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Aggregation structures of Antarctic krill (Euphausia superba) and predator interactions were investigated using active acoustic data collected for approximately one month by WBAT and Signature100 echosounders deployed on moorings in two hydrographically different sites in Bransfield Strait. Near Nelson Island, water flows from the northwest to southeast while Deception Island is influenced by the Bransfield Front with stronger net current velocities from the southwest to northeast. Krill aggregations were identified and then classified in three clusters using a swarm-identification algorithm and hierarchical clustering using aggregation morphological characteristics: acoustic density, mean depth, center of mass, inertia, equivalent area, aggregation index, and proportion occupied. A total of 693 and 736 aggregations were detected at the mooring sites close to Nelson and Deception Islands. The three aggregation categories ranged from high to low densities, evenness and dispersion and were distributed throughout the water column. Krill aggregation densities and mean thickness are influenced by current velocities, direction, mean depth, and predator foraging. The majority of observed predator dive profiles occurred over the aggregation type with highest krill densities at both Nelson and Deception Islands, and within the first 25 m of the water column. The heterogeneity of krill aggregations potentially impacts predator foraging strategies and predator-krill interactions in the hydrodynamically active Bransfield Strait.

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.

The Southern Ocean zooplankton community is diverse, yet most species are understudied, especially with respect to their overwinter feeding ecologies. Here we present body condition and trophic biomarker data (lipid content and stable isotopes of carbon and nitrogen) from 19 zooplankton species collected over five consecutive winters (August and September 2012–2016) around the northern Antarctic Peninsula. We report environmental data (percent sea-ice cover, sea-ice type, water temperature, salinity, and integrated chl-a) as well as species abundance data at each sampling location to provide additional context for interpreting the lipid and stable isotope data. For most species, these are the first winter measurements or time series of body condition, trophic position, and abundance in relation to environmental variables. These data are critical for evaluating changes in ecosystem structure and predator-prey relationships in a region of Antarctica that is warming faster than most other areas on Earth as a result of climate change., Measurement(s) lipid • stable isotope analysis • concentration of chlorophyll a in water • sea ice • species abundance • daylight conditions • carbon content • nitrogen content • temperature of water • ocean salinity Technology Type(s) chemical extraction • elemental analysis isotope ratio mass spectrometry • fluorometer • visual observation method • Count • environmental condition • Temperature Sensor Device • Sensor Device Factor Type(s) water-column chlorophyll-a Sample Characteristic - Organism Amphipoda • Copepoda • Chaetognatha • Polychaeta • Cnidarian • Ctenophora • Euphausiidae • Gastropoda • Tunicata Sample Characteristic - Environment ocean Sample Characteristic - Location West Antarctica Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13003151

Overwinter environmental conditions are a major driver of larval survival, recruitment, and reproductive pre-conditioning in Antarctic krill (Euphausia superba). Few winter studies exist from which to infer impacts of changing environmental conditions on the biology of krill. Here demographic and maturity patterns of krill during five winters are examined at an overwintering hotspot in the northern Antarctic Peninsula. Two different recruitment pulses were present over the 5 years of the study. A large recruitment event was evident in winter 2012, followed by cohort growth tracked through winter 2014. A recruitment event also occurred in 2015, despite lower abundance of larvae in 2013 and 2014. The recruitment pulse in 2015 was correlated with the intrusion of high salinity water, characteristic of the Weddell Sea, during a positive El Nino–Southern Oscillation phase, suggesting recruitment was driven by advection of recruits into the northern Antarctic Peninsula. Despite the inter-annual environmental variability, most female krill were in resting reproductive stages in all years. The lack of variability in female reproductive status during winter may impose a developmental constraint to earlier spawning than previously considered; the time necessary to advance from resting stages to reproductive maturity may limit the ability of krill to adapt to trends in the timing and magnitude of the spring bloom. The observed patterns of recruitment and variation in maturity stages highlight the complexity of krill population dynamics and should be considered when projecting population responses of Antarctic krill to climate-driven changes in the Southern Ocean.

Winter sea-ice conditions are considered important for Antarctic krillEuphausia superbasurvival and recruitment, yet few broad-scale longitudinal studies have examined the underlying relationships between winter conditions and krill recruitment. We used data from a 4 yr winter study of krill condition (lipid content), diet (stable isotopes and fatty acids), and length distributions around the northern Antarctic Peninsula to examine relationships among environmental variables (annual sea-ice cover, water column chlorophylla[chla], and upper mixed-layer water temperature), the condition and diet of krill, and recruitment success the following year. Diet indicators (lipid content, δ15N, δ13C, and the fatty acid ratios 16:1n-7/18:4n-3 and 18:1n-9/18:1n-7) in post-larvae were consistent among years regardless of sea-ice cover, suggesting that post-larval krill do not rely on sea-ice resources for overwinter survival. Diet indicators in larvae were more variable and suggest that larvae may feed on sea-ice resources when they are available but can still persist in the water column when they are not. Principal component analysis between environmental variables and diet indicators showed that water-column chlawas the only variable that significantly affected diet, regardless of annual changes in sea-ice cover. Extensive winter ice in one year did not translate into successful recruitment the following year. Krill demonstrate a high degree of flexibility with respect to overwinter habitat and diet, and the degree to which sea ice is important during different times of year and at different life stages may be more complex than previously thought.

Climate forcing is impacting polar marine ecosystems through increased variability of winter sea-ice dynamics, which likely influences the distribution, abundance and structure of zooplankton assemblages, and thereby trophodynamics of marine food webs. Due to the challenges of working in polar marine ecosystems, most knowledge on polar zooplankton community structure is derived from summer surveys. Here we examine the spatial distribution, abundance and community structure of macrozooplankton in relation to sea-ice and ocean-climate dynamics within the Antarctic Peninsula marine ecosystem over five consecutive winters. We compare the patterns revealed during winter with historical data collected in the same region during austral summer. Hydrographic and macrozooplankton data were collected from >100 standard stations off the northern Antarctic Peninsula during summer (2003–2011) and winter (2012–2016). Using multivariate methods, the environmental drivers and geographic structuring of the macrozooplankton community during winter and summer were investigated. Eight taxa made up 90% of total macrozooplankton abundance in winter including Metridia species, post-larval and larval Euphausia superba, post-larval Thysanoessa macrura, Limacina helicina, Chaetognatha, Ostracoda and Radiozoa. Eight slightly different taxa including Calanoides acutus, Salpa thompsoni, T. macrura (post-larvae and larvae), Metridia spp., E. superba larvae, Chaetognatha, and Rhincalanus spp. made up 87% of the total abundance in summer. Macrozooplankton clustered into five groups in winter and seven groups in summer. Winter macrozooplankton structure was more spatially consistent among years compared to summer regardless of sea-ice conditions. Salinity, chlorophyll a biomass, upper mixed layer depth and time of day were most strongly correlated with the multivariate ordination in winter whereas salinity, phaeopigment biomass and year had the highest correlations for summer, indicating the importance of similar physical features in both seasons. However, the importance of time scales differed among seasons. Although environmental determinants of summer and winter macrozooplankton community structure indicate that community structure and occurrence were strongly tied to regional variability of salinity and primary productivity gradients, macrozooplankton community structure is likely much more complex than only a few hydrographic variables can explain. Cluster boundaries are likely driven by dynamic locations of currents, fronts and localized eddies in any given season or year.

Three alternative forms of the spawner-recruit relationship – the Beverton-Holt and two expressions of the Ricker function - and a fourth form representing annual recruitment deviations without an explicit relationship to spawning biomass, were applied in an integrated assessment model for Antarctic krill. Model data were from the international krill fishery and U.S. AMLR surveys in the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Subarea 48.1, the area around the Antarctic peninsula where the majority of recent fishing for krill has taken place. Sensitivity tests using pre-specified values for the standard deviation of the logarithm of annual recruitment residuals ( σ R ) and productivity (steepness, h ) for the relationship between spawning biomass and subsequent recruitment indicated that neither the Beverton-Holt nor Ricker functions were superior to configurations of the model that did not assume a spawner-recruit relationship. As the predictive power of the spawner-recruit functions was reduced by increasing the value of σ R the fit to the data improved. All the functions fitted the data best when σ R was large enough so that random recruitment with respect to spawning biomass was produced. The alternative functions produced the same time series of recruitment and spawning biomass estimates when they were determined by the data rather than forced towards an assumed relationship. Although the Beverton-Holt and Ricker models were poor predictors of the data, cross-correlations of the recruitment predictions with the spawning biomass predictions from the best models indicated that recruitment was statistically correlated with spawning biomasses in the previous two to four years. The highest correlation (0.58) was with spawning biomass three years previously. Correlations in the reverse direction, with spawning biomass following recruitment were insignificant. These results suggest that recruitment variability does not influence subsequent spawning biomass in krill. They leave open the possibility that krill spawning biomass could influence subsequent recruitment, but not as modeled in the Beverton-Holt or Ricker functions with a one-year lag and the pre-specified von Bertalanffy growth.

The Antarctic krill, Euphausia superba, has evolved seasonal rhythms of physiology and behaviour to survive under the extreme photoperiodic conditions in the Southern Ocean. However, the molecular mechanisms generating these rhythms remain far from understood. The aim of this study was to investigate seasonal differences in gene expression in three different latitudinal regions (South Georgia, South Orkneys/Bransfield Strait, Lazarev Sea) and to identify genes with potential regulatory roles in the seasonal life cycle of Antarctic krill. The RNA-seq data were analysed (a) for seasonal differences between summer and winter krill sampled from each region, and (b) for regional differences within each season. A large majority of genes showed an up-regulation in summer krill in all regions with respect to winter krill. However, seasonal differences in gene expression were less pronounced in Antarctic krill from South Georgia, most likely due to the milder seasonal conditions of the lower latitudes of this region, with a less extreme light regime and food availability between summer and winter. Our results suggest that in the South Orkneys/Bransfield Strait and Lazarev Sea region, Antarctic krill entered a state of metabolic depression and regressed development (winter quiescence) in winter. Moreover, seasonal gene expression signatures seem to be driven by a photoperiodic timing system that may adapt the flexible behaviour and physiology of Antarctic krill to the highly seasonal environment according to the latitudinal region. However, at the lower latitude South Georgia region, food availability might represent the main environmental cue influencing seasonal physiology.

We compare estimates of krill density derived from gliders to those from contemporaneous and previous ship-based surveys. Our comparisons cover several temporal and spatial scales within two strata around the northern Antarctic Peninsula (off Cape Shirreff on the north side of Livingston Island and in the Bransfield Strait). Our objective is to explore the feasibility of using gliders to supplement or replace vessel-based surveys of fishery resources. We deployed two long-duration Slocum G3 gliders manufactured by Teledyne Webb Research (TWR), each equipped with a suite of oceanographic sensors and a three-frequency (38, 67.5, and 125 kHz, each single-beam) Acoustic Zooplankton Fish Profiler. We used the acoustic data collected by these gliders to estimate biomass densities (g⋅m–2) of Antarctic krill (Euphausia superba). The two gliders were, respectively, deployed for 82 and 88 days from mid-December 2018 through mid-March 2019. Off Cape Shirreff, glider-based densities estimated from two repeat small-scale surveys during mid-December and January were 110.6 and 55.7 g⋅m–2, respectively. In Bransfield Strait, the glider-based estimate of biomass density was 106.7 g⋅m–2 during December–January. Contemporaneous ship-based estimates of biomass density, from a multi-ship broad-scale krill survey (Macaulay et al., 2019) restricted to the areas sampled by the gliders, were 84.6 g⋅m–2 off Cape Shirreff and 79.7 g⋅m–2 in Bransfield Strait during January. We compared two alternative krill-delineation algorithms (dB differencing and SHAPES); differences between biomass densities estimated by applying these algorithms were small and ranged between 4 and 7%. Alternative methods of sampling krill length-frequency distributions (LFDs) (nets or predator diets), which are required to convert acoustic energy to biomass density, also influenced the glider-based results. In Bransfield Strait, net-based estimates of biomass density were 6% less than those based on predator diets. Off Cape Shirreff the biomass density of krill estimated from a net-based LFD was 20% greater than that based on predator diets. Development of a variance estimator for glider-based biomass surveys is ongoing, but our results demonstrate that fisheries surveys using acoustically-equipped gliders are feasible, can provide density estimates to inform management, and may be conducted at lower cost than ship surveys in some cases.

Estimating when and where survival bottlenecks occur in free-ranging marine predators is critical for effective demographic monitoring and spatial planning. This is particularly relevant to juvenile stages of long-lived species for which direct observations of death are typically not possible. We used satellite telemetry data from fledgling Adelie, chinstrap and gentoo penguins near the Antarctic Peninsula to estimate the spatio-temporal scale of a bottleneck after fledging. Fledglings were tracked up to 106 days over distances of up to 2140 km. Cumulative losses of tags increased to 73% within 16 days of deployment, followed by an order-of-magnitude reduction in loss rates thereafter. The timing and location of tag losses were consistent with at-sea observations of penguin carcasses and bioenergetics simulations of mass loss to thresholds associated with low recruitment probability. A bootstrapping procedure is used to assess tag loss owing to death versus other factors. Results suggest insensitivity in the timing of the bottleneck and quantify plausible ranges of mortality rates within the bottleneck. The weight of evidence indicates that a survival bottleneck for fledgling penguins is acute, attributable to predation and starvation, and may account for at least 33% of juvenile mortality., Facultad de Ciencias Naturales y Museo

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

SynopsisAntarctic krill (Euphausia superba) larval production and overwinter survival drive recruitment variability, which in turn determines abundance trends. The Antarctic Peninsula has been described as a recruitment hot spot and as a potentially important source region for larval and juvenile krill dispersal. However, there has been no analysis to spatially resolve regional-scale krill population dynamics across life stages. We assessed spatiotemporal patterns in krill demography using two decades of austral summer data collected along the North and West Antarctic Peninsula since 1993. We identified persistent spatial segregation in the summer distribution of euphausiid larvae (E. superba plus other species), which were concentrated in oceanic waters along the continental slope, and E. superba recruits, which were concentrated in shelf and coastal waters. Mature females of E. superba were more abundant over the continental shelf than the slope or coast. Euphausiid larval abundance was relatively localized and weakly correlated between the North and West Antarctic Peninsula, while E. superba recruitment was generally synchronized throughout the entire region. Euphausiid larval abundance along the West Antarctic Peninsula slope explained E. superba recruitment in shelf and coastal waters the next year. Given the localized nature of krill productivity, it is critical to evaluate the connectivity between upstream and downstream areas of the Antarctic Peninsula and beyond. Krill fishery catch distributions and population projections in the context of a changing climate should account for ontogenetic habitat partitioning, regional population connectivity, and highly variable recruitment.

The Southern Ocean near the Western Antarctic Peninsula (WAP) is strongly affected by climate change resulting in warmer air temperature, accompanied with reduced sea ice coverage, increased sea water temperature and potential changes in the abundances of two key grazer species Salpa thompsoni (salp) and Euphausia superba (Antarctic krill). While salp abundance is hypothesized to increase, krill abundance is hypothesized to decline with dramatic consequences for the entire food web of the Southern Ocean. A better understanding of the biotic interaction between krill and salps and their population dynamics is thus crucial. However, the life cycle of salps is complicated and barely understood. Therefore, we have developed an individual-based model describing the whole life cycle to better understand the population dynamics of salps and the conditions for blooms. The model has been used to explore if and under what conditions the empirical pattern of large variability in observed salp abundances at the WAP, generated by the long-term data of the US Antarctic Marine Living Resources Program (AMLR) can emerge from a small seeding population. The model reproduced this empirical pattern if daily growth rates of oozoids were higher than previously reported for the WAP (mean growth rate for oozoids ~ 1 mm d−1) and if growth rates of blastozooids were lower (mean growth rate ~ 0.2 mm d−1). The model suggests that a prerequisite for local salp blooms requires a small founding population in early spring. With climate change it has been suggested that more frequent and earlier transport of salps into the WAP or winter survival will occur. Hence, the risk of salp blooms in the WAP is likely to substantially increase. These findings highlight the importance for an improved quantitative understanding of how primary production and the southward advection of salps will be impacted by climate change.

Krill overwintering strategies vary with ontogeny and year; understanding this variability is essential to predicting how the species will respond to climate change in the future. Overwintering studies have focused on larval and adult krill, but we know little about how juvenile krill overwinter. The late winter diet of juvenile krill is important because it will determine their growth and development rates and consequently their reproductive potential the following spring. A diet rich in ice algae would promote growth and reproductive development. The Bransfield Strait (northern Antarctic Peninsula, AP) is an important overwintering ground for krill; it has been proposed this region offers a food-rich winter environment. We examined the contribution of ice algae to the energy budget of overwintering juvenile krill during 2 years with contrasting sea ice conditions. Grazing on ice algae contributed ∼146% to their winter energy budget in 2015, even though ice concentrations were ≤50% and consisted of newly formed pancake ice. However, when sea ice advanced late in the Bransfield Strait (2016), ice algae contributed significantly less (∼16%) to the winter energy budget of juvenile krill. Delayed sea ice advance may negatively affect growth and reproductive development of overwintering juvenile krill.

An oceanographic survey around the South Shetland Islands (SSI) and the South Orkney Islands (SOI) was conducted during January 2007 and February 2008, respectively, as part of the United States Antarctic Marine Living Resources (AMLR) program ecosystems surveys. At 27 stations, concentrations of dissolved labile Fe (DFe) and total acid leachable (unfiltered, TaLFe) iron (Fe) were measured in the upper 200 m (including coastal and oceanic waters) to better resolve the factors limiting primary production in these regions. Northwest of the SSI, a region influenced by Drake Passage (DP) waters, mean DFe (∼0.26 nM) and TaLFe (∼1.02 nM) concentrations were the lowest, whereas intermediate concentrations for both DFe and TaLFe were measured in the Bransfield Strait (BS). Around Elephant Island (EI), over and off the continental shelf, Fe concentrations differed between the west and the east margins. DFe and TaLFe concentrations further support the argument that the effect of the Shackleton Transverse Ridge (STR) is a crucial structure affecting both the Fe and the chlorophyll distributions in this region. The waters around the SOI had DFe concentrations higher than those in the SSI, with the area north of the South Scotia Ridge (SSR) (60°S), having the highest DFe (0.54 nM) concentrations and the waters in Powell Basin (PB) having the lowest DFe (1.17 nM) and TaLFe (4.51 nM) and concentrations. These spatial patterns of Fe suggest that there are different Fe inputs from shelf waters near the Antarctic Slope Front (ASF). The overall TaLFe:DFe ratios, used as indicator for understanding the relative distance of Fe sources, were lower around the SOI compared to those in the SSI, suggesting that the Fe source for SOI waters was more distant. The spatial patterns between Fe and chlorophyll-a (Chl-a) concentrations in relation to the hydrography highlight the complexity and variability of the oceanographic processes in the region. These results improve the knowledge on the Fe sources and inputs in the less known SOI waters during the austral summer, and they further support the importance of advective processes from the Fe-rich waters that flow from the eastern margin of the Antarctic Peninsula (AP) into the Weddell-Scotia Confluence (WSC). Copyright © 2019 Sanchez, Reiss, Holm-Hansen, Hewes, Bizsel and Ardelan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5–100 m) and mesopelagic (200–750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42 ± 4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0°S–61.5°S) to ice-covered regions (61.5°S–62.5°S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.

The discipline of ecosystem oceanography provides a framework for assessing the role of mesoscale physical processes on the formation and occurrence of biological hotspots. We used shipboard surveys over nine years to investigate environmental determinants of seabird hotspots near the Antarctic Peninsula, a region experiencing rapid climate change and an expanding krill fishery. We hypothesize that seabird hotspots are structured by mesoscale ocean conditions that reflect differences in prey distribution within oceanic and coastal waters. We used generalized additive models to quantify functional relationships of seabird hotspots with krill biomass, and a suite of remotely sensed environmental variables, such as eddy kinetic energy. The spatial organization, changes in intensity, and distribution shifts of seabird hotspots indicate different environmental drivers within coastal and oceanic domains and reflect the seasonal variability of the ecosystem. Our results indicate at least eight mesoscale hotspot zones that represent ecologically important areas where significant krill and predator biomass may be concentrated. Our ecosystem assessment of seabird hotspots identified critical foraging habitat and provided reference points to benefit research on estimating their trophic impacts on Antarctic ecosystems and potential effects from the krill fishery. Our approach is generally applicable to other pelagic ecosystems that are structured by hydrographic fronts and eddies, and containing schooling forage species shared by multiple wide-ranging predators. Furthermore, identification of biological hotspots is useful for the designation of marine protected areas most critical to potentially endangered wildlife and fisheries resources.

Low catch limits for forage species are often considered to be precautionary measures that can help conserve marine predators. Difficulties measuring the impacts of fisheries removals on dependent predators maintain this perspective, but consideration of the spatio-temporal scales over which forage species, their predators, and fisheries interact can aid assessment of whether low catch limits are as precautionary as presumed. Antarctic krill are targeted by the largest fishery in the Southern Ocean and are key forage for numerous predators. Current krill removals are considered precautionary and have not been previously observed to affect krill-dependent predators, like penguins. Using a hierarchical model and 30+ years of monitoring data, we show that expected penguin performance was reduced when local harvest rates of krill were ≥0.1, and this effect was similar in magnitude to that of poor environmental conditions. With continued climate warming and high local harvest rates, future observations of penguin performance are predicted to be below the long-term mean with a probability of 0.77. Catch limits that are considered precautionary for forage species simply because the limit is a small proportion of the species’ standing biomass may not be precautionary for their predators.

The ability of Antarctic krill Euphausia superba Dana to withstand the overwintering period is critical to their success. Laboratory evidence suggests that krill may shrink in body length during this time in response to the low availability of food. Nevertheless, verification that krill can shrink in the natural environment is lacking because winter data are difficult to obtain. One of the few sources of winter krill population data is from commercial vessels. We examined length-frequency data of adult krill (>35 mm total body length) obtained from commercial vessels in the Scotia-Weddell region and compared our results with those obtained from a combination of science and commercial sampling operations carried out in this region at other times of the year. Our analyses revealed body-length shrinkage in adult females but not males during winter, based on both the tracking of modal size classes over seasons and sex-ratio patterns. Other explanatory factors, such as differential mortality, immigration and emigration, could not explain the observed differences. The same pattern was also observed at South Georgia and in the Western Antarctic Peninsula. Fitted seasonally modulated von Bertalanffy growth functions predicted a pattern of overwintering shrinkage in all body-length classes of females, but only stagnation in growth in males. This shrinkage most likely reflects morphometric changes resulting from the contraction of the ovaries and is not necessarily an outcome of winter hardship. The sex-dependent changes that we observed need to be incorporated into life cycle and population dynamic models of this species, particularly those used in managing the fishery.

The long-term (almost 20 years) hydrographic and primary production data collected by the U.S. Antarctic Marine Living Resources (AMLR) program during the austral summer near Elephant Island and the South Shetland Islands were combined with satellite observations to assess interannual variability in environmental conditions and production of the northern Antarctic Peninsula (NAP). Correlation analyses show that interannual variability of the region is related to the dominant mode of the Southern Hemisphere extratropical climate variability, the Southern Annular Mode (SAM). Post 2000, significant correlations (r > 0.5, p 0.7, p ≤ 0.01). Significant correlations also exist between chlorophyll concentration and the spring and annual SAM indices (r > 0.6, p ≤ 0.06). The statistical relationship between chlorophyll concentration and SAM is used with the predicted variation of SAM based on CMIP5 models to make projections of biological production change over the next 50 years in the NAP and adjacent areas. With an estimated SAM trend of 0.03 yr−1, in the next 50 years the surface chlorophyll concentration over the NAP and WAP outer shelf and slope will increase by as much as 0.5 mg m−3, and the integrated chlorophyll concentration over the upper 100-m water column will increase in the NAP area by 10 mg m−2 to 50 mg m−2.

Low-power, internally-recording multi-frequency echo sounders can acquire continuous profiles of echoes throughout the water column over long periods of time, thus providing a low-cost method to study the behavior and abundance of fish and zooplankton in the ocean. Ocean gliders are growing in importance as components of ocean observing systems, extending measurements of physical oceanography beyond those possible with moorings and expensive oceanographic research vessels. Payloads are typically composed of conductivity, temperature and pressure along with optical measures of chlorophyll reflectance and dissolved organics. The small size and low power consumption of such echo sounders now makes it practical to install them in gliders, providing the means to simultaneously measure biological metrics through the water column over extended areas and linking physical properties and primary productivity to higher trophic levels such as zooplankton and fish. We have recently installed single-beam echo-sounders with up to four acoustic frequencies in gliders. The echo-sounder (ASL Environmental Sciences’ AZFP) was designed as an autonomous, moored instrument, and several modifications, mechanical, electronic and software were required to adapt it to installation and operation in gliders. The existing lower-frequency transducers were too large and heavy for the glider; their size was reduced by increasing the beam-width, and the housings were redesigned to reduce weight and improve their profile to reduce drag. The transducer housings are designed to be mounted at an angle on the vehicle body so that they are aimed vertically down during the dive phase. Several modifications were also made to the electronics chassis and shrouds to reduce weight, and the power connection was altered to operate from the vehicle battery rather than its own dedicated supply. Examples of these design alterations will be discussed. The instrument operating software was modified to allow communication and control from the vehicle, although data storage remained in the AZFP itself. In the initial case, a 200 kHz, single-beam echo-sounder was integrated and calibrated in a Slocum Webb electric ocean glider. Calibration and controlled field tests demonstrated reasonable signal-to-noise ratio, allowing for detection of plankton and fishes to greater than 75m range. The noise background was higher than that found when the AZFP is used as an autonomous unit, so improvements to noise isolation in the glider should be able to improve the signal to noise ratio in future. A data analysis workflow was developed that estimates glider position and orientation underwater to correct for depth and range of targets. Trial missions in the eastern Gulf of Mexico traveled over a submerged pipeline and a rocky reef. Acoustic backscatter signals attributed to mid-water plankton layers were co-located with oceanographic features and peaks in chlorophyll. Schools of pelagic and demersal fishes were detected and mapped over charted seafloor features. To date, three multiple frequency versions have been built and integrated into Slocum Webb G2 gliders. Several 3-frequency AZFP (38, 70 and 125 kHz) will be used to assess the spatial and temporal distribution of krill biomass in the Antarctic. Several 4-frequency instruments (125, 200, 455 and 769 kHz) have been built to study whales. A glider with a 3-frequency AZFP (38, 125 and 200 kHz) combination was deployed in Terra Nova Bay (western Ross Sea) and mapped the vertical and horizontal distribution and abundance of zooplankton and pelagic silverfish. Future developments will be aimed at adapting the AZFP to other autonomous vehicles and to reducing the effects of the vehicle’s internal noise environment. Ultimately, this glider-based acoustic technology will pave the way for cost-effective, automated examination of food webs and ecosystems in regions throughout the global ocean.

An integrated model assessing the status and productivity of Antarctic krill (Euphausia superba, hereafter krill) was configured to estimate different subsets of 118 potentially estimable parameters in alternative configurations. We fixed the parameters that were not estimated in any given configuration at pre-specified values. The model was fitted to over forty years of fisheries and survey data for krill in Subarea 48.1, a statistical reporting area around the Antarctic Peninsula used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The number of estimated parameters was gradually increased across model configurations. Configurations that estimated more parameters fitted the data better, but the order in which the parameters were estimated became more important in finding the best fit. Twenty-two configurations estimating from 48 to 107 parameters were able to obtain an invertible Hessian matrix that was subsequently used to estimate parameter uncertainty. Parameter uncertainties calculated using asymptotic approximation around the maximum likelihood estimates were often larger than uncertainties based on Markov chain Monte Carlo sampling for the same parameters. Diagnostics applied to MCMC samples in the best model of each configuration that obtained an invertible Hessian indicated that the most highly parameterized configurations did not reach stationary distributions. A 96-parameter configuration was the best fitting model of those that passed the MCMC diagnostics. The ΔAIC and ΔBIC scores indicated essentially no support relative to the best model for the alternative models that also passed MCMC diagnostics. Simulated data using the configurations as operating models showed that while all configurations passed "self-tests" for spawning biomass and recruitment, there was a small negative bias due to model penalties in the fishing mortality estimates for years with the highest fishing mortalities. "Cross-tests" of configurations that estimated different parameters often differed from the operating model values.

Somatic growth of pelagic invertebrates is controlled by temperature and food, both of which vary in space and time. Species-specific growth rate responses to environmental variabil- ity may affect populations through changes in reproductive potential; therefore, measuring spatial and temporal variability in growth rates of highly abundant zooplankton is critical to predict the impact of climate change on pelagic ecosystems. Here, we used length frequencies from bi-annual surveys conducted 1 month apart to estimate growth rates of one the most abundant euphausiids in the Southern Ocean, Thysanoessa macrura. We analyzed summer data from 4 separate years (1995, 1998, 2001, and 2004) that varied widely in temperature and primary production. Stations within the surveys were grouped by water characteristics: warm, low salinity Antarctic Circum - polar Current (ACC) water, and cold, saline Bransfield Strait and Weddell Sea (MBW) water, to assess inter-annual and spatial variability in cohort growth. Mid-summer cohort growth rates of T. macrura varied between years and water masses, ranging from �0.037 mm d �1 in MBW water in 2004 to 0.081 mm d �1 in ACC water in 1995. Growth rates were faster in ACC water than in MBW water during all years. Growth rates were strongly correlated with temperature (R 2 = 0.82) but weakly correlated with copepod density (R 2 = 0.38), and were not correlated with chl a con- centration (R 2 = 0.11). These results suggest that the growth rates of T. macrura may increase in regions exhibiting warming trends, such as the Antarctic Peninsula. This contrasts with published data on the growth rates of Euphausia superba, which is predicted to be impacted negatively by climate warming.

Specialization is a common mechanism of niche differentiation that can lead to eco- logical co-existence among species. However, species with specialized habitat or dietary require- ments often exhibit a high degree of sensitivity to environmental change. Understanding patterns of specialization and niche segregation among Antarctic marine predators is of increased impor- tance because of recent climate-driven reductions in a key prey species, Antarctic krill Euphausia superba. We examined the stomach contents and stable isotope values of sympatric chinstrap Pygoscelis antarctica and gentoo P. papua penguins across 5 breeding seasons at Cape Shirreff, Livingston Island, Antarctica. Our goal was to examine foraging niche segregation and the degree of specialization between species during the chick-rearing period. Dietary and isotopic foraging niches indicated consistent niche partitioning with higher krill consumption and greater use of off- shore foraging habitats by chinstrap relative to gentoo penguins. While chinstrap penguin diets were dominated by krill with little variation, gentoo penguins exhibited broader dietary and iso- topic niches with a higher degree of variation. There was little evidence that shifts in the availabil- ity of adult krill influenced penguin diets or foraging niches during our study, though the contrast- ing foraging strategies identified provide insight into the differing population trends observed between penguin species. The narrower foraging niche observed in declining chinstrap penguin populations indicates that they are likely highly sensitive to declines in the abundance of Antarc- tic krill. In contrast, the generalist niche exhibited by recently expanding gentoo penguin popula- tions is likely better suited to the rapidly changing environmental conditions in the Antarctic Peninsula.

We examined the feeding ecology and lipid composition of Antarctic krill Euphausia superba around the northern Antarctic Peninsula from 2001 to 2012. We used lipid biomarkers to quantify feeding patterns and relate the variability in biomarkers to environmental conditions that structure the phytoplankton community. Fatty acid profiles varied among years, with some years dominated by lipids indicative of herbivory, while other years were indicative of greater omnivory. Principal component analysis of 60 fatty acids showed that 3 principal components (PCs) explained approximately 50% of the variability in fatty acid profiles. The first PC separated the fatty acid indi- cators along a herbivoryomnivorycarnivory gradient. Correlations between the gradient of her- bivory and carnivory, as summarized by the first PC, were found with both winter sea ice extent (r = 0.55, p < 0.01) and with ENSO conditions the previous winter (r = 0.88, p < 0.001). These findings suggest that climatic conditions during the late winter precondition the pelagic ecosystem around the Antarctic Peninsula, impacting the trophic ecology of Antarctic krill the following summer.

Using known-age Antarctic krill (Euphausia superba) grown from eggs hatched at two different laboratories, we validate the annual pattern of bands deposited in the eyestalks of krill and determine the absolute age of these animals. Ages two through five years were validated, and these animals ranged from 37.1 to 62.6 mm in total length. The band counts in these individuals were either identical to their absolute ages, or only failed to agree by a few months, which demonstrates the accuracy of this method. Precision and bias were estimated graphically using Chang’s index (Coefficient of Variation = 5.03%). High accuracy and precision between readers and low ageing bias indicate that longitudinal sections of eyestalks can be used to age krill in wild samples and to develop age-based stock assessment models for krill. Archival samples preserved in formalin (5%) and stored in ambient conditions were also readable. Ageing preserved krill will provide the opportunity to examine changes in growth among krill populations within the Southern Ocean and to retrospectively examine changes in krill production over the last century to better understand the historical and future impacts of climate change on this critical Southern Ocean species.

Drake Passage, extending from the southern tip of South America to the northern Antarctic Peninsula, is a dynamic oceanographic region with well-defined habitats delineated by the three strong frontal jets of the Antarctic Circumpolar Current (ACC). Here, we describe seabird species distribution patterns across Drake Passage and test the hypothesis that species assemblages broadly reflect physical characteristics of the hydrographic fronts. Strip-transect seabird surveys were conducted between Tierra del Fuego and the South Shetland Islands (700 km track line) during January–March (austral summer) over 14 years (48 crossings). Locations of the latitudinally variable fronts were assessed using in situ shipboard data on sea surface temperature and salinity; areas of high variance were used to indicate frontal features. We quantified five distinct species assemblages that correspond to biogeographic regions and relate to the positions of the Sub-Antarctic Front, Polar Front and ACC Southern Front. Dense seabird concentrations coincided with regions characterized by highly variable sea surface temperature and salinity, suggesting that associated species assemblages reflect the mesoscale hydrographic surface as indicated by sea surface conditions.

Antarctic krill (Euphausia superba) and salps are major macroplankton contributors to Southern Ocean food webs and krill are also fished commercially. Managing this fishery sustainably, against a backdrop of rapid regional climate change, requires information on distribution and time trends. Many data on the abundance of both taxa have been obtained from net sampling surveys since 1926, but much of this is stored in national archives, sometimes only in notebooks. In order to make these important data accessible we have collated available abundance data (numerical density, no. m−2) of postlarval E. superba and salps (combined aggregate and solitary stages and species) into a central database, KRILLBASE, together with environmental information, standardisation and metadata. The aim is to provide a temporal-spatial data resource to support a variety of research such as biogeochemistry, autecology, higher predator foraging and food web modelling in addition to fisheries management and conservation. Previous versions of KRILLBASE have led to a series of papers since 2004 which illustrate some of the potential uses of this database. With increasing numbers of requests for these data we here provide an updated version of KRILLBASE that contains data from 15,194 net hauls, including 12,758 with krill abundance data and 9,726 with salp abundance data. These data were collected by 10 nations and span 56 seasons in two epochs (1926–1939 and 1976–2016). Here, we illustrate the seasonal, inter-annual, regional and depth coverage of sampling, and provide both circumpolar- and regional-scale distribution maps. Krill abundance data have been standardised to accommodate variation in sampling methods, and we have presented these as well as the raw data. Information is provided on how to screen, interpret and use KRILLBASE to reduce artefacts in interpretation, with contact points for the main data providers.

The Antarctic Peninsula (AP) shelf is an important source of dissolved iron (Fe) to the upper ocean in the southern Scotia Sea, one of the most productive regions of the Southern Ocean. Here we present results from a four-year (2003-2006) numerical simulation using a regional coupled physical-biogeochemical model to assess the Fe sources and transport on the AP shelf and toward the southern Scotia Sea. The model was validated with a suite of data derived from in situ surveys and remote sensing. Model results indicate that sediments in the AP shelf and the South Orkney Plateau (SOP) provide the dominant source of Fe to the upper 500 m in the southern Scotia Sea. Additional Fe inputs to the region are associated with the Antarctic Circumpolar Current (ACC) and the northern limb of the Weddell Gyre, deep-ocean sediment sources, dust deposition, and icebergs. Fe on the AP shelf originates primarily from sediments on the relatively shallow inner shelf and is directly injected into the water column and subsequently transported toward Elephant Island by the confluent shelf currents. Off-shelf Fe export is primarily through entrainment of shelf waters by the ACC’s Southern Boundary frontal jet along the northern edge of the AP shelf, the Hesperides Trough, and the SOP shelf. About 70% of the off-shelf export takes place below the surface mixed layer, and is subsequently re-supplied to the euphotic zone through vertical mixing, mainly during austral fall and winter. The exported shelf-derived Fe is then advected downstream by the ACC and Weddell Gyre and spread over the southern and eastern Scotia Seas. Taken together, shelf Fe export witin top 500 m meets nearly all of the Fe demand of phytoplankton photosynthesis in the southern Scotia Sea. Waters with elevated Fe concentrations in the Scotia Sea are largely restricted to south of the Southern ACC Front.

Net-based data on the abundance, distribution, and demographic patterns of Antarctic krill are quantified from a contemporaneous two ship survey of the Antarctic Peninsula during austral summer 2011. Two survey areas were sampled focussed on Marguerite Bay in the south, and the tip of the Antarctic Peninsula in the north. Data from 177 stations showed that the highest concentrations of krill were found in the southern sampling area. Differences between areas were associated with a few large catches of one year old krill found in anomalously warm and productive waters in Marguerite Bay, and small krill catches in the less-productive, offshore waters in the north. Estimated krill density across the survey area was 3.4 krill m � 2 , and was low compared to the long-term average of 45 krill m � 2 for the Elephant Island area. Overall recruitment between the two survey regions was similar, but per capita recruitment was about 60% lower than historical mean recruitment levels measured at Elephant Island since the late 1970s. Demographic patterns showed small krill concentrated near the coast, and large krill concentrated offshore on the shelf and slope all along the survey area. The offshore distribution of adult krill was delineated by the warm (� 1 1C), low salinity (33.8) water at 30 m, suggesting that most krill were present shoreward of the southern boundary of Antarctic Circumpolar Current Front. Distributions of larvae indicated that three hotspot areas were important for the production of krill: slope areas outside Marguerite Bay and north of the South Shetland Islands, and near the coast around Antarctic Sound. Successful spawning, as inferred from larval abundance, was roughly coincident with the shelf break and not with inshore waters. Given the rapid changes in climate along the Antarctic Peninsula and the lower per capita recruitment observed in recent years, studies comparing and contrasting production, growth, and recruitment across the Peninsula will be critical to better understand how climate change will impact krill populations and their dependent predators in the Scotia Sea.

Understanding the temporal variability in vital rates (e.g. growth and survivorship) of wild populations is practically and statistically difficult but crucial for connecting such variation to mechanistic drivers and their population consequences. For somatic growth, empirical estimates of variation are rare because they often require expensive long-term tag-recapture programs. In marine pelagic ecosystems, where many species are difficult to sample and not amenable to tag- ging studies, researchers have relied on estimating growth from length-frequency distributions. We developed a general approach for estimating growth from observed length-frequency samples by combining kernel density estimates of the length-frequency distribution and the von Berta- lanffy growth function. Our approach is conceptually straightforward and easy to implement. We applied the methods to Antarctic krill Euphausia superba collected from the Southern Ocean over a span of 19 yr to document among-year variation in krill growth during the austral summer. Our estimates of growth align closely with existing estimates of growth, but we provide the first esti- mates of among-year variation in krill growth. We estimate very high among-year variation in growth (annual estimates for a 30 mm krill in the Elephant Island region ranged from 0.00 to 0.17 mm d −1 ; mean = 0.073, among-year coefficient of variation ≈ 0.8). We correlated growth rate variation to estimates of ocean chlorophyll but not to other oceanographic indices, contrasting with results from previous studies. The large amount of variation in growth unexplained by envi- ronmental covariates has substantial implications for ecosystem management in the Southern Ocean ecosystem.

Antarctic krill (Euphausia superba) occupy a key position in the Southern Ocean linking primary production to secondary consumers. While krill is a dominant grazer of phytoplankton, it also consumes heterotrophic prey and the relative importance of these two resources may differ with ontogeny. We used stable isotope analyses to evaluate body size-dependent trophic and habitat shifts in krill during the austral summer around the South Shetland Islands, Antarctica. We found evidence for an asymmetric, ontogenetic niche expansion with adults of both sexes having higher and more variable δ15N values but consistent δ13C values in comparison with juveniles. This result suggests that while phytoplankton likely remains an important life-long resource, krill in our study area expand their dietary niche to include higher trophic food sources as body size increases. The broader dietary niches observed in adults may help buffer them from recent climate-driven shifts in phytoplankton communities that negatively affect larval or juvenile krill that rely predominately on autotrophic resources.

Reliable statements about variability and change in marine ecosystems and their underlying causes are needed to report on their status and to guide management. Here we use the Framework on Ocean Observing (FOO) to begin developing ecosystem Essential Ocean Variables (eEOVs) for the Southern Ocean Observing System (SOOS). An eEOV is a defined biological or ecological quantity, which is derived from field observations, and which contributes significantly to assessments of Southern Ocean ecosystems. Here, assessments are concerned with estimating status and trends in ecosystem properties, attribution of trends to causes, and predicting future trajectories. eEOVs should be feasible to collect at appropriate spatial and temporal scales and are useful to the extent that they contribute to direct estimation of trends and/or attribution, and/or development of ecological (statistical or simulation) models to support assessments. In this paper we outline the rationale, including establishing a set of criteria, for selecting eEOVs for the SOOS and develop a list of candidate eEOVs for further evaluation. Other than habitat variables, nine types of eEOVs for Southern Ocean taxa are identified within three classes: state (magnitude, genetic/species, size spectrum), predator–prey (diet, foraging range), and autecology (phenology, reproductive rate, individual growth rate, detritus). Most candidates for the suite of Southern Ocean taxa relate to state or diet. Candidate autecological eEOVs have not been developed other than for marine mammals and birds. We consider some of the spatial and temporal issues that will influence the adoption and use of eEOVs in an observing system in the Southern Ocean, noting that existing operations and platforms potentially provide coverage of the four main sectors of the region — the East and West Pacific, Atlantic and Indian. Lastly, we discuss the importance of simulation modelling in helping with the design of the observing system in the long term.Regional boundary: south of 30°S.

Laboratory and field studies on the age, growth, mortality, and recruitment dynamics of Antarctic krill (Euphausia superba; hereafter “krill”) have greatly increased knowledge of its life history over the last 30 years. The development of long, standardized time series of recruitment and abundance have provided data to test and refine conceptual models of krill recruitment, and to examine the role of environmental factors and climatic variability on recruitment and cohort size. Laboratory studies have also greatly increased knowledge about intrinsic and extrinsic factors governing growth and moulting frequency that have been used to develop more robust models for krill growth. These findings have demonstrated the plasticity of krill life history and can provide the foundation for understanding krill population response to future climate change. However, the lack of progress on the development of conservative, direct measures of krill age continue to hamper the ability to properly compare and contrast growth and recruitment across environments.

Varying the timing of reproduction in response to local environmental conditions is akey factor influencing reproductive success. We used 19 yr (1991 to 2009) of data on breedingchronology and reproductive success of Adelie penguins Pygoscelis adeliae and gentoo penguins P. papua at King George Island, Antarctica to compare clutch initiation dates (CIDs), hatch rates,and creche rates at population and individual levels. The CIDs of both species occurred earlierwith warmer October temperatures, but gentoo penguins adjusted CIDs twice as much per °C oftemperature as Adelie penguins. Gentoo penguins also exhibited lower variances in hatch andcreche rates than Adelie penguins, suggesting that greater ability to respond to ambient condi-tions can buffer reproductive success. Mixed-effects models revealed that individual- and year-specific random effects on the overall population mean best accounted for observed variation inCIDs, with some individuals routinely breeding earlier than others. However, individual differ-ences in the relative timing of breeding provided no advantage for long-term reproductive suc-cess. The results suggest that, among gentoo and Adelie penguins, plasticity in CIDs with respectto environmental conditions is primarily a population-level rather than an individual-levelresponse. Energetic constraints of contrasting migratory and fasting behaviors likely contribute tothe different abilities to respond to warming spring temperatures. Overall, larger responses ofgentoo penguin CIDs to October temperature and lower variability in reproductive success,despite reduced reproductive success with delayed breeding, suggests that gentoo penguins arebetter equipped than Adelie penguins to adjust to rapid warming in the Antarctic Peninsulaecosystem.KEY WORDS: Phenology · Adelie · Gentoo · Climate change · Migrant