Trophic interactions and energetics of key components in the Benguela Current Ecosystem: The role of calanoid copepods and pelagic decapods

Abstract

The Benguela Current is one of the major coastal upwelling systems in the world. It has changed severely in species abundance, community composition and food web dynamics during the last decades induced by environmental changes and the impact of unsustainable fisheries. While zooplankton diversity and abundance generally increased, biomass of small pelagic fish decreased. Decapods and especially copepods represent key components of the northern Benguela food web since the 1970s and play an important trophic role interlinking lower with higher trophic levels. Moreover, they enhance the vertical flux of organic matter and dissolved inorganic carbon from the euphotic zone to deeper water layers through their diel and ontogenetic vertical migrations. This study clarifies trophodynamics and energetics of dominant calanoid copepods and pelagic decapods and investigates their life strategies in terms of species-specific adaptations to upwelling conditions. Lipid storage patterns as well as metabolic rates revealed dormant stages in the two copepod species Calanoides carinatus and Rhincalanus nasutus. The dormant stages were characterised by high wax ester levels (>80% of total lipids) and a strongly reduced metabolism in both species. However, the results indicated that C. carinatus and R. nasutus follow different life strategies: C. carinatus enters the extreme form of dormancy (diapause) during non-upwelling as part of its ontogenetic vertical migration, while R. nasutus migrates into the oxygen minimum zone in times of low food supply and enters a rather moderate and more flexible dormancy, probably quiescence. The two biomarker approaches (e.g. fatty acids and stable isotopes) revealed a complex pattern of trophic positions for a variety of copepod species, but also highlighted the dietary importance of diatoms and dinoflagellates. Two copepod species, C. carinatus and Nannocalanus minor, occupied the lowest trophic level (predominantly herbivorous) corresponding to high amounts of fatty acid markers for diatoms (e.g. 16:1(n-7)) and dinoflagellates (e.g. 18:4(n-3)). These two copepod species represent the classical link between primary production and higher trophic levels. All other copepods belonged to secondary or even tertiary (e.g. some deep-sea copepods) consumers. In contrast to the complex trophic interactions of calanoid copepods, pelagic decapod species represented a more homogeneous group regarding their carnivorous feeding habits, with indices for feeding on calanid copepods. Respiration rates revealed different energy demands of copepod and decapod species. Based on these results an energy budget approach was conducted to estimate community consumption rates. The results indicated that C. carinatus is an important primary consumer in the northern Benguela system, consuming up to 30% of daily primary production. In contrast, the other herbivorous copepod N. minor has little grazing impact. Furthermore, pelagic decapods exerted a considerable predation impact on calanid copepods, consuming 2-13% of standing stock. In conclusion, the interdisciplinary approach of this study provided new and important information on trophodynamics and energetics of various copepod and decapod species. These data are essential for the development of realistic carbon budgets and food-web models of the northern Benguela upwelling region and coastal upwelling systems in general.