Predation by jellyfish on large and emergent zooplankton: implications for benthic–pelagic coupling

Stable carbon isotopes were used to determine the contribution of emergent demersal zooplankton to the diet of the scyphozoan jellyfish, Catostylus mosaicus, at Smiths Lake, New South Wales, Australia. A preliminary study in 2004 indicated that there was no difference in the δC of ectodermal tissue and mesoglea of the medusae. In 2005, medusae and zooplankton present during the day and night were sampled and isotopic signatures were modelled using IsoSource. Modelling indicated that: 1) mollusc veligers and copepods sampled during the day contributed < 13 % of the carbon to the jellyfish, 2) copepods sampled at night contributed up to 25 % and, 3) the large, emergent decapod, Lucifer, contributed 88 – 94 %. We hypothesised, therefore, that medusae derive most of their carbon from emergent species of zooplankton. In 2006, sampling done in 2005 was repeated three times over a period of 4 weeks to measure short-term temporal variation in isotopic signatures of medusae and zooplankton, and emergent demersal zooplankton was specifically sampled using emergence traps. Short-term temporal variation in isotopic signatures was observed for some taxa, however, actual variations were small (< 1.5 ‰) and the values of medusae and zooplankton remained consistent relative to each other. IsoSource modelling revealed that mysid shrimp and emergent copepods together contributed 79 – 100 % of the carbon to the jellyfish, and that the maximum possible contribution of daytime copepods and molluscs was only 22 %. Jellyfish apparently derive most of their carbon from emergent zooplankton and by capturing small numbers of relatively large taxa, such as Lucifer or mysid shrimp. Small but abundantly captured zooplankton (such as mollusc veligers) contribute only minor amounts of carbon. Jellyfish have a major role in the transfer of carbon between benthic and pelagic food webs.