What impact will deep-sea mining have on midwater habitats?

Jessica Perelman and Jesse van der Grient for the DSM Observer

The ocean’s midwaters comprise the largest habitat on Earth and contribute significant ecosystem services including nutrient supply to surface waters and carbon transport, as well as support pelagic fisheries. But midwaters (generally defined as the pelagic ocean below 200 meters depth) also remain one of the planet’s least explored habitats. As the deep-sea mining industry advances, our understanding of its environmental impacts, especially on midwater ecosystems, remains poor. This month, we published “Midwater ecosystems must be considered when evaluating environmental risks of deep-sea mining.” The article explores the significance and complexities of midwater ecosystems, how they may be influenced by deep-sea mining, and calls for the consideration of these influences in ongoing ecosystem assessments and developing mining regulations.

Benthic ecosystems have been the focus of mining-impact research since its beginnings. Ecosystem assessments in polymetallic nodule fields, hydrothermal vent fields, and the cobalt-rich crusts of seamounts prioritized seafloor habitats where direct effects from resource removal will occur. These studies focused on characterizing the abundance, diversity, and resilience of species and communities living on the seafloor in and around these areas, and provide valuable baseline data to inform management of mining activities.

Several seafloor disturbance experiments were conducted throughout the 1970s and 1980s, including the Disturbance and Recolonization Experiment (DISCOL), which occurred in 1989 in the Peru Basin of the South Pacific Ocean. Ecosystem baseline information and simulated mining studies such as the DISCOL are vital for advancing our understanding of environmental risks and informing the design of effective mining regulations to minimize harm.

But what about the resilience of deep water-column communities in the vicinity of mining? Sediment plumes and noise generated by mining activities will not be restricted to the seafloor, and baseline studies should fully assess the deep water column as well. While midwater habitats are incorporated into the recommendations for baseline studies by the International Seabed Authority for areas beyond national jurisdiction, management regulations and guidelines can be strengthened to minimize the vulnerability of these ecosystems to deep-sea mining.

The Mining Process

While deep-sea mining strategies vary among contractors and ore deposits, several aspects of the process are similar. Collectors at the seafloor will extract mineral resources, compacting and disturbing sediments (and the benthic communities they harbor). The collected minerals will be lifted to support ships via riser pipes, which will increase noise levels throughout the water column. Sediments brought to the surface along with the crushed ore will be discharged back into the water column. These sediment plumes may extend 10-100 km in length, though more modelling and experimentation is needed to better understand plume dynamics and extent. Discharge will increase pelagic sediment concentrations above background levels. As such, these plumes will have several direct effects on oceanographic conditions by influencing light levels and oxygen concentration, among other conditions. This discharge has the potential to impact midwater ecosystems and their communities.

Risks to Midwater Organisms?

By changing the physical and chemical environment, mining-related sediment plumes pose physiological risks to midwater organisms, including fish (e.g. lanternfish and bristlemouth, but also tuna and swordfish), sharks (e.g. cookiecutter shark), gelatinous animals (e.g. siphonophores, salps and comb jellies), and squid. Discharged sediments could suffocate these animals by damaging feeding structures and clogging respiratory organs. Metal-laden sediments may be toxic to a wide range of deep pelagic animals, yet very little is understood about the toxicity of these metals on deep-sea communities, and particularly the influence of pressure on acuteness of this toxicity.

Mining may have behavioral impacts on midwater communities, too. Underwater noise generated by mining operations could disrupt communication and feeding activities of many animals, including marine mammals. Some of these noise-sensitive animals are known to migrate through the Clarion-Clipperton Zone, and many dive to forage in deep pelagic waters. Increased turbidity from sentiment plumes could impact communication between many fishes and invertebrates that rely on bioluminescence to find mates, avoid predation, or attract prey.

Daily vertical migration— the ‘largest migration on earth’—occurs nightly as innumerable small midwater fishes and invertebrates, as well as smaller zooplankton, ascend from depths between 200 and 1000 meters or more to feed in surface waters. Sediment discharge to depths within these ranges has the potential to alter migratory patterns, resulting in the dispersal or reduction of these dense aggregations.

Essential Ecosystem Services

Midwater communities provide essential ecosystem services that mining could alter. Micronekton and zooplankton play a significant role in open-ocean food webs, serving as a forage base for top predators like marine mammals, tuna, and billfishes, and consequently sustain pelagic fisheries. They form a vertical food web linking surface organisms and deeper residents, and their vertical migrations allow for carbon and energy transfer to the ocean depths. Changes to populations or the migratory patterns of micronekton and zooplankton could readily influence larger ecosystem processes.

Vertically migrating communities also play a significant role in the biological pump, the processes that transport carbon and nutrients between the surface and the deep ocean. This system stores carbon in the deep ocean, isolated from the atmosphere, for years. By consuming prey in surface waters and respiring and excreting waste at depth, vertically migrating communities can mediate downward ocean carbon transport, enhancing atmospheric CO2 removal.

In low-oxygen regions of the ocean, such as the Eastern Tropical Pacific Oxygen Minimum Zone, greater proportions of midwater communities have been observed to vertically migrate. This zone overlaps with a large portion of the Clarion-Clipperton Zone, where the majority of all deep-sea mining exploration leases are located. It is possible that vertical migration by midwater micronekton and zooplankton in this massive area could play an even greater role in carbon transport and food web connectivity than in higher-oxygen regions.

There is a clear and urgent need to generate ecological baseline records for midwater ecosystems in targeted mining regions, but that is just the first step. Research must be conducted to increase our understanding about the biodiversity, species composition, behavioral dynamics, and significance of midwater communities to ecosystem services. Scientists agree that the release of dewatering plumes from mining operations should avoid depths down to 1,500-2,000 meters to minimize disturbance of ecosystem services in the upper ~1,000 meters, and that noise-inducing activities should avoid the deep sound channel (~700-1300 meters depth) which can propagate sound thousands of kilometers. In order to achieve these mitigation measures, mining regulations can be strengthened, and contractors should develop technologies and practices that aim to reduce risks to midwater ecosystems.

As the largest habitat on Earth, midwater ecosystems harbor immense biomass and biodiversity and contribute to crucial ecosystem services and functions. In light of the essential functions of midwater communities, we must fully consider them in our evaluation of the risks that deep-sea mining could present. What remains uncertain is how the pressures of deep-sea mining will affect the resilience of these communities. Ultimately, the ability of an environment to recover from disturbance depends on the scale of that disturbance and the impacted populations, and this question may be further addressed as mining technologies and strategies continue to develop. With a precautionary approach to management and enhanced midwater research efforts, it is possible to minimize environmental harm from deep-sea mining to these significant habitats.

Featured image: Midwater animal biodiversity. Squid, fish, shrimp, copepods, medusa, filter feeding jellies and marine worms are among the midwater creatures that could be affected by deep sea mining. Photos © by E. Goetze, K. Peijnenburg, D. Perrine, Hawaii Seafood Council (B. Takenaka, J. Kaneko), S. Haddock, J. Drazen, B. Robison, DEEPEND (Danté Fenolio) and MBARI.