Differential routing of ‘new’ nitrogen toward higher trophic levels within the marine food web of the Gulf of Aqaba, Northern Red Sea
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Mesozooplankton communities in the meso-oligotrophic Gulf of Aqaba, Northern Red Sea, were investigated over a 2 years period (2005–2007) 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 δ15N onto higher trophic levels. In contrast, N2-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 15N 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 δ15N 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 15N enrichment within the carnivores. The direct link between ‘new’ nitrogen utilization by primary producers and the 15N enrichment of consumers in the planktonic food web of the Gulf of Aqaba emphasizes the significant contribution of ‘new’ nitrogen to the nitrogen budget and ecosystem functions in subtropical and tropical oligotrophic oceans.
KeywordsTrophic Position Prochlorococcus Summer Stratification Harpacticoids Gelatinous Zooplankton
This study was funded by the German–Israeli Foundation for Scientific Research and Development (GIF Grant No.: I-732-54.8/2002). We are grateful to the staff of the Interuniversity Institute for Marine Sciences in Eilat, Israel, and especially Marc Chernichovsky and Roy Holzmann for their helpful support during the field campaign. Special thanks go to Dorit Golan and Sigrid Penno who helped us substantially to organize the field campaign and enabled a pleasant sojourn. Monika Claessens is thanked for her kind support and valuable ideas during the initiation of the field campaign. We are grateful to Arne Malzahn and two anonymous reviewers for their fruitful comments on earlier versions of the manuscript. Thanks are due to Dr. K. Schulz (German Centre for Marine Biodiversity Research) for the identification of calanoid copepod species. Financial support to RBS from the German Science Foundation (SCHN 455/4-1) is gratefully acknowledged.
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