Microcosm experiments
Predation by C. sowerbii on natural crustacean plankton
was investigated in laboratory experiments. Four experiments
were carried out between 9 June and 17 June
1996. Each experiment included four replicates to
record zooplankton initial densities, four replicates with
medusae (M) and four replicates without medusae (C,
details in Table I). The experiments were carried out as
follows. A container was first filled with 20 L of natural
pond water and mixed well. From this container,
12 glass jars were each filled with 1 L of water. Every
third jar (=4 replicates) was randomly chosen for the
determination of initial zooplankton densities (N0) by
filtering the water through a 55-mm net and fixing the
zooplankton in a 70% ethanol–30% water (vol : vol; plus
40 g sucrose L–1 and 40 g Glycin L–1) solution. Five
medusae were added to each of another four jars. At the
end of the experiment, these jars provided information
on zooplankton densities under the impact of jellyfish
predation (NM). The remaining four jars were employed
to assess the net growth rate of the zooplankton; i.e. at
the end of the experiment, they provided information on
zooplankton densities without predation (NC). The bell
diameter of each of the medusae used in the experiment