The use of stable isotopes to study food webs has increased rapidly, but there are still some uncertainties in their application. We examined the ␦ 15 N and ␦ 13 C values of Antarctic euphausiids and copepods from the Polar Front, Lazarev Sea, and Marguerite Bay against their foodweb baseline of particulate organic matter (POM). Interpretations of trophic level were helped by comparison with other approaches and by calibration experiments with Euphausia superba fed known diets. Results for well-known mesozooplankters (e.g., Calanoides acutus and Metridia gerlachei) were internally consistent and corresponded to those derived from independent methods. This gave confidence in the isotope approach for copepods and probably larval euphausiids. Among the dominant yet poorly known species, it suggested mainly herbivory for Rhincalanus gigas but omnivory for Calanus simillimus and furcilia larvae of Thysanoessa spp. and Euphausia frigida. The ␦ 15N values of adult copepods were up to 3‰ higher than those of early copepodites, pointing to ontogenetic shifts in diet. In the Lazarev Sea in autumn, the isotopic signals of E. superba larvae suggested pelagic, mainly herbivorous, feeding rather than feeding within the ice. In contrast to the mesozooplankton, some anomalous results for postlarval krill species indicated problems with this method for micronekton. The experiments showed that postlarval E. superba did not equilibriate with a new diet within 30 d. We suggest that the slower turnover of these larger species, partly in combination with their ability to migrate, has confounded trophic effects with those of a temporally/spatially changing food-web baseline. Interpretations of food sources of micronekton could be helped by analyzing their molts or fecal pellets, which responded faster to a new diet.

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Abstract:We developed new perspectives to identify important questions and to propose approaches for future research on marine food web lipids. They were related to (i) structure and function of lipids, (ii) lipid changes during critical life phases, (iii) trophic marker lipids, and (iv) potential impact of climate change. The first addresses the role of lipids in membranes, storage lipids, and buoyancy with the following key question: How are the properties of membranes and deposits affected by the various types of lipids? The second deals with the importance of various types of lipids during reproduction, development, and resting phases and addresses the role of the different storage lipids during growth and dormancy. The third relates to trophic marker lipids, which are an important tool to follow lipid and energy transfer through the food web. The central question is how can fatty acids be used to identify and quantify food web relationships? With the fourth, hypotheses are presented on effects of global warming, which may result in the reduction or change in abundance of large, lipid-rich copepods in polar oceans, thereby strongly affecting higher trophic levels. The key question is how will lipid dynamics respond to changes in ocean climate at high latitudes? Kattner et al. 1639Résumé : Une nouvelle approche a été développée afin d'identifier les questions importantes relatives aux lipides dans les réseaux alimentaires marins et pour suggérer de nouvelles perspectives pour les recherches futures. Celles-ci concernent (i) la structure et la fonction des lipides, (ii) les changements lipidiques durant les phases critiques du cycle biologique, (iii) le rôle des lipides comme marqueurs trophiques et (iv) l'impact potentiel du changement climatique. Le premier point touche le rôle des lipides dans les membranes, les lipides de réserve et la flottabilité et la question essentielle est de savoir comment les propriétés des membranes et des réserves sont affectées par les divers types de lipides? Le second point traite de l'importance des lipides durant les phases de reproduction, de développement et de repos, ainsi que du rôle des différents lipides de réserve durant la croissance et la dormance.

Zooplankton grazing on bacterio-and phytoplankton was studied in the Gulf of Aqaba and the Northern Red Sea during Meteor Cruise Me 44-2 in February-March 1999. Protozoan grazing on bacterioplankton and autotrophic ultraplankton was studied by the Landry dilution method. Microzooplankton grazing on phytoplankton > 6 µm was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual zooplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae < 6 µm were high (bacteria: 0.7 to 1.1 d ), while grazing rates on Synechococcus spp. were surprisingly low and undetectable in some experiments. Mesozooplankton grazing was weak, cumulative grazing rates being ca. 2 orders of magnitude smaller than the grazing rates by protozoans. Among mesozooplankton, appendicularians specialised on smaller food items and calanoid copepods on larger ones. KEY WORDS: Phytoplankton · Protozoa · Bacteria · Zooplankton · Grazing · Red Sea · Gulf of Aqaba Resale or republication not permitted without written consent of the publisher

The application of marker fatty acids to trace the feeding habits of Euphausia superba (krill) has produced contradictory results. We examined the effects of various diets on the fatty acid composition of larval, juvenile, and adult E. superba collected in April 1999 in the southwest Lazarev Sea and in April 2001 in the Bellingshausen Sea. Specimens were fed four different diets (mixed phytoplankton, mixed ice algae, the ice diatom Fragilariopsis cylindrus, and mixed copepod assemblages) or starved for up to 44 d. Total lipid content, lipid classes, and fatty acid composition showed very little variation in juvenile and adult krill with the different feeding regimes. Furcilia lipids were much more strongly influenced by the fatty acid signatures of their food. No stage-specific food preferences were detected in the larvae, and spatial patterns were mirrored by all furcilia stages. Comparison of the fatty acid profiles of the offered food with those of the subsequently excreted feces indicated preferential assimilation of polyunsaturated fatty acids by E. superba.The analysis of lipid compositions has been applied successfully to reveal food web relationships in marine ecosystems. This trophic biomarker concept is based on observations that specific dietary lipid components, particularly fatty acids, are incorporated into the consumers' lipids largely unmodified (Sargent and Whittle 1981; Sargent et al. 1987;Graeve et al. 1994b). This approach can provide information where the classical gut content analysis fails (e.g., soft-bodied organisms, advanced digestion). Instead of a snap-shot impression, biomarkers integrate the trophic information over a longer time scale of several weeks. However, lipid signatures usually do not have the precision to identify species-specific interactions. Rather, they provide trophic information on the level of larger taxonomic groups.Experimental studies have further consolidated the trophic biomarker approach. Clear changes in fatty acid compositions could be induced by different phytoplankton diets in Arctic copepods (Graeve et al. 1994b) and even traced up to secondary consumers, such as juvenile North Sea cod (St. John and Lund 1996). In recent years, an increasing number