Abstract
Anthropogenic activities have reshaped the relative supply rates of essential elements to organisms. Recent studies suggested that consumer performance is strongly reduced by food that is either very high or very low in relative phosphorus content. However, the generality of such ‘stoichiometric knife‐edge’ and its underlying mechanisms are poorly understood. We studied the response of a planktonic rotifer to a 10‐fold food carbon : phosphorus (C : P) gradient and confirmed the existence of the stoichiometric knife‐edge. Interestingly, we observed a complete homeostatic breakdown associated with strong growth reductions at high food C : P. In contrast, at low food C : P, animals maintained homeostasis despite pronounced performance reductions. Our results suggest that the mechanisms underlying adverse effects of stoichiometric imbalance are determined by both the identity of elements that are limiting and those that are present in excess. Negative effects of excess P reveal an additional way of how eutrophication may affect consumers.
Highlights
In recent decades, anthropogenic activities have strongly altered the relative supply rates of key elements, such as carbon (C), nitrogen (N) and phosphorus (P) to organisms (Smith & Schindler 2009; Elser et al 2009)
By relating consumer performance with the degree of stoichiometric plasticity along each of these directions, we evaluated to what extent homeostatic regulation implies reduced performance, and whether an apparent relaxation of such regulation reflects a capacity to deal with stoichiometric mismatch or, alternatively, an inability to maintain elemental homeostasis
The relationship between rotifer C : P and food C : P was best described by a two-segment piecewise regression model and its breakpoint was situated at a food C : P of 391 (Fig. 2a, Appendix S2 Table S2)
Summary
Anthropogenic activities have strongly altered the relative supply rates of key elements, such as carbon (C), nitrogen (N) and phosphorus (P) to organisms (Smith & Schindler 2009; Elser et al 2009). TER predicts a unimodal response of consumer performance to a gradient of food elemental ratios, with optimal growth close to the requirements of the organisms (TER) and a reduction towards the extremes of the gradient (Anderson & Hessen 2005; Frost et al 2006; Khattak et al 2018) This prediction has been supported for a wide range of taxa (coined as the ‘stoichiometric knife-edge’; Elser et al 2006; Bullejos et al 2014; Benstead et al 2014; Laspoumaderes et al 2015; Elser et al 2016). Plath & Boersma (2001) suggested that a low food C : P ratio may indirectly result in C limitation of Daphnia because they adjust their feeding rates in function of their P intake
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