Population Dynamics Of Daphnia Pulex Under Stress: Effects Of Low Ph And Food (modelling, Stochastic, Statistics, Life-tables).

The effects of low pH and food quantity on the potential and realized population dynamics of Daphnia pulex were examined in a series of laboratory and modelling experiments. Life-table experiments were conducted at pH 7.5 to 5.0 in 0.5 pH intervals. Between pH 7.5 and 6.0, pre-adult mortality increased, growth rates slowed and age at maturity increased. At pH 5.5, brood sizes decreased, life span was attenuated and juvenile mortality was high. Below pH 5.5, all individuals died within 3 d. Low pH reduced the rate of growth in length but not the maximum length attained. Variable food regimes masked effects of pH. Life-table experiments on even-aged cohorts provided a means of tracking a small population whose age, nutritional history and reproduction were precisely known. Under variable food levels, Daphnia lived up to 100 days and postponed production of neonates up to an age of 69 days. As food levels increased, the cohort shifted from ephippial to neonate reproduction. Results of another series of life-table tests were used to predict maximum size-specific harvesting rates. The predicted stable age structure and target harvest rate were used to design a 42 day factorial population experiment (3 food concentrations x 2 pH levels x 4 replicates). Populations were enumerated, classified and harvested daily. Total births increased consistently with food concentration at both pH levels. Recruits to the adult stage increased with food level at high pH but decreased with food at low pH. Differences in juvenile mortality between pH levels were greatest at high food concentrations. At pH 5.8 juvenile mortality was seven times greater than at pH 7.5. Differential survival of neonates was related to egg volume, which decreased as food concentration increased, irrespective of pH. A stochastic model, based on the results of the population experiment, yielded good agreement with observed values. Simulations suggested that a constant escapement policy reduced variability of replicate populations within a treatment and enhanced the ability to detect differences among treatments.PhDBiological SciencesLimnologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127883/2/8612606.pd