This paper details results of experiments intended to clarify the relative
effects of mineral versus biochemical limitations of algae on the food qual
ity for herbivorous zooplankton. Phosphorus limited algae (Scenedesmus) wer
e fed to Daphnia magna and the somatic growth rates of the animals were mea
sured. The animals showed an increase in growth rate when phosphorus (P) wa
s added to P-limited algae, especially when the animals were older. The add
ition of highly unsaturated fatty acids only resulted in an increase in gro
wth rates when the algae had a C:P ratio of less then 300. The main conclus
ion of this work is that both mineral and biochemical limitations of their
food play a role in the growth and population dynamics of zooplankters, but
that mineral requirements need to be met first.