individuals respire remains poorly understood. To address this, we investigated the effect of short-term temperature change on the respiration rate ofEuphausia triacantha, a common component of the Southern Ocean zooplankton and a prominent vertical migrator. We found respiration to vary in response to size, with a value of 0.84 for the scaling coefficient,b. When scaled tob, respiration varied strongly in response to transitory temperature change, ranging from 0.37 to 1.65 µl O2mg dry weight (DW)-bh-1between 0.17 and 4.74°C, resulting in aQ10of 3.6. ThisQ10is higher than found by other studies examining the short-term respiration response of euphausiids, including those taking a multi-species perspective. This indicates thatE. triacanthashows little compensation during short-term exposure to temperatures normally encountered during migration. Furthermore, it shows a distinct metabolic cost to diel vertical migration (DVM) when substantive changes in temperature are encountered over the course of the transit. This temporal variability in respiration rate has important implications for how community respiration is estimated, and for our understanding of DVM behaviour. Our results also have particular relevance to estimating the flux and sequestration of respiratory products, such as dissolved carbon dioxide, to and within the ocean interior.

Antarctic krill play an important role in biogeochemical cycles and can potentially generate high-particulate organic carbon (POC) fluxes to the deep ocean. They also have an unusual trait of moulting continuously throughout their life-cycle. We determine the krill seasonal contribution to POC flux in terms of faecal pellets (FP), exuviae and carcasses from sediment trap samples collected in the Southern Ocean. We found that krill moulting generated an exuviae flux of similar order to that of FP, together accounting for 87% of an annual POC flux (22.8 g m−2 y−1). Using an inverse modelling approach, we determined the krill population size necessary to generate this flux peaked at 261 g m−2. This study shows the important role of krill exuviae as a vector for POC flux. Since krill moulting cycle depends on temperature, our results highlight the sensitivity of POC flux to rapid regional environmental change., Antarctic krill are known to be important to the carbon cycle, but the exact contribution is not known. Here the authors show that krill moulting is a major vector of carbon export in the Southern Ocean, together with krill faecal pellets accounting for almost 90% of annual particulate organic carbon flux.

Ocean life helps keep atmospheric levels of carbon dioxide lower by taking carbon out of the atmosphere and transporting it to the deep ocean, through sinking particles. Antarctic krill live in the Southern Ocean and gather in huge swarms. Importantly, they produce large, fast-sinking poo (called fecal strings), meaning that we get a rain of poo below these swarms, which takes carbon quickly to the deep ocean. We calculate how krill lower the amount of carbon in the atmosphere by estimating how many krill there are and how many fecal strings each produces. These calculations reveal that Antarctic krill living near sea ice remove 39 million tons of carbon from the surface ocean each year. That is about the same weight as 100 million polar bears! Krill are therefore not only important food for whales and penguins, but also vital engineers of our climate and so they require protection.

Antarctic krill, Euphausia superba, supports a valuable commercial fishery in the Southwest Atlantic, which holds the highest krill densities and is warming rapidly. The krill catch is increasing, is concentrated in a small area, and has shifted seasonally from summer to autumn/winter. The fishery is managed by the Commission for the Conservation of Antarctic Marine Living Resources, with the main goal of safeguarding the large populations of krilldependent predators. Here we show that, because of the restricted distribution of successfully spawning krill and high inter-annual variability in their biomass, the risk of direct fishery impacts on the krill stock itself might be higher than previously thought. We show how management benefits could be achieved by incorporating uncertainty surrounding key aspects of krill ecology into management decisions, and how knowledge can be improved in these key areas. This improved information may be supplied, in part, by the fishery itself. Fil: Meyer, Bettina. Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung; Alemania. University of Oldenburg; Alemania Fil: Atkinson, Angus. Plymouth Marine Laboratory; Reino Unido Fil: Bernard, Kim S.. State University Of Oregon. College Of Earth, Ocean And Atmospheric Sciences.; Estados Unidos Fil: Brierley, Andrew S.. University of St. Andrews; Reino Unido Fil: Driscoll, Ryan. Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung; Alemania Fil: Hill, Simeon L.. British Antartic Survey; Reino Unido Fil: Marschoff, Enrique Ricardo. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Maschette, Dale. University of Tasmania; Australia. Australian Antarctic Division; Australia Fil: Perry, Frances A.. Plymouth Marine Laboratory; Reino Unido Fil: Reiss, Christian S.. Southwest Fisheries Science Center,; Estados Unidos Fil: Rombola, Emilce Florencia. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Tarling, Geraint A.. British Antartic Survey; Reino Unido Fil: Thorpe, Sally E.. British Antartic Survey; Reino Unido Fil: Trathan, Philip N.. British Antartic Survey; Reino Unido Fil: Zhu, Guoping. Shanghai Ocean University; China Fil: Kawaguchi, So. Australian Antarctic Division; Australia

Ecological niche models (ENMs) can be a practical approach for investigating distributions and habitat characteristics of pelagic species. In principle, to reflect the ecological niche of a species well, ENMs should incorporate environmental predictors that consider its full vertical habitat, yet examples of such models are rare. Here we present the first application of ‘3D’ ENMs to 10 Southern Ocean lanternfish species. This 3D approach incorporates depth-specific environmental predictor data to identify the distribution of suitable habitat across multiple depth levels. Results were compared to those from the more common ‘2D’ approach, which uses only environmental data from the sea surface. Measures of model discriminatory ability and overfitting indicated that 2D models often outperform 3D methods, even when accounting for reduced available sample size in the 3D models. Nevertheless, models for species with a known affinity for deeper habitat benefitted from the 3D approach, and our results suggest that species can track their ecological niche in latitude and depth leading to equatorward or poleward range extensions beyond that expected from incorporating only surface data. However, since 3D models require comprehensive depth-specific data, both data availability and the need for depth-specific model outputs must be considered when choosing the appropriate modelling approach. We advocate increased effort to include depth-resolved environmental parameters within marine ENMs. This will require collection of mesopelagic species occurrence data using appropriate temporal and depth-stratified methods, and inclusion of accurate depth information when occurrence records are submitted to global biodiversity databases.

Antarctic krill inhabit areas of the Southern Ocean that can exceed 4.0°C, yet they preferentially inhabit regions with temperatures of −1.5 to ≤1.5°C. Successful embryonic development and hatching are key to their life cycle, but despite the rapid climatic warming seen across their main spawning areas, the effects of elevated temperatures on embryogenesis, hatching success, and nauplii malformations are unknown. We incubated 24,483 krill embryos in two independent experiments to investigate the hypothesis that temperatures exceeding 1.5°C have a negative impact on hatching success and increase the numbers of malformed nauplii. Field experiments were on krill collected from near the northern, warm limit of their range and embryos incubated soon after capture, while laboratory experiments were on embryos from krill acclimated to laboratory conditions. The hatching success of embryo batches varied enormously, from 0 to 98% (mean 27%). Both field and laboratory experiments showed that hatching success decreased markedly above 3.0°C. Our field experiments also showed an approximate doubling of the percentage of malformed nauplii at elevated temperatures, reaching 50% at 5.0°C. At 3.0°C or below, however, temperature was not the main factor driving the large variation in embryo hatching success. Our observations of highly variable and often low success of hatching to healthy nauplii suggest that indices of reproductive potential of female krill relate poorly to the subsequent production of viable krill larvae and may help to explain spatial discrepancies between the distribution of the spawning stock and larval distribution.

Copepods are amongst the most abundant animals on our planet. Who knew?! These small (typically 1–10 mm) crustaceans are found in all of the world’s oceans and play an important role in regulating Earth’s climate. Like wildebeest in the Serengeti graze on grasslands and are food for lions, herbivorous copepods represent a vital link in oceanic food chains between microscopic algae and higher predators, such as fish, birds, and whales. A group of copepods called Calanus are particularly important in the Northern Hemisphere. These tiny-but-mighty animals also share the wildebeest’s need to make a large annual migration—but in their case, they sink thousands of meters downwards to spend the winter in the deep, dark ocean. Understanding the lives of marine copepods, and how their populations will respond to climate change, is crucial for predicting the future health of the marine environment and how it helps our planet.

The South Sandwich Islands (SSI) are a biologically productive archipelago situated in the eastern Scotia Sea to the south of the eastward flowing Antarctic Circumpolar Current (ACC). The islands support important populations of higher predators, including several penguin species, seals and humpback whales. Despite this, the plankton ecology of the region has been little studied and information on mesoscale structure and environmental forcing of plankton ecology is particularly limited. We conducted a comprehensive oceanographic and net sampling campaign during the CCAMLR Area 48 Survey (January and February 2019), incorporating phytoplankton, mesozooplankton and macrozooplankton/nekton. Satellite chlorophyll-a (chl-a) data showed the development of a large bloom that was initiated two months prior to our study period at the south-eastern edge of the archipelago and propagated northwards along the eastern side, limited to the east by mesoscale features associated with the southern boundary of the ACC (SB). Multivariate cluster analysis revealed distinct mesoscale structure within the plankton community, with four spatially defined groups of phytoplankton and macrozooplankton/nekton, and three cluster groups of mesozooplankton. North of the SB, we found some spatial congruence between the three plankton assemblages, with a distinct, spatially coherent, cluster in each, corresponding to a warmer water community. Here, biomass was dominated by mesozooplankton, particularly calanoid copepods Rhincalanus gigas, Calanus propinquus, C. simillimus and Euchaetidae. The corresponding phytoplankton community was dominated by small diatoms, particularly Thalassionema spp., Pseudo-nitzschia spp., Fragilariopsis spp. and Chaetoceros spp., whilst Themisto gaudichaudii, Euphausia triacantha and myctophids were the major contributors to the macrozooplankton/nekton community. South of the SB, there was some spatial congruence between phytoplankton and macrozooplankton/nekton community structure on the western side of the archipelago, as well as on the eastern side that corresponded to the location of the bloom, but less association with mesozooplankton structure. Macrozooplankton/nekton structure was strongly driven by environmental conditions 1–2 months prior to the survey, including sea-ice distribution, surface phytoplankton concentration and productivity, whilst mesozooplankton was more tightly coupled to in-situ prevailing conditions such as surface temperature and integrated chl-a. Top-down pressure between trophic levels may have also had an influence on spatial patterns although direct evidence is lacking. Antarctic krill (Euphausia superba) was found with relatively low biomass at our net sampling sites (median biomass of 0.04 mg m−3 or

Chronic oil pollution related to gas and oil drilling activities is increasing in the sea due to the rising offshore petroleum industry activity. Among marine organisms, zooplankton play a crucial role in the marine ecosystem and therefore understanding the effects of crude oil chronic exposure on zooplankton is needed to determine the impact of oil in marine environments. The present study reports on the effect of crude oil on adult northern krill, Meganyctiphanes norvegica, collected during three seasons. Their sensitivity to oil was examined with oil concentration of 0.01 versus 0.1 mg oil L− 1 and photo-modified oil in flowing seawater maintained in the dark for 2 weeks at in situ temperature. Oil (polycyclic aromatic hydrocarbons, PAHs) entered the krill (on average, 350 and 4400 μg·kg− 1 wet weight in low and medium oil treatments respectively) and a larger fraction of the krill exhibited digestive gland pathologies (enhanced apoptosis and pathology of digestive tubules) in oil treatments (27–80%) compared to a significantly lower fraction (7–13%) in treatments that received no oil. However, 2-week oil exposure at these concentrations did not significantly decrease survivorship or impair basic functioning such as feeding and respiration rates. Similarly, there were only limited changes in the transcription of 7 selected genes from head tissue. Additionally, although there was significant seasonal variation in krill total lipid content and fatty acid composition, there was no treatment effect on both these parameters, which suggests limited oxidative stress under experimental conditions. Furthermore, there was no significant treatment effect on two direct measures of oxidative stress (MDA: malondialdehyde and AOPP: advanced oxidation protein products) in any of the seasons. Nevertheless, histology clearly revealed enhanced digestive gland pathologies in krill even at low concentrations. Although krill with such pathologies continue to survive, their accumulation of PAHs may be transferred up the food chain, impacting their predators and the wider ecosystem.

The applicability of macroecological rules to patterns in body size varies between taxa. One of the most examined is Bergmann’s rule, which states that body size increases with decreasing temperature and increasing latitude, although the rule is not universal and the proposed mechanisms underpinning it are multifarious and lack congruence. This study considers the degree to which Bergmann’s rule applies to the Southern Ocean mesopelagic fish community. We studied patterns in body size, temperature, and latitude across a 12° latitudinal gradient within the Scotia-Weddell sector. Intraspecific Bergmann’s rule was found to apply to 8 of the 11 biomass-dominant species in the family Myctophidae. The rule was also apparent at an interspecific level. Our study suggests that greater attainable body size in this community is a necessary attribute to reach colder regions further south. The adherence of these taxa to Bergmann’s rule enables such species to act as sentinels for identifying the drivers and consequences of ocean warming for the Southern Ocean ecosystem.

Rising oil and gas activities in northern high latitudes have led to an increased risk of petroleum pollution in these ecosystems. Further, seasonal high UV radiation at high latitudes may elevate photo-enhanced toxicity of petroleum pollution to marine organisms. Zooplanktons are a key ecological component of northern ecosystems; therefore, it is important to assess their sensitivity to potential pollutants of oil and gas activity. As ontogenetic development may be particularly sensitive, the aim of this study was to examine the impact of chronic exposure to oil water dispersion (OWD) on development and feeding of early life stages of the Northern krill, Meganyctiphanes norvegica. In a range of experiments, embryonic, nonfeeding, and feeding larval stages were exposed to concentrations of between 0.01 and 0.1 mg/L of oil or photo-modified oil for 19 and 21 d. No significant effects on egg respiration, hatching success, development, length and larval survival were observed from these treatments. Similarly, evolution of fatty acid composition patterns during ontogenetic development was unaffected. The results indicates a high degree of resilience of these early developmental stages to such types and concentrations of pollutants. However, feeding and motility in later calyptopis-stage larvae were significantly impaired at exposure of 0.1 mg/L oil. Data indicate that feeding larval stage of krill was more sensitive to OWD than early nonfeeding life stages. This might be attributed to the narcotic effects of oil pollutants, their direct ingestion, or accumulated adverse effects over early development.

The biogeographic response of oceanic planktonic communities to climatic change has a large influence on the future stability of marine food webs and the functioning of global biogeochemical cycles. Temperature plays a pivotal role in determining the distribution of these communities and ocean warming has the potential to cause major distributional shifts, particularly in polar regions where the thermal envelope is narrow. We considered the impact of long-term ocean warming on the spatial distribution of Southern Ocean mesozooplankton communities through examining plankton abundance in relation to sea surface temperature between two distinct periods, separated by around 60 years. Analyses considered 16 dominant mesozooplankton taxa (in terms of biomass and abundance) in the southwest Atlantic sector of the Southern Ocean, from net samples and in situ temperature records collected during the Discovery Investigations (1926-1938) and contemporary campaigns (1996-2013). Sea surface temperature was found to have increased significantly by 0.74°C between the two eras. The corresponding sea surface temperature at which community abundance peaked was also significantly higher in contemporary times, by 0.98°C. Spatial projections indicated that the geographical location of community peak abundance had remained the same between the two eras despite the poleward advance of sea surface isotherms. If the community had remained within the same thermal envelope as in the 1920s-1930s, community peak abundance would be 500 km further south in the contemporary era. Studies in the northern hemisphere have found that dominant taxa, such as calanoid copepods, have conserved their thermal niches and tracked surface isotherms polewards. The fact that this has not occurred in the Southern Ocean suggests that other selective pressures, particularly food availability and the properties of underlying water masses, place greater constraints on spatial distributions in this region. It further demonstrates that this community is thermally resilient to present levels of sea surface warming.

Food webs in high-latitude oceans are dominated by relatively few species. Future ocean and sea-ice changes affecting the distribution of such species will impact the structure and functioning of whole ecosystems. Antarctic krill (Euphausia superba) is a key species in Southern Ocean food webs, but there is little understanding of the factors influencing its success throughout much of the ocean. The capacity of a habitat to maintain growth will be crucial and here we use an empirical relationship of growth rate to assess seasonal spatial variability. Over much of the ocean, potential for growth is limited, with three restricted oceanic regions where seasonal conditions permit high growth rates, and only a few areas around the Scotia Sea and Antarctic Peninsula suitable for growth of the largest krill (>60 mm). Our study demonstrates that projections of impacts of future change need to account for spatial and seasonal variability of key ecological processes within ocean ecosystems.