The diet, digestion times, and densities of Chrysaora quinquerirrha medusae, and densities of their prey were quantified in order to evaluate the importance of this conspicuous scyphomedusan on zooplankton populations in Chesapeake Bay, USA, and its tributaries. Medusae consumed a variety of zooplankton prey, but copepods, mostly Acartia tonsa, averaged 55 % of the prey items in gut contents collected during July and August from 2 tributaries of Chesapeake Bay in 1987 and 1988, and 71 % of the prey in gut contents from the main bay in 1987-1990. Medusae showed positive selection for copepods, but negative selection for copepod nauplii. Digestion times of copepods by medusae averaged 3.5 h. A multiple regression equation related digestion times to temperature and the number of ingested copepods, but not to medusa size. Predation rates (numbers of copepods consumed medusa-1 d-1) increased with increasing prey density, medusa diameter, and temperature. These results provide a means of predicting C. quinquecirrha feeding in situ. Greater numbers of prey in medusae at nighttime than during the day probably were due to greater prey densities at night. Daily predation effects (percentage of the copepod standing stock consumed d-1) were calculated from feeding rates, and the densities of medusae and copepods. Medusa densities (peaks of 11 to 16 m-3) and predation effects (peaks of 42 to 94 % d-1 of the copepods consumed) were much greater in the 2 tributaries than at a nearshore station in the bay (peaks of 2 medusae m-3 and 3 % d-1 of the copepods consumed). Ingestion of zooplankton provided an average of 145 % of the nitrogen needed to balance excretion in medusae <45 mm in diameter, but supplied only 65 % of the nitrogen needs of medusae >45 mm. Other prey, including ctenophores, fish eggs and larvae, and large polychaete worms are additional contributions to the diet. Predation by C. quinquecirrha medusae may control copepod populations in some tributaries of Chesapeake Bay, as suggested by high predation effects and declining copepod populations throughout the summer.
