1.1 Planet Ocean
We call our planet ‘Earth’, but 70 per cent of the Earth is covered by oceans, with oceans constituting more than 95 per cent of Earth’s living space. Fifty per cent of these oceans are below 3000 meter (m) depth and the average ocean depth is 3800 m. The largest biome on Earth is, thus, composed by deep marine ecosystems of about 1 billion km3 of deep water and 326 million km2 of deep seafloor. The deep sea is considered to start at 200 m depth, where solar energy cannot support primary productivity through photosynthesis. This depth changes regionally depending on water turbidity, but it often coincides with the shelf break where the seafloor transitions to the continental slope and is marked by a significant increase of the slope angle.1 Although large-scale bathymetry (depth topographic maps) exists for the whole ocean floor, deep-sea ecosystems are still of the least explored on Earth, with less than 0,0001% physically sampled or visually observed.2 In the last 170 years, twenty two new deep-sea habitats and associated fauna have been discovered. The deep seafloor, long believed to be a featureless and stable environment, has been shown to support one of the highest biodiversities in the planet in a wide array of interconnected habitats.3 These ecosystems sustain important functions and derived ecosystem services, spanning from nutrient regeneration and carbon sequestration to biological and mineral resources, not to mention cultural and educational services,4 many of which are key to the health of the planet. Deep-sea research is rapidly progressing in parallel to technological development, in parallel to an increase in the exploration for and exploitation of deep-sea resources.5 However, the limited understanding of the composition, diversity and functioning of many deep-sea ecosystems restricts our capacity to develop robust ecosystem-based management measures that are necessary if we are to balance resource use and ecosystem conservation.6
1.2 Aim and Structure
The aim of this chapter is to provide a general overview of deep-sea ecosystems, their faunal composition and the key functions and services that they provide. This will allow for a better understanding of the current and potential impacts derived from an increasing number of human activities and indirect stressors.
The first part of the chapter (Section 2) briefly describes the habitat, general biological features and key ecosystem functions of the main deep-sea ecosystems (Figure 2.1), from the shelf break to the deepest trenches. Starting from the shelf break (ca. 250 m deep), where the deep sea starts, the key ecological aspects of the different habitats found on continental margins are discussed, highlighting the heterogeneity of a system that was before supposed to be featureless and poor in life. The chapter then describes the vast abyssal plains that support a very high biodiversity of small fauna and the rich underwater mountains, or seamounts, where filter feeders such as corals and sponges thrive. The composition and functioning of hydrothermally active and inactive habitats of the mid-ocean ridges, back-arc basins and some active seamounts are explained, finishing with a short introduction to the deepest habitats on Earth, the hadal trenches.
The chapter then briefly introduces the main human activities that affect, directly or indirectly, deep-sea habitats and their ecosystems (Section 3). These anthropogenic impacts are grouped into two main categories: waste dumping and resource exploitation. Climate change, although a critical issue globally that affects the oceans as a whole, including deep-sea ecosystems,7 has not been included in these discussions. However, the importance of cumulative impacts on deep-sea ecosystems and the role played by climate change is discussed in the last paragraph.
2 Deep-Sea Ecosystems
2.1 A History of Exploration
The development of deep-sea research as a science is associated with the development of new techniques of navigation, sampling and measuring, and follows the path of great oceanic expeditions. It was during the last two centuries that these expeditions obtained the first scientific results, which would fascinate and inspire a whole new branch of oceanographers. The cruise of H.M.S Beacon to the Aegean (1841–1842) could be considered one of the first biological deep-sea cruises. Prof. Edward Forbes, from Edinburgh University, joined the ship as a naturalist and made around 100 dredge hauls down to a depth of 420 m. As the Beacon crew dredged deeper, fewer species were found, leading to Forbes’ ‘Azoic Theory’8 where he proposed that no life existed at great depths. However, the British admiral Sir John Ross had already collected a deep-sea echinoderm while dredging at 1460 m during his exploration for the Northwest Passage in 1818.9 Later, the Norwegian Michael Sars (1850) published a list with 19 species from waters deeper than 550 m, and his son, George Ossian Sars extended the list to 92 species. With evidence accumulating of a diverse deep-water fauna, C.W. Thomson and W.B. Carpenter encouraged the Royal Society and the Admiralty to organise a deep-sea expedition, resulting in the H.M.S. Lightning cruise in 1868 to the NE Atlantic, and the H.M.S. Porcupine cruise (1869) to NE Atlantic and Mediterranean Sea (Rice 1986). With the important discoveries of the Lightning and Porcupine, W.B. Carpenter’s application for a scientific circumnavigation expedition was accepted in April 1872. H.M.S. Challenger set sail from Sheerness on December 7th 1872 for her three and a half years cruise with C.W. Thomson as chief scientist. The Challenger expedition was set up to study the physical, chemical and biological processes in the deep ocean. This circumglobal oceanographic voyage has been considered by many to be the true birth of modern oceanography.10
The Challenger expedition was followed by an era of pioneering deep-sea research, involving numerous ships from several countries. But it was in the 1960s and 1970s, that an important change in the approach of deep-sea biological research took place. Descriptive biology was complemented with a more ecological, evolutionary and experimental approach, led by North American researchers such as Profs. Hessler, Sanders and Grassle.11 However, the conquest of the oceans would not have been complete if humans had not developed the ways of entering the deep-sea environment, to observe, explore and experiment in situ. Therefore, parallel to the remarkable developments in navigation and oceanographic technologies, there is the history of diving, deeper and longer. Beebe’s Bathysphere in 1930 was the first deep-water vehicle for observation of the seabed. From there, in little more than 50 years, the advances in deep-sea technology have led to a variety of novel instruments. Some of these include maned submersibles, Remote Operated Vehicles (ROV), Autonomous Underwater Vehicles (AUVs), new chemical and physical sensors and cabled observatories (Figure 2.2). These instruments are used in combination with other sampling gear, such as multicorers, boxcorers, trawls and sledges to sample benthic fauna; plankton nets with several opening and closing mechanisms; Conductivity-Temperature-Depth devices (CTDs) that measure conductivity, temperature and depth; multibeam echosounders and sidescan sonars to map the seafloor, etc. The use and continuous development of these technologies provides a wealth of novel information on the composition, structure and functioning of deep-sea ecosystems.12 This comprehensive knowledge is essential for the development of robust management and conservation measures to be applied to deep-sea ecosystems. Below, the main characteristics of the major deep-sea habitats and their communities are briefly considered, to set the scene for the discussion on anthropogenic impacts upon these ecosystems.
2.2 Continental Margins
The continental shelf expands from the coastline to the shelf break and is an area of relatively shallow water, mostly less than 250 m, with the exception of some large shelves like the Norwegian shelf, that has depth down to 500 m. The start of the deep sea is often considered to coincide with the shelf break, from which the continental margin descends along a slope from about 250 m to 3000 m depth (Figure 2.1). The continental margins cover about 11% of the ocean floor (ca. 40 million km2) and can be passive or active. Passive margins are found where an ocean rift has split a continent in two, generating an ocean basin in between, while active margins are found where the ocean floor is so dense that it sinks back into the Earth forming trenches along subduction zones.13 Continental margins are characterised by high habitat heterogeneity, including sedimentary slopes, submarine canyons, cold-water corals, cold seeps, mud volcanoes, pockmarks and oxygen minimum zones.14 These habitats support a variety of faunal communities that support a wide array of functions.
2.2.1 Sedimentary Slopes
Sedimentary slopes are often characterised by high biodiversity of small meiofauna (organisms retained on a 32 micron sieve, such as nematodes) and macrofauna (organisms retained on a 0.3 to 0.5 millimetre sieve, mostly small crustaceans and polychaete worms). This infauna (organisms that live in the surface layers of the sediment), together with the microorganisms in the seafloor, play a key role in the biological pump, where carbon fixed by shallow-water organisms through photosynthesis and subsequently falling to the seafloor is remineralised and carbon and nutrients that are upwelled fuel again primary productivity in the surface layers. The margin megafauna (animals identifiable from seafloor videos and photos) are often dominated by echinoderms and crustaceans as well as fish, depending on the region (Figure 2.3A). Some of these groups include valuable commercial species and, thus, sedimentary slopes are subjected to increasingly intense fisheries in certain regions of the world.15
2.2.2 Submarine Canyons
Submarine canyons are large geomorphological features covering 11.2% of continental margins globally.16 The topography of canyons intercepts regional hydrographic patterns resulting in modified local currents that trap particles. Canyons thus act as conduits for particles from the fertile coast and shelves to the deep basins, fueling the deep faunal communities.17 Canyons provide also a variety of habitats that support diverse faunal types. The canyon head and walls are characterised by rocky outcrops that provide substratum for filter feeders such as crinoids, gorgonians or corals that use the currents in the canyon to filter seawater and capture food (Figure 2.3B). The axis of the canyon is filled with fine sediment that support rich benthic communities like the ones found on the sedimentary slopes. Canyons have been described as ‘essential habitats’18 because they can provide refuge and habitat for spawning species and juveniles, as well as feeding grounds for certain species.19 The rough topography of canyons has limited fisheries, but technological developments are opening new fishing grounds in areas that were before difficult to access. Additionally, the modified currents in canyons enhance the transportation of chemical pollutants and litter that can accumulate at the base of the canyon.
2.2.3 Cold Water Corals
Cold-water corals are found at temperatures ranging from 4 to 13 °C and depths between 50 and 6000 m depths.20 Most of the reef-forming cold-water corals, such as the Lophelia pertusa reefs in the NE Atlantic, are found on the upper part of the continental slope and on seamounts. Reef-forming corals are estimated to cover an area of ca. 280 000 km2 worldwide. The 3-dimensional structure of cold-water corals can form long-lived reefs or gardens, providing habitat and refuge to a large variety of organisms, both in the adult and juvenile stages. These ecosystems support a high biodiversity and high biomass along continental margins (Figure 2.3C). Extensive damage on cold-water corals from trawling has occurred, resulting in highly productive systems being transformed into coral rubble. The recovery of damaged cold-water corals is likely to be slow (decades to centuries) and when the habitat has been altered and the corals eliminated, recovery is unlikely.21
2.2.4 Cold Seeps
Cold seeps are found both at active and inactive margins and the estimated global area is 10 000 km2. These habitats are characterised by the cold seepage of fluid with high concentrations of methane and hydrogen sulphide. These reduced chemicals are used by microorganisms as source of energy to produce organic matter, in a process called chemosynthesis. Chemosynthetic-based ecosystems, such as cold-seeps or hydrothermal vents, are the only communities in the deep-ocean where the faunal communities are supported by in situ primary productivity. But here, this productivity is based on chemical energy instead of solar energy used by plants in the sunlit zone. These chemoautotrophic microorganims in cold seeps are found both free living and in symbiosis with benthic fauna.22 The primary productivity at cold seeps supports communities of relatively low biodiversity but high biomass of highly specialised fauna. Some of the key organisms often found at cold seeps include bivalves, gastropods, siboglinid tubeworms, decapod crustaceans and cladorhizid sponges23 (Figure 2.3D).
2.3 Abyssal Plains
Abyssal plains are vast regions of relatively flat seafloor extending from 3000 to 6000 m depth (Figure 2.1), covered by a layer of fine sediment that can reach thousands of meters in thickness. The abyssal plains cover a total area of 245 million km2, about 75% of the deep seafloor, representing one of the largest ecosystems on Earth. Their vastness and remoteness makes abyssal plains one of the least explored regions of the oceans.24 As for the rest of the deep-sea fauna, excluding chemosynthetically-based ecosystems, the lack of light to fuel photosynthesis results in the abyssal fauna being heterotrophic. This means that the organisms rely fully on the arrival of organic matter from the surface layers, falling as ‘marine snow’ through the water column or advected along the margin. Abyssal plains are thus often food limited,25 but these habitats support one of the highest biodiversities on Earth. This high biodiversity is mostly composed of small organisms, from microbes to meiofauna and macrofauna.26 Abyssal plains are subjected to relative extreme ecosystem parameters, including very high pressures (1 atmosphere for each 10 m depth), low temperatures (about 2 °C), usually very slow bottom currents and usually very low annual organic matter input.27 The quantity and quality of this flux of organic matter varies seasonally depending on the geographic region and the productivity of the surface oceanic layers. Thus, ecosystem composition, structure and function vary regionally at abyssal plains. A major characteristic of abyssal fauna is that rare is common. This means that most organisms collected from abyssal depths have been recorded as a few individuals (typically less than 5) from one or two sampling sites.28 Technological development has greatly increased our sampling activity, providing a wealth of samples with a high number of species new to science, most of them represented by small, single individuals. The rate at which potentially new species are being collected together with the decrease in expert taxonomists (specialists in species identification and naming) have led to what has been termed ‘taxonomic impediment’.29 This results in a significant delay between the discovery of a new species (when it is collected and identified as new) and the scientific description of the species (when it is given a name and published, thus becoming available).30 Addressing this issue is thus essential if we are to obtain a thorough understanding of abyssal community composition, structure and function.
Although remote, abyssal plains are subjected to different environmental stressors. In particular, some abyssal plains (e.g. Pacific Ocean) include important mineral resources in the form of polymetallic manganese nodules (see below Section 3.5.1) which are currently under exploration licenses. Thus, improving scientific understanding of the structure and function of these ecosystems at the local and regional scales is essential prior to the signature of exploitation contracts. Climate change will also have an impact on abyssal faunal word wide, mainly related to changes in organic matter fluxes caused by changes in surface primary productivity, as well as potential water column stratification and changes in global circulation.31
Seamounts and knolls are underwater mountains rising from 100 to over 1000 m from the surrounding seafloor (Figure 2.1). The number of seamounts and knolls has been estimated to be ca. 100 000, covering an area of 8.5 million km2, which represents 2.6% of the seafloor.32 However, the biological communities of only less than 300 seamounts have been studied with enough detail to provide a thorough description of their composition, let alone functioning. The topography of seamounts modifies locally the prevailing currents and results in the retention of particles above the seamount, providing an enhanced food supply to the seamount fauna. The available rocky substratum, elevation from the seafloor and modified hydrography of seamounts support high abundances and biomass of often distinct faunal communities.33 The dominant fauna includes sessile, filter-feeder organisms such as corals and sponges, which in turn provide habitat for a variety of other species, such as fish, echinoderms and crustaceans. Seamounts have often been described as isolated habitats supporting hot spots of species richness with high degrees of endemism. However, knowledge is still scarce and recent evidence does not support these widely accepted paradigms.34 They are also proposed to serve as stepping stones for dispersal of species across the abyssal plains. The high abundance of commercially-valuable fishes that may aggregate over seamounts has attracted industrial interest to these distinctive topographic habitats, with, in some cases, devastating impacts on the sessile fauna and the long-lived populations of target fish (see below Section 3.3).
2.5 Mid-Ocean Ridges and Hydrothermal Vents
2.5.1 Mid-Ocean Ridges
Mid-ocean ridges form a 65 000 km long, semi-continuous, linear range of volcanic mountains where new oceanic crust is being formed and hydrothermal vents are found (Figure 2.1). Mid-ocean ridges support a wealth of habitats, from rocky substratum that includes hills and seamounts to deep axial valleys that can reach 4000 m depth and are covered with fine sediment.35 The rocky seafloor supports communities dominated by filter feeders such as crinoids, sponges, corals and gorgonians and attracts motile fauna such as fish, galatheid crustaceans and cephalopods. This fauna contrast with the sediment communities, which are like those found in abyssal plains.36
2.5.2 Hydrothermal Vents
Hydrothermal vents and their associated fauna, discovered in 1977 in the Galapagos Rift (Pacific Ocean), are one of the major discoveries of the last decades.37 A total of ca. 2000 vents has been estimated to occur globally,38 although recent models have suggested a number 3 to 6 times higher.39 Vents are found on mid-ocean ridges and back-arc basins where cold oxygenated deep seawater penetrates through the cracks of the ocean crust and reacts with the hot rock close to the magma chamber underlying the ridge. During this process, the fluids can exceed 350 °C, dissolving metals and sulphur from the rocks. The heated fluid rises back to the surface of the seafloor and, when it mixes with the cold oxygenated water, the dissolved metals and sulphides precipitate, appearing as black smokers. The deposition of these particles forms the vent chimneys and can accumulate as massive sulphide deposits. Hydrothermal vents support unique faunal communities based on chemosynthetic primary productivity. As in cold seeps (see Section 2.2.4), chemoautotrophic microbes use the reduced chemicals (e.g. hydrogen sulphide) from the vent fluid as source of energy to produce organic matter.40 These microorganisms are found free living forming bacterial mats over the vent chimneys, but also in tight symbiosis with benthic fauna. The availability of primary productivity on the seafloor supports high abundance and biomass of highly specialised megafauna communities. At the same time, the extreme environmental conditions found at vents (high temperature gradients, high levels of toxic chemicals, dynamism of vents) result in a low biodiversity with a high proportion of endemic species41 (see Figure 2.4). The deposition of metals from the vent fluids can result in large accumulations of commercially-interesting minerals, in what is known as seafloor massive sulphide deposits (see Section 3.5.3.).
The trenches are the deepest areas of the seafloor, extending from 6000 m to 11 km, in what is known as the hadal zone (Figure 2.1). The deepest point on Earth is in the Marianas Trench, in the western Pacific, with a maximum-recorded depth of 11 033 m in the Challenger Deep. There are 33 trenches around the world, covering an area of 0.2% of the seafloor.42 Trenches are covered with fine sediment and their main characteristic is the very high hydrostratic pressure (600 to 1100 atmospheres), while temperature and oxygen variables are similar to those found on abyssal plains. The trench macro- and megafauna communities are composed by diverse fauna with a high degree of endemism, including hadal fish, large amphipods, shrimp, polychaetes, bivalves and holothurians.43 The smaller faunal fraction, the meiofauna (32–63 microns) is dominated by soft-bodied foraminifera.44
3 Anthropogenic Impacts to the Deep Seafloor
Technological development in the last half century has facilitated access to deep-sea ecosystems. This has provided evidence of a wealth of undiscovered biodiversity and ecosystem functions as well as important resources, both mineral (hydrocarbons, minerals) and biological (fisheries, genetic resources). Interest in the exploration for and exploitation of these resources is rapidly increasing, paralleling the increasing demand for raw materials and the depletion of resources on land and in the coastal area.45 Additionally, the remoteness of the deep seafloor has promoted for centuries the disposal of waste and, even under the current restrictive regulations on dumping waste in the seas and oceans, the issue of marine litter continues to increase. Below, we briefly describe the major activities that can have a significant impact on deep-sea ecosystems.
3.1 Marine Litter
Marine litter is defined by the United Nations Environmental Programme (UNEP) as ‘any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment’. Although dumping litter in the sea was banned by the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972 (known as the London Convention) and subsequent London Protocol (1996), litter continues to be a major threat to marine ecosystems. Major sources of marine litter are heavily populated coastal areas and rivers, as well as illegal dumping from boats.46 About 6,4 Million tonnes of litter have been reported to enter the oceans each year.47 Litter can float in the surface or water column, eventually sinking and accumulating on the seafloor. Plastics are the most abundant litter type observed on the deep seabed, followed by metal and glass48 (Figure 2.5). The impacts of marine litter on the benthic fauna have not been studied in detail, but effects such as suffocation, entanglement, physical damage, ghost fishing of discarded/lost nets and chemical pollution from decomposing materials (e.g. plastics additives, microplastics, paints) have been suggested as major issues requiring further investigation.
3.2 Submarine Tailing Disposal
Tailings are the fine waste produced by mining activities after extraction of the target metals from the ore. Most industrial mines dispose the vast amounts of tailing waste in land-based dams. However, in countries were the topography or climate do not allow for safe management of dams (e.g. Norway, Indonesia, Papua New Guinea), the disposal of tailings in the sea is used as a suitable option. There are currently two main types of tailing disposal in the sea.49 In submarine tailing disposal (STD), tailings are disposed through an underwater pipeline at relatively shallow depths (<100 m). Tailings create a gravity flow that deposits the waste on the seafloor. In deep-seat tailing disposal (DSTD), tailings are disposed via a submerged pipeline below the mixing zone (>100 m). The tailings create a gravity flow that deposits the waste on the deep seafloor below 1000 m depth.
The main impacts of STDs and DSTDs, reviewed in50 include: 1) smothering of the benthic communities by hyper-sedimentation at the local scale; 2) potential toxic effects from heavy metals or added chemicals (flocculants, floatation); 3) impact of changes in grain, which can modify the organic content in the sediment, and grain structure, with some tailing particles having very sharp edges that can physically damage feeding structures or the settlement of larvae/juveniles; and 4) plume dispersal, upwelling and slope failure, which can re-distribute tailings far from the original settling area, thus affecting communities at the regional scale.
Acknowledging the urgent need for further research and robust management measures, the International Maritime Organisation (IMO), together with the Deep-Ocean Stewardship Initiative (DOSI), the International Network for Scientific Investigations of the Deep Sea (INDEEP) and the Norwegian Research Council (NRC) funded-project MITE-DEEP, co-organised a workshop to discuss current knowledge on DSTD processes and environmental impacts. The discussions and conclusions have been synthesised in a report to be discussed by the parties of the London Convention/London Protocol for future action.51 In parallel, the European Commission is in the process of updating the first ‘Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities’. The revised ‘Best Available Techniques Reference Document for the Management of Waste from Extractive Industries’ (MWEI BREF) is subject to the EU Directive on the management of waste from extractive industries (2006/21/EC) and has been published in December 2018.52
Increased demand for marine biological resources and technological development have promoted the continuous increase of deep-sea fisheries exploitation,53 with fishing grounds commonly found below 1500 m depth. Bottom trawling (i.e. the towing of a trawl net along the seafloor) has the highest impact, both on the fauna (target and non-target species) and the habitat. Deep-sea target species are often long-lived and have delayed maturity, so the exploitation of such communities, which depletes the population of reproductively-active adults, has rarely proven sustainable.54 The trawling gear has also a major impact on the seafloor and the benthic communities. In sedimentary slopes, where most trawling takes place, recent studies in the Mediterranean have shown that the regular trawling of the seabed triggers sediment flows downslope, with unknown effects on the faunal communities.55 Furthermore, long-term trawling activity in a region can modify the shape of the submarine landscape, reducing the original complexity of the seafloor in the same way that agriculture does on land.56 Fishing over seamounts has resulted in significant impacts, again on the target species and the ecosystem.57 Seamounts are characterised by rich communities of sessile fauna, including sponges and corals that provide habitat to other fauna (see Section 2.3). These communities are heavily impacted by bottom trawling and their recovery is in the order of decades or centuries (see Section 2.2.3).
Fishing regulations are implemented by coastal states and RFMOs (Regional Fisheries Management Organisations). These regulations may include quota managements, licensing systems and protection of specific habitats (e.g. seamounts, ridges, cold-water corals, upper slope) by closing the areas to fishing activities.58 In recent years, certain RFMOs have extended their regulations to the protection of vulnerable benthic marine ecosystems in international waters.59
3.4 Oil and Gas Exploitation
The decrease in land-based resources and developing technology has promoted the increase of oil and gas exploitations in deep waters, with routine drilling below 200 m depth in many regions. In well explored areas, such as the Gulf of Mexico, ultra-deep water drilling (>1000 m depth) activities, which reach 3000 m depth, are expanding.60 Impact of oil and gas exploitation can come from various activities related to offshore oil and gas development. Some of the major direct impacts are relatively local, including the physical damage to the benthic habitat and community caused by the installation of the drilling infrastructure (ca. 100 m radius), and the discharge of drilling muds and produced water that can affect benthic communities at distances of about 300 m from the source.61 Effects of drill muds on all size classes of the benthic community (meio-, macro- and megafauna) include changes in density, biomass and diversity, but little is known on the effects on the microbial community. These potential effects of oil and gas exploitation activities are particularly important in operations close to cold-water corals, where coral polyps mortality can increase by burial from thin layers (6.5 mm) of drill cuttings.62 Additionally, large environmental impacts can occur during accidental oil spills, such as the Deepwater Horizon blowout accident of the Macondo well in the Gulf of Mexico in 2010.63 Impacts to the deep benthic fauna have been detected in an area of 300 km2, with significant impacts to the cold-water coral communities 22 km away from the well and at depths of 1950 m.64
Although experiments of the toxic effects on deep-sea fauna of chemical dispersants used during oil spills are limited, there is evidence that dispersants can affect larval development, cause tissue degradation in invertebrates and damage cold-water corals. Management of oil and gas exploration and exploitation licenses falls under national regulations and should include activity management, where some processes or technologies are restricted, temporal management, where temporal variations in feeding, breeding or migration of key species is considered, and spatial management, where exploitation may be restricted in relation to the proximity of sensitive species or habitats.65
3.5 Deep-Sea Mining
The commercial exploitation of deep-sea mineral resources has not started yet. However, in the last 15 years, interest in exploration for and exploitation of these resources has greatly increased Currently, there are four major resource types that are being considered for commercial exploitation from habitats deeper than 200 m depth: manganese nodules, cobalt-rich crusts, seafloor massive sulphides and phosphorite nodules. Each of these resources is found in a specific habitat with particular geochemical and biological characteristics, which will define the significance of the mining impact and the ecosystem recovery potential.66 Below, we briefly describe each of these mineral resources, their associated ecosystems and main expected impacts and recovery potential form mining activities.
3.5.1 Manganese Nodules
Manganese nodules are polymetallic concretions made of manganese and iron sulphides which form by precipitation from the ambient sea-water over millions of years.67 Manganese nodules are rich in manganese, copper, cobalt and nickel and are found on abyssal plains, particularly in the Pacific Ocean. The sediments support rich communities of meio- and macrofauna, with larger animals such as holothurians, sea urchins, sea stars, polycahetes and octocorals also present, but in lower abundance. The nodules are colonised by large single-celled foraminifera.68 The processes in these abyssal plains are very slow, with very slow sedimentation rates and very weak bottom currents. Additionally, nodules are formed at geological-time scales. Thus, the recovery and recolonization of these ecosystems will be extremely slow and not at the ecological time-scales that mining-licenses will operate, making robust spatial management plans more valuable than possible restoration measures.69
3.5.2 Cobalt-Rich Ferromanganese Crusts
Cobalt-rich ferromanganese crusts form by precipitation from the seawater over millions of years over all rocky surfaces free of sediment in the deep oceans. Potentially exploitable crusts are found on the flanks of seamounts, knolls and ridges at depths of 800–2500 m.70 These crusts are rich in cobalt, nickel and platinum. Although little is known of the fauna specifically on cobalt-rich crusts (in comparison to that of seamounts), these geomorphological structures provide substrate for a variety of sessile filter feeders, such as corals and sponges, and other motile fauna including crustaceans and echinoderms.
3.5.3 Seafloor Massive Sulphides
Seafloor massive sulphides (SMS) form through the precipitation of metals from the fluids at hydrothermal vents, typically at depths between 1000 and 3000 m. SMS are sources of copper, gold, silver, zinc and lead.71 Vent communities are characterised by very high abundances and biomass of highly adapted species, with a high degree of endemism, supported by microbial chemoautotrophy (see Section 2.4.2).72 These systems are very dynamic and subjected to sporadic volcanic eruptions, particularly in fast-spreading ridges, as well as changes in the activity of individual chimneys and sources of diffuse flow. There are two scientifically documented cases where naturally impacted vent communities from volcanic eruptions recovered one decade after the eruption.73 However, these processes took place in fast-spreading ridges, while the major SMS identified to date are on slow-spreading ridges, which are much less dynamic systems. The recovery of such ecosystems from mining depends, thus, on the habitat itself, as well as on the availability of larvae, juveniles or mobile adults from intact populations that are able to disperse to and colonise the new vents systems post-mining.74 However, mining will add on to the existing natural loss of critical habitat, and cumulative impacts may result in significant changes in the abundance and distribution of vent species.75 Because of the rarity of active hydrothermal vent systems, their unique fauna and the challenges of identifying representative systems for area-based management, it has been proposed that active hydrothermal vents are protected legally from direct and indirect mining impacts.76
3.5.4 Phosphorite Nodules
Phosphorite nodules are formed from limestone deposits following chemical reactions in areas with upwelling and high surface productivity on upper continental slopes (200–400 m). Phosphorite nodules contain products used to make phosphate fertiliser and they have recently been explored off New Zealand and Namibia. In these regions, the dominant fauna includes echinoderms, galatheid crabs, sponges, corals and bryozoans, and abundant amphipods in the sediment. However, the impacts of potential mining of the mineral resources on the upper continental margin have been little investigated.
3.5.5 Impacts of Deep-Sea Mining
The main impacts of deep-sea mining on the seafloor include the depletion or physical damage to the habitat and fauna by the mining equipment, changes in seafloor topography and geochemical characteristics, creation of sediment plumes and potential toxicity from metal and/or process chemicals release (Figure 2.6). Additionally, light and noise may be an issue for deep-water fauna and sediment plumes may impact pelagic life, including larvae and juveniles. These processes will affect the composition, structure and functioning of the faunal communities in different ways depending on the ecosystem considered. For example, mining manganese nodules at abyssal plains, where processes such as nodule formation and sedimentation are extremely slow (millennia), will have a very significant and long-lasting impact on the ecosystem.
With a new deep-sea mining industry emerging, regulatory bodies, both for areas within and beyond national jurisdiction, need to develop regulations and licenses where potential economic gains need to be balanced against impacts on the environment, other ocean users and civil society. The need to ensure the protection of the environment requires a robust scientific understanding of what can cause a significant adverse change to deep-sea biodiversity, ecosystem structure and function that will cause serious harm to the affected ecosystem.77 The International Seabed Authority (ISA) is responsible for the regulations and license contracts for exploration and exploitation of minerals on the seabed beyond national jurisdiction (The Area), under the principle that The Area and its mineral resources are ‘common heritage of mankind’.78 Within territorial waters, regulations are often lacking, but interested nations are currently developing such regulations, which, for parties of the UN Law of the Sea Convention, must be at least as restringing as the ISA regulations.
3.6 Cumulative Impacts
The deep ocean is experiencing increasing pressure from human activities targeting its resources or receiving and accumulating synthetic waste and chemical pollution. These different impacts may have synergies on single ecosystems if acting together, with a magnified effect on the structure and functioning of the faunal communities.79 In particular, climate change-related stressors such as warming water masses, de-oxygenation, changes in primary productivity and ocean stratification, can affect the oceans globally.80 These global climatic stressors will add to direct impacts from other human activities, such as fishing or mining (Figure 2.7), possibly reducing resilience and recovery potential of the affected ecosystems. Different extractive industries may also be in spatial conflict. For example, in New Zealand and Namibia, phosphorite nodule reserves on the upper continental margin coincide with existing fishing grounds.81 Based on the still limited scientific understanding of the composition and functioning of many deep-sea ecosystems, several stakeholders recommend the development of precautionary and ecosystem-based management systems. These measures should balance the use of mineral and biological resources with the maintenance of healthy marine systems and the ecosystems services they provide.82
ERLL was funded by the MarMine project (grant no. 247626/O30) and with support from the Norwegian Institute for Water Research, NIVA, Oslo.
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