Explore the vast range of titles available.

Two species of salps, Salpa thompsoni and Ihlea racovitzai , were sampled during three cruises to the Lazarev Sea, Southern Ocean, in summer (December–January) 2005/2006, Autumn (April–May) 2004 and Winter (July–August) 2006. Dry weight, carbon, nitrogen, protein, lipid and carbohydrate contents were measured to characterize the potential value of salps as a food source for predators in the Antarctic ecosystem. Biochemical composition measurements showed that despite having a high percentage of water (~94% of wet weight), both species had relatively high carbon and protein contents in their remaining dry weight (DW). In particular I. racovitzai showed high carbon (up to 22% of DW) and protein (up to 32% of DW) values during all seasons sampled, compared to lower values for S. thompsoni (carbon content only about 15% of the DW, protein content about 10% of the DW). At the same time, carbohydrates (CH) and lipids (Lip) only accounted for a small portion of salp DW in both species (1.4% CH and 3.6% Lip for I. racovitzai ; 2.1% CH and 2.9% Lip for S. thompsoni ). There was little variability in the biochemical composition of either salp species between the seasons sampled. Both biochemical composition and life cycle characteristics suggest that Antarctic salps, especially I. racovitzai, may be important prey items for both cold and warm-blooded predators in an environment where food is often very scarce.

Trophic dynamics of 2 abundant macrozooplankton species Salpa thompsoni and Themisto gaudichaudii were studied during the austral summer at 2 locations near the Antarctic Polar Front with contrasting low and high chlorophyll a (chl a) concentrations. Compound-specific stable isotope analysis, complemented by gut content and bulk isotope analyses, were used to investigate trophic interactions between species, and to assess their trophic positions in the pelagic food web. The results of the compound-specific stable isotope analysis placed S. thompsoni at the second trophic level and approx. 1 trophic level below T. gaudichaudii. Two forms of T. gaudichaudii appeared to feed at different trophic levels, with T. gaudichaudii bispinosa feeding at a higher trophic level (~3.3) than T. gaudichaudii compressa (~2.8). Isotope data coupled with gut content analysis indicated a regular consumption of salps in both areas, although a higher contribution of gelatinous prey was encountered in a chl a poor area. The food web baseline values (bulk δ13C) varied regionally, highlighting 2 independent food webs albeit with a similar trophic structure. Overall, our findings suggested that in areas where S. thompsoni is highly abundant, T. gaudichaudii may be a significant predator of this species.

We estimate the energy flow through meso- and bathypelagic chaetognaths in the Atlantic sector of the Southern Ocean from (1) depth-structured chaetognath abundance and body mass data, (2) a general chaetognath respiration model driven by body mass, temperature, water depth and taxon, and (3) published relationships between respiration, production and consumption in chaetognaths. In the 500 to 2000 m depth layer chaetognaths have a mean biomass of 0.109 mg C m–3in summer and 0.146 mg C m–3in winter. Chaetognaths respiration and consumption amount to 282 and 563 mg C m–2yr–1, respectively. Thus, Antarctic midwater chaetognaths consume 0.05% of the copepod standing stock per day or 1% of the daily copepod production in summer. About 2.8% (= 1.9 g C m–2yr–1) of the net annual primary production is required to fuel the midwater chaetognath community via herbivorous copepods. When assuming a 1:1 diet of herbivorous and carnivorous copepods, this share increases to 6.1% (= 4.1 g C m–2yr–1) of annual primary production. It is estimated that chaetognath consumption for the whole water column is 1.4 g C m–2yr–1. This corresponds to 7.1% (= 4.8 g C m–2yr–1) and 15.5% (= 10.4 g C m–2yr–1) of the primary production channeled through herbivorous copepods and through herbivorous and carnivorous copepods, respectively. Hence, chaetognaths represent an important link between lower and higher trophic levels. To further unravel their role in the ecosystem, additional studies on the meso- and bathypelagic zooplankton community are needed.

Distribution, density, and feeding dynamics of the pelagic tunicate Salpa thompsoni have been investigated during the expedition ANTARKTIS XVIII/5b to the Eastern Bellingshausen Sea on board RV Polarstern in April 2001. This expedition was the German contribution to the field campaign of the Southern Ocean Global Ocean Ecosystems Dynamics Study (SO-GLOBEC). Salps were found at 31% of all RMT-8 and Bongo stations. Their densities in the RMT-8 samples were low and did not exceed 4.8 ind m-2 and 7.4 mg C m-2. However, maximum salp densities sampled with the Bongo net reached 56 ind m-2 and 341 mg C m-2. A bimodal salp length frequency distribution was recorded over the shelf, and suggested two recent budding events. This was also confirmed by the developmental stage composition of solitary forms. Ingestion rates of aggregate forms increased from 2.8 to 13.9 micro g (pig) ind-1 day-1 or from 0.25 to 2.38 mg C ind-1 day-1 in salps from 10 to 40 mm oral-atrial length, accounting for 25-75% of body carbon per day. Faecal pellet production rates were on average 0.08 pellet ind-1 h-1 with a pronounced diel pattern. Daily individual egestion rates in 13 and 30 mm aggregates ranged from 0.6 to 4.8 micro g (pig) day-1 or from 164 to 239 micro g C day-1. Assimilation efficiency ranged from 73 to 90% and from 65 to 76% in 13 and 30 mm aggregates, respectively. S. thompsoni exhibited similar ingestion and egestion rates previously estimated for low Antarctic (~50 degrees S) habitats. It has been suggested that the salp population was able to develop in the Eastern Bellingshausen Sea due to an intrusion into the area of the warm Upper Circumpolar Deep Water[PUBLICATION ABSTRACT]

The overwintering success of Euphausia superba is a key factor that dictates population size, but there is uncertainty over how they cope with the scarcity of pelagic food. Both nonfeeding strategies (reduced metabolism, lipid use, or shrinkage in size) and switching to other foods (carnivory, ice algae, or detritus) have been suggested. We examined these alternatives in the southwest Lazarev Sea in autumn (April 1999), when sea ice was forming and phytoplankton was at winter concentrations. Both juveniles and adults had a very high lipid content (36% and 44% of dry mass, respectively) of which >40% was phospholipid. However, their low O : N ratios suggested that these reserves were not being used. Results from gut contents analysis and large volume incubations agreed that juveniles fed mainly on phytoplankton and adults fed on small (<3 mm) copepods. This dietary difference was supported possibly by elevated concentrations of 20 : 1 and 22 : 1 fatty acids in the adults. The feeding methods also confirmed that feeding rates were low compared with those in summer. Even when acclimated to high food concentrations, clearance and ingestion rates were <30% of summer rates. Respiration and ammonium excretion rates of freshly caught krill were 60%-80% of those in summer and declined significantly during 18 d of starvation. These findings suggest both switch feeding and energy conservation strategies, with a trend of reduced and more carnivorous feeding with ontogeny. This points to a \"compromise\" strategy for postlarvae, but there are alternative explanations. First, the krill may have reduced their feeding in an autumn transition to a nonfeeding mode, and, second, some of the population may have maintained a high feeding effort whereas the remainder was not feeding.

Physiological condition and feeding behavior of furcilia larvae were investigated in autumn (April 1999) in the southwestern Lazarev Sea prior to the critical overwintering period. Furcilia stage III (FIII) larvae were most abundant, so only these were used for all analyses (dry mass [DM], elemental and biochemical composition, gut content) and experiments (metabolic and ingestion rates, selective feeding behavior). Chlorophyll a (Chl a) concentrations in the mixed layer were <0.1 µg L-1. Respiration rates of freshly caught FIII larvae were between 0.4 and 1.2 µl$O_2\\>mg^{-1}\\, DM\\,h^{-1}$, similar to larvae fed for 7 d on high food concentrations (4 µg Chl a L-1). Excretion rates ranged between 0.01 and 0.02 µg$NH_4\\>mg^{-1}\\,DM\\,h^{-1}$. Their atomic O : N ratio of 72 indicated that lipids were the main metabolic substrate of FIII larvae in the field. The daily C ration ranged from 0.4% at the lowest food concentration of 3 µg C L-1to 28% at the highest enriched food concentration of 216 µg C L-1, whereas clearance rates decreased with increasing food concentrations. In natural seawater, 115 ml mg-1C h-1, and in natural seawater enriched with ice biota, 24 ml mg-1C h-1, the clearance rates on specific phytoplankton taxa revealed no significant difference across a food size range of 12-220 µm. The study suggests that during periods of low food supply in the water column, larvae have to exploit ice biota to cover their metabolic demands.

Detailed determination of Salpa thompsoni elemental composition has been carried out on specimens collected in the Eastern Bellingshausen Sea and at the northern edge of the Weddell Gyre during austral autumn (April and May) of 1996 and 2001. More than 170 Antarctic tunicates S. thompsoni were analysed to determine wet weight (WW), dry weight (DW), ash-free dry weight (AFDW) and elemental composition (C, N content, proteins, carbohydrates and lipids) of different sizes and stages. Dry weight comprised 6.4% (aggregate form) to 7.7% (solitary form) of the WW. AFDW amounted to ~44% of the DW. Carbon and nitrogen contents (Carbon: 17-22%, Nitrogen: 3-5% of the DW) of both aggregate and solitary forms were found to be high relative to data reported in the literature. Although some unidentified organic compounds are not included in our carbon budget, the findings of this study show higher than previously reported nutritional values of S. thompsoni. In spite of this, a shift from a krill-dominated towards a salp-dominated ecosystem would have dramatic consequences for organisms at higher trophic levels.[PUBLICATION ABSTRACT]

We investigated the causes of a large increase in abundance of small copepods, in particularOithona similis, that was observed during the iron fertilisation experiment EisenEx in the Southern Ocean.Oithonaspp. individuals showed a pronounced migratory response and shifted their vertical distribution towards the progressively phytoplankton-enriched surface layer in the bloom area, while outside, in the area with dilute food concentration, a substantial number of individuals resided in deeper layers. This deep-dwelling behaviour affected an increased drift relative to scarce food in the surface layer, whereas upward migration led to a gradual accumulation of animals in the bloom area. Our simulation study takes into account the particular flow field and the migratory response ofOithonaspp. and shows that it can explain most of the abundance increase inOithonaspp. observed during EisenEx. The migratory behaviour ofOithonaspp. may be considered as a food-finding strategy to cope with the patchy, mostly poor food environment of the Southern Ocean.

The physiological condition and feeding activity of the dominant larval stages of Euphausia superba (calyptopis stage III, furcilia stages I and II) were investigated from February to March 2000 at the Rothera Time Series monitoring station (67 degree 34' S, 68 degree 07' W, Adelaide Island, Western Antarctic Peninsula). A dense phytoplankton bloom (5 to 25 mu g chl a/L) occupied the mixed layer throughout the study period. The feeding of larvae was measured by incubating the animals in natural seawater. Food concentrations ranged from 102 to 518 mu g C/L across experiments, and the mean daily C rations were 28% body C for calyptosis stage III (CIII), 25% for furcilia stage I (FI) and 15% for FII. The phytoplankton, dominated by diatoms and motile prey taxa, ranged from 8 to 79 mu m in size. Across this size spectrum of diatoms, CIII cleared small cells most efficiently, as did FI to a lesser degree. FII, however, showed no clear tendency for a specific cell size. Across the measured size spectrum of the motile taxa, all larvae stages showed a clear preference towards the larger cells. Estimated C assimilation efficiencies were high, from 70 to 92% (mean 84%). Respiration rates of freshly caught larvae were 0.7 to 1.1 kl O sub(2) mg/ DM/h. The calculated respiratory C loss showed a significant increase with increasing food concentration in all larval stages, ranging from 0.9 to 2.4% body C/d. These respiratory losses, combined with the high assimilation efficiencies, thus give the larvae ample capacity for growth at these food concentrations. Critical concentrations of food to achieve maximum daily rations were in the range of 100 to 200 mu g C /L (~62 to 4 mu g chl a/L). Thus productive shelf sites along the Antarctic Peninsula, such as Rothera, may act as good 'nursery' areas for krill larvae.

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0-2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m(-2) in summer and autumn, and 2.7 individuals m(-2) in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0-2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0-2 m layer were higher than corresponding values from the 0-200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0-200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.