Feeding interactions between the introduced ctenophore Mnemiopsis leidyi and juvenile herring Clupea harengus in the Wad
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ORIGINAL PAPER
Feeding interactions between the introduced ctenophore Mnemiopsis leidyi and juvenile herring Clupea harengus in the Wadden Sea Florian Kellnreitner • Moritz Pockberger Ragnhild Asmus • Harald Asmus
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Received: 11 August 2011 / Accepted: 30 August 2012 / Published online: 10 October 2012 Ó Springer Science+Business Media B.V. 2012
Abstract We analysed feeding interactions between Mnemiopsis leidyi and juvenile Clupea harengus in the Wadden Sea. Biomass, diet overlap, prey selectivity, predation impact and stable isotope composition (C, N) of both species were assessed from June to September 2010. High biomass of C. harengus was found in June and July (wet weight 3.0 ± 1.8 g m-3) followed by a steep decline from August to September (wet weight 0.01 ± 0.01 g m-3), coinciding with a dramatic increase in M. leidyi biomass (wet weight 18.3 ± 16.1 g m-3 during August). These two species showed a high overlap in their respective diets (copepods, meroplankton) during the study period. Predation impact of C. harengus on calanoid copepods was highest in June and July where 84 and 41 % of the standing stock were eaten per day in June and July, respectively. Predation impact of M. leidyi on calanoid copepods was highest in September (16 %). Based on stable isotope analysis C. harengus and M. leidyi were assigned to a trophic level of 3.08 and 2.47, respectively. Furthermore, we assessed the potential of competition between M. leidyi and C. harengus in a mesocosm experiment. Results indicated that at present zooplankton densities intraspecific competition in
F. Kellnreitner (&) M. Pockberger R. Asmus H. Asmus Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, Germany e-mail: [email protected]
C. harengus seemed to be greater than interspecies competition with M. leidyi. Due to the low predation impact of M. leidyi and the reduced temporal overlap, competition between M. leidyi and C. harengus during the study period was estimated as low. Nevertheless, considering the high dietary overlap and the interannual variation in biomass and occurrence of both species and their zooplankton prey, competition in the Wadden Sea area cannot be excluded. Keywords Mnemiopsis leidyi Clupea harengus Food overlap Competition Invasive species
Introduction The increase of abundances of jellyfish in marine ecosystems all over the world is currently a matter of concern (Attrill et al. 2007; Lynam et al. 2006; Pauly et al. 2009; Richardson et al. 2009, but see Condon et al. 2012) and there have been numerous discussions on the causes and consequences of jellyfish blooms (Bilio and Niermann 2004; Oguz et al. 2008; Parsons and Lalli 2002; Purcell 2005; Purcell et al. 2007). Since the introduction of Mnemiopsis leidyi A. Agassiz 1865 into the Black Sea and consequent mass occurrences in the late 1980s, this ctenophore and its impact on ecosystems has been the focus of many studies (e.g. Bilio and Niermann 2004; Gucu 2002;
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