Monte Carlo simulations for the determination of the acceptable subsampling simplification

Laboratory processing of macroinvertebrates samples is the most labor-intensive step within the biological monitoring. Its simplification by subsampling was systematically tested by Monte Carlo simulations in order to identify a minimum number of subsampler cells necessary to be processed for a certain accuracy of the benthic community characteristics and common indexes. The acceptable level of uncertainty was set to enable a safe identification of significant disturbances of urban streams (max. 30% difference in total abundance, 20% difference in number of species, 10% difference in indexes). Results show that laboratory subsampling is very suitable for the determination of the total number of individuals, Saprobity index and diversity index, whose uncertainties due to subsampling are symmetrically distributed around the reference value (total sample). It is less suitable for the number of species, BMWP index and ASPT index exhibiting an asymmetric distribution of uncertainties affected by a systematic error (underestimation). Only 8 cells have to be processed to reach the uncertainty criterion for the number of individuals whereas at least 48 cells for the number of species. 5 cells have to be sorted out for Saprobity index, 16 cells for diversity, 33 cells for ASPT index and 76 cells for BMWP index.