An evaluation of accuracy, precision, and speed of several zooplankton subsampling techniques

Abstract

Several sources of error associated with subsampling zooplankton were investigated, using five subsampling devices on “pure” (single-taxon) samples. Bias occurred in 8·8 % of these experiments. Coefficients of variation (CV) were generally similar and ranged from 4·8 to 30·5%, in agreement with previous research. For fish eggs the Stempel pipette was most precise and very fast, though it is often impractical for normal samples because of clogging. For the fish larvae and invertebrates the Folsom splitter was the most precise and the Huntsman Marine Laboratory (HML) beaker technique was the fastest. The HML beaker technique appeared to cause less damage to delicate organisms than the other devices examined. In five of six subsampling comparisons with the pure-sample experiments, the CVs for an artificial “wild” zooplankton sample decreased, indicating that the pure-sample CVs are conservative estimates of subsampling error. For the wild sample, again the Folsom splitter was the most accurate and precise, and the HML beaker technique was the fastest. Both the pure- and wild-sample experiments indicated an occasional, inherent unpredictability in subsampling zooplankton in terms of bias, precision, and time. The literature clearly showed an inverse relationship between subsample N and subsampling error. Using the HML beaker technique and a pure sample of fish eggs it was determined that error increased with the number of splits; the results were inconclusive as to whether or not error is reduced by a device which gives multiple subsamples. The difference in speed between repeated halving and obtaining multiple subsamples was negligible. From the literature it was determined that subsampling error is not always small compared with variation among samples, as is generally assumed. It is suggested that in any zooplankton study which employs subsampling, the contribution of subsampling error to total variance should be determined for specific target taxa and considered when sample abundance estimates are made.