# Influence of Sampling Effort and Taxonomic Resolution on Benthic Macroinvertebrate Taxa Richness and Bioassessment in a Non-Wadable Hard-Bottom River (China)

## TL;DR

This study finds that six hand-net samples provide a reliable and cost-effective way to assess river health using benthic macroinvertebrates in non-wadable hard-bottom rivers.

## Contribution

Proposes a practical sampling strategy for non-wadable hard-bottom rivers using six hand-net replicates and family-level taxonomy.

## Key findings

- Six hand-net replicates provide stable and reliable BMWP assessment results.
- Family-level taxonomy reduces workload and increases consistency in bioassessment.
- Sampling effort significantly affects taxa richness and accuracy of river health assessments.

## Abstract

River health is usually assessed by monitoring benthic macroinvertebrate assemblage. However, it is a challenge to collect representative samples in non-wadable hard-bottom rivers. The purpose of this study is to determine how many sample replicates need to be collected in this river type in order to explore a reliable monitoring method of benthic macroinvertebrates. We chose a study reach of Danjiang river, China, which met the requirements of non-wadable hard substrates, and analyzed the changes in taxa richness at family and genus/species levels with different sample efforts. The results showed that the richness of benthic macroinvertebrates increased with the increase in sample replicates at both taxonomic levels. We found that six hand nets could provide an economical and stable assessment result of BMWP index. The coarser taxonomic resolution (i.e., family level) also makes the results more consistent and reduces the workload. This study initially provides a practical, economical, and reliable sampling strategy for monitoring the health status of similar rivers in China, and provides a reference for exploring bio-monitoring methods for other types of rivers.

Benthic macroinvertebrates are widely used for river ecosystem health monitoring, yet challenges remain in non-wadable rivers, particularly regarding sampling effort. We evaluated hand-net sampling efficiency at three sites along the Danjiang River (a Yangtze River tributary) by analyzing taxa richness across taxonomic levels under varying replicate numbers. In total, 61 taxa (41 families) of benthic macroinvertebrates were identified. Non-metric multidimensional scaling analysis indicated no significant spatiotemporal variation in community composition. However, sampling effort increased, and the benthic macroinvertebrate taxa richness at both genus/species and family levels also increased. At eight sample replicates, the taxa accumulation curve at the genus/species level did not show an asymptote, with the observed richness reaching 67–80% of the predicted values calculated by Jackknife 1. In contrast, the family-level curve exhibited a clear asymptotic trend, with the observed richness reaching 82–100% of the predicted values. As sampling effort increased, bias decreased and accuracy improved, particularly for family-level taxa. Additionally, the BMWP scores also increased with the sampling effort. When the replicate number was no less than six, the BMWP reached stable assessment grades for all cases. From the perspective of bioassessment in non-wadable rivers, the hand net is suitable for collecting benthic macroinvertebrates. However, there is a risk of underestimating taxa richness due to insufficient sampling effort. Using family-level taxa can partially mitigate the impacts caused by insufficient sampling efforts to a certain extent, but further validation is needed for other non-wadable rivers (e.g., those with soft substrates). In conclusion, our research results indicate that six replicate hand-net samplings in non-wadable hard-bottom rivers can be regarded as a cost-effective and reliable sampling method for benthic macroinvertebrate BMWP assessment. This strategy provides a relatively practical reference for the monitoring of benthic macroinvertebrate in the same type of rivers in China.