Predator-mediated landscape structure: seasonal patterns of spatial expansion and prey control by Chrysaora quinquecirrha and Mnemiopsis leidyi

The scyphomedusa Chrysaora quinquecirrha and lobate ctenophore Mnemiopsis leidyi are dominant consumers in the planktivorous food web in Chesapeake Bay, USA, and are important predators throughout much of their ranges. Our studies in the Patuxent River (a subestuary of Chesapeake Bay) and its tributary creeks suggest successive waves of population spread and trophic influence of these 2 gelatinous species in opposing directions across the aquatic landscape. In years when both species were abundant, Mnemiopsis appeared first in the main channel of the Patuxent River and initially was most abundant in the bottom layer of the water column. Mnemiopsis densities then rapidly increased in shallow tributaries and coves, with distributions likely caused by a combination of transport and temporally and spatially varying patterns of growth and reproduction. In contrast, densities of Chrysaora ephyrae were initially highest in small coves and tributary creeks, with densities of Chrysaora medusae spreading outward from these small systems to the main river as summer progressed. We found no conclusive evidence for tidally-cued vertical migrations of either species or directional swimming by Chrysaora that would create these differing spatio-temporal patterns. As Chrysaora increased and spread, it likely reduced or eliminated Mnemiopsis by direct predation, and possibly through the effect that partial predation could have on Mnemiopsis reproduction. Because of differences in diets and feeding rates, these shifting temporal and spatial patterns of medusa and ctenophore dominance potentially influence spatial distributions and temporal patterns of survival of ichthyoplankton, oyster larvae, and copepods.