Food-­web structure and trophic interactions in the northern Benguela upwelling system

Thorsten.Werner [ at ]


In the framework of the GENUS –“Geochemistry and Ecology of the Namibian Upwelling System” research program, trophic interactions and carbon pathways throughout the food web of the coastal upwelling system are being quantified. In contrast to earlier studies, special focus is been given to lower trophic levels in higher taxonomic resolution. Energy demands of various zooplankton taxa, including copepods, euphausiids, decapods and fish larvae, have been quantified with standardized methodology via optode respirometry. Dietary spectra and trophic levels were analyzed by trophic biomarker approaches based on fatty acid composition and stable isotopes (15N, 13C), respectively. All empirical data are assembled for an Ecopath with Ecosim (EwE) food-web model. The EwE model distinguishes between shelf and offshore communities. The conceptual food‐web model consists of three groups of primary producers, i.e. diatoms, dinoflagellates, and cyanobacteria, as well as many consumers such as Calanoides carinatus as the key herbivorous copepod in the Benguela upwelling system, other copepods, Euphausia hanseni, other zooplankton, and pelagic fishes including sardine, anchovy, and horse mackerel. Empirical data show that zooplankton and particularly copepods encompass a wider range of trophic levels from herbivory to secondary or even tertiary consumers (δ15N from 4 to 12‰), while anchovy had rather low δ15N of about 7‰. Respiration rates and metabolic activities of copepods could be parameterized for the model by an energy budget approach based on ambient temperature, body mass, and activity level. Calanoid copepods consumed 78mg C m‐2 d‐1 in shelf regions and 21mg C m‐2 d‐1 in oceanic regions. Locally, C. carinatus could remove up to 90% of the diatom biomass per day. The community consumption of pelagic decapods ranged from 7 mg C m‐2 d‐1 to >20mg C m‐2 d‐1 with highest values in the northernmost part of the study area. Overall, pelagic decapods apparently play a more prominent role in the northern Benguela ecosystem than previously assumed and may exert a substantial predation pressure on calanoid copepods. GENUS results emphasize that the trophic interactions within zooplankton and lower trophic levels are more complex than just linking primary producers with pelagic fish and should be taken into account in the process of developing realistic food‐web models of coastal upwelling systems. Keywords: foodweb, zooplankton, trophic interactions, energy flux

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IMBER OSC - Future Oceans, 22 Jun 2014 - 27 Jun 2014, Bergen, Norway.
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Auel, H. , Bode, M. , Hagen, W. , Moroff, N. , Koppelmann, R. , Martin, B. , Werner, T. , Bohata, K. , Geist, S. , Ekau, W. , Buchholz, F. , Wasmund, N. , Schmidt, M. and Lahajnar, N. (2014): Food-­web structure and trophic interactions in the northern Benguela upwelling system , IMBER OSC - Future Oceans, Bergen, Norway, 22 June 2014 - 27 June 2014 .

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