Direct and indirect trophic interactions of Aurelia sp. (Scyphozoa) in a stratified marine environment (Mljet Lakes, Adriatic Sea)

The pattern of diel vertical migration and the trophic interactions of moon jelly (Aurelia sp.) were investigated in the sea lakes of Mljet Island (Adriatic Sea) where this scyphomedusa is present throughout the year. Water column characteristics, plankton and in situ behaviour of Aurelia were followed over several 24-h cycles (6–8 times during each cycle) from the surface to the bottom (44 m). Aurelia exhibited a consistent pattern of diel vertical migration. Most of the time Aurelia were located at the bottom of the thermocline layer at temperatures lower than 19 C. Aurelia migrated towards the surface at dusk when the majority was found within the thermocline or just above it. During the night the medusae sank into the deepest layers below 25 m. The main medusa food items inferred from stomach contents were small adult copepods like Oithona nana and Paracalanus parvus and copepodites of small calanoids and cyclopids. In addition, in situ feeding experiments indicated high clearance rates for nauplii and naked ciliates and clear response of bacterial populations pointing to indirect cascade effects of Aurelia on microbial in addition to classical food web. Introduction In many areas worldwide there is strong evidence for changes in plankton communities, over recent decades (Hays et al. 2005), an increase in the frequency and intensity of jellyfish outbreaks is among most evident (Purcell et al. 2001; Dumont et al. 2004). Some studies have indicated a correlation between high jellyfish abundance and increased eutrophication (Papathanasiou et al. 1987) but Purcell et al. (1999) argued that the connections between jellyfish population size and eutrophication are difficult to make because of the lack of appropriate historical data. Arai (2001) concluded that increases in coelenterates could rarely be unequivocally linked to eutrophication. Long-term fluctuations in jellyfish abundance have been associated with climatic changes (Brodeur et al. 1999; Mills 2001; Molinero et al. 2005), which, in turn, mediated trophic regime shifts (Lynam et al. 2004). The decline of fish competitors due to over-fishing has also been suggested as factor favouring jellyfish and contributing to regime shift (Daskalov 2002; Sommer et al. 2002). When very numerous, jellyfish play an important trophic role (Mills 1995; CIESM 2001), if massive occurrences persist, ecosystem structure and functioning may change (Kideys 2002). Dramatic changes in zooplankton and a strong decline of the pelagic fishery have been attributed to predation and competition by pelagic coelenterates (Hay et al. 1990; Niermann 2004), although the significance of such interactions to either fish or jellyfish populations is still poorly understood (Purcell and Arai 2001). Most studies of the trophic interactions of jellyfish and their impact concentrated on mesozooplankton. Despite some evidence of jellyfish preying on microCommunicated by R. Cattaneo-Vietti, Genova. A. Malej (&) V. Turk Marine Biological Station, National Institute of Biology, Fornace 41, 6330 Piran, Slovenia e-mail: [email protected] D. Lučić A. Benović Institute for Marine and Coastal Research, University of Dubrovnik, D. Jude 12, 20001 Dubrovnik, Croatia