How well do laboratory experiments explain field patterns of zooplankton emergence?

1. We conducted a laboratory experiment to explore potential mechanisms dri ving variation in zooplankton emergence from diapausing eggs observed in On eida Lake, NY, U.S.A. We hypothesized that variation in timing of ice-out ( date of thawing of ice) between 1994 and 1995, which resulted in variation in photoperiod-temperature cues, contributed to the differences in the obse rved field patterns. Environmental chambers were used to establish weekly p hotoperiod-temperature combinations that reproduced natural conditions in O neida Lake in 1994 and 1995. In addition, a third treatment ('dark') expose d eggs only to the changes in temperature. We recorded zooplankton emergenc e for 2.5 simulated ice-free seasons. 2. Nine cladoceran taxa were found to hatch, but only Daphnia pulicaria in large numbers. Hatching of D. pulicaria was recorded throughout the season in the two light treatments and sporadically in the dark treatment. The ear ly ice-out treatment had the highest emergence, followed by the late ice-ou t and dark treatments. Among taxa, there was temporal segregation with five hatching in the early weeks of sampling and two taxa hatching during the m iddle weeks. Alona hatched late in the first year, but earlier in the secon d year. 3. We compared our laboratory results of D. pulicaria hatching with the fie ld data obtained by Caceres (1998). Hatching was continuous in the laborato ry, whereas a synchronous spring emergence was found in the field. However, in both the laboratory and the field, more D. pulicaria hatched under cond itions reflecting ice-out occurring in March as opposed to April. Because d ifferences in rates and timing of emergence can affect the population and c ommunity dynamics of pelagic systems, we suggest caution when applying labo ratory results to field populations.