Estudiante De Biologia

Limnol. Oceanogr., 50(1), 2005, 376–387
2005, by the American Society of Limnology and Oceanography, Inc.

Effects of climate on relative predation by scyphomedusae and ctenophores on
copepods in Chesapeake Bay during 1987–2000
J. E. Purcell 1 and M. B. Decker 2
University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland 21613
Abstract
Themesohaline 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, thefeeding 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 andtemperature 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 climaticfactors. 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. The
North Atlantic Oscillation Index was significantly inverselycorrelated with medusa numbers from 1960–1995.
Climate clearly affects gelatinous predator abundances, with consequences that cascade throughout the plankton
food web.

When jellyfish and ctenophores occur in large populations, their predation can have substantial effects on the
plankton food web. For example, because the predominant
species in Chesapeake Bay, the scyphomedusan Chrysaoraquinquecirrha and the ctenophore Mnemiopsis leidyi, are voracious consumers of zooplankton and ichthyoplankton, they
could be detrimental to fish populations (e.g., Cowan and
Houde 1993; Purcell et al. 1994a,b; Purcell and Arai 2001).
Food web analyses suggest that because of their high trophic
positions and great abundances, these gelatinous species are
extremely important to plankton dynamics andto planktivorous fish populations during summer in Chesapeake Bay
(Baird and Ulanowicz 1989).
The mesohaline Chesapeake Bay has large populations of
C. quinquecirrha medusae and M. leidyi ctenophores. New

C. quinquecirrha medusae (ephyrae) are budded from benthic polyps when water temperature reaches 17 C in spring.
Large medusae first are seen in May or June in the tributaries
of themesohaline region, and a month later in the mainstem
bay (Purcell 1992; Purcell et al. 1994b). The greatest population densities (up to 16 m 3) occur in the tributaries in
July and August (Purcell 1992). Usually, few medusae remain by October. During the 1960s, medusae were so abundant that a congressional act was passed to provide funding
in 1968–1972 for research on them. Ctenophores, which areholoplanktonic, proliferate in spring, and greatest biovolumes are found in July and August (Purcell et al. 1994b). So
that the vast quantities of this species would not interfere
with plankton sampling, methods were developed to exclude
them from plankton nets and to dissolve them if collected
(Heinle 1965; Burrell and Van Engel 1970).
Hydrographic conditions affect the timing, distributions...