The study examines ecological conditions 44 years after a 1979 deep-sea polymetallic nodule mining test in the Clarion–Clipperton Zone (CCZ). It finds that although some biological groups have partially recovered, substantial physical disturbance persists. Megafaunal communities remain altered, but mobile and some sessile species have begun re-establishing. Sediment plume impacts appear limited with time, yet physical track features remain clearly visible.

Collection tracks

Collector tracks remain identifiable after four decades, showing persistent physical alteration. Disturbance varied from complete nodule removal to partial retention, reflecting mechanical variability during 1979 operations. Sediments show similar grain size but lower and more variable organic carbon relative to controls.

Sediment macrofauna densities in disturbed areas are slightly lower but broadly comparable to controls. Dominant groups include annelids (43% in controls), arthropods (34%) and molluscs (18%). Nodule-dwelling macrofauna showed similar densities where nodules remained, though full-scale mining would likely reduce these due to more comprehensive nodule extraction.

Xenophyophores were abundant in the tracks (up to eight individuals per m²), indicating early-stage recolonisation. They reached sizes above 50 mm and colonised even heavily disturbed areas, though overall xenophyophore richness was lower than in control sites. Sessile megafauna remain scarce; only two large sponges were observed, both attached to undisturbed nodules. Mobile deposit feeders such as Psychronaetes hanseni were present, yet megafauna densities in tracks remained markedly lower (<0.1 individuals per m²) compared with controls (0.33 individuals per m²).

Microbial biomass was similar inside and outside tracks (92 mg C m² vs 52 mg C m²). Estimated carbon assimilation on tracks was higher (0.032 mg C m² day⁻¹) relative to outside (0.012 mg C m² day⁻¹), though based on low replication.

Vehicle propulsion tracks

Propulsion tracks remain largely unchanged from 1979 due to extremely slow sedimentation rates (1.5–11 mm kyr⁻¹). Furrows 0.2–0.8 m deep and 1–3 m wide persist, with berms up to 0.5 m forming along track edges. Organic detritus and occasional plastic debris accumulated in some depressions.

These tracks host a distinct megafaunal assemblage of 35 taxa across three phyla, compared with 76 taxa in undisturbed areas. Assemblages were dominated by Plesiodiadema globulosum (42%) and Elpidiidae holothurians (33%). Several taxa aggregated in track depressions, likely due to localised organic enrichment. Some sessile black corals had recolonised, though overall richness remained low. Physical disturbance in propulsion tracks is notably greater than expected from modern tracked collector systems.

Plume area

Modelling indicates sediment deposition of 0–10 mm within tens of metres of tracks. By 2023, plume-affected areas were visually indistinguishable from controls and retained high nodule abundance. Photogrammetry showed sediment infill of up to 10 mm between nodules near tracks, lower micro-relief and similar organic content to controls.

Megafaunal densities were elevated in plume areas relative to both tracks and controls, particularly bryozoans and echinoderms. Sediment plume effects appear dispersed or redistributed through bioturbation and current activity.

Conclusions

Physical disturbance from mining persists for decades, with limited remediation. Some mobile fauna and macrofauna have recolonised, and early sessile species are re-establishing, but communities remain altered. Plume effects diminish over time, though detectable in sediment micro-relief. The study suggests that reducing direct physical collector impacts will be crucial to limiting long-term ecological change if mining proceeds.