The enigma of Daphnia death rates

Abstract

Birth rates, rates of population change, and mortality rates were computed for Daphnia pulex and Daphnia rosea collected at three separate stations in 2.5-ha meromictic Crawford Lake. Birth and death rates for the same species at the three separate stations, or at the same station but living at slightly different mean depths were substantially different. A correction for tow net efficiency for young and adult animals increased birth rate values by 40%. Mean finite birth rate, B, for Daphnia in a thermally stratified lake is calculated from the relation [see article for formula] where for each stratum, s, E is number of eggs, D is egg development time, N is number of animals, and n is number of strata. An assumption of this formulation is that all individuals in the population behave similarly, but at Crawford Lake D. pulex behaves like two separate populations for at least part of the year. High death rates were calculated for D. rosea in midsummer. A comparison of observed neonates with expected neonates during this period led to the conclusion that most mortality in the population occurs either in late embryos or at hatching.