Jellyfish are often neglected in ecological studies and not much is known about their role in the energy flow in the marine ecosystems. A complete understanding of carbon flux through the food web demands detailed knowledge of the bioenergetics of the organism. The present study deals with the bioenergetics, especially respiration and growth, of Aurelia aurita and selected hydromedusae. Effects of size, temperature, prey species and concentrations are studied and energy budgets established. The field work was conducted in Limfjorden, Denmark in 2003-2005. Collections of jellyfish for determination of size (and thereby growth) and abundance was combined with samples taken of zooplankton (incl. fish larvae and eggs), phytoplankton, temperature and nutrients. When combining the energy budgets from the laboratory experiments with field data (actual growth / zooplankton biomass) it is possible to estimate if the jellyfish exploit their growth- and predator-potential in the field.

The present study has provided energetic tools that can be used to evaluate the role of jellyfish in the ecosystem; not only in Limfjorden but also at other localities. The study has demonstrated the importance of studying bioenergetics when evaluating both direct and indirect effects of mass occurrences of jellyfish. The effect of mass occurrences of Aurelia aurita has probably been under estimated so far since the respiration and ammonia excretion has been underestimated while the assimilation efficiency might have been over estimated. It is concluded that Aurelia aurita in certain years may be very abundant in Limfjorden where it may exert a considerable predatory impact on zooplankton and fish larvae during the summer period. Knowledge about the zooplankton-predatory impact exerted by jellyfish may explain otherwise unaccountable large variations in both zoo- and phytoplankton biomasses. Such insight is of great importance, not only for a general basic understanding of the plankton dynamics in fjords and coastal ecosystems, but also for marine monitoring programmes using traditional parameters routinely in the assessment of environmental conditions.