Predation patterns and prey quality of medusae in a semi-enclosed marine lake: implications for food web energy transfer in coastal marine ecosystems

Veliko Jezero (Mljet, Croatia) is a nearly enclosed karstic depression filled with saltwater, where jellyfish and prey exchange with the Adriatic Sea is negligible, making this small ecosystem ideal for the controlled study of medusae in food webs. Based on the analysis of their gut contents, medusae appeared to ingest less carbon than expected on the basis of their carbon content. To accurately define carbon ingestion by medusae, we determined the diet of Aurelia sp. 5 (Dawson and Jacobs, 2001) (Scyphozoa: Semaeostomeae) from Veliko Jezero by combining gut content and stable isotope analyses. During daytime, gut contents identified a mix of small copepods as the dominant prey (62%). In contrast, feeding models based on stable isotope values of medusae and their potential prey analyzed using Stable Isotope Analysis in R indicated appendicularians could have made the greatest contribution to Aurelia sp. 5 diet (14–78%), followed by calanoid copepods (0–50%) and fish larvae (0–43%). Because appendicularians and fish larvae are abundant with medusae at night near the bottom of the “lake” and contain more carbon than small copepods, we suggest diel movements of Aurelia sp. 5 and carbon content of prey determine the carbon assimilated by medusae, with night-time prey making a