Ecker, Ursula (2015) Gelatinous zooplankton food web in the southern North Sea: Trophic interactions between species and importance of food quality under varying temperature and CO2. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 48 pp.

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Abstract

Species composition and trophic interactions in the gelatinous zooplankton food web were investigated around the offshore island Helgoland in the southern North Sea.
Four scyphozoan and five ctenophore species were observed and sampled from April until October 2014 resulting in two distinct gelatinous zooplankton food webs over the year. The first dominated by scyphozoan species and the second consisting mainly of ctenophores with few advected specimens of scyphozoan species.
Based on stable isotope and fatty acid analyses, species interactions such as potential competitions between Aurelia aurita and Pleurobrachia pileus and between P. pileus and Mnemiopsis leidyi as well as predator prey relationships between Mnemiopsis leidyi and P. pileus with the occurring Beroe species were found.
The competition between the native P. pileus and invasive M. leidyi was further examined with grazing and starvation experiments. A species-specific temperature dependent response agrees with temporal separation of both species in the field enabling both species to be established in the area. The native species was found to be competitive superior and forced the invasive species to be present at more unfavorable conditions during the season.
The importance of unsaturated fatty acids, as a proxy of food quality, at different temperature and CO2 levels were investigated for the ctenophore P. pileus. No direct effect of food quality or pH on growth was observed, while increasing temperatures affected growth negatively. Composition changes of fatty acids indicate the relevance of food quality and therefore indirect effect of climate change on the performance of this ctenophore species.

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