Implications of taxonomic and numerical resolution on DNA metabarcoding-based inference of benthic macroinvertebrate responses to river restoration

It is essential to explore and define the level of taxonomic identification and quantification approaches for diversity and biomonitoring studies, given their potential influence on assessing and interpreting ecological outcomes. This study assessed the response of benthic macroinvertebrate communities to restore and construct gravel bars in a dam-impacted river, with non-dam influenced tributaries serving as reference sites. Additionally, we evaluated the performance of different taxonomic and numerical (i.e., abundance vs. presence/absence data) resolutions of DNA metabarcoding with consequent comparison to morphology-based identification and how it affects assessment outcomes. DNA metabarcoding detected 93% of the morphologically identified individuals and provided finer taxonomic resolution. We also detected significant correlations between morphological sample abundance, biomass, and DNA metabarcoding-read abundance. Furthermore, we observed a relatively high and significant congruence in macroinvertebrate community structure and composition between different taxonomic and numerical resolutions of both methods. This indicates a satisfactory surrogacy between the two approaches and their varying identification levels and data transformation. Additionally, community-environmental associations were significant for all datasets but showed significant varying associations with physicochemical parameters. Our observations imply that coarser taxonomic resolution could be highly advantageous for DNA metabarcoding-based applications in situations where the lack of taxonomic information, e. g., poor reference database, might severely affect biological assessment quality.