Christiansen, Svenja (2016) Distribution, eddy association and biogeochemical importance of the pelagic polychaete Poeobius sp. in the tropical Atlantic. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 77 pp.

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Abstract

The distribution and ecological function of gelatinous zooplankton in the vast habitat of the pelagic oceans is mostly unknown and new species and their roles in the ecosystem are still being discovered. Nets undersample and destroy this highly diverse group of organisms, thus, the use of underwater optical systems is the most appropriate way to observe them. In 2015, the deployment of the towed pelagic in situ video camera system (PELAGIOS) revealed high abundances of a holopelagic polychaete of the genus Poeobius in a mesoscale eddy in the tropical Atlantic, where it co-occurred with very low particle concentrations. Its sister species, the flux-feeder Poeobius meseres, is only known from the Pacific Ocean. A negative correlation of the polychaete with particle concentrations was described there. Several publications emphasize the role of mesozooplankton in remineralisation, but little is known about the impact of gelatinous particle-feeders on the flux in mesoscale eddies. After the discovery of Poeobius sp. in the Atlantic by PELAGIOS, it was also identified on images of the Underwater Vision Profiler 5 (UVP5), a high-resolution camera that quantitatively records both particle concentrations and mesozooplankton. Here I use UVP5 and environmental data, including more than 1.8 million images from 13 cruises, to identify the horizontal and vertical distribution of Poeobius sp. in the tropical Atlantic Ocean. I test the hypothesis that Poeobius sp. is associated with mesoscale eddies and investigate its biogeochemical role in these features. In total, 481 individuals were observed between 5°S and 20°N and between 16°W to 46°W with the shallowest observation at around 22 m and the deepest at around 1000 m depth. A comparison between non-eddy stations and three different eddy types revealed elevated abundances and a restricted depth distribution of Poeobius sp. especially in anticyclonic mode water eddies (ACMEs), confirming the hypothesis of eddy association. The results from this thesis suggest that mesoscale eddies, especially ACMEs, may serve as an aggregation and dispersal mechanism for Poeobius sp. and possibly other low-oxygen tolerant zooplankton species. These in turn influence the environment of the eddy: High Poeobius sp. abundances could be related to strongly reduced particle concentrations and fluxes in the layers directly below the polychaetes. It is shown that the worms take up almost the entire vertical particle flux by feeding with their mucus nets. Poeobius sp. may play a significant role in the development of particle fluxes, and thus the biological carbon pump, in ageing mesoscale eddies. These results should be considered when discussing or modelling mesoscale eddy ecology and biogeochemistry.

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