Limnol. Oceanogr., 44(6), 1999, 1549–1556

Recent studies have demonstrated that Daphnia, a cladoceran (anomopod) zooplankton species with a high phosphorus content, can become P limited when fed algae deficient in phosphorus. Bosmina is another common anomopod zooplankter, but its body has a lower percentage of P than does that of Daphnia, and Bosmina might therefore be less susceptible to P mineral limitation. To test for this hypothesized interspecific difference in nutrition, Bosmina and Daphnia were raised on two food types: P-deficient or P-sufficient Scenedesmus acutus in two concentrations (0.2 or 1.0 mg C L21). As predicted by its lower P content, Bosmina growth and fecundity were not affected by the P content of algal food, even though P-deficient food caused significant declines in these life history parameters for Daphnia. Results suggest that Bosmina has a lower P requirement than does Daphnia and that Bosmina may process phosphorus more efficiently or be better able to survive losses in body P content. The outcome of competition between Bosmina and Daphnia therefore might shift in favor of low-P-content zooplankton such as Bosmina under conditions of seston P mineral limitation. ‘‘Food quality’’ can be a nebulous term. Historically, investigators categorized an algal species as high in food quality if zooplankton grew well on a laboratory-cultured clone of that species (e.g., Taub and Dollar 1968; Schwartz and Ballinger 1980). Clearly many factors are important in determining algal food quality for zooplankton. For example, algal size (e.g., Burns 1968; Holm et al. 1983; Knisely and Geller 1986), edibility or digestibility (Porter 1973, 1975; Horn 1981; Burns et al. 1989; van Donk and Hessen 1993), toxin production (Porter and Orcutt 1980; Lampert 1981; Burns et al. 1989), and biochemical composition (fatty acids, Ahlgren et al. 1990; Müller-Navarra 1995; DeMott and Müller-Navarra 1997; Weers and Gulati 1997; Weers et al. 1997) have all been shown to influence food quality for zooplankton. Recently, several studies have demonstrated that algal mineral phosphorus (P) content is another potentially important contributor to zooplankton food quality (e.g., Sterner 1993; Sterner and Hessen 1994). Several observations led to the hypothesis that the P content of algal food is important for zooplankton. First, despite a large (10-fold) variation in freshwater algal carbon : phosphorus (C : P) ratio (Hecky and Kilham 1988), individual zooplankton species sampled at different locations and seasons have relatively constant body C : P (Andersen and Hessen 1991; Sterner and Hessen 1994). Furthermore, the intraspecific differences in zooplankton C : P are less than the interspecific differences (Hessen and Lyche 1991), suggesting that individual zooplankton species maintain relatively stable and unique C : P body ratios despite widely varying algal stoichiometry (Sterner 1990).