Background/Question/Methods: Resource elemental limitation impacts consumer ingestion rates which can have important consequences for trophic interactions. However, the effects of stoichiometric imbalance have primarily been quantified at high resource densities and there is a dearth of investigations into its effect across an ecologically relevant range. Additionally, it is unclear if consumers modify ingestion by directly assessing resource elemental content or whether they rely on other properties that are an indirect result of elemental limitation. A consumer’s functional response describes the change in consumption rate that occurs in response to changes in resource density and provides a framework to describe the strength and stability of trophic interactions. To investigate the effect of resource elemental limitation on a herbivore consumer’s functional response, and the cues used to modify this behaviour, we conducted ingestion trials across a range of resource densities with the microzooplankton consumer Brachionus calyciflorus and the motile green algae Chlamydomonas reinhardtii as a resource. We used algae with both high (HP) and low phosphorus (LP) content, as well as a third resource quality created by supplementing LP-algae with inorganic P (LP+P). The LP+P treatment increased the P-content of the algae but retained the other biochemical properties associated with growth in P-limited conditions, allowing us to disentangle the direct and indirect effects of P-limitation.
Results/Conclusions: Rotifers consuming LP algae exhibit compensatory feeding (i.e. elevated ingestion rates) at high but not low food densities, and density-dependent search rates (i.e. Holling type III functional response) in both treatments. These results suggest that in systems with high C availability, resource P-limitation may increase top-down control. As type III responses were observed in both food treatments, consumer-resource interactions are likely to be stable. We observed no differences in the functional response of consumers provided with LP and LP+P algae, despite the former being P-limited and the latter being P-replete. Thus, the rotifers' food ingestion rate appears to respond to properties of the resource that P-limitation indirectly determines (e.g., biochemical composition, morphology) rather than to the mineral P-content of the algae itself. Our study demonstrates that for herbivore consumers there is a significant interaction between resource quality and quantity on food ingestion rates, however, such dynamics are rarely taken into consideration. The incorporation of stoichiometrically explicit functional responses, especially in highly productive systems, will aid in our understanding of trophic interactions, and the impacts of anthropogenic changes in nutrient cycling.
