Does excess carbon affect respiration of the rotifer Brachionus calyciflorus Pallas?

1. Herbivorous zooplankton maintain a rather constant elemental composition in their body mass as compared with the variability commonly encountered in their food. Furthermore, their high phosphorus (P) and nitrogen (N) content means that they often face an excess of carbon (C) in their diet. Regulation of this surplus of energy may occur via modulation of assimilation efficiency, or postassimilation by increased respiration (CO₂) and/or excretion dissolved organic carbon, DOC. Whereas several studies have examined the effect of elemental imbalance in the genus Daphnia, few have examined other zooplankton taxa. 2. We investigated whether the rotifer Brachionus calyciflorus uses increased respiration as a means of stoichiometrically regulating excess dietary C. Growth rate and respiration were measured under different food qualities (C : N and C : P ratios). 3. Both C : N and C : P ratios in food had strong effects on growth rate, demonstrating strong nutrient limitation of rotifer growth when nutrient elements were depleted in the diet and indicating the need for stoichiometric regulation of excess ingested C. 4. Respiration measurements, supported by a stoichiometric model, indicated that excess C was not released as CO₂ in B. calyciflorus and that nutrient balance must therefore be maintained by other means such as excretion of DOC or egestion in faecal material.