Life at the Edge: Is Food Quality Really of Minor Importance at Low Quantities?

There is increasing evidence that the quality of nutrient-limited algae is suboptimal for zooplankton production. These effects of nutrient limitation are supposed to be important mainly at higher concentrations of food because at lower quantities the overall energetic limitation of body growth should hide the effect of mineral limitations. This has been hypothesized in a variety of papers, but experimental evidence is still weak. In the present study we carried out a set of growth experiments investigating the effect of food quality at very low food levels ranging from 30 μg C/L up to 150 μg C/L. In all of the experiments, the growth rates of Daphnia magna neonates were lower when grown on P-limited Scenedesmus obliquus. This effect disappeared when phosphorus was added to the P-limited algae prior to feeding, indicating that mineral limitation can occur even at very low levels of food. Neonates born to mothers raised on either high- or low-P Scenedesmus were analyzed for body mass and lipid content as well as mass-specific phosphorus content to check for possible differential investment into neonates in different environments. Although mass-specific phosphorus content was higher in animals born from mothers grown on high-P algae, when fed low quantities of P-limited algae, growth rates of neonates born under low-P conditions were higher than those of animals born under high-P conditions. This can be explained by an increased body lipid content of low-P neonates, even though there were no differences in neonate body mass between treatments. These results illustrate the importance of the incorporation of low food concentrations in ecological stoichiometry models.