A critical assessment of the stoichiometric knife-edge: no evidence for artifacts caused by the experimental P-supplementation of algae.

The stoichiometric knife-edge refers to the reduced performance of consumers encountering food with excess phosphorus (P) relative to carbon (C) or nitrogen (N). Studies that provide evidence for such knife-edge in aquatic systems often apply phosphate supplementation to create P-rich food treatments. However, this method may suffer from artifacts, because after uptake algae may store P in a form different from the P-rich biomolecules typically consumed by zooplankton. Our aim was to test if P supplementation results in potential biases. We experimentally exposed populations of the herbivore rotifer species, Brachionus calyciflorus (Pallas), to four different food quality treatments: algae grown under P-saturating (HP_chem, molar C:P ratio = 59.7 ± 2.7) and P-sufficient (MP_chem, molar C:P = 116.3 ± 5.2) conditions in chemostats, and algae grown under P-limiting conditions, but with molar C:P ratios equal to HP_chem and MP_chem treatments, respectively (HP_LP+P, molar C:P = 59.8 ± 0.14; MP_LP+P, molar C:P = 121.0 ± 4.3). The latter two treatments were achieved through P-supplementation of P-limited algae. Results show that for rotifers fed algae with either excess or intermediate P content, population growth rates were consistently higher on algae grown in chemostats than algae treated with the P supplementation method. Importantly, growth rates were also consistently lower in HP than in MP treatments and the magnitude of this negative impact was independent on algal growth history. The latter result confirms the existence of a stoichiometric knife-edge and indicates that P supplementation is a reliable method to study the relative effect of excess P on zooplankton performance in a standardized way. [ABSTRACT FROM AUTHOR]