Abstract
In 2007, the invasive ctenophore Mnemiopsis leidyi was observed for the first time in Limfjorden (Denmark) where it exhibited mass occurrence in late summer while the indigenous and usually dominating common jellyfish, Aurelia aurita, was nearly absent. Both species were further studied in 2008 and 2009 and it was found that the additional predation pressure by M. leidyi caused the zooplankton stocks to be severely depressed. Here, we report on the population dynamics and predation impact of M. leidyi and A. aurita in Limfjorden in 2010 and 2011. In 2010, M. leidyi was observed in Limfjorden for the first time in August with the highest density and largest size in the central parts (Skive Fjord). The estimated half-life of zooplankton (copepods) was only important in Skive Fjord in mid August 2010 when the joint-predation impact of A. aurita and M. leidyi was 2.3 d. In 2011, no M. leidyi were observed on the first cruise (3 August), while during the second cruise (17 November) it was observed in large numbers. The western most location (Veno Bugt) was dominated by large sized (≤ 60 mm) M. leidyi, while the average size decreased towards the central parts of the fjord-system. The proportion of cydippid larvae increased from west to the central parts thus suggesting rapid reproduction and population-size expansion. The bio-volumes of ctenophores were highest in the central part with 85 ml m-3 in Logstor Bredning, which may be compared to the greatest mean bio-volume of about 184 ml m-3 observed in the Black Sea in 1989 when the zooplankton and fish stocks collapsed. Analysis of available hydrographic data and model calculations indicates that re-invasion of M. leidyi from the North Sea seeded the autumn population in Limfjorden in mid-September.
Highlights
Species translocation and population expansion of invasive species of ctenophores gained large public and scientific attention especially after the introduction of the American comb jelly Mnemiopsis leidyi
We report on the distribution, densities, and estimated zooplankton predation impacts of these gelatinous predators as well as the importance of hydrographic pattern for their population dynamics
In 2010, the half-life of zooplankton in Limfjorden was only able to be calculated in Skive Fjord 18 August 2010 when the jointpredation impact by A. aurita and M. leidyi was tot-t1/2 = 2.3 d (Table 2)
Summary
Species translocation and population expansion of invasive species of ctenophores gained large public and scientific attention especially after the introduction of the American comb jelly Mnemiopsis leidyi During the late summer of 2006, Mnemiopsis leidyi was observed along the coast of the Netherlands for the first time (Faasse and Bayha 2006), presumably introduced with ballast water. In 2007, M. leidyi was observed for the first time in Limfjorden (Denmark) where it exhibited mass occurrence in late summer while the usually predominant common jellyfish, Aurelia aurita (Linnaeus, 1758), was nearly absent (Riisgård et al 2007). A follow-up study conducted from autumn 2008 through summer 2009 monitored the invasive ctenophore and the indigenous A. aurita (Riisgård et al 2012a). Low densities of M. leidyi in early July increased to high densities by late August, and the estimated time to clear the central part of Limfjorden of zooplankton by the joint-predation impact of A. aurita and M. leidyi was between 0.5 and 2.9
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