Nutrient limitation on a planktonic rotifer: Life history consequences and starvation resistance

Nutrient content of plants is low relative to that of herbivores. Dietary nutrients can limit the growth and reproduction of herbivores. We studied life history consequences and starvation resistance for a common planktonic rotifer, Keratella cochlearis, feeding on algae grown on nutrient-limited media. A strain of Cryptomonas was grown on three types of growth medium with nitrogen (N) and phosphorus (P) adjusted to produce N-limiting, P-limiting, and nutrient-sufficient conditions. The analysis of nutrient content of Cryptomonas cells grown on nutrient-limited conditions showed that those algae exhibited higher P content than algae growing on a nutrientsufficient medium. However, Cryptomonas reached lower densities in nutrient-limited media. The life history responses of Keratella to food quality were examined with low and high food availability. Our study showed that the interaction of food quantity and quality had a significant effect on rotifer growth rates. We observed, at low food levels, that Keratella growth rates were highest when the algae were grown under P limitation. The lowest rotifer growth rates were obtained when the rotifer fed on Cryptomonas grown in nutrient-sufficient conditions. Our study also showed that starvation resistance was higher for those rotifers fed on N-limited Cryptomonas. In natural situations, rotifers could even grow better under nutrient-limited conditions if this group of algae is dominant in the phytoplankton community. Moreover, differences in starvation resistance could be critical in determining competitive outcome and community structure in nutrient-variable environments. Like higher plants, algae have relatively high ratios of C : P and C : N, and under N or P limitation, the C : N : P ratio of phytoplankton varies considerably (Goldman et al. 1979; Moal et al. 1987). Numerous studies have shown that N limitation, and particularly P limitation, reduce the quality of algae as food for zooplankton (Sommer 1992; Sterner et al. 1993; Rothhaupt 1995; Lürling and Van Donk 1997; DeMott 1998; DeMott et al. 1998; Schulz and Sterner 1999; Conde-Porcuna 2000; DeMott et al. 2001; Plath and Boersma 2001; Villar-Argaiz and Sterner 2002). Traditionally in these experiments, Nor P-limited media have been used to alter the elemental ratios of algal food and to exploit this feature to create food supplies of differing quality for zooplankton. Most of these studies have focused largely on crustacean zooplankton (usually Daphnia) feeding on a single species of green algae (usually Scenedesmus). The extrapolation of these results to actual field conditions is quite limited because natural populations are usually made up of a variety of crustacean and noncrustacean zooplankton that feed on an algal community made up of a frequently diverse array of algae. However, few experimental studies have examined nutrient limitation for rotifer species (Rothhaupt 1995; Conde-Porcuna 2000), and/or nutrient limitation for zooplankton using nongreen algal species as food resources (Lürling and Van Donk 1997; Boersma 2000). There is no a priori reason to assume that Scenedesmus, a relatively rare alga in the wild, is generally representative of all other algae that zooplankton are likely to feed on in natural lakes (Brett et al. 2000). Therefore, more studies that examine how nutrient availability affects the elemental composition of nongreen algal food items and their quality as food for different zooplankton are clearly necessary. Rotifer species are more susceptible than Daphnia or copepods to nutrient limitation, especially P limitation (Morales-Baquero and Conde-Porcuna 2000; Conde-Porcuna et al. 2002). Conde-Porcuna et al. (2002) observed that the abundances of some rotifer species were not correlated with food availability but showed a strong dependence on P availability in a reservoir. Different susceptibilities of zooplankton species to nutrient limitation could be important in explaining the dynamics of these organisms in natural situations. Rothhaupt (1995) and Conde-Porcuna (2000) showed that P limitation significantly reduces the growth rates of the rotifers Brachionus and Anuraeopsis. Rothhaupt (1995) also showed that N-limited Scenedesmus allowed similar rotifer growth rates at low to moderate concentrations of food, but maximum growth rates at high food concentration were significantly reduced. However, no attempt has been made on life history consequences for rotifers feeding on nutrient-limited phytoplankton. Starvation resistance reflects the ability of a species to store energy and control its allocation during periods of extreme