2005. Effects of climate on relative predation by scyphomedusae and ctenophores on copepods in Chesapeake Bay during 1987–2000

The mesohaline region of Chesapeake Bay had high densities of the scyphomedusan Chrysaora quinquecirrha and low densities of the ctenophore Mnemiopsis leidyi in 1987–1990 and 1995. By contrast, 1996–2000 had much lower medusa and higher ctenophore densities. Predation on copepods (Acartia tonsa) was intense in 1996–2000, and copepod densities were low when ctenophores were abundant. At equivalent sizes, the feeding potentials of ctenophores were greater than those of medusae, with clearance rates about 1.2 times greater by volume and about 3 times greater by carbon biomass. M. leidyi ctenophores more negatively influence copepod populations than C. quinquecirrha medusae because they are more effective predators of copepods and have broader spatial and temporal occurrence, wider salinity and temperature ranges, greater densities, and a more flexible life history. Because they consume ctenophores, C. quinquecirrha medusae positively affect copepod abundance. C. quinquecirrha and M. leidyi could be considered keystone predators because of their far-reaching effects on the plankton food web. The balance between medusae and ctenophores in Chesapeake Bay was greatly affected by climatic factors. Medusa abundances were high when dry years prevailed prior to and during 1987–1990 and 1995. By contrast, medusa abundances were low when dry and wet years alternated before and during 1996–2000. Significant variables that favored medusae in 1987–1990 and 1995 were high salinity, warm temperature, and high solar irradiance. Th