Urabe, J., J. Clasen, R.W. Sterner
Abstract:
The possibility of P limitation for zooplankton has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resources. There have no direct test, however, to determine whether the putative P limitation is real or not. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus actus, and then examined changes in the body mass. In each day from age 1 to 5 d, D. magna was fed on live algae for 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h. We used P-free water as a control. Body mass at age 6 d of Daphnia fed on P deficient algae was significantly larger in the P treatment than the control treatment, while significant difference was not detected between the treatments in the animals fed on P sufficient algae (C:P atomic <300 ). The results clearly demonstrate that Daphnia growth is in fact limited by P itself when they fed on P deficient algae. P limitation of Daphnia growth is, therefore, real rather than due to some other biochemicals related with P-deficiency.
(Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, USA; Present address: Center for Ecological Research, Kyoto University, Shimosakamoto, Otsu, Shiga 520-01, Japan. E-mail: urabe@ecology.kyoto-u.ac.jp)
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