Impacts of an industrial deep-sea mining trial on macrofaunal biodiversity.

In 2022 a large-scale test of a commercial deep-sea mining machine was undertaken on the abyssal plain of the eastern Pacific Ocean at a depth of 4,280 m, recovering over 3,000 t of polymetallic nodules. Here, using a quantitative species-level sediment-dwelling macrofaunal dataset, we investigated spatio-temporal variation in faunal abundance and biodiversity for 2 years before and 2 months after test mining. This allowed for the separation of direct mining impacts from natural background variation, which we found to be significant over the 2-year sampling period. Macrofaunal density decreased by 37% directly within the mining tracks, alongside a 32% reduction in species richness, and significantly increased community multivariate dispersion. While species richness and diversity indices within the tracks were reduced compared with controls, diversity was not impacted when measured by sample-size independent measures of accumulation. We found no evidence for change in faunal abundance in an area affected by sediment plumes from the test mining; however, species dominance relationships were altered in these communities reducing their overall biodiversity. These results provide critical data on the effective design of abyssal baseline and impact surveys and highlight the value of integrated species-level taxonomic work in assessing the risks of biodiversity loss.