Biological stoichiometry of Daphnia growth: An ecophysiological test of the growth rate hypothesis

The growth rate hypothesis (GRH) proposes that variation in organism C : P and N : P ratios reflects variation in P content associated with altered allocation to P-rich ribosomal RNA under different growth rates. We tested the GRH by examining the effects of food quantity and stoichiometric quality (differing carbon : nitrogen : phosphorus [C : N : P] ratios) on juvenile growth and chemical composition (C : N : P, RNA, and DNA contents) of two species of Daphnia (D. galeata, D. pulicaria). Daphnia in P-limited experiments were fed saturating and limiting concentrations of food (Scenedesmus acutus) of high P (C : P 5 110 6 7.3), medium P (456 6 20.7), and low P (934 6 23.6), and in an N-limited experiment D. pulicaria was fed saturating concentrations of high N (C : N 5 6.31 6 0.35), medium 1 and medium 2 (9.0 6 0.42; 15.0 6 0.49, respectively), and low N (18.22 6 0.56) food. In Plimited experiments, both Daphnia species grew fastest under P-rich, high food conditions and grew slowest under P-deficient, low food conditions, showing effects of both food quality and quantity. Daphnia body percentage P, C : P, N : P, and percentage RNA were tightly correlated with growth rates, and RNA contributed a significant fraction of total body P (48.8% [62.0%]). This strong three-way (growth–RNA–P) set of correlations supports the GRH. In the N-limited experiment, food C : N had a moderate effect on Daphnia growth. While there was a good linear correlation between P and RNA, growth rate was uncorrelated with RNA content and P content, suggesting that the three-way coupling of growth, RNA, and P content is broken under N limitation of growth, but more data for these conditions are needed. These data help in delineating the physiological conditions under which the GRH holds and may be useful in interpreting variation in body stoichiometry of zooplankton from field and lab studies. Growth rate is a critical parameter for most animals, since it affects age at first reproduction with consequences for reproductive output, adult body size, predation risk, and other key aspects of a species’ life history (Arendt 1997). Indeed, for the crustacean zooplankter Daphnia, instantaneous somatic growth rate of juveniles varies in the same way as intrinsic rate of population increase (Lampert and Trubetskova 1996) and can be used as a direct correlate of overall fitness (McCauley et al. 1990). This close association occurs because Daphnia allocates a fixed proportion of its total production to reproduction regardless of variations in absolute growth rate due to environmental conditions. While population increase has often been used as a measure of fitness of genotypes under certain environmental conditions (e.g., Weider 1993), field and laboratory measurements of population demography are time consuming and laborious. Thus, development of approaches for reliable estimation of individual growth rates would represent a major step forward in the study of the ecology of Daphnia and allow a better understanding of how Daphnia growth responds to a variety of important environmental factors. Growth of Daphnia is generally determined by the availability and quality of food, in addition to several abiotic factors (such as temperature and pH). Phosphorus (P) is one 1 Present address: Department of Biology and Center for Watershed Research, University of Louisville, Louisville, Kentucky