Trophic ecology of Mnemiopsis leidyi in the southern North Sea: a biomarker approach
The non-indigenous ctenophore Mnemiopsis leidyi A. Agassiz 1865 was first observed in the southern North Sea in 2006 and has since then frequently been encountered. Knowledge on the diet, trophic position and interactions with other components of the pelagic food web will largely contribute to assess the impact of this species on the ecosystem. Using both stable isotope (SI) and fatty acid (FA) analysis, this study revealed spatial and temporal variation in the trophic ecology of M. leidyi in different ecosystems in the southern North Sea. Based on the isotopic composition, spatial differences were largely driven by variation at the base of the food web rather than diet changes of M. leidyi in the different ecosystems. Temporal variation in M. leidyi SI composition was also influenced by shifting baseline values and driven by seasonal changes in the associated plankton communities. This study provides first data on the FA composition of M. leidyi as compared to FA concentrations of two indigenous ctenophores. Total FA concentration in M. leidyi was three to four times lower compared to Pleurobrachia pileus and Beroe sp., categorising it as a lipid-poor organism. Trophic interactions between M. leidyi and two co-occurring ctenophores (P. pileus and Beroe sp.) showed considerable resource differentiation, which could be the result of competition or different diets. A mixture of zooplankton was identified as potential food sources for M. leidyi. FA markers supported the carnivorous diet of Beroe sp., but its SI composition did not confirm the predatory relation with M. leidyi.