Potential climate change effects on the habitat of antarctic krill in the weddell quadrant of the southern ocean

Transcriptome analysis reveals genes connected to temperature adaptation in juvenile antarctic krill Euphausia superba

BACKGROUND

The Antarctic krill, Euphausia superba (E. superba), is a key organism in the Antarctic marine ecosystem and has been widely studied. However, there is a lack of transcriptome data focusing on temperature responses.

METHODS

In this study, we performed transcriptome sequencing of E. superba samples exposed to three different temperatures: -1.19 °C (low temperature, LT), - 0.37 °C (medium temperature, MT), and 3 °C (high temperature, HT).

RESULTS

Illumina sequencing generated 772,109,224 clean reads from the three temperature groups. In total, 1,623, 142, and 842 genes were differentially expressed in MT versus LT, HT versus LT, and HT versus MT, respectively. Moreover, Kyoto Encyclopedia of Genes and Genomes analysis revealed that these differentially expressed genes were mainly involved in the Hippo signaling pathway, MAPK signaling pathway, and Toll-like receptor signaling pathway. Quantitative reverse-transcription PCR revealed that ESG037073 expression was significantly upregulated in the MT group compared with the LT group, and ESG037998 expression was significantly higher in the HT group than in the LT group.

CONCLUSIONS

This is the first transcriptome analysis of E. superba exposed to three different temperatures. Our results provide valuable resources for further studies on the molecular mechanisms underlying temperature adaptation in E. superba.

Juvenile emperor penguin range calls for extended conservation measures in the Southern Ocean

To protect the unique and rich biodiversity of the Southern Ocean, conservation measures such as marine protected areas (MPAs) have been implemented. Currently, the establishment of several additional protection zones is being considered based on the known habitat distributions of key species of the ecosystems including emperor penguins and other marine top predators. However, the distribution of such species at sea is often insufficiently sampled. Specifically, current distribution models focus on the habitat range of adult animals and neglect that immatures and juveniles can inhabit different areas. By tracking eight juvenile emperor penguins in the Weddell Sea over 1 year and performing a meta-analysis including previously known data from other colonies, we show that conservation efforts in the Southern Ocean are insufficient for protecting this highly mobile species, and particularly its juveniles. We find that juveniles spend approximately 90% of their time outside the boundaries of proposed and existing MPAs, and that their distribution extends beyond (greater than 1500 km) the species' extent of occurrence as defined by the International Union for Conservation of Nature. Our data exemplify that strategic conservation plans for the emperor penguin and other long-lived ecologically important species should consider the dynamic habitat range of all age classes.

WOS, Steinberg DK, Swadling KM, Tarling GA, Thorpe SE, Veytia D, Ward P, Weldrick CK, Yang G. Status, Change, and Futures of Zooplankton in the Southern Ocean

In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research.

Stepping stones towards Antarctica: Switch to southern spawning grounds explains an abrupt range shift in krill

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.

Southern Ocean Food Web Modelling: Progress, Prognoses, and Future Priorities for Research and Policy Makers

Graphical summary of multiple aspects of Southern Ocean food web structure and function including alternative energy pathways through pelagic food webs, climate change and fisheries impacts and the importance of microbial networks and benthic systems.

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Productivity and Change in Fish and Squid in the Southern Ocean

Southern Ocean ecosystems are globally important and vulnerable to global drivers of change, yet they remain challenging to study. Fish and squid make up a significant portion of the biomass within the Southern Ocean, filling key roles in food webs from forage to mid-trophic species and top predators. They comprise a diverse array of species uniquely adapted to the extreme habitats of the region. Adaptations such as antifreeze glycoproteins, lipid-retention, extended larval phases, delayed senescence, and energy-conserving life strategies equip Antarctic fish and squid to withstand the dark winters and yearlong subzero temperatures experienced in much of the Southern Ocean. In addition to krill exploitation, the comparatively high commercial value of Antarctic fish, particularly the lucrative toothfish, drives fisheries interests, which has included illegal fishing. Uncertainty about the population dynamics of target species and ecosystem structure and function more broadly has necessitated a precautionary, ecosystem approach to managing these stocks and enabling the recovery of depleted species. Fisheries currently remain the major local driver of change in Southern Ocean fish productivity, but global climate change presents an even greater challenge to assessing future changes. Parts of the Southern Ocean are experiencing ocean-warming, such as the West Antarctic Peninsula, while other areas, such as the Ross Sea shelf, have undergone cooling in recent years. These trends are expected to result in a redistribution of species based on their tolerances to different temperature regimes. Climate variability may impair the migratory response of these species to environmental change, while imposing increased pressures on recruitment. Fisheries and climate change, coupled with related local and global drivers such as pollution and sea ice change, have the potential to produce synergistic impacts that compound the risks to Antarctic fish and squid species. The uncertainty surrounding how different species will respond to these challenges, given their varying life histories, environmental dependencies, and resiliencies, necessitates regular assessment to inform conservation and management decisions. Urgent attention is needed to determine whether the current management strategies are suitably precautionary to achieve conservation objectives in light of the impending changes to the ecosystem.

Facing Southern Ocean warming: Temperature effects on whole animal performance of Antarctic krill (Euphausia superba)

The ongoing environmental changes in the Southern Ocean may cause a dramatic decrease in habitat quality. Due to its central position in the food web, Antarctic krill (Euphausia superba) is a key species of the marine Antarctic ecosystem. It is therefore crucial to understand how increasing water temperatures affect important krill life-cycle processes. Here, a long-term (August - March) laboratory acclimation experiment at different temperature scenarios (0.5 °C, 1.5 °C, 2.5 °C, 3.5 °C, 5 °C, 7 °C) was performed and the effects of elevated temperatures on whole animal parameters (O₂ consumption, body length, length of the digestive gland) were analyzed. The response of krill oxygen consumption to different experimental temperatures differed between acute/short-term and long-term acclimation. After 8 months, krill oxygen consumption remained unchanged up to temperatures of 3.5 °C and was significantly higher at temperatures > 3.5 °C. Krill acclimated to temperatures ≥ 3.5 °C were significantly smaller at the end of the experiment. Limited food intake and/or conversion may have contributed to this effect, especially pronounced after the onset of the reproductive period. In addition, the seasonal growth pattern in males differed from that of females. Together, our findings indicate that warming Southern Ocean waters are likely to increase metabolic rate in krill, possibly altering the amount of energy available for other important life-cycle processes, a finding directly related to future population dynamics and fisheries management.

Continuous moulting by Antarctic krill drives major pulses of carbon export in the north Scotia Sea, Southern Ocean

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.

Long-term changes in habitat and trophic level of Southern Ocean squid in relation to environmental conditions

Long-term studies of pelagic nekton in the Southern Ocean and their responses to ongoing environmental change are rare. Using stable isotope ratios measured in squid beaks recovered from diet samples of wandering albatrosses Diomedea exulans, we assessed decadal variation (from 1976 to 2016) in the habitat (δ¹³C) and trophic level (δ¹⁵N) of five important Southern Ocean squid species in relation to indices of environmental conditions—Southern Oscillation Index (SOI) and Southern Annular Mode (SAM). Based on δ¹³C values, corrected for the Suess effect, habitat had changed over the last 50 years for Taonius sp. B (Voss), Gonatus antarcticus, Galiteuthis glacialis and Histioteuthis atlantica but not Moroteuthopsis longimana. By comparison, mean δ¹⁵N values were similar across decades for all five species, suggesting minimal changes in trophic levels. Both SAM and SOI have increased in strength and frequency over the study period but, of the five species, only in Taonius sp. B (Voss) did these indices correlate with, δ¹³C and δ¹⁵N values, indicating direct relationships between environmental conditions, habitat and trophic level. The five cephalopod species therefore changed their habitats with changing environmental conditions over the last 50 years but maintained similar trophic levels. Hence, cephalopods are likely to remain important prey for top predators in Southern Ocean food webs, despite ongoing climate change.

Comparing feedback and spatial approaches to advance ecosystem-based fisheries management in a changing Antarctic

To implement ecosystem-based approaches to fisheries management, decision makers need insight on the potential costs and benefits of the policy options available to them. In the Southern Ocean, two such options for addressing trade-offs between krill-dependent predators and the krill fishery include “feedback management” (FBM) strategies and marine protected areas (MPAs); in theory, the first adjusts to change, while the latter is robust to change. We compared two possible FBM options to a proposed MPA in the Antarctic Peninsula and Scotia Sea given a changing climate. One of our feedback options, based on the density of Antarctic krill (Euphasia superba), projected modest increases in the abundances of some populations of krill predators, whereas outcomes from our second FBM option, based on changes in the abundances of penguins, were more mixed, with some areas projecting predator population declines. The MPA resulted in greater increases in some, but not all, predator populations than either feedback strategy. We conclude that these differing outcomes relate to the ways the options separate fishing and predator foraging, either by continually shifting the spatial distribution of fishing away from potentially vulnerable populations (FBM) or by permanently closing areas to fishing (the MPA). For the krill fishery, we show that total catches could be maintained using an FBM approach or slightly increased with the MPA, but the fishery would be forced to adjust fishing locations and sometimes fish in areas of relatively low krill density–both potentially significant costs. Our work demonstrates the potential to shift, rather than avoid, ecological risks and the likely costs of fishing, indicating trade-offs for decision makers to consider.

Sharing menus and kids' specials: Inter- and intraspecific differences in stable isotope niches between sympatrically breeding storm-petrels

Species sharing resources are predicted to compete, but co-occurring species can avoid competition through niche partitioning. Here, we investigated the inter- and intra-specific differences using stable isotope analyses in the black-bellied storm-petrel (Fregetta tropica) and the Wilson's storm-petrel (Oceanites oceanicus), breeding sympatrically in maritime Antarctica. We analysed stable carbon, nitrogen and oxygen isotopes in samples representing different life stages; chick down (pre-laying females), chick feather (chick), and adult blood (chick-rearing adults). Pre-laying females had wider stable isotope niches than chicks or chick-rearing adults, due to pre-laying females being free roaming while chick-rearing adults were central-place-foragers. Chicks were fed at a higher trophic level than the adults (higher δ¹⁵N), likely to compensate for the high nutritional demands of the growing chicks. Wilson's storm-petrels showed substantial overlap in stable isotope niches between all life stages, while the black-bellied storm-petrel chicks showed very little overlap. Wilson's storm-petrel niches significantly overlapped with those of pre-laying and chick-rearing black-bellied storm-petrels, suggesting negligible niche partitioning. Chick growth rate was negatively correlated with chick δ¹⁵N values, suggesting nutritional stress resulting in the use endogenous instead of dietary amino acids in protein synthesis. The higher trophic level of the relatively larger black-bellied storm-petrel chicks may be due to their longer stay in the nest, and relatively larger body mass gain, despite chick growth rates being similar to the smaller Wilson's storm-petrel chicks. Despite breeding sympatrically, the studied storm-petrel species showed considerable overlap in isotopic niches, which may be explained by sharing the same main prey species, reducing the detectability of foraging niche partitioning through stable isotope analyses. We found dietary shifts in black-bellied storm-petrels that are absent in Wilson's, showing different chick provisioning strategies, and shows that the high productivity of the Antarctic marine ecosystem may facilitate foraging niche overlap of sympatrically living species.

Antarctic Futures: An Assessment of Climate-Driven Changes in Ecosystem Structure, Function, and Service Provisioning in the Southern Ocean

In this article, we analyze the impacts of climate change on Antarctic marine ecosystems. Observations demonstrate large-scale changes in the physical variables and circulation of the Southern Ocean driven by warming, stratospheric ozone depletion, and a positive Southern Annular Mode. Alterations in the physical environment are driving change through all levels of Antarctic marine food webs, which differ regionally. The distributions of key species, such as Antarctic krill, are also changing. Differential responses among predators reflect differences in species ecology. The impacts of climate change on Antarctic biodiversity will likely vary for different communities and depend on species range. Coastal communities and those of sub-Antarctic islands, especially range-restricted endemic communities, will likely suffer the greatest negative consequences of climate change. Simultaneously, ecosystem services in the Southern Ocean will likely increase. Such decoupling of ecosystem services and endemic species will require consideration in the management of human activities such as fishing in Antarctic marine ecosystems.

The importance of Antarctic krill in biogeochemical cycles

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.

Future recovery of baleen whales is imperiled by climate change

Historical harvesting pushed many whale species to the brink of extinction. Although most Southern Hemisphere populations are slowly recovering, the influence of future climate change on their recovery remains unknown. We investigate the impacts of two anthropogenic pressures-historical commercial whaling and future climate change-on populations of baleen whales (blue, fin, humpback, Antarctic minke, southern right) and their prey (krill and copepods) in the Southern Ocean. We use a climate-biological coupled "Model of Intermediate Complexity for Ecosystem Assessments" (MICE) that links krill and whale population dynamics with climate change drivers, including changes in ocean temperature, primary productivity and sea ice. Models predict negative future impacts of climate change on krill and all whale species, although the magnitude of impacts on whales differs among populations. Despite initial recovery from historical whaling, models predict concerning declines under climate change, even local extinctions by 2100, for Pacific populations of blue, fin and southern right whales, and Atlantic/Indian fin and humpback whales. Predicted declines were a consequence of reduced prey (copepods/krill) from warming and increasing interspecific competition between whale species. We model whale population recovery under an alternative scenario whereby whales adapt their migratory patterns to accommodate changing sea ice in the Antarctic and a shifting prey base. Plasticity in range size and migration was predicted to improve recovery for ice-associated blue and minke whales. Our study highlights the need for ongoing protection to help depleted whale populations recover, as well as local management to ensure the krill prey base remains viable, but this may have limited success without immediate action to reduce emissions.

Impacts of rising sea temperature on krill increase risks for predators in the Scotia Sea

Climate change is a threat to marine ecosystems and the services they provide, and reducing fishing pressure is one option for mitigating the overall consequences for marine biota. We used a minimally realistic ecosystem model to examine how projected effects of ocean warming on the growth of Antarctic krill, Euphausia superba, might affect populations of krill and dependent predators (whales, penguins, seals, and fish) in the Scotia Sea. We also investigated the potential to mitigate depletion risk for predators by curtailing krill fishing at different points in the 21st century. The projected effects of ocean warming on krill biomass were strongest in the northern Scotia Sea, with a ≥40% decline in the mass of individual krill. Projections also suggest a 25% chance that krill biomass will fall below an established depletion threshold (75% of its unimpacted level), with consequent risks for some predator populations, especially penguins. Average penguin abundance declined by up to 30% of its unimpacted level, with up to a 50% chance of falling below the depletion threshold. Simulated krill fishing at currently permitted harvest rates further increased risks for depletion, and stopping fishing offset the increased risks associated with ocean warming in our model to some extent. These results varied by location and species group. Risk reductions at smaller spatial scales also differed from those at the regional level, which suggests that some predator populations may be more vulnerable than others to future changes in krill biomass. However, impacts on predators did not always map directly to those for krill. Our findings indicate the importance of identifying vulnerable marine populations and targeting protection measures at appropriate spatial scales, and the potential for spatially-structured management to avoid aggravating risks associated with rising ocean temperatures. This may help balance tradeoffs among marine ecosystem services in an uncertain future.

Persistent organic pollutants in krill from the Bellingshausen, South Scotia, and Weddell Seas

Persistent organic pollutants (POPs) reach Antarctica through atmospheric transport, oceanic currents, and to minor extent, by migratory animals. The Southern Ocean is a net sink for many POPs, with a key contribution of the settling fluxes of POPs bound to organic matter (biological pump). However, little is known about POP transfer through the food web in the Southern Ocean and Antarctic waters, where krill is an important ecological node. In this study, we assessed the occurrence of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs) in Antarctic krill (Euphausia superba) from the Bellingshausen, South Scotia and Weddell Seas around the Antarctic Peninsula. The concentrations of PCDD/Fs, PBDEs and PCBs in krill showed a large variability and the average were higher (generally within a factor 3) than those previously reported for eastern Antarctica. This result highlights regional differences related to atmospheric transport and deposition, and also probable regional sources due to human activities. Bioaccumulation and biomagnification factors for PCBs in krill were estimated using previously reported phytoplankton and seawater concentrations for this region. These suggested a near water-krill equilibrium for PCBs, which was not observed for water-phytoplankton partitioning. The estimated removal settling fluxes of PCBs due to the biological pump were several orders of magnitude higher than the estimated fluxes of PCBs transferred from phytoplankton to krill.

The Euphausia superba transcriptome database, SuperbaSE: An online, open resource for researchers

Antarctic krill (Euphausia superba) is a crucial component of the Southern Ocean ecosystem, acting as the major link between primary production and higher trophic levels with an annual predator demand of up to 470 million tonnes. It also acts as an ecosystem engineer, affecting carbon sequestration and recycling iron and nitrogen, and has increasing importance as a commercial product in the aquaculture and health industries. Here we describe the creation of a de novo assembled head transcriptome for E. superba. As an example of its potential as a molecular resource, we relate its exploitation in identifying and characterizing numerous genes related to the circadian clock in E. superba, including the major components of the central feedback loop. We have made the transcriptome openly accessible for a wider audience of ecologists, molecular biologists, evolutionary geneticists, and others in a user-friendly format at SuperbaSE, hosted at http://www.krill.le.ac.uk.

Restricted regions of enhanced growth of Antarctic krill in the circumpolar Southern Ocean

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.

Validation of band counts in eyestalks for the determination of age of Antarctic krill, Euphausia superba

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.

Divergent ecological histories of two sister Antarctic krill species led to contrasted patterns of genetic diversity in their heat-shock protein (hsp70) arsenal

The Arctic and the Antarctic Peninsula are currently experiencing some of the most rapid rates of ocean warming on the planet. This raises the question of how the initial adaptation to extreme cold temperatures was put in place and whether or not directional selection has led to the loss of genetic variation at key adaptive systems, and thus polar species’ (re)adaptability to higher temperatures. In the Southern Ocean, krill represents the most abundant fauna and is a critical member at the base of the Antarctic food web. To better understand the role of selection in shaping current patterns of polymorphisms, we examined genetic diversity of the cox‐1 and hsp70 genes by comparing two closely related species of Euphausiid that differ in ecology. Results on mtcox‐1 agreed with previous studies, indicating high and similar effective population sizes. However, a coalescent‐based approach on hsp70 genes highlighted the role of positive selection and past demographic changes in their recent evolution. Firstly, some form of balancing selection was acting on the inducible isoform C, which reflected the maintenance of an ancestral adaptive polymorphism in both species. Secondly, E. crystallorophias seems to have lost most of its hsp70 diversity because of a population crash and/or directional selection to cold. Nonsynonymous diversities were always greater in E. superba, suggesting that it might have evolved under more heterogeneous conditions. This can be linked to species’ ecology with E. superba living in more variable pelagic conditions, while E. crystallorophias is strictly associated with continental shelves and sea ice.

Assessing the structure and temporal dynamics of seabird communities: the challenge of capturing marine ecosystem complexity

Understanding interspecific interactions, and the influences of anthropogenic disturbance and environmental change on communities, are key challenges in ecology. Despite the pressing need to understand these fundamental drivers of community structure and dynamics, only 17% of ecological studies conducted over the past three decades have been at the community level.

Here, we assess the trophic structure of the procellariiform community breeding at South Georgia, to identify the factors that determine foraging niches and possible temporal changes. We collected conventional diet data from 13 sympatric species between 1974 and 2002, and quantified intra‐ and inter‐guild, and annual variation in diet between and within foraging habits. In addition, we tested the reliability of stable isotope analysis (SIA) of seabird feathers collected over a 13‐year period, in relation to those of their potential prey, as a tool to assess community structure when diets are diverse and there is high spatial heterogeneity in environmental baselines.

Our results using conventional diet data identified a four‐guild community structure, distinguishing species that mainly feed on crustaceans; large fish and squid; a mixture of crustaceans, small fish and squid; or carrion. In total, Antarctic krill Euphausia superba represented 32%, and 14 other species a further 46% of the combined diet of all 13 predators, underlining the reliance of this community on relatively few types of prey. Annual variation in trophic segregation depended on relative prey availability; however, our data did not provide evidence of changes in guild structure associated with a suggested decline in Antarctic krill abundance over the past 40 years.

Reflecting the differences in δ¹⁵N of potential prey (crustaceans vs. squid vs. fish and carrion), analysis of δ¹⁵N in chick feathers identified a three‐guild community structure that was constant over a 13‐year period, but lacked the trophic cluster representing giant petrels which was identified using conventional diet data.

Our study is the first in recent decades to examine dietary changes in seabird communities over time. Conventional dietary analysis provided better resolution of community structure than SIA. However, δ¹⁵N in chick feathers, which reflected trophic (level) specialization, was nevertheless an effective and less time‐consuming means of monitoring temporal changes.

Trophodynamics of Protomyctophum (Myctophidae) in the Scotia Sea (Southern Ocean)

This study investigated spatial and temporal patterns in distribution, population structure and diet of Bolin's lanternfish Protomyctophum bolini, Tenison's lanternfish Protomyctophum tenisoni and gaptooth lanternfish Protomyctophum choriodon in the Scotia Sea using data collected by midwater trawl during spring, summer and autumn. Protomyctophum bolini was the most abundant species of the genus encountered throughout the Scotia Sea with the greatest concentrations occurring around the Antarctic Polar Front (APF). This species had a life cycle of 2+ years, but spatial differences in population structure were apparent as the I-group was absent from all regions south of the APF, suggesting that the species does not recruit in the Scotia Sea. Protomyctophum tenisoni occurred mostly in waters characteristic of the APF and was absent from the southern Scotia Sea. It had a limited size range, but there was clear size-related sexual dimorphism with males significantly larger than females. The species had a life cycle of c. 2 years, but the I-group (c. 1 year old, 1 November to 31 October the next year) occurred only in regions close to the APF suggesting that recruitment is restricted to these waters. A seasonal southward migration for P. choriodon is likely as the species occurred mostly to the south-west of South Georgia in summer, but extended to the sea-ice sectors in autumn. Protomyctophum choriodon had a life cycle of 4+ years in the Scotia Sea and the population was dominated by age classes >3 years old. Larval stages were absent during the surveys for all species. Diurnal variations in vertical distribution were apparent for all three species. Interspecific variations in diet were evident, but all species were primarily copepod feeders, with Metridia spp., Rhincalanus gigas and Calanus simillimus generally dominating their diet. Small euphausiids, principally Thysanoessa spp., were also an important component of their diets, particularly for P. choriodon which had the largest body size. The spatial and temporal variations in diet for both P. bolini and P. tenisoni were broadly consistent with underlying abundance patterns within the mesozooplankton community.

OCEANOGRAPHY. Contrasting futures for ocean and society from different anthropogenic CO₂ emissions scenarios

The ocean moderates anthropogenic climate change at the cost of profound alterations of its physics, chemistry, ecology, and services. Here, we evaluate and compare the risks of impacts on marine and coastal ecosystems—and the goods and services they provide—for growing cumulative carbon emissions under two contrasting emissions scenarios. The current emissions trajectory would rapidly and significantly alter many ecosystems and the associated services on which humans heavily depend. A reduced emissions scenario—consistent with the Copenhagen Accord's goal of a global temperature increase of less than 2°C—is much more favorable to the ocean but still substantially alters important marine ecosystems and associated goods and services. The management options to address ocean impacts narrow as the ocean warms and acidifies. Consequently, any new climate regime that fails to minimize ocean impacts would be incomplete and inadequate.

Diversification, evolution and sub-functionalization of 70kDa heat-shock proteins in two sister species of antarctic krill: differences in thermal habitats, responses and implications under climate change

BACKGROUND

A comparative thermal tolerance study was undertaken on two sister species of Euphausiids (Antarctic krills) Euphausia superba and Euphausia crystallorophias. Both are essential components of the Southern Ocean ecosystem, but occupy distinct environmental geographical locations with slightly different temperature regimes. They therefore provide a useful model system for the investigation of adaptations to thermal tolerance.

METHODOLOGY/PRINCIPAL FINDING

Initial CTmax studies showed that E. superba was slightly more thermotolerant than E. crystallorophias. Five Hsp70 mRNAs were characterized from the RNAseq data of both species and subsequent expression kinetics studies revealed notable differences in induction of each of the 5 orthologues between the two species, with E. crystallorophias reacting more rapidly than E. superba. Furthermore, analyses conducted to estimate the evolutionary rates and selection strengths acting on each gene tended to support the hypothesis that diversifying selection has contributed to the diversification of this gene family, and led to the selective relaxation on the inducible C form with its possible loss of function in the two krill species.

CONCLUSIONS

The sensitivity of the epipelagic species E. crystallorophias to temperature variations and/or its adaptation to cold is enhanced when compared with its sister species, E. superba. These results indicate that ice krill could be the first of the two species to be impacted by the warming of coastal waters of the Austral ocean in the coming years due to climate change.

Climate change and Southern Ocean ecosystems I: how changes in physical habitats directly affect marine biota

Antarctic and Southern Ocean (ASO) marine ecosystems have been changing for at least the last 30 years, including in response to increasing ocean temperatures and changes in the extent and seasonality of sea ice; the magnitude and direction of these changes differ between regions around Antarctica that could see populations of the same species changing differently in different regions. This article reviews current and expected changes in ASO physical habitats in response to climate change. It then reviews how these changes may impact the autecology of marine biota of this polar region: microbes, zooplankton, salps, Antarctic krill, fish, cephalopods, marine mammals, seabirds, and benthos. The general prognosis for ASO marine habitats is for an overall warming and freshening, strengthening of westerly winds, with a potential pole-ward movement of those winds and the frontal systems, and an increase in ocean eddy activity. Many habitat parameters will have regionally specific changes, particularly relating to sea ice characteristics and seasonal dynamics. Lower trophic levels are expected to move south as the ocean conditions in which they are currently found move pole-ward. For Antarctic krill and finfish, the latitudinal breadth of their range will depend on their tolerance of warming oceans and changes to productivity. Ocean acidification is a concern not only for calcifying organisms but also for crustaceans such as Antarctic krill; it is also likely to be the most important change in benthic habitats over the coming century. For marine mammals and birds, the expected changes primarily relate to their flexibility in moving to alternative locations for food and the energetic cost of longer or more complex foraging trips for those that are bound to breeding colonies. Few species are sufficiently well studied to make comprehensive species-specific vulnerability assessments possible. Priorities for future work are discussed.

Winter and spring controls on the summer food web of the coastal West Antarctic Peninsula

Understanding the mechanisms by which climate variability affects multiple trophic levels in food webs is essential for determining ecosystem responses to climate change. Here we use over two decades of data collected by the Palmer Long Term Ecological Research program (PAL-LTER) to determine how large-scale climate and local physical forcing affect phytoplankton, zooplankton and an apex predator along the West Antarctic Peninsula (WAP). We show that positive anomalies in chlorophyll-a (chl-a) at Palmer Station, occurring every 4-6 years, are constrained by physical processes in the preceding winter/spring and a negative phase of the Southern Annular Mode (SAM). Favorable conditions for phytoplankton included increased winter ice extent and duration, reduced spring/summer winds, and increased water column stability via enhanced salinity-driven density gradients. Years of positive chl-a anomalies are associated with the initiation of a robust krill cohort the following summer, which is evident in Adélie penguin diets, thus demonstrating tight trophic coupling. Projected climate change in this region may have a significant, negative impact on phytoplankton biomass, krill recruitment and upper trophic level predators in this coastal Antarctic ecosystem.