David Billett (Deep Sea Environmental Solutions Ltd, UK), Daniel Jones (National Oceanography Centre, Southampton) and Phil Weaver (Seascape Consultants, UK)
The deep-sea mining industry is moving slowly towards the exploitation of minerals on the deep-sea floor. A number of test mining experiments are being planned in international waters and within the Exclusive Economic Zones of a few States. In addition, the UN International Seabed Authority (ISA) is in the process of formulating new Regulations for commercial mining, including issues relating to the protection of the marine environment. Consequently, there is a new focus on the restoration of marine ecosystems as part of Environmental Management, Monitoring and Closure Plans. Ecosystem restoration is a key element of environmental management approaches, such as the ‘Mitigation Hierarchy’ of Avoid → Minimise → Restore → Compensate (Billett, Jones and Weaver, 2019).
A number of recent publications have reviewed the importance of using the ‘Mitigation Hierarchy’ in deep-sea management (Niner et al., 2018; Cuvelier et al., 2018; Billett et al., 2019). In addition, the Cross-Sector Biodiversity Initiative (CSBI) (2015), which includes the International Council of Mining and Metals (ICMM), has provided practical guidance on the implementation of the mitigation hierarchy for extractive industries. While the examples given in the CSBI guidance document relate almost exclusively to land-based mining, the principles espoused for one part of the mining industry are clearly transferrable to all mining sectors.
Addressing the Mitigation Hierarchy in relation to deep-sea mining specifically (Billett et al. 2019):
Avoid
Many approaches are possible for avoiding mining impacts. This includes avoiding working in areas of conservation importance, avoiding mining practices that are particularly harmful, and avoiding mining at specific times, such as during breeding seasons (Cross Sector Biodiversity Initiative, 2015). These avoidance measures may be combined in a way that is beneficial for environmental management, but at the same time have little or no impact on the costs and planning of mining operations. Avoidance can operate at many scales, from the setting aside of large regional areas where no mining is permitted, to precluding mining in small areas where particular species may occur in high abundances.
Minimise
The nature of mining means that few impacts can be avoided altogether. Environmental management considerations then move to how specific impacts from deep-sea mining can be minimised; for instance, whether engineering solutions can limit the creation and spread of plumes during operations. Exploring how impacts can be minimised is a vital part of an Environmental Impact Assessment (EIA).
An EIA should document how approaches for minimising impacts have been considered and costed. This will include balancing the cost of introducing various engineering solutions against the benefits derived from the true value of the ecosystem services which might be affected. Unless regulators include ecosystem services, and the costs associated with their loss or impairment, as part of their decision-making, and as part of an ethical approach to ensuring the health of the oceans for the Common Heritage of Mankind, it is unlikely that business resource and engineering managers will be stimulated to devise technical solutions to reduce environmental harm.
Weaver and Billett (2019) detail the main impacts that will be caused by mining in the deep sea; namely 1) habitat destruction and/or modification, 2) the dispersal of sediment laden plumes generated by mining systems, 3) impacts related to plumes released in the water column after dewatering of the ores on the ship and transport barges and 4) impacts related to other factors such as sound and light. For seafloor massive sulphides, mining also has the potential to alter the natural conduits and flow rates of the hydrothermal fluids and to introduce toxic metals. The importance of these main impacts may change depending on the resource being considered and the ecosystems affected (Washburn et al., 2019).
Niner et al. (2018) suggest that the use of shrouds on collecting and cutting systems and the development of methods to reduce the creation of fine particulate materials might reduce the impacts of plumes both in terms of duration and spatial extent. The management of plumes is probably the single most important impact that could be solved through design innovations of mining vehicles and equipment. Methods for enhancing the flocculation of particles within plumes using non-toxic materials may also be possible (Cuvelier et al., 2018). For each mine site the engineering solutions introduced to minimise impacts will influence the time, and therefore the cost, required to monitor sites following mining. In addition, the time and costs of long-term monitoring may also be reduced by undertaking restoration activities following mine closure, as occurs in many cases on land (Cross Sector Biodiversity Initiative, 2015)
Restore
The practice of restoring ecosystems has advanced rapidly in recent years with the publication of “International Standards for the Practice of Ecological Restoration – Including Principles and Concepts” by the Society for Ecological Restoration (McDonald et al., 2016). While the Standards have been formulated mainly from experiences in restoring habitats on land they are broadly applicable to terrestrial, freshwater, coastal and marine environments. However, the restoration of marine ecosystems in a fashion analogous to the restoration of land-based mine sites is still in its infancy (Van Dover et al., 2014). There are considerable challenges in the restoration of marine ecosystems as a whole and especially in the deep sea at locations distant from land, at great ocean depths, and involving ecosystems with exceptionally slow recovery speeds.
The slow recovery rates of deep-sea ecosystems, and, in the case of nodule mining, the large areas over which mining will occur, oblige contractors and regulators to consider how elements of restoration should be included in Environmental Management and Monitoring Plans and Closure Plans. The scale of some mining impacts and the time required for recovery of ecosystems will be of particular societal concern (Billett et al. 2019).
Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed and seeks to ‘assist recovery’ of a natural or semi-natural ecosystem (McDonald et al., 2016). Restoration therefore has a focus on helping nature to recover naturally, but perhaps at a faster rate. While the aim might be to re-establish an ecosystem to the state of biodiversity and functioning that existed prior to its impairment, ecosystems are always in a state of flux and so restoration really aspires to reinstate natural ecological progression (Aronson et al., 2016). This may require introducing active methods, such as the transplantation of fauna or the use of artificial reefs to kick-start or speed up recovery. Billett et al. (2019) consider these specific restoration actions that might be undertaken in relation to 1) cobalt crust mining and restoring ecosystems on seamounts, 2) polymetallic sulphide mining and restoring ecosystems on mid-ocean ridges, and 3) polymetallic nodule mining and restoring ecosystems on abyssal plains.
Compensate
Offsetting and compensation for biodiversity and habitat loss through man’s impacts is the last consideration of the Mitigation Hierarchy, once all approaches in avoiding and minimising, and then restoring ecosystems have been exhausted. Niner et al. (2018) identified several ways in which compensation is achieved in other environments. “Like for like” compensation might include the protection of a similar deep-sea habitat to that mined. “Like for like” habitat restoration might also include the creation or restoration of new, additional and equivalent biodiversity of a similar type in a different location. “Out of kind” habitat restoration to create new biodiversity of a different type, such as in shallow water, might also be considered. Compensation might also be achieved through capacity building. Niner et al. (2018) argue that all of these strategies have major barriers to application in the deep sea. For example, in the deep sea like-for-like offsetting may only be possible in relation to seamounts and cobalt crust mining, where the restoration of seamount ecosystems impacted by bottom trawling in the past might be considered.
Conclusion
All the possible active methods of ecosystem restoration require rigorous testing and experimentation in order to avoid unintended consequences and to test their practicality and cost effectiveness. It is surprising therefore that very little attention has been given to identifying potential restoration measures in the deep sea. Owing to the time required to measure significant changes between restoration actions and natural processes, experiments are required to be organised sooner than later during exploration for minerals, and test mining, in order to be included in Environmental Impact Statements. Good, cost effective and simple restoration actions have the potential to lead to considerable cost savings for contractors over the life time of a mining project and will be important for the maintenance of vital ecosystem services in the deep sea.
Figure 1. The benthopelagic holothurian Peniagone leanderfeeding on the seabed among polymetallic nodules in the Clarion Clipperton Fracture Zone.
Figure 2. A large sponge on the flanks of a seamount on the Mid Atlantic Ridge at a depth of about 800 m
Figure 3. An acorn worm (Hemichordata, Enteropneusta) at a depth of about 2500m on the Mid Atlantic Ridge.
References
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