The role of dormancy and emergence patterns in the dynamics of a freshwater zooplankton community

Emergence rates of two cladoceran species (Ceriodaphnia reticulata and Eubosmina longispina) and two Calanoid copepod species (Epischura nordenskioldi and Diaptomus sanguineus) from sediments were monitored weekly in Bullhead Pond (Rhode Island) over a 3.5-yr period with permanently deployed plastic, inverted-funnel traps. Emergence rates of C. reticulata decreased over the study period, dropping to zero in the third year, and determined timing of first appearance as well as fluctuations in population size. Emergence of E. Zongispina was sporadic and correlated poorly with population abundance variations, but continued throughout the study. Significantly fewer ephippia of C. reticzdata were recovered from under traps compared to adjacent sediments, suggesting that reserves of ephippia had been depleted during the study. Reserves of E. longispina ephippia were not significantly depleted under traps during the study. Patterns of emergence for E. nordenskioldi and D. sanguineus were very similar to each other in timing and importance in determining planktonic standing stocks. For each, nauplii emerged synchronously and initiated the population each year. Both species hatched during every year of the study, suggesting that eggs can remain viable in the sediments for at least 3 yr. Dormancy enables many animals and plants to survive harsh environmental periods (Harper 1977; Wiggins et al. 1980; Fryxell 1983; Henis 1987). Many planktonic species living in variable habitats produce dormant eggs that can persist for extended periods (see Hutchinson 1967). It is known that rotifers produce resting eggs that can remain viable for at least 35 yr (Nipkow 196 1). Some cladocerans have been reported to produce resting eggs (ephippia) that can persist for decades (e.g. Moritz 1987; Moghraby 1977), and recent work has shown that Calanoid copepods can make diapausI Current address: Center for Limnology, 680 N. Park Street, University of Wisconsin, Madison 53706. Acknowledgments This research was supported in part by NSF grants BSR 83-07350 and BSR 85-16724 to N. G. Hairston, Jr., and by a Sigma Xi Grant-in-Aid of Research. Support and facilities were provided by the Zoology Department, University of Rhode Island. I thank N. G. Hairston, Jr., for his support, discussions, and practical assistance during the study. E. Olds and A. Rothchild were a help in the field, and D. Miller helped with the computer work. This paper has benefited from the reading of earlier drafts by E. De Stasio, N. G. Hairston, Jr., K. D. Hambright, P. Jumars, M. L. Keefe, S. Twombly, T. Whitesel, and two anonymous reviewers. This manuscript was part of a disscrtation submitted in partial fulfillment of the requirements for a Ph.D. degree at Cornell University. ing eggs that last for at least 3 yr and probably > 15 (Hairston and De Stasio 1988; De Stasio 1989). The traditional emphasis on dormancy as an adaptation to avoid harsh conditions perhaps has resulted in little appreciation for its role during periods of “normal” environmental conditions. Recent studies have suggested that pools of dormant eggs in sediments have potentially important effects on the ecology and evolution of zooplankton species in both freshwater and marine environments (Kankaala 1983; Marcus 1984; Hairston and De Stasio 1988; De Stasio 1989). These studies have shown for individual species that dormancy and emergence patterns can play a major role in determining the timing of occurrence of a species (see also Tauber et al. 1986). Many studies have analyzed the dynamics of zooplankton communities that include species that undergo dormancy (e.g. Wiggins et al. 1980; Frey 1982), but there is a general lack of information on the species-specific patterns of dormancy and their direct effects on the temporal dynamics of zooplankton communities. There is a clear need for both basic field studies and theoretical investigations of dormancy during normal environmental variations to supplement the extensive laboratory data