BIASES ASSOCIATED WlTH FOUR STREAM SUBSTRATE SAMPLERS

A. Wesche. 1991. Biases associated with four stream substrate samplers. We compared samples collected from 10 substrates of various compositions with a single-probe freeze-core sampler, a triple-probe freeze-core sampler, a McNeil sampler, and a shovel. The accuracy with which these devices sampled particles larger than 50 mm in diameter varied; they were oversampled by the freeze-core devices, sampled in proportion to their availability by a shovel, and sampled inconsistently by the McNeil Sam-' pler. The geometric mean particle size and variance of single-probe freeze-core samples consistently exceeded those of samples collected with the other devices. Most sample means also exceeded the test substrate means. By excluding the proportions of particles larger than 50 mm in diameter in our analyses, we found that proportions of several particles sizes in samples collected by different methods differed significantly from the actual proportions in test substrates. There were few differences between the single-and triple-probe freeze-core samples or between McNeil and shovel samples. All four samplers were biased, but the McNeil sampler most frequently produced samples that approximated the true substrate composition. Nous avons compare des echantillons preleves dans dix substrats de composition variable au moyen d'un echan-tillonneur A carottes (freeze core) 2 sonde unique, d'un 6chantillonneur A carottes (freeze core) 5 sonde triple, d'un echantillonneur McNeil et d'une pelle. La precision du prekvement d'une particule ayant un diamPtre superieur A 50 mm a varie selon les dispositifs utilises; ces particules etaient sur-representees dans les preleve-ments faits avec les carrotteurs, etaient echantillonnes en proportion de leur disponibilite avec la pelle, et etaient preleves de maniere inconsistante avec I'echantillonneur McNeil. La taille moyenne geometrique des particules contenues dans les bchantillons du carotteur A sonde unique, ainsi que la variance, etaient constamment sup&