Food quality of freshwater consumers is often defined as the relative supply of carbon (C) to phosphorus (P) in diet. The growth rate hypothesis makes mechanistic links between P supply, ribosome biogenesis, and growth, with subsequent impacts on other life history traits such as age at maturity and clutch size. However, we know surprisingly little about the role and importance of other elements in impacting life histories of freshwater zooplankton. Because there is much evidence indicating a pivotal role for iron (Fe) in oogenesis, we hypothesized that dietary Fe content will invoke distinct effects on consumer life history compared to P. We tested this hypothesis in four species of Daphnia, by characterizing the relative impact of P and Fe on life history traits, and also measuring changes in Fe kinetics in response to dietary P and Fe. We found that while P had the largest influence on growth rate, Fe was particularly important in impacting reproductive traits. Radiotracer (Fe55) analyses revealed differences in the acquisition and retention rates of Fe between two species of Daphnia. Finally, we found that P‐ and Fe‐supply driven differences in growth and reproduction had significant effects on population growth. These results indicate that Fe can constrain production of freshwater zooplankton. Understanding the interaction between the supply of P in relation to trace elements should provide a clearer picture on how stoichiometric constraints are realized in lakes.
The growth rate hypothesis posits that the rate of protein synthesis is constrained by phosphorus (P) supply. P scarcity invokes differential expression of genes involved in processing of most if not all elements encompassing an individual (the ionome). Whether such ionome‐wide adjustments to P supply impact growth and trophic interactions remains unclear. We quantified the ionomes of a resource‐consumer pair in contrasting P supply conditions. Consumer growth penalty was driven by not only P imbalance between trophic levels but also imbalances in other elements, reflecting complex physiological adjustments made by both the resource and the consumer. Mitigating such imbalances requires energy and should impact the efficiency at which assimilated nutrients are converted to biomass. Correlated shifts in the handling of multiple elements, and variation in the supplies of such elements could underlie vast heterogeneity in the rates at which organisms and ecosystems accrue biomass as a function of P supply.
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