Modeling calcium-limitation in Daphnia with consideration of food quantity and phosphorus content.

Low concentrations of calcium have been found to slow growth and reproduction of the freshwater zooplankton, Daphnia. Despite experimental evidence linking poor animal performance and low calcium concentrations, there are no theoretical models that link calcium uptake dynamics to Daphnia growth. Here I created a multi-element (carbon (C), phosphorus (P), and calcium (Ca)) model of Daphnia growth that incorporates information on animal Ca uptake and loss, feeding rates, C and P absorption efficiencies, and body nutrient content. This model examines how the relative supplies of Ca, C, and P in food and water affect Daphnia's acquisition of these elements and subsequently, their growth rates. The model demonstrates that animal limitation by Ca switches to C or P with changes in food abundance and C:P ratios above a threshold Ca concentration. While modelled threshold Ca concentrations were remarkably similar to previous empirical estimates, Daphnia growth rates predicted by the model were generally much lower than those previously reported for animals experimentally raised under low Ca concentrations. Further, the model predicts Daphnia to be primarily food quantity or P-limited in lakes of south-central Ontario due to relatively low supply of food C and/or P compared to Ca. While this model would benefit from additional data on Ca uptake parameters, it nonetheless shows the utility of multi-element mass-balance modeling and provides an approach to determine the frequency and strength of Ca-limitation in zooplankton populations in lake ecosystems.