Phosphorus Fluxes in Limnetic Cladocerans: Coupling of Allometry and Compartmental Analysis

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Abstract
A size-dependent two-compartment model was developed for estimation of32P turnover and fluxes by limnetic cladocerans in steady state. After feeding on radioactively labelled food, uniformly labelled animals were fed unlabelled cells and the time course of release of tracer followed. Rates of turnover and size-specific fluxes were subsequently fitted to a two-compartment model. The model predicted that steady-state turnover and size-specific fluxes for32P excretion declined with body weight and that the exponent of weight did not significantly differ from −0.25, suggesting the relationships between total P turnover or flux rates and body size in cladocerans follow the same allometry observed for other organisms and other metabolic activities. However, rate constants for intercompartmental exchanges declined faster than weight−0.25, indicating that their turnover and flux declined much faster with increasing body size than would be expected from general allometry. Size-specific ingestion and assimilation rates of32P by cladocerans decreased with increasing body size with a slope of the allometric function similar to −0.25.