Conventional simultaneous CNS stable isotope abundance measurements of solid samples usually require high sample amounts, up to 1 mg carbon, to achieve exact analytical results. This rarely used application is often impaired by high C:S element ratios when organic samples are analyzed and problems such as incomplete conversion into sulphur dioxide occur during analysis. We introduce, as a technical innovation, a high sensitivity elemental analyzer coupled to a conventional isotope ratio mass spectrometer, with which CNS-stable isotope ratios can be determined simultaneously in samples with low carbon content (<40 microg C corresponding to approximately 100 microg dry weight). The system includes downsized reactors, a temperature program-controlled gas chromatography (GC) column and a cryogenic trap to collect small amounts of sulphur dioxide. This modified application allows for highly sensitive measurements in a fully automated operation with standard deviations better than +/-0.47 per thousand for delta15N and delta34S and +/-0.12 per thousand for delta13C (n = 127). Samples collected from one sampling site in a Baltic fjord within a short time period were measured with the new system to get a first impression of triple stable isotope signatures. The results confirm the potential of using delta34S as a stable isotope tracer in combination with delta15N and delta13C measurements to improve discrimination of food sources in aquatic food webs.

Mesozooplankton communities in the mesooligotrophic Gulf of Aqaba, Northern Red Sea, were investigated over a 2 years period (2005)(2006)(2007) with emphasis on the trophodynamic relations among different taxonomic groups ranging from primary consumers to carnivorous predators. Based on stable isotope analyses, we present evidence for a strong contribution of 'new' nitrogen mainly derived from the utilization of aerosol nitrate by unicellular cyanobacteria especially during summer stratification and the propagation of exceptionally low d 15 N onto higher trophic levels. In contrast, N 2 -fixation by diazotrophs seemed to play a minor role, while the utilization of deep water nitrate by cyanobacteria and eukaryotic algae might be of importance during winter mixing. Based on 15 N enrichment of consumers, clear differences between exclusively herbivorous organisms (doliolids, appendicularians, pteropods) and those with omnivorous feeding modes were detected. The category of omnivores comprised a large variety of taxons ranging from small meroplanktonic larvae to non-calanoid copepods (harpacticoids, cyclopoids and poecilostomatoids) that together form a diverse and complex community with overlapping feeding modes. In addition, distinct seasonality patterns in d 15 N of copepods were found showing elevated trophic positions during periods of winter mixing, which were most pronounced for non-calanoid copepods. In general, feeding modes of omnivores appeared rather unselective, and relative contributions of heterotrophic protists and degraded material to the diets of non-calanoid copepods are discussed. At elevated trophic positions, four groups of carnivore predators were identified, while calanoid copepods and meroplanktonic predators showing lowest 15 N enrichment within the carnivores. The direct link between 'new' nitrogen utilization by primary producers and the 15