Calculations of results in grazing experiments using the counting method

Abstract

Different formulae used in the literature for calculation of ingestion rates are reviewed. Their applicability is discussed in relation to two criteria: the occurrence of saturation of the feeding and/or growth of the food particles. It is shown by simulations that the differences in results obtained by calculating ingestion following a linear or an exponential model are restricted to a few percent. Calculation of ingestion rates by subtraction of particle concentrations from control and grazing bottles can result in substantial over- or underestimation depending on the growth constant of the food particles and the grazing time.

The formule l=Vk(C_zt−C_zoe^kt)/N(l−e^kt) is proposed for the calculation of ingestion rates under saturated circumstances.

It is demonstrated that, with data obtained by multi-size class analysis, calculations on total particle concentrations are only feasible when growth constants and grazing coefficients are equal for all size classes considered.

Attention is drawn to calculation problems resulting from non-normally distributed results and from non equal particle concentrations in control and grazing bottles at the beginning of the experiment. For the latter problem, two correction methods are proposed.