Phosphorus limitation of Daphnia growth: Is it real?

The possibility of P limitation for zooplankton growth has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resoutxes. However, there have been no direct tests to determine whether the putative P limitation is real. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus acutus, and then evamined changes in body mass. During the period from birth to age 6 d, D. magna were fed on live algae For 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h each day. We used P-free water as a control treatment. Growth rate estimated from initial and final body mass during the 6-d incubation was significantly larger in the P treatment than in the control treatment when Daphnia fed on P-deficient algae, whereas a significant dij’ference was not detected between the treatments for Daphnia 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 feed on P-deficient algae. Recently, controversy has sparked over the possibility of direct P limitation for zooplankton production. Several studies have demonstrated lower individual and population growth rates of Daphnia when they feed on algae with low P content relative to carbon, i.e. high C : P ratios (Sommer 1992; Sterner 1993; Sterner et al. 1993). Using mass-balance models, Olsen et al. (1986) and Urabe and Watanabe (1992) estimated the threshold food C : P ratio, above which net production of cladocerans is limited by P content rather than C in the food. In both these studies, the threshold was calculated to be -300 (atomic ratio) for Daphnia, although the ratio changes according to food concentration and digestibility of carbon in the food (Hessen 1992; Urabe and Watanabe 1992) and because of changes in the proportion of metabolic cost (respiration) within assimilated carbon (Sterner and Robinson 1994; Sterner 1997). Seston has C : P ratios >300 in many north temperate lakes (Hecky et al. 1993; Elser and Hassett 1994; Sterner et al. 1997), implying that P limitation of Daphnia growth may be common. Direct P limitation of zooplankton growth, however, has been called into question (Brett 1993; Mtiller-Navarra 1995a, b) in part because of the lack of direct evidence showing whether P is in fact the actual substance limiting the growth rate of Daphnia. Although direct demonstrations of the precise limiting factors for zooplankton feeding on low-quality foods have been lacking, several studies suggest that the growth rate of Daphnia is affected by the availability of individual long-chain unsaturated fatty acids such as EPA (eicosapentaenoic acid, 20 : 503) and DHA (docosahexaenoic acid, 22 : 603) (Ahlgren et al. 1989; Miiller-Navarra 1995b). Fatty acids such as these are essential substances for I Present address: Center for Ecological Research, Kyoto University, Shimosakamoto 4-l-23, Otsu 520-01, Japan. Acknowledgments We thank N. George for technical assistance and M. T Brett, A. Galford, K. Schulz, and an anonymous reviewer for comments. S. Kilham kindly provided us the recipe of COMBO medium. most animals. In some specific instances (Mtiller-Navarra 1995b), but perhaps not in all, fatty acids may be correlated with P content in algae. Thus, the alternative hypothesis has been suggested that the observed lowered growth rate of Daphnia fed on P-limited algae is due to a correlated deficiency of these essential biochemicals rather than due to the P itself. Because elemental limitation of zooplankton production has many implications in ecological processes such as the success of a zooplankton species in a given environment, feedback dynamics between consumers and resources, nutrient cycling, and ecological transfer efficiency (Sterner and Hessen 1994; Urabe 1995; Urabe and Sterner 1996), it is important determine whether elemental limitation for zooplankton growth occurs. This study was designed to answer the question whether P limitation is real or not. A variety of indirect approaches to testing for dietary limitation are possble and have been used in zooplankton and elsewhere. Indirect approaches include correlations between animal growth rate and concentration of chemical substances in the food (P: Sterner 1993; fatty acids: Miiller-Navarra 1995b). A direct experimental approach is to control completely for diet but to arrange for an experimental group of animals to receive an additional supplement of only a single dietary substance hypothesized to limit growth. Differences in growth between this experimental group and a control group can then be taken to be direct evidence that a given single substance: indeed limits the growth rate. This direct approach has been utilized for example by microencapsulation techniques used in studies of growth limitation in benthic invertebrates (Kreeger and Langdon 1994). Unfortunately, inorganic P is difficult to encapsulate successfully due to its high solubility in water. In the present study, we supplied inorganic P in solution apart from food algae directly to Daphnia feeding either on P-deficient or P-sufficient food. If the growth of Daphnia feeding on P-deficient food is stimulate,d by supplemental P additions, but animals feeding on P-sufficient food show no similar response, this should be taken as direct evidence for mineral P limitation