Effects of fixed-count size on macroinvertebrate richness, site separation, and bioassessment of Chinese monsoonal streams

Subsampling that counts and identifies a random subset of individuals from field samples is widespread yet controversial. We evaluated the effects of fixed-count size on macroinvertebrate richness, site separation, and performance of modeled and null (i.e., natural variation adjusted and unadjusted, respectively) biological indices in Chinese monsoonal stream sites. To do so, we estimated the fixed-count size that was adequate to collect 95% of the estimated true regional macroinvertebrate taxa richness, and we also evaluated the effects of fixed-count size on site and group (reference vs test) separation, and the precision, sensitivity and responsiveness of modeled and null multimetric indices (MMI) and observed/expected (O/E) indices. Random subsamples of individual fixed-count sizes ranged from 50 to 500. Mean cumulative taxa richness continued to increase with increasing fixed-count size. We found that 150 and 200 individuals were needed to collect 75% of estimated true species richness 75% and 95% of the time, respectively. We estimated that at least 1500 individuals per site were required for collecting 95% of estimated true species richness. Site and group separation and classification strength also improved with increased fixed-count size. Larger fixed-count sizes improved the performance of modeled and null O/E-50 (O/E calculated using taxa with probabilities of capture >= 0.5); however, they showed no significant difference for modeled and null MMIs and O/E-0 (O/E calculated using all taxa). Overall, we found that fixed-counts affected richness and site/group separation, but not index performance. Until China develops standard sampling methods, we recommend using fixed-count sizes of 500 individuals and rarefaction of >= 200 individuals to limit the effects of sampling error for site and group separation and for precise and accurate bioassessment, respectively. (C) 2015 Elsevier Ltd. All rights reserved.

从野外样本中对个体进行计数并识别其随机子集的子采样方法应用广泛但存在争议。我们评估了固定计数数量对中国季风性河流区域大型无脊椎动物丰富度、样点区分以及建模和零（即分别经过和未经过自然变异调整）生物指数性能的影响。为此，我们估算了足以收集到估计的真实区域大型无脊椎动物分类群丰富度95%的固定计数数量，还评估了固定计数数量对样点和群组（参照组与测试组）区分以及建模和零多度量指数（MMI）以及观测/预期（O/E）指数的精密度、敏感度和响应度的影响。个体固定计数数量的随机子样本范围从50到500不等。平均累积分类群丰富度随着固定计数数量的增加而持续增加。我们发现，分别需要150和200个个体才能在75%和95%的情况下收集到估计的真实物种丰富度的75%。我们估计每个样点至少需要1500个个体才能收集到估计的真实物种丰富度的95%。样点和群组的区分以及分类强度也随着固定计数数量的增加而提高。较大的固定计数数量提高了建模和零O/E - 50（使用捕获概率≥0.5的分类群计算的O/E）的性能；然而，对于建模和零MMI以及O/E - 0（使用所有分类群计算的O/E），它们没有显示出显著差异。总体而言，我们发现固定计数影响丰富度和样点/群组区分，但不影响指数性能。在中国制定出标准采样方法之前，我们建议分别使用500个个体的固定计数数量以及≥200个个体的稀疏化处理，以分别限制采样误差对样点和群组区分以及对精确生物评估的影响。（C）2015爱思唯尔有限公司。保留所有权利。