A new study in Nature Ecology & Evolution reveals an unprecedented two-year, species-level record of abyssal macrofauna, shedding light on both natural deep-sea dynamics and the rapid ecological effects of experimental nodule mining.
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The deep-sea mining industry is moving toward commercial exploitation, particularly polymetallic nodules in the Clarion-Clipperton Zone (CCZ). This transition has created an urgent need for direct assessment of the impacts of deep-sea mining on faunal abundance and biodiversity at the seafloor.
In 2022, a large-scale industrial test of a prototype commercial deep-sea mining machine was conducted on the abyssal plain of the eastern Pacific Ocean, recovering over 3,000 tons of polymetallic nodules from a depth of approximately 4280 m.
Previous, smaller deep-sea disturbance experiments have reported significant effects on benthic communities, including decreases in macrofaunal density. However, fully understanding and assessing the environmental impacts of a disturbance event, such as deep-sea mining, requires sufficient baseline data to separate the effect of interest from inherent natural spatial and temporal variability.
Knowledge gaps related to the effective environmental management of deep-seabed mining persist. The study site was the NORI-D exploration contract area in the southeastern CCZ, licensed by the International Seabed Authority (ISA). The collector test involved tracking over approximately 80 km of abyssal seafloor within a designated 2 x 4 km test field.
The Current Study
The research employed an asymmetrical Before-After-Control-Impact (BACI) style experimental design, incorporating several control sites to effectively disentangle mining impacts from natural variability, following principles that informed the initial ISA exploration regulations.
A random stratified sampling design was implemented, collecting data from four control sites (FFE, FFW, NFE, and PRZ) and one impacted site, the CTA.
Sampling occurred at three time points before the collector test (and again two months after the test). Following the impact, samples taken at the CTA were categorized into "track" (directly impacted area) and "plume" (area affected by resuspended sediment) categories. A total of 80 boxcore samples were collected across the four campaigns.
The researchers emphasized the need for a minimum of five, and ideally 10, cores per site and time point, spatially randomized within strata, for baseline studies. Macrofaunal specimens were sorted and identified, yielding 4,350 sediment macrofauna from which 3,826 specimens (88%) were identified to a total of 788 species.
Results and Discussion
The study successfully determined significant impacts of the large-scale deep-sea mining test on several aspects of benthic abundance and diversity, despite a background of strong natural variation.
Immediately following the impact, macrofaunal densities decreased significantly by 37 % within the path of the nodule collector (the "track"), a finding consistent with patterns seen in previous smaller deep-sea disturbance experiments. The samples collected from within the mining track had significantly lower macrofaunal densities compared to control sites.
In contrast, the control sites either maintained or increased their densities. This reduction in faunal density is expected since most abyssal infauna inhabit the top 2 cm of sediment, which is directly disturbed during the nodule removal process.
The mining track also experienced a 32 % reduction in species richness and significantly increased community multivariate dispersion. Increased ecological variability (dispersion) within the track is common following disturbance, potentially caused by the destabilization of community dynamics, leading to altered competitive interactions and spatial aggregation of species. The even reduction in abundance across broad taxonomic groups within the track suggests the collector test caused an equal removal of individuals from the top sediment layers, rather than disproportionately affecting specific taxonomic groups.
Regarding the sediment plumes generated by the mining vehicle, no significant change in macrofaunal abundance or species richness was found in the area affected two months after deposition, aligning with reports from previous studies on meiofauna and macrofauna. However, the communities in the plume area showed altered species dominance relationships, which reduced community diversity as measured by evenness. This change in dominance relationships is a similar ecological response observed in benthic communities impacted by turbidity flows.
Conclusion
The results provide critical quantitative data on the immediate impacts of a large-scale industrial deep-sea mining trial on abyssal macrofaunal biodiversity. Directly within the mining tracks, there was a substantial and significant reduction in macrofaunal density and species richness, alongside increased variability in community composition.
In areas affected only by sediment plumes, while faunal abundance and richness remained unchanged, the community structure and overall diversity were negatively impacted due to shifts in species dominance. These findings underscore the importance of collecting multi-year baseline data to effectively separate the impacts of mining from significant natural spatiotemporal variation. Furthermore, the study emphasizes the importance of integrated species-level taxonomic work in accurately assessing the risks of biodiversity loss associated with deep-sea mining.
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Journal Reference
Stewart E.C.D., Wiklund H., et al. (2025). Impacts of an industrial deep-sea mining trial on macrofaunal biodiversity. Nature Ecology & Evolution. DOI: 10.1038/s41559-025-02911-4, https://www.nature.com/articles/s41559-025-02911-4