Southern Ocean Marine Research Articles

Following the Antarctic Circumpolar Current: patterns and processes in the biogeography of the limpetNacella(Mollusca: Patellogastropoda) across the Southern Ocean

AbstractAimWe use an integrative biogeographical approach to further understand the evolution of an important Southern Ocean marine benthic element, the limpet genusNacella(Mollusca: Patellogastropoda).LocationSouthern Ocean.MethodsWe used multi‐locus time‐calibrated phylogeny ofNacellaat the scale of the whole Southern Ocean to elucidate the underlying processes involved in the origin and diversification of the genus.ResultsDivergence‐time estimates suggest that soon after its origin during the mid‐Miocene (c. 12.5 Ma),Nacellaseparated into two main lineages currently distributed in (1) South America and (2) Antarctica and the sub‐Antarctic islands. We identified two pulses of diversification, during the late Miocene (8 to 5.5 Ma) and the Pleistocene (< 1 Ma).Main conclusionsMajor periods of climatic and oceanographical change strongly affected the biogeography ofNacellaand demonstrate both the long‐ and short‐term influence of the Antarctic Circumpolar Current across the Southern Ocean. Our analyses support the validity of all currently recognizedNacellaspecies and reveal a new South‐American lineage. This work constitutes the most detailed molecular‐based study of an ecologically important, near‐shore invertebrate Southern Ocean group and in so doing contributes to the improved understanding of the underlying patterns and processes in the origin and diversification of marine benthic fauna across this globally important region.

Erratum: Corrigendum: Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs

Nature Geoscience 9, 219–221 (2016); published online 11 January 2016; corrected after print 16 August 2016. In the version of this Letter originally published, it was incorrectly stated in Figure 1 that Chlorophyll-a concentrations were recorded on 12 January 2013, when the date should have been 1 December 2013.

Patterns of genetic diversity and structure in Antarctic and sub-Antarctic Nacella (Patellogastropoda: Nacellidae) species

The biogeography of the Southern Ocean reflects complex interactions between major macro-evolutionary forces and biotic elements. Major gateway openings, the establishment of the Antarctic Circumpolar Current and climate cooling are deeply connected to the composition, abundance and distribution of the Southern Ocean marine benthic fauna. Glacial episodes of the Quaternary heavily impacted the distribution of the genetic variation of the Southern Ocean biota. The genus Nacella includes 12 nominal species in different provinces of the Southern Ocean. In this study, we compared patterns of mitochondrial DNA diversity in three Nacella species from Antarctic Peninsula, Kerguelen Island and Patagonia. Low levels of genetic diversity and absence of genetic structure characterise each one of them showing the strong impact of ice advances and retreats over their respective demographics. Haplotype diversity, short genealogies and demographic inference recorded suggest the occurrence of a more dramatic demographic process in Antarctic Peninsula than in the sub-Antarctic.

Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs

Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean.

Observation of sea‐salt fraction in sub‐100 nm diameter particles at Cape Grim

AbstractVolatility‐hygroscopicity tandem differential mobility analyzer measurements were used to infer the composition of sub‐100 nm diameter Southern Ocean marine aerosols at Cape Grim in November and December 2007. This study focuses on a short‐lived high sea spray aerosol (SSA) event on 7–8 December with two externally mixed modes in the Hygroscopic Growth Factor (HGF) distributions (90% relative humidity (RH)), one at HGF > 2 and another at HGF~1.5. The particles with HGF > 2 displayed a deliquescent transition at 73–75% RH and were nonvolatile up to 280°C, which identified them as SSA particles with a large inorganic sea‐salt fraction. SSA HGFs were 3–13% below those for pure sea‐salt particles, indicating an organic volume fraction (OVF) of up to 11–46%. Observed high inorganic fractions in sub‐100 nm SSA is contrary to similar, earlier studies. HGFs increased with decreasing particle diameter over the range 16–97 nm, suggesting a decreased OVF, again contrary to earlier studies. SSA comprised up to 69% of the sub‐100 nm particle number, corresponding to concentrations of 110–290 cm−3. Air mass back trajectories indicate that SSA particles were produced 1500 km, 20–40 h upwind of Cape Grim. Transmission electron microscopy (TEM) and X‐ray spectrometry measurements of sub‐100 nm aerosols collected from the same location, and at the same time, displayed a distinct lack of sea salt. Results herein highlight the potential for biases in TEM analysis of the chemical composition of marine aerosols.

Statistical modeling of Southern Ocean marine diatom proxy and winter sea ice data: Model comparison and developments

We compare the performance of the modern analog technique (MAT), the Imbrie and Kipp transfer function (IKTF), the generalized additive model (GAM) and weighted averaging partial least squares (WA PLS) on a southern hemisphere diatom relative abundance and winter sea ice concentration training data set. All relevant model assumptions are tested with a random 10-fold cross-validation, whilst a hold out cross-validation tested the explanatory power of each model on spatially independent validation data. We used auto correlograms on model residuals, variance partitioning, and principal coordinates analysis of neighbor matrices (PCNM) to investigate the importance of the spatial structure of our training database. A set of hierarchical logistic regression models (or Huisman–Olff–Fresco models) are used to infer the response of each diatom species along the winter sea ice gradient. Our analyses suggest that IKTF is an inappropriate sea ice estimation approach as its underlying statistical assumptions do not hold and the fit of IKTF to our data under cross-validation was poor. We conclude that MAT may be biased by spatial autocorrelation, and together with IKTF fails to provide unbiased estimates of winter sea ice. We find GAM and WA PLS are more appropriate than IKTF and MAT for the estimation of paleo winter sea ice cover throughout the Southern Ocean. However, as WA PLS is based on a unimodal species response, which is rarely exhibited by diatoms along the winter sea ice gradient, we ultimately advocate the application of GAM. GAM only uses diatoms with a statistically significant association, and ecologically based link, with sea ice. GAM outperformed all other models under cross-validation in terms of performance statistics, the fit of GAM to the training dataset and diagnostic tests for model assumptions. We also demonstrate that GAM provides a more detailed and potentially more accurate (based on a comparison with New Zealand and southeast Australian paleo climatic records) paleo winter sea ice record for the southwestern Pacific Ocean in comparison with IKTF, MAT and WA PLS.

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.

Combining bio-logging and fatty acid signature analysis indicates spatio-temporal variation in the diet of the southern elephant seal, Mirounga leonina

Quantifying the foraging ecology of apex predators is crucial for understanding the predator–prey interactions in marine ecosystems. This is particularly important in the Southern Ocean ecosystem, where year-round studies are logistically and financially impractical. Detailed investigations into the diet of wide-ranging southern hemisphere marine predators, therefore, need to be quantified spatio-temporally. We coupled tracking data with fatty acid signature analysis (FASA) to investigate the foraging ecology of a wide-ranging Southern Ocean marine predator, the southern elephant seal (Mirounga leonina). Seal foraging areas varied spatially and over time, but seals concentrated their activities in three broad geographic regions (i) off the East Antarctic Continental Shelf (S-ACC), (ii) at the edge of the pack ice north of the Ross Sea (SE-RS) and (iii) north of the Sub-Antarctic Front (SE-SAF). There were significant differences in the fatty acid (FA) composition of the blubber from the seals that used these different regions. Seals foraging in the SE-SAF had blubber high in short chained mono-unsaturated fatty acids (SC-MUFAs), compared to those from the S-ACC and SE-RS habitats, which contained more poly-unsaturated fatty acids (PUFAs). Comparisons with FAs of known prey species in the region indicated that blubber collected from seals using the shelf and pelagic habitats (i.e. S-ACC and SE-SAF) were likely to have higher proportions of fish in the diet and, conversely, those from the pack-ice habitat (i.e. SE-RS) were more likely to have a cosmopolitan diet, i.e. an evenly mixed diet of fish and squid. Seal diet also varied annually, within the shelf and pelagic habitats changing from a diet relatively high in fish, to a diet relatively high in squid. Coupling tracking data with FASA is a powerful technique to investigate the spatio-temporal variations in the diet of wide-ranging marine predators.

Ground/satellite observations and atmospheric modeling of dust storms originating in the high Puna‐Altiplano deserts (South America): Implications for the interpretation of paleo‐climatic archives

AbstractThis study provides a detailed description of the sources, transport, dispersion, and deposition of two major dust events originating from the high‐altitude subtropical Puna‐Altiplano Plateau (15–26°S; 65–69°W) in South America. A long and severe drought provided the right conditions for the onset of both events in July 2009 and 2010. Dust was transported SE and deposited over the Pampas region and was observed to continue to the Atlantic Ocean. Dust monitoring stations located downwind recorded both events, and samples were characterized through chemical and textural analysis. Through a combination of meteorological data and satellite observations (CALIPSO and MODIS detectors), we estimate the emission flux for the 2010 event. This estimate was used to constrain the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) transport model and simulate the dust event. Both satellite imagery and model results agree in the location and extension of the dust cloud. CALIPSO detected dust between ~6000 and ~8500 m a.s.l., which remained at this height during most of its trajectory. The dust cloud mixed with a strong convective system in the region, and the associated precipitation brought down significant amounts of dust to the ground. Dust particle size analysis for both events indicates that near the sources dust samples show median modes of 12.4–14.1 µm, similar to modes observed 1300 km away. Chemical composition of sediments from potential dust sources shows distinct signatures within the Puna‐Altiplano Plateau, the Puna sector being clearly different from the Altiplano area. In addition, both sources are markedly different from the Patagonian chemical fingerprint. These results have important implications to improve the interpretation of paleo‐environmental archives preserved on the Argentine loess, Antarctic ice cores, and Southern Ocean marine sediments.

Investigation of the Natural Carbon Cycle since 6000 BC using an Intermediate Complexity Model: The Role of Southern Ocean Ventilation and Marine Ice Shelves

The mechanisms behind the 20 ppm pre-industrial rise in atmospheric CO2 since 6000 BC have been the focus of considerable debate in recent years. Some studies suggest that natural processes, such as a decline in global forests, calcite compensation, and warming ocean temperatures, can explain the increase in CO2. Others have argued that, because the CO2 increase did not occur during previous interglacial periods, it is an indication of an early human influence on the climate. In this paper, we investigate several facets of the natural carbon cycle of the past 8000 years related to ocean circulation patterns and ice shelf configuration using the University of Victoria Earth System Climate Model (v. 2.9), which includes a representation of the climate system with dynamic vegetation and an interactive carbon cycle. The fully simulated earth system for various freely evolving atmospheric carbon scenarios since 6000 BC failed to recreate the observed rise in CO2 and consistently produced a decline in CO2 throughout the Holocene, in keeping with projections related to previous interglacial periods. However, the extent and timing of the decline was strongly dependent on the initial state of the ocean's meridional overturning circulation and the location of marine ice shelves off the coast of Antarctica. For simulations with ice shelves, carbon-poor North Atlantic Deep Water (NADW) dominated the deep Atlantic basin, with less production of (relatively) carbon-rich deep water from the Southern Ocean. This setup led to lower net ocean carbon storage and thus elevated atmospheric CO2 levels. These deep water distributions, which are relatively independent of orbital forcing and CO2 concentrations but strongly dependent on Antarctic marine ice shelves, suggest that greater ice shelf extent during the Holocene relative to previous interglacial periods (Pollard & DeConto, 2009) may have contributed 5–6 ppm CO2 to the atmosphere. The results from this study also indicate that multi-centennial scale changes in the meridional overturning circulation on the order of 2–3 Sv may lead to quasi-periodic increased atmospheric CO2 concentrations (approximately 6 ppm) through enhanced ventilation of Pacific deep waters. RÉSUMÉ [Traduit par la redaction] Depuis quelques années, les mécanismes qui sont intervenus dans l'augmentation du CO2 atmosphérique de l'ordre de 20 ppm, survenue à l'ère préindustrielle font l'objet d'un vaste débat. Selon certaines études, ce phénomène serait attribuable à des processus naturels, comme le recul des forêts dans le monde, la compensation du calcite et le réchauffement de la température des océans. D'après d'autres études, en l'absence d'augmentation de la concentration de CO2 pendant les périodes interglaciaires précédentes, il marquerait le début de l'influence des activités humaines sur le climat. Nous explorons ici plusieurs facettes du cycle naturel du carbone des 8000 dernières années dans la perspective de la configuration de la circulation océanique et de celle des plates-formes de glace flottante au moyen du modèle du système climatique de la Terre (version 2.9), qui comprend une représentation des systèmes climatiques avec une végétation dynamique et un cycle du carbone interactif. Le système terrestre entièrement simulé pour différents scénarios de carbone atmosphérique à évolution libre depuis 6000 ans avant Jésus-Christ n'a pas permis de recréer l'augmentation de la concentration de CO2 observée; il a plutôt produit systématiquement une diminution du CO2 pendant l'Holocène, ce qui va dans le sens des projections se rapportant aux périodes interglaciaires précédentes. Cependant, l'ampleur de la baisse et le moment où le phénomène s'est produit dépendaient fortement de l'état initial de la circulation méridienne de retournement de l'océan ainsi que de l'emplacement des plates-formes de glace flottante au large de la côte de l'Antarctique. Dans le cas des simulations avec des plates-formes de glace flottante, l'Eau profonde de l'Atlantique Nord (NADW) pauvre en carbone prédominait dans les eaux des grands fonds du bassin de l'Atlantique, la production d'eaux des grands fonds (relativement) riches en carbone en provenance de l'océan Austral étant réduite. Dans le cas de cette configuration, le stockage net de carbone dans les océans diminuait, ce qui augmentait la concentration de CO2 dans l'atmosphère. D'après la répartition des eaux profondes, qui est relativement indépendante du forçage orbital et des concentrations de CO2, mais qui dépend énormément des plates-formes de glace flottante dans l'Antarctique, le fait que les plates-formes de glace flottante étaient plus étendues pendant l'Holocène que durant les périodes interglaciaires précédentes (Pollard & DeConto, 2009) expliquerait peut-être un apport de 5 ou 6 ppm de CO2 dans l'atmosphère. L’étude nous apprend également que des changements à l’échelle pluricentennale dans la circulation méridienne de retournement de l'ordre de 2 à 3 Sv peuvent se traduire par des augmentations presque systématiques des concentrations de CO2 atmosphérique (de l'ordre d'environ 6 ppm) grâce à une meilleure ventilation des eaux profondes du Pacifique.

Developing integrated models of Southern Ocean food webs: Including ecological complexity, accounting for uncertainty and the importance of scale

The Southern Ocean supports diverse and unique ecosystems that have been impacted by more than two centuries of exploitation and are now experiencing rapid changes in ocean temperature and seasonal ice cover due to climate warming. Understanding and projecting responses of Southern Ocean marine ecosystems to changing climate conditions and direct human impacts, such as fisheries, requires integrated ecosystem analyses at scales previously unexplored. Here we consider the main ecological and modelling challenges in predicting the responses of Southern Ocean ecosystems to change, and propose three interlinked focus areas that will advance the development of integrated models for Southern Ocean ecosystems. The first focus area is development of fundamental understanding of the factors that determine the structure and function of the food webs at multiple scales. Ecological research in the Southern Ocean is often centred on key species or localised systems, a tendency which is reflected in existing food web and ecosystem models. To build on this, a systematic analysis of regional food web structure and function is required. The second focus area is development of a range of mechanistic models that vary in their resolution of ecological processes, and consider links across physical scales, biogeochemical cycles and feedbacks, and the central role of zooplankton. These two focus areas underlie the third, which is development of methodologies for scenario testing across a range of trophic levels of the effects of past and future changes, which will facilitate consideration of the underlying complexity of interactions and the associated uncertainty. The complex nature of interactions determining Southern Ocean ecosystem structure and function will require new approaches, which we propose should be developed within a scale-based framework that emphasises both physical and ecological aspects.

Possible effects of global environmental changes on Antarctic benthos: a synthesis across five major taxa.

Because of the unique conditions that exist around the Antarctic continent, Southern Ocean (SO) ecosystems are very susceptible to the growing impact of global climate change and other anthropogenic influences. Consequently, there is an urgent need to understand how SO marine life will cope with expected future changes in the environment. Studies of Antarctic organisms have shown that individual species and higher taxa display different degrees of sensitivity to environmental shifts, making it difficult to predict overall community or ecosystem responses. This emphasizes the need for an improved understanding of the Antarctic benthic ecosystem response to global climate change using a multitaxon approach with consideration of different levels of biological organization. Here, we provide a synthesis of the ability of five important Antarctic benthic taxa (Foraminifera, Nematoda, Amphipoda, Isopoda, and Echinoidea) to cope with changes in the environment (temperature, pH, ice cover, ice scouring, food quantity, and quality) that are linked to climatic changes. Responses from individual to the taxon-specific community level to these drivers will vary with taxon but will include local species extinctions, invasions of warmer-water species, shifts in diversity, dominance, and trophic group composition, all with likely consequences for ecosystem functioning. Limitations in our current knowledge and understanding of climate change effects on the different levels are discussed.

Fine-scale spatial genetic structure in the brooding sea urchin Abatus cordatus suggests vulnerability of the Southern Ocean marine invertebrates facing global change

The Southern Ocean benthic communities are characterized by their levels of endemism and their diversity of invertebrate brooding species. Overall, biological processes acting within these species remain poorly understood despite their importance to understand impacts of ongoing global change. We take part in filling this gap by studying the genetic structure over different spatial scales (from centimeters to tens of kilometers) in Abatus cordatus, an endemic and brooding sea urchin from the Kerguelen Islands. We developed three microsatellites and two exon-primed intron crossing markers and conducted a two-scale sampling scheme (from individuals to patches) within two dense localities of Abatus cordatus. Between patches, all pairwise comparisons, covering distances from few meters (between patches within locality) to 25 km (between localities), revealed significant genetic differentiation, a higher proportion of the molecular variance being explained by the comparisons between localities than within localities, in agreement with an isolation by distance model. Within patches, we found no significant correlation between individual pairwise spatial and genetic distances, except for the most polymorphic locus in the patch where the largest range of geographical distances had been analyzed. This study provides an estimation of the dispersal capacities of Abatus cordatus and highlights its low recolonization ability. Similar low recolonization capacities are thus expected in other Antarctic and Subantarctic brooding invertebrate species and suggest a high vulnerability of these species facing global change.

Short Note: Natural seasonal variability of aragonite saturation state within two Antarctic coastal ocean sites

. The observed threefold changes in &#937 ARAG shown here over a few months are of the same order of magnitude as those conditions used in artificial laboratory CO2 experiments. This makes the Antarctic coastal oceans a perfect natural mesocosm to investigate resilience of Southern Ocean marine ecosystems to ocean acidification.

Molecular phylogeny and historical biogeography of Nacella (Patellogastropoda: Nacellidae) in the Southern Ocean

The evolution and the historical biogeography of the Southern Ocean marine benthic fauna are closely related to major tectonic and climatic changes that occurred in this region during the last 55 million years (Ma). Several families, genera and even species of marine organisms are shared between distant biogeographic provinces in this region. This pattern of distribution in marine benthic invertebrates has been commonly explained by vicariant speciation due to plate tectonics. However, recent molecular studies have provided new evidence for long-distance dispersion as a plausible explanation of biogeographical patterns in the Southern Ocean. True limpets of the genus Nacella are currently distributed in different biogeographic regions of the Southern Ocean such as Antarctica, Kerguelen Province, southern New Zealand Antipodean Province, North-Central Chile and South American Magellanic Province. Here, we present phylogenetic reconstructions using two mitochondrial DNA markers (Cytochrome Oxidase I and Cytochrome b) to look into the relationships among Nacella species and to determine the origin and diversification of the genus. Phylogenies were reconstructed using two methods, Maximum Parsimony and Bayesian Inference, while divergence time among Nacella species was estimated following a relaxed Bayesian approach. For this purpose, we collected inter- and subtidal species belonging to four biogeographic regions in the Southern Ocean: Antarctica, Kerguelen Province, Central Chile, and Magellanic Province. Our molecular results agree with previous morphological and molecular studies supporting the monophyly of Nacella and its sister relationship with Cellana. Two rounds of diversification are recognized in the evolution of Nacella. The first one occurred at the end of the Miocene and gave rise to the main lineages, currently distributed in Antarctica, South America or Kerguelen Province. Large genetic divergence was detected among Nacella species from these distant biogeographic provinces emphasizing the significance of trans-oceanic discontinuities and suggesting long-distance dispersal was relatively unimportant. The second diversification round consisted of a more recent Pleistocene radiation in the Magellanic region. In this province, different morphological species of Nacella exhibit extreme low levels of genetic divergence with absence of reciprocal monophyly among them. According to our time estimation, the origin and diversification of Nacella in the Southern Ocean is more recent (<15 MY) than the expected under the hypothesis of vicariant speciation due to plate tectonics. The evolution of this genus seems to be closely related to drastic climatic and oceanographic changes in the Southern Ocean during the middle-Miocene climatic transition. In spite of the high number of species described for the Magellanic Province, molecular results indicate that these species are the most derived ones in the evolution of the genus and therefore that the Magellanic region does not need to correspond to the origin center of Nacella. The absence of genetic divergence among these species supports a very recent radiation process accompanied by rapid morphological and ecological diversification.

Ecological relevance of laboratory determined temperature limits: colonization potential, biogeography and resilience of Antarctic invertebrates to environmental change

AbstractThe relevance of laboratory experiments in predicting effects of climate change has been questioned, especially in Antarctica where sea temperatures are remarkably stable. Laboratory studies of Southern Ocean marine animal capacities to survive increasing temperature mainly utilize rapid temperature elevations, 100 ×–10 000 × faster than sea temperature is predicted to rise. However, due to small‐scale temperature fluctuations these studies may be crucial for understanding colonization patterns and predicting survival particularly through interactions between thermal tolerance and migration. The colonization of disjunct shelves around Antarctica by larvae or adult drift requires crossing or exposure to, rapid temperature changes of up to 2–4 °C over days to weeks. Analyses of responses to warming at varying rates of temperature change in the laboratory allow better predictions of the potential species have for colonizing disjunct shelf areas (such as the Scotia Arc). Inhabiting greater diversities of localities increases the geographic and thermal range species experience. We suggest a strong link between short‐term temperature tolerance, environmental range and prospects for surviving changing environments.

Temporal variation in Adélie penguin diet at Béchervaise Island, east Antarctica and its relationship to reproductive performance

Diet, and in particular, food quality and quantity can influence the reproductive performance of marine predators. Also, the diet of specialist predators is often monitored in programmes that model and manage ecosystems. We examined the diet of Adelie penguins (Pygoscelis adeliae), an important consumer of Southern Ocean living resources, at Bechervaise Island, east Antarctica, during the chick-rearing periods for 11 years between 1991–1992 and 2002–2003. We also investigated the relationship between diet and annual reproductive performance. Substantial inter- and intra-annual variation in both meal mass and composition was evident: adults generally returned with larger food loads during the creche compared with the guard stages, and diet composition was dominated by two prey types, krill and fish, which combined contributed to >90% of the diet by mass in 7 out of 11 years. Females generally brought back meals dominated by krill; males generally consumed fish-dominated meals. However, both sexes returned with a high proportion of krill when annual mean meal mass was also high, suggesting that more food was available in high krill years. There was also evidence that years of high reproductive performance were positively correlated with years of both high meal and krill mass. We believe that our results indicate that there is significant long-term inter- and intra-annual variability in the amount of food available to Adelie penguins and that this was reflected in their diet and measures of reproductive performance. Coupled with the observation that penguins did not switch prey, this indicates that Adelie penguins from Bechervaise Island are dependent predators of krill. This contrasts with populations in other locations but supports the notion that Adelie penguins are an informative species to monitor the management of Southern Ocean marine living resources in this region.

Reduced calcification in modern Southern Ocean planktonic foraminifera

The oceans are becoming more acidic as they absorb anthropogenic carbon dioxide—this may limit the ability of marine organisms to secrete carbonate. A sediment-trap study shows that in the Southern Ocean the shell weights of a surface-dwelling single-celled organism with a calcite shell are lower than pre-industrial values, probably as a result of increasing ocean acidity. Anthropogenic carbon dioxide has been accumulating in the oceans, lowering both the concentration of carbonate ions and the pH (ref. 1), resulting in the acidification of sea water. Previous laboratory experiments have shown that decreased carbonate ion concentrations cause many marine calcareous organisms to show reduced calcification rates2,3,4,5. If these results are widely applicable to ocean settings, ocean acidification could lead to ecosystem shifts. Planktonic foraminifera are single-celled calcite-secreting organisms that represent between 25 and 50% of the total open-ocean marine carbonate flux6 and influence the transport of organic carbon to the ocean interior7. Here we compare the shell weights of the modern foraminifer Globigerina bulloides collected from sediment traps in the Southern Ocean with the weights of shells preserved in the underlying Holocene-aged sediments. We find that modern shell weights are 30–35% lower than those from the sediments, consistent with reduced calcification today induced by ocean acidification. We also find a link between higher atmospheric carbon dioxide and low shell weights in a 50,000-year-long record obtained from a Southern Ocean marine sediment core. It is unclear whether reduced calcification will affect the survival of this and other species, but a decline in the abundance of foraminifera caused by acidification could affect both marine ecosystems and the oceanic uptake of atmospheric carbon dioxide.

Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO 2

Southern Ocean acidification via anthropogenic CO(2) uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate ion (CO(3)(2-)) to levels where calcium carbonate (both aragonite and calcite) shells begin to dissolve. Natural seasonal variations in carbonate ion concentrations could either hasten or dampen the future onset of this undersaturation of calcium carbonate. We present a large-scale Southern Ocean observational analysis that examines the seasonal magnitude and variability of CO(3)(2-) and pH. Our analysis shows an intense wintertime minimum in CO(3)(2-) south of the Antarctic Polar Front and when combined with anthropogenic CO(2) uptake is likely to induce aragonite undersaturation when atmospheric CO(2) levels reach approximately 450 ppm. Under the IPCC IS92a scenario, Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038. Some prominent calcifying plankton, in particular the Pteropod species Limacina helicina, have important veliger larval development during winter and will have to experience detrimental carbonate conditions much earlier than previously thought, with possible deleterious flow-on impacts for the wider Southern Ocean marine ecosystem. Our results highlight the critical importance of understanding seasonal carbon dynamics within all calcifying marine ecosystems such as continental shelves and coral reefs, because natural variability may potentially hasten the onset of future ocean acidification.

Short Note: Paradigms need hypothesis testing: no evidence for top-down forcing on Adélie and emperor penguin populations

In their recent review article “Paradigm lost, or is top-down forcing no longer significant in the Antarctic marine ecosystem?” Ainley et al. (2007) questioned why Southern Ocean marine ecologists apparently have shifted to a central paradigm where bottom-up forcing by physics and climate change has become the single most important driver of food web dynamics in the Southern Ocean. Ainley et al. (2007) suggest that top-down forcing (forcing by biotic processes) is no longer considered in the interpretation of ecological research results aimed at understanding ecosystem processes of the Southern Ocean. Based on two examples from the literature they suggest that population trends could better be explained by including species interactions in the modelling rather than by changes in climate related physical processes alone. Nicol et al. (2007) questioned the paradigm shift proposed by Ainley et al. (2007) and made a broad review of the ecological research conducted in the Southern Ocean ecosystems. They concluded that there has been considerable research effort into ecosystem interactions over the last 25 years in the Southern Ocean, and that there seems little evidence that there has been an almost complete shift in paradigms; rather both bottom-up and top-down processes are recognized to govern ecosystems functioning.