Abstract
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. Deep-sea mining within the CCZ is at a critical juncture, as the industry looks to move beyond the exploration phase and into commercial exploitation. Consequently, there is a clear need for direct assessment of the impacts of mining on faunal abundance and biodiversity at the seafloor. Our results show that against a background of strong natural variation we can determine significant impacts of a large-scale deep-sea mining test on several aspects of benthic abundance and diversity. Immediately following impact, macrofaunal densities decreased significantly by 37% within the path of the nodule collector, while densities within the control sites either increased or remained unchanged. This is in line with patterns seen in previous smaller deep-sea disturbance experiments which have reported 38–63% decreases in macrofaunal density. Parallel with the observed reduction in faunal abundance, we found a significant decrease in species richness and diversity within the mining tracks. The observed number of species within a sample is intrinsically linked and sensitive to underlying differences in abundance. Therefore, the significantly lower abundance of macrofauna within the mining tracks is the most likely driver of the reduced diversity per sample unit.
Key findings
- Large-scale deep-sea mining test occurred in 2022 on eastern Pacific abyssal plain at 4,280 m depth.
- Over 3,000 tonnes of polymetallic nodules were recovered.
- Macrofaunal density fell by 37% inside mining tracks; species richness fell by 32%.
- Community multivariate dispersion increased after mining.
- Within mining tracks, diversity declined; outside tracks, background natural variation remained significant.
- Sediment plumes did not reduce faunal abundance in affected areas, but altered species dominance relationships.
- Results inform abyssal baseline design and underscore value of species-level taxonomic work.
Method overview
- Quantitative, species-level, sediment-dwelling macrofaunal dataset used to assess spatio-temporal variation.
- Comparisons made across 2 years before mining and 2 months after, distinguishing mining effects from natural variability.
- Analyses included density, species richness, diversity indices, and community dispersion.
Implications
- Demonstrates detectable impacts of a large-scale deep-sea mining test on benthic communities amid natural variation.
- Highlights need for robust baseline surveys and long-term monitoring in the Clarion-Clipperton Zone (CCZ).
- Supports integrated taxonomic work to assess biodiversity risk in deep-sea mining contexts.
Author's summary
This study provides empirical evidence that commercial deep-sea mining can reduce macrofaunal density and species richness within mined tracks, even when natural background variation is considered, underscoring the importance of robust baselines and taxonomic detail for impact assessments.
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