The quality of natural seston as food for zooplankters can be highly variable. Thus far, experimental evidence on the factors affecting food quality under natural conditions is scarce. Hence, in this study, we set out to investigate how Daphnia galeata × hyalina responded to qualitative variation in natural seston. This was done in laboratory experiments where we supplement natural seston from a mesotrophic lake with dissolved phosphorus and emulsions of highly unsaturated "essential" fatty acids. The growth rate of juveniles increased upon the supply of both phosphorus and fatty acids. These results suggest that these phosphorus and highly unsaturated fatty acids are substitutable and thus challenge our existing interpretation/understanding of how herbivore growth is "limited".