Description: Deep-seabed polymetallic nodule mining can have multiple adverse effects on benthic communities, such as permanent loss of habitat by removal of nodules and habitat modification of sediments. One tool to manage biodiversity risks is the mitigation hierarchy, including avoidance, minimization of impacts, rehabilitation and/or restoration, and offset. We initiated long-term restoration experiments at sites in polymetallic nodule exploration contract areas in the Clarion-Clipperton Zone that were (i) cleared of nodules by a preprototype mining vehicle, (ii) disturbed by dredge or sledge, (iii) undisturbed, and (iv) naturally devoid of nodules. To accommodate for habitat loss, we deployed 〉2000 artificial ceramic nodules to study the possible effect of substrate provision on the recovery of biota and its impact on sediment biogeochemistry. Seventy-five nodules were recovered after eight weeks and had not been colonized by any sessile epifauna. All other nodules will remain on the seafloor for several years before recovery. Furthermore, to account for habitat modification of the top sediment layer, sediment in an epibenthic sledge track was loosened by a metal rake to test the feasibility of sediment decompaction to facilitate soft-sediment recovery. Analyses of granulometry and nutrients one month after sediment decompaction revealed that sand fractions are proportionally lower within the decompacted samples, whereas total organic carbon values are higher. Considering the slow natural recovery rates of deep-sea communities, these experiments represent the beginning of a ~30-year study during which we expect to gain insights into the nature and timing of the development of hard-substrate communities and the influence of nodules on the recovery of disturbed sediment communities. Results will help us understand adverse long-term effects of nodule removal, providing an evidence base for setting criteria for the definition of “serious harm” to the environment. Furthermore, accompanying research is needed to define a robust ecosystem baseline in order to effectively identify restoration success.

Description: The eastern Clarion Clipperton Fracture Zone (CCZ) is a heterogeneous abyssal environment harbouring relatively low abundances of highly diverse megafauna communities. Potential future mining of polymetallic nodules threatens these benthic communities and calls for detailed spatial investigation of megafauna. Based on the predicted probability of occurrence of 68 megafauna morphotypes, a seabed area extending over 62,000 km 2 was divided into three assemblages covering an eastern plain area, a deeper western plain area and an area covering both seamount and abyssal hill sites. Richness, estimated as the sum of morphotypes with a predicted probability of occurrence larger than 0.5, amounts to 15.4 of 68 morphotypes. Highest richness was predicted at seamount sites, and lowest richness in the western part of the study area. Combining the predicted probability of megafauna occurrences with bathymetric variables, two seamount habitats and two plain habitats could be defined. One of these megafauna plain habitats corresponds with contiguous nodule fields of high abundance that may be targeted for future mining, showing that prospective nodule fields have a clearly differentiated megafauna assemblage. Monitoring and management schemes, including the delineation of preservation and protection areas within contract areas, need to incorporate this geological and biological heterogeneity.