Calculations of zooplankton grazing rates according to a closed, steady-state, three-compartment model applied to different¹⁴C methods

Abstract

A re-examination of the numerical example of the three-compartment model by CONOVER & FRANCIS (1973) showed that the warning by these authors for the misuse of radio isotopes in transfer studies within food chains is incorrect and based on a misinterpretation of their results. There is no difference in the estimate of transfer rate by use of specific activities or by use of total radioactivities observed in each compartment.

After adapting the formulae developed by CONOVER & FRANCIS, their model was used to illustrate deviations of the programmed grazing rate in 3 types of grazing experiments; a) with¹⁴C present only in the phytoplankton at the start of the experiment, b) with¹⁴C only in the water, and c) with¹⁴C in both phytoplankton and water. Up to a duration of the grazing experiment of 2 hours, and at various light conditions and grazing pressures, deviations were small and did not exceed 4%. These results are not directly applicable to practical work because the quantitatively important loss by egestion of radioactive material was not accounted for, only losses by respiration were incorporated in the closed, steady-state model.

Best calculations of the community filtering rate (fraction of the volume of the grazing vessel swept clear per day) were generally obtained with the formula (with t in hours) ‘((DPM zoo at time t)/(DPM phyto at 0+DPM phyto at t)/2)×24/t’, applicable to all three types of grazing experiments considered.