Anthropogenic activities have strongly reshaped the amounts and ratios in which essential elements, such as carbon (C) and phosphorus (P) are available to organisms. Recent studies in the framework of ecological stoichiometry have suggested that organisms may be living on a ‘stoichiometric knife edge’, where either too high or too low food C:P ratios cause performance reductions of consumers. However, the generality of the ‘stoichiometric knife edge’, as well as the associated physiological and behavioral responses of consumers are still poorly understood. In this study, we subjected populations of a planktonic rotifer to a 10-fold food C:P gradient, and studied the responses of population as well as somatic growth rates. In addition, with the aim to assess how and to what extent the consumers maintain homeostasis, we also measured somatic elemental composition along with two key physiological rates, i.e. P intake and P release rates.
Results/Conclusions
The highest growth rate of animals was observed at an intermediate food C:P level of 170 and any deviations towards higher or lower molar C:P ratios were associated with reduced consumer performance, confirming the existence of a stoichiometric knife edge. When fed P-limited food, animals showed increased P ingestion and reduced P loss rates, whereas the opposite was found with P-rich food. Consumers were better able to maintain elemental homeostasis when confronted with P-rich than P-poor food. A complete homeostatic breakdown was observed above food C:P ratios of 390. In contrast, reductions of food C:P below 165 only resulted in relatively modest changes in animal C:P levels. Homeostatic breakdown at high food C:P was associated with pronounced growth reductions, indicating that animals were unable to cope with excess C and limitation of P. Interestingly, despite the absence of major deviations from homeostasis, we still observed strong performance reductions at the low C:P end of the food quality gradient, this is indicative for important costs associated with the maintenance of homeostasis, possibly also combined with toxicity effects of excess P.