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Reconstructing long-term trends in the biomass and trophic relations of jellyfish in a coastal sea

2011Reconstructing long-term trends in the biomass and trophic relations of jellyfish in a coastal sea.WINTER Laura Melanie (Canada)
Organisation:Christian Albrechts University, Kiel (DE)Supervisor :
Summary: There is growing evidence that jellyfish (including Cnidaria and Ctenophora) are increasing in biomass and abundance in marine ecosystems. In this study, a long-term time series of zooplankton abundance at Helgoland in the German Bight, North Sea is analysed with respect to key ctenophore species and their prey. Body size, or the mean thereof for species or groups, is an important controlling factor in marine systems and strongly influences predator-prey interactions. Based on the existing count data, representative annual time series of average group and individual biomass are created for the groups Beroe spp., Pleurobrachia pileus and copepods. Correlations between the variables in these time series will be investigated. Two time periods are considered for all analyses, 1974-1987 and 1988-2006, reflecting an increase in the North Atlantic Oscillation in 1988. It is expected that ctenophore biomass will increase after 1988 while average individual copepod biomass will decrease. Furthermore, a strong correlation between P. pileus and Beroe spp. is predicted, possibly weakening after 1988. A much weaker relationship is predicted between P. pileus and copepods, but it is expected to increase after 1988. In order to relate the existing jellyfish counts to biomass, jellyfish were regularly sampled at Helgoland in June 2011. Regression analyses yielded strong relationships between size and Equivalent Spherical Diameter (ESD) for P. pileus, B. cucumis and one species of scyphomedusae, Cyanea lamarckii. Equations relating size to each of carbon, nitrogen and phosphorus content were also derived for B. cucumis and C. lamarckii. This study thus yields important new biomass conversions for jellyfish. Furthermore, the time series analysis will provide fresh insights into how mean body size influences the trophic interactions of jellyfish. This knowledge can be incorporated into models to predict how jellyfish will respond to global warming, overfishing and other pressures.