Diel vertical migration by zooplankton: Constant and fluctuating temperature effects on life history parameters of Daphnia1

Abstract

The thermal demographic advantage of vertical migration was tested using life‐table studies of Daphnia parvula cohorts fed on Chlamydomonas reinhardi. At a high food level, lifespan and body size at death were relatively insensitive to temperature differences. Fecundity parameters (brood size, brood number, total number of young per reproductive female, and net reproduction rate) were highest at an optimal midrange temperature (15°C). However, age at onset of reproduction, instar at first reproduction and brood duration decreased with increasing temperature, resulting in an overall increase in the realized rate of increase ( r ) with increasing temperature. Realized rates of increase were higher in fluctuating temperatures than at comparable average constant temperatures but were always lower than those at the highest constant temperature in each range. A low food level reduced the effect of temperature on some growth and fitness parameters from those at a high food level, but r was still greater at the higher temperatures. Therefore, there is no apparent thermal demographic advantage to vertical migration at either a high or low food level within the natural range experienced by D. parvula. Maximum fitness is achieved by remaining in the warmest surface waters at all times.