Juvenile Krill Research Articles

Omnivorous summer feeding by juvenile Antarctic krill in coastal waters

AbstractThe Antarctic krill Euphausia superba is often considered an herbivore but is notable for its trophic flexibility, which includes feeding on protistan and metazoan zooplankton. Characterizing krill trophic position (TP) is important for understanding carbon and energy flow from phytoplankton to vertebrate predators and to the deep ocean, especially as plankton composition is sensitive to changing climate. We used repeated field sampling and experiments to study feeding by juvenile krill during three austral summers in waters near Palmer Station, Antarctica. Our approach was to combine seasonal carbon budgets, gut fluorescence measurements, imaging flow cytometry, and compound‐specific isotope analysis of amino acids. Field measurements coupled to experimentally derived grazing functional response curves suggest that phytoplankton grazing alone was insufficient to support the growth and basal metabolism of juvenile krill. Phytoplankton consumption by juvenile krill was limited due to inefficient feeding on nanoplankton (2–20 μm), which constituted the majority of autotrophic prey. Mean krill TP and the metazoan dietary fraction increased in years with higher mesozooplankton biomass, which was not coupled to phytoplankton biomass. Comparing TP estimates using δ15N of different amino acids indicated a substantial and consistent food‐web contribution from heterotrophic protists. Phytoplankton, metazoans, and heterotrophic protists all were important contributors to a diverse krill diet that changed substantially among years. Juvenile krill fed mostly on heterotrophic prey during summer near Palmer Station, and this food web complexity should be considered more broadly throughout the changing Southern Ocean.

The energetic cost of early reproductive development in juvenile Antarctic krill at the Western Antarctic Peninsula

The overwinter survival mechanisms of Antarctic krill, Euphausia superba, are poorly characterized, especially for juveniles. It has been suggested that juveniles adopt a mix of strategies characteristic of both larvae and adults. Like larvae, they may feed opportunistically throughout winter when food is available, and like adults they may be able to suppress their metabolism when food is scarce. In this study we look at the overwinter strategies of juvenile krill and how their reproductive development changes when energy input exceeds what is necessary for survival. We take a closer look at how the sexual maturation of juvenile krill progresses in response to different environmental conditions throughout the fall and winter. We exposed juvenile Antarctic krill to four different “food environment scenarios”, supplementing them with various diets from May to September 2019 that were representative of environmental conditions that they may encounter in different regions of the Western Antarctic Peninsula during autumn and winter. Each month, we measured the physiology and condition of the krill, and assessed the reproductive development of females. We found that when female juvenile krill have greater energy reserves than what is needed to survive the winter, they will begin to sexually mature. Further, when there are sufficient levels of the fatty acids eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and 16:4 (n-1), krill are likely to be in a more reproductive advanced stage. However, when lipids, EPA, DHA and 16:4 (n-1) are depleted throughout the winter, juvenile female krill lose their ability to develop reproductively. We also found that sexual development is an energy intensive process that requires high respiration rates in juvenile krill. Furthermore, when juvenile females expend energy maturing, their physiological condition declines. This trade-off between early reproductive development and condition in juvenile female krill has important implications for individual health and population fecundity. Gaining a better understanding of the mechanisms behind juvenile krill winter survival strategies and their consequences will allow us to predict how future change at the western Antarctic Peninsula may affect krill population dynamics, especially in light of a warming climate.

Winter condition, physiology, and growth potential of juvenile Antarctic krill

In recent years, substantial efforts have been made to understand the implications of climate change on Antarctic krill, Euphausia superba, because of their pivotal role in the Southern Ocean food web and in biogeochemical cycling. Winter is one of the least studied seasons in Antarctica and we have limited understanding about the strategies Antarctic krill use to survive the winter. In particular, data on the winter physiology and condition of juvenile Antarctic krill are severely lacking. From May to September (the austral autumn-winter) of 2019, we maintained juvenile Antarctic krill in large (1,330 L) aquarium tanks at Palmer Station, Antarctica and, at monthly time intervals, measured their physiology and condition. Each tank served as a “food environment scenario”, representing possible food environments the krill may encounter during winter along the Western Antarctic Peninsula. We found that, unlike adults, juvenile krill maintain relatively high respiration rates through the winter and respond positively to increased food concentrations by increasing their ingestion rates. Unlike larval krill, juveniles use lipid stores accumulated during the summer and autumn to sustain themselves through periods of starvation in the winter. We used our empirically derived measurements of physiology and condition to estimate the energy budget and growth potential of juvenile krill during the winter. We found that, given their comparatively high respiration rates, small juvenile krill (20 mg dry weight) would need to encounter food at concentrations of ~ 0.15 mg C L-1 daily to avoid loss of body carbon. Without sufficient lipid reserves, this value increases to ~ 0.54 mg C L-1, daily. The health of juvenile krill in the wintertime is dependent on their ability to accumulate lipid stores in the summer and autumn and to find sufficient food during the winter. Changes in food availability to Antarctic krill throughout the year may become problematic to juvenile krill in the future. Understanding the variability in the winter energy budget of juvenile Antarctic krill will allow us to improve population models that make assumptions on seasonal growth patterns.

Size structure of Antarctic krill inferred from samples of Pygoscelid penguin diets and those collected by the commercial krill fishery

In the Antarctic Peninsula, during the chick-rearing period Adélie (Pygoscelis adeliae) and gentoo (P. papua) penguins feed primarily on Antarctic krill (Euphausia superba), which is also exploited by the commercial fishery. Krill length and proportion of juvenile krill consumed by these predators that breed at Stranger Point, South Shetland Islands (from 2007/2008 to 2015/2016), and those collected by the krill fishery in the Mar de la Flota/Bransfield Strait (from 2009/2010 to 2015/2016) were compared to evaluate the potential of each data source as an indicator of changes in the size composition and so, in the demographic structure of the krill population. Overall, the mean krill size taken by gentoo penguins was larger than that ingested by Adélie penguins, which consumed higher proportions of juvenile (≤ 35 mm) and one-year-old (≤ 25 mm) krill. Although the krill size caught by the fishery was statistically similar to that taken by both penguin species, there were differences in the frequency distributions of krill size among the three databases. Furthermore, when only adult krill (> 35 mm) was considered, the three sources of krill data showed a similar inter-annual variation in the availability of adult krill cohorts. Our findings suggest that each database analysed here can potentially provide different information (although complementary) about krill size composition. In addition, inter-annual fluctuations in the smaller size classes of krill likely reflect their first year of recruitment and, therefore, may be used as an indicator of shifts in local krill availability.

Linking Antarctic krill larval supply and recruitment along the Antarctic Peninsula.

Antarctic 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.

Состав популяции и транспорт молоди антарктического криля в районе бассейна Пауэлла (северо-западная часть моря Уэдделла) в январе 2020 г.

This paper presents the data on distribution and size composition of the Antarctic krill (Euphausia superba) population in the northwestern Weddell Sea (Powell Basin) and adjacent areas in summer 2020. Juvenile krill dominated in the western Powell basin and the eastern Bransfield Strait as a result of transport with the advected Weddell Sea water and drifting sea ice. Using a distribution mixture analysis made possible to identify up to 9–10 size groups of krill. Groups 1–4 corresponds to juveniles (average size about 22, 25, 27–28 and 32 mm respectively). They may reflect the transport of different “waves” of recruitment of the Atlantic sector krill population with the waters of the western branch of Weddell Gyre as well as the presence of juvenile krill of local origin. Groups 5–7 are intermediate in size (average length within 38–43 mm) and include mostly immature specimens. Larger groups 8–10 comprise of mostly mature females and mature and immature males. The maximum average size of the largest group reaches about 53 mm. the Wedell Front zone in the north of Powell Basin plays an in important role for retention and growth of krill, originating from different sources: western Weddell Sea, Bransfield Strait and the zone of the southern branch of Antarctic Circumpolar Current.

Patterns of total mercury and methylmercury bioaccumulation in Antarctic krill (Euphausia superba) along the West Antarctic Peninsula

We examined mercury (Hg) accumulation in juvenile and adult subpopulations of Antarctic krill (Euphausia superba) collected west of the Antarctic Peninsula. Samples were collected along a northern cross-shelf transect beginning near Anvers Island and farther south near the sea ice edge in the austral summers of 2011, 2013, 2014, and 2015. Regardless of geographical position, mean concentrations of total Hg and methylmercury (MeHg), the form of Hg that biomagnifies in marine food webs, were significantly higher in juvenile than adult krill in all years. In 2013, juvenile Antarctic krill collected along the coast near Anvers Island had significantly higher MeHg concentrations than krill collected farther offshore, and in 2013 and 2014, coastal juvenile krill exhibited some of the highest MeHg concentrations of all subpopulations sampled. Across all sampling years, collection in northern (sea ice-free) or southern (sea ice edge) transects did not affect MeHg concentrations of juvenile or adult krill, suggesting similar levels and routes of MeHg exposure across the latitudes sampled. Developmental stage, feeding near the coast, and annual variations in sea ice-driven primary and export production were identified as potentially important factors leading to greater MeHg accumulation in juvenile than adult krill. Krill-dependent predators feeding primarily on juveniles may thus accumulate more MeHg than consumers foraging on older krill. These results report MeHg concentrations in Antarctic krill and will be useful for predicting Hg biomagnification in higher-level consumers in this productive Antarctic ecosystem.

Krill (Euphausia superba) distribution contracts southward during rapid regional warming

High-latitude ecosystems are among the fastest warming on the planet1. Polar species may be sensitive to warming and ice loss, but data are scarce and evidence is conflicting2–4. Here, we show that, within their main population centre in the southwest Atlantic sector, the distribution of Euphausia superba (hereafter, ‘krill’) has contracted southward over the past 90 years. Near their northern limit, numerical densities have declined sharply and the population has become more concentrated towards the Antarctic shelves. A concomitant increase in mean body length reflects reduced recruitment of juvenile krill. We found evidence for environmental controls on recruitment, including a reduced density of juveniles following positive anomalies of the Southern Annular Mode. Such anomalies are associated with warm, windy and cloudy weather and reduced sea ice, all of which may hinder egg production and the survival of larval krill5. However, the total post-larval density has declined less steeply than the density of recruits, suggesting that survival rates of older krill have increased. The changing distribution is already perturbing the krill-centred food web6 and may affect biogeochemical cycling7,8. Rapid climate change, with associated nonlinear adjustments in the roles of keystone species, poses challenges for the management of valuable polar ecosystems3. As the southwest Atlantic sector of the Southern Ocean has warmed, the distribution of a key species, Antarctic krill, has contracted southwards. This has occurred in tandem with a decline in recruitment of juveniles, linked to increasingly positive anomalies of the Southern Annular Mode.

The contribution of ice algae to the winter energy budget of juvenile Antarctic krill in years with contrasting sea ice conditions

AbstractKrill 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.

Modelling dispersal of juvenile krill released from the Antarctic ice edge: Ecosystem implications of ocean movement

Abstract Sea ice retreat is a key event affecting Southern Ocean ecosystems during spring and summer. The impacts of this change can be seen in these ecosystems from primary producers to top predators, through biological, chemical and physical systems. We apply a Lagrangian particle tracking method to investigate transport processes from the retreating sea ice edge in the Southern Ocean during spring and summer. The resulting distribution of surface krill patches is used as a case study for our modelling approach. Antarctic krill (Euphausia superba), a secondary producer, plays a key role in the Southern Ocean ecosystems. Antarctic krill are highly abundant in the Southern Ocean with a complex distribution pattern both in the horizontal and vertical dimensions. Observations dating back to the 1930s show that juvenile krill often form surface patches – high density clusters of krill at the ocean surface – throughout the Southern Ocean during the spring and summer seasons. We develop a hypothesis, based on historical observations, that surface krill patches composed of juvenile krill move passively with ocean currents after their release from the sea ice edge zone in spring. Applying this hypothesis and method to the Southern Hemisphere spring/summer, leads to results that indicate that the observed changes in distribution of krill patches from historical to contemporary records could be related to the southward shift of the sea ice edge over the last century.

The carapace matters: refinement of the instantaneous growth rate method for Antarctic krill Euphausia superba Dana, 1850 (Euphausiacea)

Growth of Antarctic krill Euphausia superba Dana, 1850 is commonly calculated using the Instantaneous Growth Rate (IGR) method based on the difference between the uropod lengths of the moulted exoskeleton and the uropod lengths after moulting. To date, this method has not accounted for sex-dependent differences in body proportion, relying only upon uropod measurements. We measured the carapace, uropod, and total body lengths of gravid females, non-gravid females, males, and juvenile krill from the Indian Sector of the Southern Ocean. Growth rates derived using a combination of the carapace and uropod measurements for gravid females were different from those derived through the traditional uropod-only based IGR, whereas non-gravid females, male, and juvenile growth rates showed no significant difference between methods. The refined method we propose successfully reflects dimorphism in growth between the sexes of krill, with gravid females having enlarged carapaces during the reproductive season. The interaction between growth and reproduction must be considered to improve the reliability of predictions from krill life history models, which is possible through the use of sex-dependent IGR measurements. We propose that, whenever possible, measurements of carapace and total length should be made along with uropod measurements. Together with assessments of maturity stages of krill that did not moult during experiments, these measurements will aid in further informing krill stock assessments.

Ice Algae-Produced Carbon Is Critical for Overwintering of Antarctic Krill Euphausia superba

Antarctic krill Euphausia superba (‘krill’) constitute a fundamental food source for Antarctic seabirds and mammals, and a globally important fisheries resource. The future resilience of krill to climate change depends critically on the winter survival of young krill. To survive periods of extremely low production by pelagic algae during winter, krill are assumed to rely partly on carbon produced by ice algae. The true dependency on ice algae-produced carbon, however, is so far unquantified. This confounds predictions on the future resilience of krill stocks to sea ice decline. Fatty acid (FA) analysis, bulk stable isotope analysis (BSIA) and compound-specific stable isotope analysis (CSIA) of diatom- and dinoflagellate-associated marker FAs were applied to quantify the dependency of overwintering larval, juvenile and adult krill on ice algae-produced carbon (αIce) during winter 2013 in the Weddell-Scotia Confluence Zone. Our results demonstrate that the majority of the carbon uptake of the overwintering larval and juvenile krill originated from ice algae (up to 88% of the carbon budget), and that the dependency on ice algal carbon decreased with ontogeny, reaching less than 56% of the carbon budget in adults. Spatio-temporal variability in the utilization of ice algal carbon was more pronounced in larvae and juvenile krill than in adults. Differences between αIce estimates derived from short- versus long-term FA-specific isotopic compositions suggested that ice algae-produced carbon gained importance as the winter progressed, and might become critical at the late winter-spring transition, before the phytoplankton bloom commences. Where the sea ice season shortens, reduced availability of ice algae might possibly not be compensated by surplus phytoplankton production during wintertime. Hence, sea ice decline could seriously endanger the winter survival of recruits, and subsequently overall biomass of krill.

Diet of Adélie penguins (Pygoscelis adeliae) at Stranger Point (25 de Mayo/King George Island, Antarctica) over a 13-year period (2003–2015)

Fil: Juares, Mariana Alejandra. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Direccion Nacional del Antartico. Instituto Antartico Argentino; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; Argentina

Condition of larval (furcilia VI) and one year old juvenile Euphausia superba during the winter–spring transition in East Antarctica

Antarctic krill, Euphausia superba, is an important species in the Southern Ocean ecosystem. Information on krill condition during winter and early spring is slowly evolving with our enhanced ability to sample at this time of year. However, because of the limited spatial and temporal data, our understanding of fundamental biological parameters for krill during winter is limited. Our study assessed the condition of larval (furcilia VI) and one year old juvenile krill collected in East Antarctica (115°E–130°E and 64°S–66°S) from September to October 2012. Krill condition was assessed using morphometric, elemental and biochemical body composition, growth rates, oxygen uptake and lipid content and composition. Diet was assessed using fatty acid biomarkers analysed in the krill. The growth rate of larvae was 0.0038mm day with an inter-moult period of 14 days. The average oxygen uptake of juvenile krill was 0.30±0.02μl oxygen consumed per mg dry weight per hour. Although protein was not significantly different amongst the krill analysed, the lipid content of krill was highly variable ranging from 9% to 27% dry weight in juveniles and from 4% to 13% dry weight in larvae. Specific algal biomarkers, fatty acids ratios, levels of both long-chain (≥C20) monounsaturated fatty acids and bacterial fatty acids found in krill were indicative of the mixed nature of dietary sources and the opportunistic feeding capability of larval and juvenile krill at the end of winter.

Stable isotopes identify an ontogenetic niche expansion in Antarctic krill (Euphausia superba) from the South Shetland Islands, Antarctica

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.

Diet and growth rates of Meganyctiphanes norvegica in autumn

Fatty acid biomarkers analysis was performed on juvenile and adult northern krill (Meganyctiphanes norvegica) to determine variability in diet between individuals and the consequences for their growth rates. Animals were sampled from Gullmarsfjorden, western Sweden during September 2004 and incubated for 4 days to measure instantaneous growth rate (IGR) before subsequent fatty acid analysis. We found positive linear relationships between IGR and certain fatty acid concentrations within adult specimens. In particular, highest growth was seen in individuals containing high concentrations of 18:2(n-6) and 18:3(n-6), indicative of filamentous algae and/or terrestrial carbon dietary sources, and 16:4(n-1), a biomarker fatty acid for diatoms. Similar relationships were not evident for juvenile krill. In an accompanying study, stomach content analyses established that adult krill had been feeding on a diverse range of red, brown and green benthic filamentous algae, which contain fatty acids of the (n-6) series. Incidence of filamentous algae in the stomachs of juvenile krill was much lower, possibly reflecting the shallower water depths inhabited by this maturity stage. Although a benthic feeding mode has previously been observed for M. norvegica, this is the first direct evidence to indicate that benthic food sources can be important in supporting the growth of this euphausiid. Furthermore, high variability in growth rates and fatty acid concentrations between individuals suggest a diversity of feeding strategies within this fjordic population.

Krill growth and condition in Western Indian Ocean sector of the Southern Ocean 30-80°E in austral summer 2006

Krill growth and physiological condition were measured during the Baseline Research on Oceanography, Krill and the Environment - West (BROKE-West) large-scale survey of the Western Indian Ocean sector (30-80 �E) of the Southern Ocean. Krill growth and condition were related to sea-surface temperature (SST) and Chlorophyll a levels. Estimates of Antarctic krill growth were obtained using the Instantaneous Growth Rate (IGR) technique, which reflects in situ growth of krill during the intermoult period prior to capture. A total of 10,362 krill were incubated in the IGR experiments conducted on board the research vessel. The average growth rate ranged between 1.8�9.6% of body length throughout the study area (n=1557). Juvenile and sub-adult krill showed higher growth rates compared to adults. Growth decreased with size for krill of all maturity classes, with the largest krill (total length >50 mm) expressing negative growth. On average, mean growth was higher (6.5%) in the western section (30-55� E) of the study area compared to the east (56-80� E) (4.2%). In the western section significantly higher growth rates were recorded for krill sampled close to the ice-edge (8.4%) compared to the open ocean (4.2%), which may be due to the presence of phytoplankton blooms associated with the retreating sea ice. However, in the eastern sector higher growth rates occurred in krill from the open ocean compared to the krill caught in areas closer to the coast (5.4% and 2.6% respectively). Krill condition was measured in terms of digestive gland size as a percentage of carapace length. The size of krill digestive gland relative to carapace length ranged from 0.46 to 0.66 (n=2403). A significant positive correlation was found between growth rate and size of the digestive gland (r2=0.7), indicating that growth is related to feeding events. Both digestive gland size and krill growth significantly increased with increasing Chlorophyll a levels and declining SST.

Cycles of Euphausia superba recruitment evident in the diet of Pygoscelid penguins and net trawls in the South Shetland Islands, Antarctica

Size and sex of Antarctic krill taken from chinstrap and gentoo penguin diet were compared to those from scientific net surveys in the South Shetland Islands from 1998 to 2006 in order to evaluate penguin diet as a sampling mechanism and to look at trends in krill populations. Both penguin diet and net samples revealed a 4–5 year cycle in krill recruitment with one or two strong cohorts sustaining the population during each cycle. Penguin diet samples contained adult krill of similar lengths to those caught in nets; however, penguins rarely took juvenile krill. Penguin diet samples contained proportionately more females when the krill population was dominated by large adults at the end of the cycles; net samples showed greater proportions of males in these years. These patterns are comparable to those reported elsewhere in the region and are likely driven by the availability of different sizes and sexes of krill in relation to the colony.

Transport of Antarctic krill ( Euphausia superba) across the Scotia Sea. Part II: Krill growth and survival

A time-dependent, size-structured, physiologically based krill growth model was used in conjunction with a circulation model to test the hypothesis that Antarctic krill ( Euphausia superba) populations at South Georgia are sustained by import of individuals from upstream regions. Surface phytoplankton concentrations along the simulated drifter trajectories were extracted from historical Coastal Zone Color Scanner (CZCS) measurements and sea ice biota concentrations were calculated from sea ice concentration and extent extracted along drifter trajectories from Special Sensor Microwave/Imager measurements. As additional food sources, a time series of heterotrophic food was constructed from historical data, and time series of detritus concentrations were calculated using phytoplankton concentrations extracted from CZCS measurements together with measured particulate organic carbon to chlorophyll a ratios. These food resources along specified drifter trajectories were then input to the krill growth model to determine the size and viability of krill during transport from the source region to South Georgia. The krill growth model simulations showed that no single food source can support continuous growth of krill during the 58–306 days needed for transport to South Georgia. However, under the current assumptions results indicate that combinations of food sources during the transport time enhanced krill survival, with heterotrophic food and detritus being particularly important during periods of low phytoplankton concentrations. The growth model simulations also showed that larval and juvenile krill originating along the western Antarctic Peninsula can grow to 1 + (14–36 mm) and 2 + (26–45 mm) age and size classes observed at South Georgia during the time needed for transport to this region. Krill originating in the Weddell Sea need 20 months for transport, which allows retention in a potentially high food environment, provided by sea ice, for almost 1 year. Krill then complete transport to South Georgia in the following year and larval and juvenile krill grow to 2 + (26–45 mm) and 3 + (35–60 mm) age and size classes during transport. The results of this study show that the successful transport of krill to South Georgia depends on a multitude of factors, such as the location of the spawning area and timing of spawning, food concentrations during transport, predation, and variations in the location of the Southern Antarctic Circumpolar Current Front (SACCF) and in sea ice extent.

Large scale importance of sea ice biology in the Southern Ocean

Despite being one of the largest biomes on earth, sea ice ecosystems have only received intensive study over the past 30 years. Sea ice is a unique habitat for assemblages of bacteria, algae, protists, and invertebrates that grow within a matrix dominated by strong gradients in temperature, salinity, nutrients, and UV and visible radiation. A suite of physiological adaptations allow these organisms to thrive in ice, where their enormous biomass makes them a fundamental component of polar ecosystems. Sea ice algae are an important energy and nutritional source for invertebrates such as juvenile krill, accounting for up to 25% of total annual primary production in ice-covered waters. The ability of ice algae to produce large amounts of UV absorbing compounds such as mycosporine-like amino acids makes them even more important to organisms like krill that can incorporate these sunscreens into their own tissues. Furthermore, the nutrient and light conditions in which sea ice algae thrive induce them to synthesize enhanced concentrations of polyunsaturated fatty acids, a vital constituent of the diet of grazing organisms, especially during winter. Finally, sea ice bacteria and algae have become the focus of biotechnology, and are being considered as proxies of possible life forms on ice-covered extraterrestrial systems. An analysis of how the balance between sea ice and pelagic production might change under a warming scenario indicates that when current levels of primary production and changes in the areas of sea ice habitats are taken into account, the expected 25% loss of sea ice over the next century would increase primary production in the Southern Ocean by approximately 10%, resulting in a slight negative feedback on climate warming.