Carbon and phosphorus linkages in Daphnia growth are determined by growth rate, not species or diet

Ecological stoichiometry provides a framework for understanding how fitness relates to acquisition and use of elements. This framework predicts a trade-off between the ability to grow quickly under good food conditions and to minimize growth depression on low-phosphorus (P) diets. These traits should both be related in some way to organism P content, a commonly used proxy of P demand. Previous tests using Daphnia species did not consistently support this prediction, whereas comparisons made across genera or higher taxonomic groups did. One explanation could be that body P content measured at one point in time is a poor indicator of P demand for growth over an interval. We measured the rate of both mass and phosphorus gain of seven Daphnia species under contrasting food conditions, including low- and high-P algal diets. Similar to other studies, we found no significant relationship between body P content and maximum growth rate or sensitivity to P limitation. On the other hand, Carbon (C) and P specific growth rates were linearly related (R² = 0.83) to all treatments and species with a positive intercept and slope less than one. This relationship indicates that rapid growth does require high-P accrual as predicted by the growth rate hypothesis. These results reveal a fundamental and unexpected relationship between growth and P demand that transcends species and diets. C and P coupling in growth represents an alternative measure of P demand that confirms theoretical predictions concerning maximum growth rate, sensitivity to P limitation and P demand as well as revealing unpredicted linkages between P demand and the degree of stoichiometric homeostasis. The straight line coupling of C and P in growth represents a new framework for understanding C and P linkages that does not assume strict homeostasis, but does allow generalization across species and diets.