To assess the risk of all nanomaterials (NMs) on a case-by-case basis is challenging in terms of financial, ethical and time resources. Instead a more intelligent approach to knowledge gain and risk assessment is required.

Deep-sea polymetallic nodule mining is in the exploration phase at present with some groups proposing a move towards extraction within years¹. Management of this industry requires evidence of the long-term effects on deep-sea ecosystems², but the ability of seafloor ecosystems to recover from impacts over decadal scales is poorly understood³. Here we show that, four decades after a test mining experiment that removed nodules, the biological impacts in many groups of organisms are persistent, although populations of several organisms, including sediment macrofauna, mobile deposit feeders and even large-sized sessile fauna, have begun to re-establish despite persistent physical changes at the seafloor. We also reveal that areas affected by plumes from this small-scale test have limited detectable residual sedimentation impacts with some biological assemblages similar in abundance compared to control areas after 44 years. Although some aspects of the modern collector design may cause reduced physical impact compared to this test mining experiment, our results show that mining impacts in the abyssal ocean will be persistent over at least decadal timeframes and communities will remain altered in directly disturbed areas, despite some recolonization. The long-term effects seen in our study provide critical data for effective management of mining activities, if they occur, including minimizing direct impacts and setting aside an effective network of protected areas^4,5.