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Dissertation zugänglich unter
Zooplankton dynamics, fish zonation and trophic interactions at two seamounts in contrasting regimes of the Eastern Atlantic
Zooplanktondynamik, Fischzonierung und trophische Interaktionen an zwei Sebergen in gegensätzlichen Regionen des Ostatlantiks
Dokument 1.pdf (10.528 KB)
Freie Schlagwörter (Englisch):
benthopelagic fish , seamount , trophic interactions , tropic-subtropical Eastern Atlantic , zooplankton
Möllmann, Christian (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
More than 125000 large seamounts are predicted to occur on the ocean floor. Compared to the flat areas of the deep-sea plains, seamounts provide different habitats of hard substrata and soft bottom and have special geochemical and biological features. Recent seamount research has shown that seamount ecosystems may be highly variable. The reasons why some appear to be much more productive than the surrounding ocean, while others do not show conspicuous differences, are still poorly understood. However, with regard to the sustainable exploitation and the management of natural resources at seamounts, such as fish and minerals, further knowledge of the driving forces and mechanisms in seamount ecosystems is absolutely necessary.
The present study compares the zooplankton communities and the benthopelagic fish fauna at two shallow seamounts, Ampère and Senghor, in relation to the large-scale and local current fields and hydrographic conditions of the contrasting regimes of the subtropical and tropical NE Atlantic gyres. Spatial and temporal distribution patterns of different zooplankton size fractions were elucidated in terms of biomass, abundance and taxonomic composition. In order to detect possible seamount effects on the zooplankton due to current-topography interactions an unaffected open ocean reference site was always sampled for comparison. The study addressed several specific objectives. (1) The diel vertically migrating zooplankton was assessed to identify a possible influence of the topography, and how this affects the food supply to seamount residents. (2) Respiratory carbon demand was estimated for distinct zooplankton size fractions, in particular to assess the role of microzooplankton for the phytoplankton control and for the carbon conversion. (3) Another question was, whether a seamount may be considered as larval source in the open ocean for benthic invertebrates. (4) Within the pelagic communities nutritional sources and trophic pathways were elucidated and linked to the benthopleagic fish fauna of the seamounts and the adjacent abyssal plains. (5) Furthermore the fish communities were characterised in a zoogeographical context in comparison with other NE Atlantic seamounts.
Zooplankton standing stocks and respiratory carbon demand were three to six times higher at Senghor than at Ampère, indicating nutritional and productive differences between both seamount systems associated with the different trophic realms. While the zooplankton community at Ampère reflects the oligotrophic character of the NE Atlantic subtropical gyre, with microzooplankton attaining an important role for carbon conversion and the control on phytoplankton, the nutrient-rich waters of the tropical gyre feature a higher zooplankton abundance of different composition at Senghor. Surveys across Ampère show clear differences in the mesozooplankton distribution between day and night, reflecting the pattern of diel vertical migration. At Senghor no day-night related distribution patterns were observed, pointing to a high proportion of non-migrating zooplankton. Evidence for a general build-up of higher zooplankton biomass was not found as compared to the open ocean, neither at Ampère nor at Senghor. Only a higher abundance of dinoflagellates suggests possible interactions between current and topography over Senghor, such as upward displacement of isopleths and associated enhanced vertical mixing. Senghor was confirmed as a hotspot for meroplanktonic larvae, with significantly enhanced larval abundance in the seamount surrounding flows as compared to the open ocean.
For the identification of nutritional sources and pathways a combination of stomach content and stable isotope ratio (δ13C and δ15N) analyses was used based on epipelagic particulate organic matter. Feeding types of fish species comprised zooplanktivores and mixed feeders, benthivores, piscivores, and predator-scavengers. Differences in stomach contents and stable isotope signatures indicate a resource partitioning among the benthopelagic fishes through distinct habitat choice, vertical feeding positions and prey selection. The lack of larger diel vertically migrating zooplankton over the summit plateau and the presence of non-migrating taxa in the stomach contents of the zooplanktivorous bottom associated fishes indicate that topographic trapping of diel vertical migrators on the summit plateau seemed to be of minor importance for the food supply at the seamounts. Rather, horizontal current-driven advection of the planktonic prey from the surrounding ocean was assumed as major food source for the benthopelagic fish fauna at Ampère and Senghor. Despite differences in production and trophic pathways in the surface layers, the remaining nutritional sources seem to be comparable in the deep-sea among all sampling sites, as were biomass, abundance and respiration.