Euphausia Crystallorophias Research Articles

The Oceanography and Ecology of the Ross Sea

The continental shelf of the Ross Sea exhibits substantial variations in physical forcing, ice cover, and biological processes on a variety of time and space scales. Its circulation is characterized by advective inputs from the east and exchanges with off-shelf regions via the troughs along the northern portions. Phytoplankton biomass is greater there than anywhere else in the Antarctic, although nitrate is rarely reduced to levels below 10 μmol L(-1). Overall growth is regulated by irradiance (via ice at the surface and by the depths of the mixed layers) and iron concentrations. Apex predators reach exceptional abundances, and the world's largest colonies of Adélie and emperor penguins are found there. Krill are represented by two species (Euphausia superba near the shelf break and Euphausia crystallorophias throughout the continental shelf region). Equally important and poorly known is the Antarctic silverfish (Pleuragramma antarcticum), which is also consumed by most upper-trophic-level predators. Future changes in the Ross Sea environment will have profound and unpredictable effects on the food web.

Erratum to: Mesozooplankton distribution patterns and grazing impacts of copepods and Euphausia crystallorophias in the Amundsen Sea, West Antarctica, during austral summer

Erratum to: Mesozooplankton distribution patterns and grazing impacts of copepods and Euphausia crystallorophias in the Amundsen Sea, West Antarctica, during austral summer

Mesozooplankton distribution patterns and grazing impacts of copepods and Euphausia crystallorophias in the Amundsen Sea, West Antarctica, during austral summer

The rapid melting of glaciers as well as the loss of sea ice in the Amundsen Sea makes it an ideal environmental setting for the investigation of the impacts of climate change in the Antarctic on the distribution and production of mesozooplankton. We examined the latitudinal distribution of mesozooplankton and their grazing impacts on phytoplankton in the Amundsen Sea during the early austral summer from December 27, 2010 to January 13, 2011. Mesozooplankton followed a latitudinal distribution in relation to hydrographic and environmental features, with copepods dominating in the oceanic area and euphausiids dominating in the polynya. Greater Euphausia crystallorophias biomass in the polynya was associated with lower salinity and higher food concentration (chlorophyll a, choanoflagellates, and heterotrophic dinoflagellates). The grazing impact of three copepods (Rhincalanus gigas, Calanoides acutus, and Metridia gerlachei) on phytoplankton was low, with the consumption of 3 % of phytoplankton standing stock and about 4 % of daily primary production. Estimated daily carbon rations for each of the three copepods were also relatively low (<10 %), barely enough to cover metabolic demands. This suggests that copepods may rely on food other than phytoplankton and that much of the primary production is channeled through microzooplankton. Daily carbon rations for E. crystallorophias were high (up to 49 %) with the grazing impact accounting for 17 % of the phytoplankton biomass and 84 % of primary production. The presence of E. crystallorophias appears to be a critical factor regulating phytoplankton blooms and determining the fate of fixed carbon in the coastal polynyas of the Amundsen Sea.

Diatom-Specific Highly Branched Isoprenoids as Biomarkers in Antarctic Consumers

The structure, functioning and dynamics of polar marine ecosystems are strongly influenced by the extent of sea ice. Ice algae and pelagic phytoplankton represent the primary sources of nutrition for higher trophic-level organisms in seasonally ice-covered areas, but their relative contributions to polar marine consumers remain largely unexplored. Here, we investigated the potential of diatom-specific lipid markers and highly branched isoprenoids (HBIs) for estimating the importance of these two carbon pools in an Antarctic pelagic ecosystem. Using GC-MS analysis, we studied HBI biomarkers in key marine species over three years in Adélie Land, Antarctica: euphausiids (ice krill Euphausia crystallorophias and Antarctic krill E. superba), fish (bald notothens Pagothenia borchgrevinki and Antarctic silverfish Pleuragramma antarcticum) and seabirds (Adélie penguins Pygoscelis adeliae, snow petrels Pagodroma nivea and cape petrels Daption capense). This study provides the first evidence of the incorporation of HBI lipids in Antarctic pelagic consumers. Specifically, a di-unsaturated HBI (diene) of sea ice origin was more abundant in ice-associated species than in pelagic species, whereas a tri-unsaturated HBI (triene) of phytoplanktonic origin was more abundant in pelagic species than in ice-associated species. Moreover, the relative abundances of diene and triene in seabird tissues and eggs were higher during a year of good sea ice conditions than in a year of poor ice conditions. In turn, the higher contribution of ice algal derived organic matter to the diet of seabirds was related to earlier breeding and higher breeding success. HBI biomarkers are a promising tool for estimating the contribution of organic matter derived from ice algae in pelagic consumers from Antarctica.

Spatial distribution of Antarctic minke whales (Balaenoptera bonaerensis) in relation to spatial distributions of krill in the Ross Sea, Antarctica

AbstractThe spatial distribution of Antarctic minke whales in the Ross Sea with relation to spatial distributions of their prey – krill – was investigated in this study using generalized additive models (GAMs). Spatial distributions of two species of krill (ice and Antarctic krill) were estimated by GAMs. Three abiotic factors – distance from the continental shelf break (800 m isobaths), the mean temperature and salinity from the surface to 200 m (MTEM‐200 and MSAL‐200), and latitude and longitude – were used as covariates for models of krill. Estimated spatial distributions of krill were then used with other covariates to model the spatial distribution of Antarctic minke whales. In the selected model of Antarctic minke whales, Antarctic krill were more influential than ice krill. The number of Antarctic minke whales increased as the density of Antarctic krill increased to around 1.5 g m−2. Beyond that, the number of Antarctic minke whales decreased as the density of Antarctic krill increased. High densities of the Antarctic minke whales were estimated along the sea ice edge in the eastern part of the Ross Sea. Specifically, the densities were high in the north of the continental shelf break where low MTEM‐200 and MSAL‐200 and intermediate densities of Antarctic krill were observed. Further data collection is needed to investigate interannual variations and trends in their relationship. The results show that the spatial distribution of Antarctic minke whales is a function of longitude, distance from the shelf break, oceanographic condition (temperature and salinity), and densities of ice and Antarctic krill.

Summertime grazing impact of the dominant macrozooplankton off the Western Antarctic Peninsula

The Western Antarctic Peninsula (WAP) is a region of rapid climate change that is altering plankton community structure. To investigate how these changes may impact carbon and energy transfer in the pelagic food web, grazing rates of the five dominant macrozooplankton species (euphausiids Euphausia superba, Euphausia crystallorophias, and Thysanöessa macrura; the pteropod Limacina helicina, and the salp Salpa thompsoni) in the WAP were measured in January 2009 and 2010 as part of the Palmer Antarctica Long-Term Ecological Research (LTER) study. Measurements were made across the coastal-shelf-offshore and north–south gradients of the LTER survey grid. Highest grazing rates occurred offshore in both years, and in the south during 2009 and north during 2010, all associated with the presence of large localized salp blooms. During both years, E. superba was the major grazer at the coast, while S. thompsoni dominated grazing offshore. L. helicina was an important grazer throughout the study area during both years, but especially so over the shelf during 2009. During 2009, there was little difference in the relative importance of the macrozooplankton grazers along the north–south gradient. The presence of a salp bloom in the north during 2010, though, resulted in a distinct shift in the relative importance of major grazers from the euphausiids and L. helicina in the south to salps in the north. Grazing impact was low in coastal waters (≤0.3% of phytoplankton standing stock and ≤0.6% of primary productivity). In contrast, in the offshore waters, where salp blooms were observed, grazing impacts of up to 30% of standing stock and 169% of primary productivity were recorded. If S. thompsoni and L. helicina continue to expand their ranges and increase in abundance, the associated shift in the food web dynamics of the WAP will alter the regional flow of carbon through the WAP food webs and the export of carbon to depth.

Distribution and density of Antarctic krill ( Euphausia superba) and ice krill ( E. crystallorophias) off Adélie Land in austral summer 2008 estimated by acoustical methods

From January to February 2008 the training research vessel TRV Umitaka Maru conducted a comprehensive oceanographic survey of the waters around the 140°E meridian off Adélie Land as part of the Collaborative East Antarctic Marine Census (CEAMARC) project. The acoustic component of this survey was conducted using a scientific echosounder operating at 38 and 70 kHz to estimate the distribution and density of Antarctic krill ( Euphausia superba) and ice krill ( E. crystallorophias). In addition, the relationship between the vertical distribution of Antarctic krill and the water temperature structure along the 140°E meridian was investigated. Antarctic krill were distributed in the waters of the continental slope at 65–66°S and the maximum value of the mean areal density ρ in 1 nautical mile (nmi) intervals was 4344 inds. m −2. Ice krill were distributed in the neritic waters of the continental shelf to the south of the 66°S and the maximum ρ in 1 nmi intervals was 23,669 inds. m −2. Along the 140°E meridian, Antarctic krill were mainly distributed at the water temperatures below 0.5 °C. Although they were mostly distributed shallower than approximately 100 m, dense aggregations at approximately 180–200 m were also observed, which coincided with a depression of the water temperature structure.

Interannual variations in euphausiid life stage distribution in the Dumont d’Urville Sea from 2004 to 2008

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.

Interannual variability of zooplankton in the Dumont d’Urville sea (139°E − 146°E), east Antarctica, 2004–2008

Spatial and temporal variability of zooplankton was studied during five summers (2004–2008) in the Dumont d’Urville Sea, east Antarctica. The species recorded, based on the catch of a 500 μm-mesh Bongo net, were typical of southern continental shelf communities in Antarctica, including Euphausia crystallorophias, polychaetes, pteropods and biomass-dominant copepods. There was a strong degree of temporal variation in abundance, possibly related to the thickness and extent of the sea ice cover during each spring prior to the surveys. Total mean abundance was highly variable between years, with a minimum of 961 ind. 1000 m −3 in 2004 (range 65–3407 ind. 1000 m −3) and a maximum of 15,627 ind. 1000 m −3 in 2005 (range 5109–33,869 ind. 1000 m −3). Spatially, within each year, abundances were also variable, and there were no uniform patterns in abundance from year to year. Water column physical characteristics (temperature and salinity) were relatively constant and did not contribute substantially to variation between the years. It is likely that variation in zooplankton distribution was largely related to a combination of localised features, such as the thickness and extent of sea ice cover, the position and extent of the Mertz Polynya, local wind conditions and bathymetric features.

Invertebrate micronekton and macrozooplankton in the Marguerite Bay region of the Western Antarctic Peninsula

Invertebrate micronekton and macrozooplankton in the Marguerite Bay region of the Western Antarctic Peninsula (WAP) were sampled using a 10-m 2 MOCNESS as part of the Southern Ocean Global Ecosystem Dynamics (SO GLOBEC) program. A total of 62 trawls were completed during four separate cruises in the austral fall (April–June) and winter (July–September) of 2001 and 2002. Crustaceans dominated the system in both seasons, accounting for 32 of the 55 species captured in the fall and 30 of the 48 species captured in winter. In both seasons, a very few species made up the majority of the catch. In the fall, the euphausiids Euphausia crystallorophias, Euphausia superba, and Thysanoessa macrura, and the mysid Antarctomysis ohlinii numerically dominated the assemblage, contributing over 85% of the total. In the winter, the same three euphausiids and the chaetognath Pseudosagitta gazellae were the numerical dominants, comprising over 90% of the catch. A significant increase in total abundance and biomass was observed from 2001 to 2002. The invertebrate micronekton/macrozooplankton communities found in the Marguerite Bay region of the WAP were a mixture of oceanic and neritic fauna: a direct result of local hydrographic conditions. Near the shelf break and in the outer reaches of the Marguerite Trough, a deep canyon transecting the shelf in a south-southeast direction, the communities were more diverse, dominated by oceanic species such as Euphausia triacantha, Salpa thompsoni, and Themisto gaudichaudi. The assemblages present in the nearshore fjords exhibited lower diversity and were dominated by neritic species such as E. crystallorophias and A. ohlinii. At the mid-shelf and mid-trough locations, the assemblages were composed of a variable mixture of oceanic and neritic fauna. The faunal mixing and overall species composition in those areas is the result of episodic Circumpolar Deep Water (CDW) intrusions onto the shelf via deep bathymetric features such as the Marguerite Trough. Species diversity and integrated abundance for the upper 200 m of the water column were similar between seasons in the WAP study region, but integrated biomass was nearly three times greater in fall than in winter. Integrated estimates from the WAP study region were similar to those from other studies conducted in the Scotia and Weddell Seas, but were orders of magnitude lower than estimates from a study in Croker Passage, primarily due to a large catch of E. superba. In contrast, species diversity in the WAP was higher than recorded in any of the previously mentioned studies, which is due to the mixing of typical oceanic fauna with endemic nearshore fauna.

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m2 Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.

The zooplankton of Marguerite Bay, western Antarctic Peninsula—Part II: Vertical distributions and habitat partitioning

The vertical distribution patterns of the dominant zooplankton in the vicinity of Marguerite Bay on the west side of the Antarctic Peninsula were studied during austral fall of 2001 and 2002, using net and concurrent environmental data. Vertical distributions of zooplankton usually were similar to those reported for other Antarctic regions. Maximum abundances of the copepods Ctenocalanus spp. and Calanus propinquus, the euphausiids Euphausia superba, Euphausia crystallorophias, and Thysanoessa macrura, and appendicularians primarily occurred in shallow Antarctic Surface Water (<100 m) or the upper pycnocline. The copepod, Oncaea spp., mysids, and ostracods had the deepest distributions (>250 m), in warmer modified Circumpolar Deep Water. Other dominant copepods ( Calanoides acutus, Metridia gerlachei, Oithona spp., Paraeuchaeta spp., and Rhincalanus gigas), pteropods, and chaetognaths had depths of maximum abundance within the pycnocline or in deeper warmer waters. Overlapping depth distributions suggest that E. superba would have the highest prey encounter rates with M. gerlachei, Ctenocalanus spp., C. propinquus, and Oithona spp. during fall, although most of the copepod community was deeper than the euphausiid community. Even though the three euphausiid species occupied similar depth ranges on average, at any given location E. superba, E. crystallorophias, and T. macrura depths of maximum abundance often did not overlap, suggesting vertical habitat partitioning behavior. The vertical patterns of copepods, euphausiids, amphipods, and mysids did not have a consistent association with the distributions of pigments, temperature, salinity, or density. Instead, the observed vertical distributions are mainly attributed to different behaviors, including seasonal vertical migration to deeper water for overwintering (i.e., C. acutus, R. gigas, ostracods, chaetognaths, pteropods) and vertical habitat partitioning to reduce competition (i.e., euphausiids). Migration into deep water and aggregation behavior (i.e., euphausiids) also reduce the risk of predation.

Horizontal and vertical distribution of euphausiid species on the Western Antarctic Peninsula U.S. GLOBEC Southern Ocean study site

The Western Antarctic Peninsula (WAP) is a site of high krill abundance and a likely source region for krill populations found to the north and east of the area. The U.S. GLOBEC Southern Ocean program studied factors that contribute to the overwintering success of krill in the region of Marguerite Bay, WAP. A MOCNESS net system was used to sample the vertical distribution and abundance of zooplankton relative to physical features (hydrography and circulation) during four broad-scale survey cruises in the fall and winter of 2001 and 2002. Four species were found throughout the study area on all four cruises: Euphausia superba, Euphausia crystallorophias, Euphausia triacantha, and Thysanoessa macrura. The species had significantly different horizontal and vertical distributions. Both E. superba and T. macrura were broadly distributed throughout the area, but the central 50% of their vertical distributions were distinct with E. superba most abundant in the upper 100 m in the coldest, freshest water (average temperature and salinity: −1.13 °C; 33.80) and T. macrura occurring between 100 and 250 m (at 0.01 °C; 34.228). E. crystallorophias had a more coastal distribution and was usually found deeper in slightly warmer and saltier water (−0.44 °C; 33.9982) than E. superba and either overlapped or was above T. macrura in depth. E. triacantha was much rarer and sporadically distributed in the study area and was found substantially deeper (center of distribution about 300 m) in the warmest saltiest water (1.40 °C; 34.65) than the other three euphausiid species. Larval distributions for E. superba indicated that at least some proportion of the populations resulted from reproduction and development on the continental shelf, and not solely from offshore reproduction and transport onto the shelf. A neutral particle tracking model was used to gain insight into the relative importance of shelf versus off-shelf origins for the larvae. The results suggest that a combination of offshore and onshore reproduction can account for the observed E. superba larval distributions in the U.S. SO GLOBEC study site.

The zooplankton of Marguerite Bay, Western Antarctic Peninsula—Part I: Abundance, distribution, and population response to variability in environmental conditions

The zooplankton community of Marguerite Bay was studied during austral fall of 2001 and 2002 using net and concurrent environmental data. Interannual differences were observed in zooplankton species composition, developmental stages, and abundances, which were linked to unusually high chlorophyll concentrations in the Bellingshausen Sea and Marguerite Bay during spring–summer 2000/2001. Copepod abundance was significantly higher in 2001 than in 2002 (46.3 and 28.3 ind m −3 in 2001 and 2002, respectively). During 2001, the copepod community was dominated by two species. Calanoides acutus, a herbivore, and Metridia gerlachei, an omnivore, accounted for 46% and 45% of the community, respectively. During 2002, however, several species were relatively abundant, including M. gerlachei, Ctenocalanus spp., C. acutus, Oithona spp., and Paraeuchaeta spp. Euphausiids also showed a rapid population response to high chlorophyll levels in 2001. Even though average total euphausiid (juvenile/adult) abundances were similar during both years (0.20 and 0.15 ind m −3 for 2001 and 2002, respectively), species composition showed marked interannual differences due to varying life history strategies among species. Thysanoessa macrura, which has a relatively rapid development from larval to juvenile stages between spring and fall of the same year, was the most abundant euphausiid in 2001. In contrast, Euphausia crystallorophias and Euphausia superba juvenile/adult populations increased in 2002, owing to a slower development in which larval stages overwinter and recruit to juveniles during the following spring/summer. Other zooplankton groups those were abundant in Marguerite Bay, but showed little variability between years, included ostracods, pteropods, chaetognaths, medusae, amphipods, and mysids. Summer phytoplankton concentrations strongly influenced copepods and euphausiids; however, there were no clear associations between zooplankton distributions and fall environmental conditions (i.e., pigment concentrations and surface salinity) or bottom depth. It is notable that ostracods and pteropods had the highest abundances of non-copepod zooplankton.

Larval feeding of Chionodraco hamatus (Pisces, Channichthyidae) in the Ross Sea and its relation to environmental conditions

Feeding habits of early life stages of the channichthyid Chionodraco hamatus were investigated on samples collected in the western Ross Sea in early summer of 1996 and 2004. The stomach content analysis was carried out on larval and postlarval specimens ranging from 14 to 39 mm SL. Unlike larvae and juveniles of other channichthyids, which elsewhere largely rely on early life stages of Antarctic krill or ice krill, no euphausiids were found in the stomach contents of C. hamatus, except for a single large individual of Thysanoessa macrura. In both years, dietary composition consisted almost exclusively of notothenioid fish larvae. Early larvae of Pleuragramma antarcticum overwhelmingly dominated the diet in terms of abundance, biomass and frequency of occurrence, accounting for 98.4% of the index of relative importance (IRI). Other fish larvae consumed occasionally in small amount were Trematomus lepidorhinus, T. scotti and C. hamatus itself. As a result, the feeding strategy of C. hamatus was considerably shifted toward specialization, relying on relatively few taxa of prey, each of them showing a high prey-specific abundance. Comparing both dietary composition and feeding strategy of C. hamatus in the two different sampling years, several differences were observed, probably due to different environmental conditions, mainly linked to a significant delay of ice retreat and formation of the Ross Sea polynya verified in the 2003–2004 summer season.

Antarctic mesozooplankton community structure during BROKE-West (30°E–80°E), January–February 2006

Abstract The distribution of mesozooplankton, based on the catch by a RMT1 net, in the upper 200 m of the South Western Indian Ocean sector (30°E–80°E) of the Southern Ocean was examined during the large-scale BROKE-West survey in summer 2006. Multivariate analyses revealed four groups of zooplankton that could be broadly linked to oceanographic features. The first group, occurring south of the southern boundary of the Antarctic Circumpolar Current (sbACC) and west of 40°E, was associated with waters from the eastern limb of the Weddell Gyre. The group was typified by moderate abundance (mean: 11,251 ind. 1000 m−3) and included foraminiferans, appendicularians and small copepods. Group 2 was composed of stations found in the deeper, warmer waters lying between the sbACC and the southern ACC front (sACCf), along with a few stations north of the sACCf. Sites in this group exhibited the highest mean zooplankton abundance (81,750 ind. 1000 m−3) and comprised large numbers of small copepods and appendicularians. Sites in group 3 fell south of the sbACC, were situated east of 50°E, and generally located near the Antarctic slope current (ASC), a strong and narrow jet of westward flowing water. Typical species for this group included Euphausia crystallorophias, Fritillaria spp. and Metridia gerlachei. Finally, group 4 represented neritic sites in the far southwestern corner of the survey region. Abundances were lowest (mean: 2588 ind. 1000 m−3) and the assemblage was dominated by E. crystallorophias, Neogloboquadrina pachyderma and Fritillaria spp. The four groups differed more by abundance than by composition. The suite of environmental variables that best correlated with patterns in the species data included chlorophyll a concentration, proximity to the ASC and length of time without an ice cover, all features which indicated that large scale oceanographic processes were underpinning the patterns in mesozooplankton distribution.

Spatio-temporal distribution and abundance of Euphausia crystallorophias in Terra Nova Bay (Ross Sea, Antarctica) during austral summer

Average abundance values for Euphausia crystallorophias in the shelf region of Terra Nova Bay were generally high. Mean number of adults and juveniles were 20 and 87 ind/m2, respectively, while larval concentrations were much higher, reaching a mean of 14,764 ind/m2. Euphausia crystallorophias were very patchy in their distribution during all stages of their life cycle. The distribution patterns of E. crystallorophias in TNB seem to be strictly correlated both to hydrological features such as the presence of polynyas, pack-ice zone and certain temperatures values. According to our data, E. crystallorophias spawned from early October to early January. For the two age groups (0+ and 1+) an average growth rate of 0.074 and 0.076 mm/day was found, respectively. The mean biomass of larval E. crystallorophias (from Metanauplius to Furcilia I) in January was estimated as 84 mg WW/m2.

Palmer LTER: Patterns of distribution of five dominant zooplankton species in the epipelagic zone west of the Antarctic Peninsula, 1993–2004

Abstract Variability in the temporal-spatial distribution and abundance of zooplankton was documented each summer on the Palmer Long-Term Ecological Research (LTER) grid west of the Antarctic Peninsula between Anvers and Adelaide Islands during a 12-yr time series. Oblique tows to 120 m with a 2×2 m fixed-frame net were made at about 50 stations each January/February between 1993 and 2004. The numerically dominant macro- and mesozooplanktonic species >2 mm included three species of euphausiids (Euphausia superba, Antarctic krill; Thysanoessa macrura; Euphausia crystallorophias, ice krill), a shelled pteropod (Limacina helicina), and a salp (Salpa thompsoni). Life cycles, life spans, and habitat varied among these species. Abundance data from each year were allocated to 100 km by 20 km (alongshore by on/offshore) grid cells centered on cardinal transect lines and stations within the Palmer LTER grid. The long-term mean or climatology and means for each year were used to calculate annual anomalies across the grid. Principal components analysis (PCA) was used to analyze for patterns and trends in the temporal-spatial variability of the five species. Questions included whether there are groups of species with similar patterns, and whether population cycles, species interactions or seasonal sea-ice parameters were correlated with detected patterns. Patterns in the climatology were distinct, and matched those of physical parameters. Common features included higher abundance in the north than in the south, independent of the cross-shelf gradients, and cross-shelf gradients with higher abundance either inshore (E. crystallorophias) or offshore (S. thompsoni). Anomalies revealed either cycles in the population, as episodic recruitment in Antarctic krill, or changes in anomaly pattern between the first and second half of the sampling period. The 1998 year, which coincided with a rapid change from a negative to a positive phase in the SOI, emerged as a year with either significant anomalies or that marked a change in anomaly patterns for different species. PCA analysis showed that the pattern of cumulative variance with increasing number of modes was distinctly different for shorter-lived versus longer-lived species; the first mode accounted for nearly 50% of the variance in the shorter-lived species and less than 25% in the longer-lived species. This suggested that the mechanisms driving variability in the temporal-spatial distribution of the shorter-lived, more oceanic species were less complex and more direct than those for the longer-lived euphausiids. Evidence from both the anomaly plots and the trend analysis suggested that salps have been more consistently present across the shelf from 1999 to present, and that the range of L. helicina has been expanding. With shorter life spans, these two species can respond more quickly to the increasing heat content on the shelf in this region. The cross-correlation analysis illustrated the negative correlation between salps and ice retreat and the number of ice days, and the positive correlation between the presence of ice krill and the day of ice retreat. These results suggest that for these species, several environmental controls on distribution and abundance were linked to seasonal sea-ice dynamics.

Isotopic niches of emperor and Adélie penguins in Adélie Land, Antarctica

The emperor and Adelie penguins are the only two species of penguins that co-occur at high-Antarctic latitudes. We first measured and compared their isotopic niches on the same year in Adelie Land in spring, when the two species co-exist. Emperor and Adelie penguins segregated by their blood isotopic signatures, with adult δ13C values (−24.5 ± 0.2 and −25.4 ± 0.2‰, respectively) suggesting that emperor penguins foraged in more neritic waters than Adelie penguins in spring. At that time, difference in their δ15N values (4.1‰, 12.0 ± 0.4 vs. 7.9 ± 0.1‰) encompassed more than one trophic level, indicating that emperor penguins preyed mainly upon fish (and squids), while Adelie penguins fed exclusively on euphausiids. Second, we compared the food of breeding adults and chicks. The isotopic signatures of adults and chicks of emperor penguins were not statistically different, but δ15N value of Adelie penguin chicks was higher than that of adults (10.2 ± 0.8 vs. 9.0 ± 0.2‰). The difference showed that adult Adelie penguins captured higher trophic level prey, i.e. higher-quality food, for their chicks. Third, the isotopic signatures of Adelie penguins breeding in Adelie Land showed that adults fed on Antarctic krill in oceanic waters in spring and shifted to neritic waters in summer where they preyed upon ice krill for themselves and upon fish and euphausiids for their chicks. A comparison of isotopic niches revealed large overlaps in both blood δ13C and δ15N values within the community of Antarctic seabirds and pinnipeds. The continuum in δ15N values nevertheless encompassed more than one trophic level (5.2‰) from Adelie penguin and crabeater seal to the Weddell seal. Such a broad continuum emphasizes the fact that all Antarctic seabirds and marine mammals feed on varying proportions of a few crustacean (euphausiids) and fish (Antarctic silverfish) species that dominate the intermediate trophic levels of the pelagic neritic and oceanic ecosystems.

Fine-scale habitat selection of crabeater seals as determined by diving behavior

Abstract Previous studies within the Marguerite Bay region of the Antarctic Peninsula (∼67°S, ∼67°W) demonstrated that during winter, crabeater seals (Lobodon carcinophagus) were not randomly distributed across available habitat, but instead were more likely to be located in nearshore waters where bathymetric gradients and ice concentrations were high. Here, we investigate how the diving patterns of crabeater seals vary in response to these habitat characteristics, and interpret seal behaviors in light of information on the distribution of their primary prey, krill (Euphausia superba or Euphausia crystallorophias). Diving and movement patterns were obtained from 34 seals (16 male, 18 female) fitted with satellite-relayed data loggers (SRDLs) during the 2001 and 2002 Southern Ocean GLOBEC cruises. Tags transmitted position and dive information for 4–174 days, during which time we received an average of 21 positions/day, and information on a total of 124,681 dives. A series of generalized linear mixed-effect models (GLMM) were used to evaluate the relationship between diving behavior and temporal and physical features of the habitat, and models contrasted using AICc and BIC weights. Overall, we found that the most parsimonious models included year, month, and period (day, dusk, night). In general, seals dived deeper (158 vs. 73 m) and longer (432 vs. 360 s) during the day than at night. In addition, daytime dives included slightly more time at the foraging depths (142 vs. 102 s), and were slightly more efficient (24% vs. 21% of the dive cycle spent at the bottom). When dive patterns were examined with respect to bathymetry, models indicated that seals were foraging in shallower waters (366 vs. 410 m) and closer to the bottom (dives were 50.3% vs. 26.3% of bathymetric depth) during the day than at night. In combination, these findings suggest that crabeater seals foraging during the day exploited zooplankton schools compressed along the bottom. At night, when zooplankton were dispersed and light levels low, foraging activity was less frequent and seals concentrated their diving closer to the surface over a broader range of habitat depths. As individual seals moved an average of only 4.1±1.4 km between daytime and nighttime positions, these results suggest that crabeater seals diving along the Western Antarctic Peninsula select areas of high bathymetric gradients so that they can maximize foraging success over a 24-h cycle without the need to travel long distances. However, annual differences in behavior and the generally low amount of deviation explained by models also suggests that seals vary their diving behavior in response to finer-scale biological, temporal, and/or physical features that were not monitored as part of this study.