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High latitude oceans are strongly seasonal ecosystems where winter conditions are marked by periods of low primary productivity. These oceans tend to have shortened food webs with relatively few species linking primary production to upper trophic levels. In the case of the Southern Ocean, a single species, Euphausia superba, is thought to be this link between trophic levels. The polar regions in both hemispheres are also among the ecosystems heavily impacted by climate change. For example, the western Antarctic Peninsula is experiencing some of the most rapidly changing climate on the planet with changes in temperature, wind, and sea ice durations and extent (Vaughn et al 2003, Stammerjohn et al 2008). Understanding how climate change will affect these ecosystems requires knowledge of trophic structure, its key species, their life history, and their plasticity to environmental variability. In this thesis, I explore the seasonal life history strategies of Antarctic euphausiids. In chapter one, I introduce the high latitude marine ecosystem of the Southern Ocean, Antarctic euphausiids, and climate change and its impact on the ecosystem in this region. In chapter two, I fill in gaps in our knowledge of the life history strategy of the highly abundant, but relatively understudied, Thysanoessa macrura, by synthesizing distribution, maturity, and diet data from summer and winter surveys in the Antarctic Peninsula region. I find that krill show knife edge maturity, can spawn in their first year, are more dispersed and offshore in winter, and are more predatory with increasing size and in winter. In the third chapter I investigate E. superba from the perspective of optimal foraging theory to examine when krill generalize or specialize to unify seasonal and regional differences in the diet and feeding behavior of E. superba under the Trophic Wave Hypothesis. I predict that krill have a more specialized yet higher mortality risk diet in summer and more of a generalist diet in winter. In chapter four, I ask whether E. superba found in benthic and mesopelagic habitats are an aberration or reflect an important part of their life history strategy. In this chapter, I use a Stochastic Dynamic Programming model (VertiKrill) to explore how food, predation, and respiration drive vertical habitat selection across a range body conditions throughout the year. I find that for both juveniles and adults, deep water habitats provide important refuges for avoiding predation and starvation in winter as well as during the transition between seasons. In chapter five I summarize the main findings of each chapter, discuss their implications, and offer future directions for this research.

Growth, which is intrinsically linked to environmental conditions including temperature and food availability are highly variable both temporally and spatially. Estimates of growth rates of the Southern Ocean euphausiid Thysanoessa macrura are currently restricted to limited studies which rely upon repeated sampling and length-frequency analysis to quantify growth rates. The instantaneous growth method (IGR) was used to measure the growth rate of T. macrura successfully in the southern Kerulen Plateau region during summer, providing the first IGR parameters for the Southern Ocean euphausiid species. Results of the four-day IGR incubation indicate a period of low somatic growth for adult T. macrura. Males had a longer intermoult period (IMP) (62 days) than females (42 days), but the sexes exhibited similar daily growth rates of 0.011 mm day−1 and 0.012 mm day−1 respectively. Juveniles exhibited the fastest growth, with an IMP of 13 days and daily growth rate of 0.055 mm day−1 indicating a prolonged growth season, similar to the Antarctic krill E. superba. Consequently, we highlight the usability of the IGR method and strongly encourage its use in developing a comprehensive understanding of spatial and seasonal growth patterns of T. macrura.

The Southern Ocean provides strong contrasts in rates and directions of change in temperature and sea ice between its sectors, but it is unknown how these affect plankton species that are distributed right around Antarctica. Here, we quantify the changing circumpolar distributions of Antarctic krill, based on the CHINARE 2013/14 circum‐Antarctic expedition, plus independent analyses of compiled abundance data (KRILLBASE: 1926–2016). In the 1920s–1930s, average krill densities in the Atlantic‐Bellingshausen sector were eight times those in the other sectors. More recently, however, the concentration factor has dropped to only about twofold. This reflects a rebalancing broadly commensurate with climatic forcing: krill densities declined in the Atlantic‐Bellingshausen sector which has warmed and lost sea ice, densities may have increased in the Ross‐Pacific sector which showed the opposite climatic trend, while densities showed no significant changes in the more stable Lazarev‐Indian sectors. Such changes would impact circumpolar food webs, so to better define these we examined circumpolar trends of isotopic values in krill and other zooplankton based on the CHINARE cruise and a literature meta‐analysis. Krill δ15N values ranged significantly between sectors from 2.21‰ (Indian) to 3.59‰ (Ross‐Pacific), about half a trophic level lower than another key euphausiid, Thysanoessa macrura. These isoscapes form a baseline for interpreting the reliance of predators on euphausiids, within the varying food webs around the continent. Overall, we suggest that the Indo‐Pacific sector has acted as a refuge for the circumpolar krill stock while conditions for them deteriorated rapidly in the Atlantic sector.

Southern Ocean krill (Thysanoessa macrura) is one of the most abundant euphausiid species in the Southern Ocean. Despite a wide distribution and high abundance this species remains poorly understood. Unique ecological traits of this species, including a late-winter reproductive period, highly omnivorous diet and large lipid deposits within the carapace, suggest T. macrura is attuned to the seasonal patterns of the Southern Ocean. The absence of the descriptions of T. macrura larval stages limits the development of a comprehensive understanding of the early life-history of this species. Here, I provide the first detailed descriptions of the post-naupliar larval developmental stages of T. macrura with accompanying photographic documentation. Three calyptopis and 6 furcilia stages were identified and described for T. macrura, providing an accessible guide for their identification in both historical samples and for future investigations.

Euphausiid larvae were collected in the Weddell Scotia Confluence region in summer 2011, in the West Antarctic Peninsula and Scotia Sea in 2012, and in the South Orkneys Shelf in 2014. From 2011 to 2014 the larval stages geographic distribution and the distribution of water masses were consistent with published information. The densities observed were compared between the cruises and with the 1981 and 1995 values by means of a kernel estimate of absolute and relative densities at a fixed grid of points. The values of Thysanoessa macrura were higher in 2011 and 2012 and those of Euphausia superba were lower than those observed in 1981 and 1995. Euphausia frigida did not show significant variations in absolute density but their relative values were higher than those of E. superba. No significant variations were observed in absolute values between 1981 and 1995. The opposite pattern was found in 2014, with significantly higher densities of E. superba than those of T. macrura. No single factor could be identified to explain these variations in density, suggesting the existence of complex mechanisms coupling the reproductive biology with oceanographic phenomena. Their description will require ample spatial and temporal monitoring rather than locally detailed observations. Fil: Rombola, Emilce Florencia. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Franzosi, Claudio Atilio. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Tosonotto, Gabriela Viviana. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Alder, Viviana Andrea. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentina Fil: Marschoff, Enrique Ricardo. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina

Macrozooplankton/micronekton in the epipelagic zone (0–100 m) of the NW Weddell Sea were sampled with a 10 m2 multiple opening-closing net environmental sampling system (MOCNESS) trawl during three cruises in 1995 and 1996. A total of 40 species were collected during all cruises. Community composition, as measured using Bray-Curtis analysis and multi-dimensional scaling, permitted separation into two groups: group 1 included samples collected during September–October 1995 and group 2 contained samples collected during April-May 1996 and November–December 1996. Zooplankton collections were dominated by Thysanoessa macrura in September–October 1995 (high pack-ice cover) and by both Euphausia superba and Salpa thompsoni in April–May 1996 (intermediate pack-ice cover) and November–December 1996 (open water). Copepods were not sampled quantitatively by the MOCNESS trawl and were not included in this analysis. Trends in measured parameters of community structure are discussed with respect to environmental characteristics during each sampling period.

International audience; Euphausiid larvae were caught by oblique tows of a double-framed bongo net at 118 stations along the coast of Terre Adélie during every summer from 2004-2008. Larvae of Euphausia crystallorophias Holt and Tattersall and Thysanoessa macrura G.O. Sars were observed in the study area every year in contrast to larvae of Euphausia superba which were absent during the entire period of sampling. Highest abundances of both species were observed during the summer of 2005 and the lowest in the summer of 2004. Larvae of E. crystallorophias dominated every year and comprised more than 80% of the total abundance of euphausiid larvae. Within species, the calyptopis stages especially calyptopis 1 of E. crystallorophias dominated compared to furcilia stages of T. macrura which were less abundant. For both species, spatio-temporal variations in abundances of developmental stages were significantly correlated to density and salinity of the deep water layer. In the case of calyptopis 3 of E. crystallorophias, spatio-temporal variations were correlated with the number of days between the end of the sea ice retreat and the sampling day and for T. macrura inversely correlated to temperature of surface water layer. These variations are discussed and related to the timing of sea ice retreat, the sea ice concentration as well as metabolic conditions of adults of both species during their spawning period.

The results of morphometric and biochemical measurements of samples of Euphausia superba and Thysanoessa macrura were used to study krill longevity. On each individual, different measurements were taken, a differentiation index was calculated, and lipids and carotenoids analysed; the data were processed by principal component analysis (PCA). in Euphausia superba the analysis of six morphometric characteristics shows a separation into two groups for the males, but not for females. The analysis of a larger sample, using only four variables, shows an analogous, though less marked separation, suggesting a morphological evolution linked with age. When biochemical data were added, PCA shows a good separation into two groups for both males and females, and the same result was obtained using only the biochemical data and the differentiation index. No possible age groups, other than juveniles and adult males and females, could be identified by PCA for Thysanoessa macrura .

In the Scotia Sea, the larvae of Thysanoessa macrura G.O. Sars, 1885 were scarce in spring (September 27 to October 6, 1971) and mainly present at the Calyptopis I stage, while in summer (December 17, 1974 to January 3, 1975) they were much more abundant and much further developed (up to Furcilia VI). The spawning season commences as early as September and, as indicated by the presence of numerous nauplii and metanauplii in summer, is rather long. T. macrura begins to spawn much earlier in West-Wind-Drift waters than in waters of the Weddell Sea. It starts to spawn simultaneously with Euphausia frigida and E. triacantha, but significantly earlier than E. superba. These timings of the breeding process are related to differential biogeographical characteristics of these species. Plankton collections made at depths of 500 or 1000 m have, for T. macrura, revealed a “developmental ascent” analogous to that of E. superba. Ontogenetical deep-water migrations are, consequently, characteristic not only for E. superba, but also for other Antarctic euphausiids (in particular E. frigida, but possibly not for inshore E. crystallorophias) and, as correctly postulated by several other investigators, for all oceanic euphausiid species.

Krill was sampled in the Weddell Sea, farther south than ever before, and off Bouvet-oya. Five species were found: Euphausia superba, E. crystallorophias, E. frigida, E. triacantha, and Thysanoessa macrura. E. superba and E. crystallorophias were the most abundant species north and south of 74° S respectively. Length frequency analyses suggest that E. superba from the inner Weddell Sea takes three years to reach maturity. This population differed in several aspects from the populations farther north. E. crystallorophias and T. macrura were registered south to 77°45′ S, the southernmost localisation investigated. The material gives no clear evidence whether these species take one or two years to reach maturity. Their spawning activity seems to start earlier in the season than for E. superba. E. frigida and E. triacantha were only-found off Bouvetoya. Continuous echosounder recordings revealed that the total abundance of krill in the southern parts of the Weddell Sea was low, as was also the abunda...