Population dynamics, growth and predation impact of the common jellyfish Aurelia aurita and two hydromedusae, Sarsia tubulosa, and Aequorea vitrina in Limfjorden (Denmark)

The population dynamics, growth and predation impact of the common jellyfish Aurelia aurita and 2 hydromedusae, Sarsia tubulosa and Aequorea vitrina, were studied in Limijorden (Denmark), with the main focus on an inner branch of the fjord (Skive Fjord) from 2003 through 2005. We evaluated the degree to which the jellyfish realise their growth potential in Skive Fjord, and 2 other sites in Limfjorden (Nissum Bredning and Logstor Bredning). We also evaluated whether the jellyfish were controlling the zooplankton, or if the biomass of zooplankton was controlling the growth of the jellyfish, by comparing prey concentrations in the sea with laboratory estimations of the minimum concentrations of prey needed for sustaining maximum growth of the jellyfish. In all 3 yr, A. aurita ephyrae and hydromedusae were present during spring, and in 2003 A. aurita was present through the entire study period (February to August). In April 2004, all jellyfish disappeared, but in August 2004 A. vitrina became abundant. In both 2004 and 2005, disappearance and reappearance of jellyfish coincided with sudden changes in the salinity caused by incoming water from the North Sea. By applying energy budgets from laboratory experiments, we calculated that the jellyfish realised their growth potential until August in 2003, after which negative growth was observed. In 2003, A. aurita in Skive Fjord had the potential to control the zooplankton from mid-May to August (half-life times between 0.8 and 6.2 d). In this period, almost no zooplankton or fish larvae were present. Release of phytoplankton from zooplankton grazing-control due to the predation pressure (on zooplankton) from A. aurelia resulted in a pronounced phytoplankton bloom (with up to 60 mu g l(-1) chl a in 2003) triggered by the release of nutrients from the sediment during a period of oxygen depletion in the near-bottom water. A. vitrina had no effect on the zooplankton population, but probably preyed on ctenophores. We conclude that in some years A. aurita may be very abundant in Limfjorden and might exert a considerable predatory impact on zooplankton and fish larvae during the summer. The abundance, species composition and population dynamics of jellyfish in Limfjorden may, at irregular time intervals, be strongly influenced by dramatic, but apparently rather unusual hydrographic events, and hydraulic processes may explain otherwise unaccountably large variations in the plankton (including jellyfish).