Comparative study of traditional and DNA-based methods for environmental impact assessment: A case study of marine...

Environmental impact assessments of marine aggregate extraction are traditionally conducted based on morphological characteristics of macrobenthos, which is time-consuming, labour-intensive and requires specific taxonomic expert knowledge. Bulk DNA metabarcoding is suggested as a promising alternative. This study compares the traditional morphological and the bulk DNA metabarcoding method to assess the impact of sand extraction activities on three sandbanks in the Belgian North Sea. Substantial differences in the detected species were observed between methods: Abundant and/or large macrobenthos species were detected by both methods, while small species or species with an exoskeleton were usually only detected by the morphological method. Taxa uniquely detected by bulk DNA metabarcoding could be explained by specimens identified at a higher taxonomic level by morphology, or by specimens with very low read numbers, probably representing species missed in the morphological sorting process, DNA traces on the specimens or false positives during PCR amplification efficiency. Despite the difference in detected species, comparable alpha and beta diversity patterns were observed by both methods, indicating that bulk DNA metabarcoding can effectively detect the overall ecological changes associated with sand extraction. We further demonstrate that bulk DNA metabarcoding reduces sample processing both in time (44 % faster) and cost (26 % cheaper) compared to the morphology-based identification. However, biomass quantification remains challenging for bulk DNA metabarcoding since of the ten most abundant genera, only two genera (Echinocardium and Ophelia) showed a significant positive correlation between biomass and read numbers. Additionally, bulk DNA metabarcoding does not provide information on life stages or size of the identified specimens. As such, our results underpin the complementary nature of both methods, wherein DNA-based analyses allow for rapid detection of community changes (as similar patterns in alpha and beta diversity and biotic index were observed), while morphology-based analyses provide additional information on e.g. secondary production (biomass) and size composition. We show how the strengths of both methods can be combined to assess the impact of sand extraction.