are inadequate to test whale falls as chemosynthetic stepping-stones using network analysis: faunal overlaps do support a stepping-stone role.

Whale bones and wood on the deep-sea floor provide resource pulses that support characteristic faunal assemblages in an otherwise food-poor environment. To isolate the role of bathymetric and geographical drivers of organic-fall diversity and community structure, the study of organic-rich substrates of similar sizes, qualities, and seafloor durations is necessary. We used a comparative experimental approach to examine the roles of depth, location, and substrate type in structuring organic-fall faunal assemblages. Four free-vehicle landers containing replicate wood, whale-bone, and inorganic hard (control) substrates were deployed for 15 mo at depths of ~1600 and ~2800 m, spaced at ~400 km along the Washington-Oregon (USA) margin. The landers collected a total of ~84890 macrofaunal individuals of 144 species. Wood, bone, and control substrates supported assemblages with different community structures on all landers. Community composition was significantly different between depths and between locations at similar depths, indicating variability on regional and bathymetric scales. Wood blocks at ~1600 m were heavily degraded by wood-boring xylophagaid bivalves. Xylophagaid colonisation was lower in deeper wood blocks, which we hypothesise results partly from lower propagule supply as wood falls decrease in abundance with distance from terrestrial sources of wood. Bone-eating Osedax colonised whale bones, but bone degradation was low compared to some NE Pacific whale falls of similar duration; nonetheless, bones exhibited reducing conditions and supported sulphophilic species. Our study demonstrates quantitatively that co-located wood falls and whale bones support highly distinct, species-rich assemblages and thus promote biodiversity on the deep-sea floor.

The International Seabed Authority (ISA) is developing regulations to control the future exploitation of deep-sea mineral resources including sulphide deposits near hydrothermal vents, polymetallic nodules on the abyssal seafloor, and cobalt crusts on seamounts. Under the UN Convention on the Law of the Sea the ISA is required to adopt are taking measures to ensure the effective protection of the marine environment from harmful effects arising from mining-related activities. Contractors are required to generate environmental baselines and assess the potential environmental consequences of deep seafloor mining. Understandably, nearly all environmental research has focused on the seafloor where the most direct mining effects will occur. However, sediment plumes and other impacts (e.g., noise) from seafloor mining are likely to be extensive in the water column. Sediment plumes created on the seafloor will affect the benthic boundary layer which extends 10s to 100s of meters above the seafloor. Separation or dewatering of ore from sediment and seawater aboard ships will require discharge of a dewatering plume at some depth in the water column. It is important to consider the potential impacts of mining on the ocean’s midwaters (depths from ~200 m to the seafloor) because they provide vital ecosystem services and harbor substantial biodiversity. The better known epipelagic or sunlit surface ocean provisions the rest of the water column through primary production and export flux (This was not the focus at this workshop as the subject was considered too large and surface discharges are unlikely). It is also home to a diverse community of organisms including commercially important fishes such as tunas, billfish, and cephalopods that contribute to the economies of many countries. The mesopelagic or twilight zone (200-1000 m) is dimly lit and home to very diverse and abundant communities of organisms. Mesopelagic plankton and small nekton form the forage base for many deep-diving marine mammals and commercially harvested epipelagic species. Furthermore, detritus from the epipelagic zone falls through the mesopelagic where it is either recycled, providing the vital process of nutrient regeneration, or sinks to greater depths sequestering carbon from short-term atmospheric cycles. The waters below the mesopelagic down to the seafloor (both the bathypelagic and abyssopelagic) are very poorly characterized but are likely large reservoirs of novel biodiversity and link the surface and benthic ecosystems. Great strides have been made in understanding the biodiversity and ecosystem function of the ocean’s midwaters, but large regions, including those containing many exploration license areas and the greater depths where mining plumes will occur, remain very poorly studied. It is clear that pelagic communities are distinct from those on the seafloor and in the benthic boundary layer. They are often sampled with different instrumentation. The fauna have relatively large biogeographic ranges and they are more apt to mix freely across stakeholder boundaries, reference areas and other spatial management zones. Pelagic organisms live in a three-dimensional habitat and their food webs and populations are vertically connected by daily or lifetime migrations and the sinking flux of detritus from the epipelagic. The fauna do not normally encounter hard surfaces, making them fragile, and difficult to capture and maintain for sensitivity or toxicity studies. Despite some existing general knowledge, ecological baselines for midwater communities and ecosystems that likely will be impacted by mining have not been documented. There is an urgent need to conduct more research and evaluate the midwater biota (microbes to fishes) in regions where mining is likely to occur. Deep-sea mining activities may affect midwater organisms in a number of ways, but it is still unclear at what scale perturbations may occur. The sediment plumes both from collectors on the seafloor and from midwater discharge will have a host of negative consequences. They may cause respiratory distress from clogged gills or respiratory surfaces. Suspension feeders, such as copepods, polychaetes, salps, and appendicularians, that filter small particles from the water and form an important basal group of the food web, may suffer from dilution of their food by inorganic sediments and/or clogging of their fragile mucous filter nets. Small particles may settle on gelatinous plankton causing buoyancy issues. Metals, including toxic elements that will enter the food web, will be released from pore waters and crushed ore materials. Sediment plumes will also absorb light and change backscatter properties, reducing visual communication and bioluminescent signaling that are very important for prey capture and reproduction in midwater animals. Noise from mining activities may alter the behaviors of marine mammals and other animals. Small particles have high surface area to volume ratios, high pelagic persistence and dispersal and as a result greater potential to result in pelagic impacts. All of these potential effects will result in mortality, migration (both horizontal and vertical), decreased fitness, and shifts in community composition. Depending on the scale and duration of these effects, there could be reduction in provisioning to commercial fish species, delivery of toxic metals to pelagic food webs and hence human seafood supply, and alterations to carbon transport and nutrient regeneration services. After four days of presentations and discussions, the workshop participants came to several conclusions and synthesized recommendations. 1. Assuming no discharge in the epipelagic zone, it is essential to minimize mining effects in the mesopelagic zone because of links to our human seafood supply as well as other ecosystem services provided by the mesopelagic fauna. This minimization could be accomplished by delivering dewatering discharge well below the mesopelagic/bathypelagic transition (below ~1000 m depth). 2. Research should be promoted by the ISA and other bodies to study the bathypelagic and abyssopelagic zones (from ~1000 m depths to just above the seafloor). It is likely that both collector plumes and dewatering plumes will be created in the bathypelagic, yet this zone is extremely understudied and contains major unknowns for evaluating mining impacts. 3. Management objectives, regulations and management actions need to prevent the creation of a persistent regional scale “haze” (enhanced suspended particle concentrations) in pelagic midwaters. Such a haze would very likely cause chronic harm to deep midwater ecosystem biodiversity, structure and function. 4. Effort is needed to craft suitable standards, thresholds, and indicators of harmful environmental effects that are appropriate to pelagic ecosystems. In particular, suspension feeders are very important ecologically and are likely to be very sensitive to sediment plumes. They are a high priority for study. 5. Particularly noisy mining activities such as ore grinding at seamounts and hydrothermal vents is of concern to deep diving marine mammals and other species. One way to minimize sound impacts would be to minimize activities in the sound-fixing-and-ranging (SOFAR) channel (typically at depths of ~1000 m) which transmits sounds over very long distances. 6. A Lagrangian (drifting) perspective is needed in monitoring and management because the pelagic ecosystem is not a fixed habitat and mining effects are likely to cross spatial management boundaries. For example, potential broad-scale impacts to pelagic ecosystems should be considered in the deliberations over preservation reference zones, the choice of stations for environmental baseline and monitoring studies and other area-based management and conservation measures. 7. Much more modeling and empirical study of realistic mining sediment plumes is needed. Plume models will help evaluate the spatial and temporal extent of pelagic (as well as benthic) ecosystem effects and help to assess risks from different technologies and mining scenarios. Plume modeling should include realistic mining scenarios (including duration) and assess the spatial-temporal scales over which particle concentrations exceed baseline levels and interfere with light transmission to elucidate potential stresses on communities and ecosystem services. Models should include both near and far field-phases, incorporating realistic near field parameters of plume generation, flocculation, particle sinking, and other processes. It is important to note that some inputs to these models such as physical oceanographic parameters are lacking and should be acquired in the near-term. Plume models need to be complemented by studies to understand effects on biological components by certain particle sizes and concentrations.

Fjordic systems in temperate and Arctic regions often feature extensive kelp forests at their shallow coastal margins as well as extensive terrestrial forests. Detrital export from these shallow-water and terrestrial ecosystems is an important source of carbon for deep-sea communities in the form of kelp and wood falls. Benthic landers with experimental substrates (wood blocks and kelp parcels) were deployed for 10 mo at a depth of 530 m in a deep Norwegian fjord to investigate and compare macro- and megabenthic community structure, biodiversity and ecosystem functioning on kelp and wood falls. Results revealed that while wood and kelp falls can support a similar number of species and abundance of fauna, they support significantly different faunal communities. Biomass and secondary production on both wood and kelp substrates were significantly greater than in the control samples. Secondary production estimates were similar or higher than those reported from soft-sediment ecosystems at shallower European marine sites. Biological trait analysis showed that macrofaunal assemblages were distinct between the kelp and wood, providing evidence for differences in ecosystem function between the substrates. This case study from a deep-sea fjord in Norway provides clear evidence that while wood and kelp organic falls can support similar abundances of fauna, the associated benthic biodiversity, community structure and ecosystem functioning can be dramatically different between these substrates. The work presented here aims to provide information that is useful in assessing the extent of anthropogenic impacts on deep fjord ecosystems with respect to informing future conservation and management strategies.