The hydrodynamic regime of any mined habitat is extremely pertinent to deep-sea mining because it will partly dictate the spatial and temporal extent of sediment plumes. Generally, hydrodynamic processes (advection, mixing, upwelling, eddy diffusivity) in the shallower portions of the water column (surface to ~1000 m) are better known than in the bathypelagic or benthic boundary layer (BBL). An exception may be the dynamic equatorial region where repeated broad surveys revealed a circulation dominated by slowly evolving jets (Cravatte et al. 2017) and large intra-seasonal to interannual fluctuations in the form of eddies and equatorial waves (e.g. Kessler and McCreary 1993, Marin et al. 2010). There have also been a number of studies using drifters below 1000 m in and near the CCZ (WOCE Subsurface Float Data Assembly Center Woods Hole, Mass.; http://wfdac.whoi.edu/table.htm). Generally, however, hydrodynamic characterization in the bathypelagic, especially below 2000 m, is a major knowledge gap. Water column characteristics, hydrodynamics as well as nutrients, chemistry, and suspended particle background levels, are relatively homogenous at the mesoscale level and could reasonably be extrapolated at scales of 10-100 km in abyssal regions. However, it is clear that a full dynamic characterization of physical advection and eddy diffusion regime is needed throughout the water column from the surface to the seafloor. The few surveys covering the entirety of the water column have been limited to single repeat lines prone to aliasing spatial and temporal complexities (e.g. Firing and Lukas 1985), and those observations covering broader spatial and temporal scales, like satellites (e.g. Chelton et al. 2007) or ARGO float programs, clearly indicate large variations in both those dimensions. In turn, the lack of bathypelagic and abyssopelagic observations make it difficult to evaluate state-of-the-art ocean numerical models. For instance, the fact that only a few models represent mean jets or eddy energy levels recorded in anecdotal surveys indicates that caution is warranted in their use to infer tracer diffusion.

Near the seafloor, bottom topography is a strong determinant of flow regimes. Importantly, there are many seamounts (rising >1000 m about the surrounding plain) and smaller hills across the abyssal plain of the CCZ, some reaching 3000 m above the abyssal plain. From studies of seamounts in shallower waters (see seamount section below), we know that flows will be affected and we can expect that local hydrodynamic conditions will vary substantially around hills and seamounts.

There was some discussion about how best to characterize the hydrodynamics of a mining region given that there are gaps in our knowledge at the scales required in almost all cases. For the CCZ region, it was concluded that the most revealing and constraining method for physical characterization would be two repeated synoptic latitudinal sections of vertical profiles across the region. Such an approach would characterize a large amount of the physics. Fixed instrument moorings would provide information on temporal dynamics (seasonal to interannual time scales), if embedded in a larger program, including modeling, to characterize the regional hydrodynamics. Given the immense size of the CCZ and its variation from gyre circulation features to the north and equatorial influence to the south, a number of moorings across the region were deemed necessary. However, the location and number of such moorings needed to provide high resolution data should be determined by first creating a regional circulation model and identifying the zones of greatest dynamic importance. Gliders were also discussed as an effective way to survey large areas, measuring basic properties such as turbidity, turbulence, and currents.

The working group discussed the importance of integrating existing and new data into models of physical circulation. Such models should include a region much larger than a single mining site or license area if the plume is expected to expand beyond this region. Although it does not currently exist, an important goal would be to create a more generic base model for the CCZ region that can be manipulated site-specifically by each contractor or party interested in a particular site or mining area. Further, physical models should integrate chemical properties (dissolved and particulate) with optical properties since these have a great influence on biological communities (see below). It was noted that the currently supplied EIA template lacks a great amount of detail on modeling. In particular, it was noted that contractor efforts thus far have focused on the near seafloor environment; however, to develop an understanding of both benthic and dewatering plumes, hydrodynamic measurements and modeling from 200-4000 m in the water column will be required. Furthermore, the ISA should encourage contractors to construct comparable models because the CCZ is a HUGE region that will benefit from an integrated approach.

To characterize the chemical environment, knowledge of the distributions of dissolved nutrients, oxygen, and metals, as well as redox chemistry (including pH), will be needed to evaluate baseline conditions and mining effects. Mining will lead to fragmentation of nodules during the processes of separation from sediments and hydraulic lifting to the surface. Nodule surfaces that have long been in contact with water are in steady state, but newly exposed fractures have the potential to mobilize chemical constituents and allow for reactions. The GEOTRACES program has provided substantial general information about the distribution of many trace elements and metals in the various ocean basins (i.e. Atlantic, Indian, Arctic, and Pacific Oceans; www.eGEOTRACES.org; (Schlitzer et al. 2018). Soon, information that is more detailed will be provided in the CCZ region by an on-going GEOTRACES Pacific Meridional Transect cruise. Further measurements are needed from the seafloor upwards to the potential vertical extent of both benthic disturbance and midwater dewatering sediment plumes, including assessment of the concentrations of bioactive metals and nutrients, and redox regimes (which influence metal mobility).

In addition to dissolved parameters, natural levels of particle flux and suspended particle concentrations are important environmental characteristics. Some data already exist for particular locations, depths and times in the CCZ that may provide good background information. For instance a recent review of global suspended particle concentrations determined that the CCZ bathypelagic and benthic boundary layer have among the lowest measured particle concentrations in the world ocean (Gardner et al. 2018). Particulate organic carbon (POC) flux in the CCZ region varies greatly, being lower in northern central-gyre-influenced waters and higher in the south near equatorial upwelling (Lutz et al. 2007, Wedding et al. 2013). POC flux also increases from the west to east in the CCZ. To gain more data, sediment traps should be deployed at several depths, particularly at the base of the euphotic zone (which is typically measured in many past studies) and also below the level where sediment plumes are expected so that alterations to natural flux can be ascertained. Unlike physical parameters, chemical parameters such as suspended and sinking particle concentrations and oxygen are likely to be variable across the CCZ and not generalizable over large (several hundred km) spatial scales, as suggested by satellite oceanographic data products and sporadic measurements of some of these variables (see data sources in Watling et al. 2013, Wedding et al. 2013).

The baseline acoustic regime is another important ecological characteristic. Mining activities will create noise from the ship to the seafloor, including from pumps, hydraulic lifting and rattling of nodules in riser pipes and from mining vehicles at the seafloor. Little is known about background noise volumes, sources and frequencies in remote abyssal areas such as the CCZ. However, we do know that a number of pelagic animals such as marine mammals, which are sensitive to noise, migrate through this region at least seasonally. It was noted that no acoustic measurements are required in the ISA EIS template.

The light environment was discussed briefly. Light levels control the depth of vertical migration and mediate many predator-prey interactions throughout the top 1000 m of the pelagic. Sunlight visible to animals is generally absent below 1000 m. However, light penetration in the mesopelagic varies regionally, depending on primary productivity and water clarity. Thus, it was concluded that light profiles in the area should be determined. Furthermore, bioluminescent light is present throughout the water column and could be measured with systems such as the “splat cam.” Bioluminescence is correlated with organismal biomass (Martini and Haddock 2017) and is thus a physical measurement that elucidates an important biological characteristic.

After considering pelagic environmental characteristics within the abyssal CCZ, the group considered how similar these pelagic characteristics would be to those near seamounts and vents, the other two main habitats that have mineral resources. The conclusion was that there is minimal similarity. The abyssal plains are much deeper (to 5500 m vs depths of 1000-3000 m for vents and seamounts), with slower and less variable currents. Furthermore, the seafloor on the plains is largely covered by fine clay sediments whereas many areas on seamounts and in vent regions have hard substrates (though not exclusively) which is likely to influence the nature of the sediment plumes in habitat type. Thus, it will be important to assess environmental characteristics in a habitat-type framework.

The group also discussed the preferred discharge depth for dewatering plumes because this key variable will determine where particular knowledge of pelagic ecosystem characteristics will be needed. Two positions were explored and considered. One option would be to discharge dewatered sediments a few hundred meters above the abyssal seafloor. Because the density of organisms decreases with depth until reaching the benthic boundary layer, one thought was that it is better to discharge plumes as deep as possible but still a few hundred meters above the bottom to reduce smothering the benthos and suspended particle loads in the BBL. Keeping the plumes above the BBL might reduce impacts to benthic and demersal communities, which have relatively high biomass and biodiversity. The persistent component of the plume would then be focused as deep as possible where biomass/diversity is low before it increases again in the BBL, a very rich layer of the pelagic. The second option considered was to discharge the dewatering plume as close to the seafloor as possible where the benthic ecosystem would already be greatly affected by the benthic plume and direct disturbance generated by the collector itself; a dewatering plume discharged close to the seafloor would blanket already smothered communities, thus “minimizing” impact. In comparison to discharge much higher in the water column, discharge close to the bottom, would likely minimize the horizontal spread of the dewatering plume. This second option was ultimately favored. Some doubt was expressed about whether contractors would bring the discharge pipe all the way back to the seafloor due to cost, but it was thought that this was technologically possible. A final comment was that we cannot confidently speculate on the best depth for plume release until we have a better understanding of the physical and chemical properties of the baseline ecosystem and of the plumes (including thermal characteristics, density, and buoyancy).

For environmental characteristics, the workshop participants decided to combine the vent and seamount habitats into a single discussion group due to some similarities in habitat depths, elevated bathymetries (vent deposits often occur on ridge systems or on raised topography and seamounts in arc/back arc systems), and complex and sometimes enhanced flow environments.

This group began its discussion by first evaluating “What is the mining technology like and how does this affect the seamounts and vents of interest?” The goal was to provide context to the discussion and help narrow our understanding of what information is needed. Mining on seamounts was considered challenging due to the extremely rough terrain but still possible. Contractors are mostly considering mining seamount flanks where the topography is a bit more regular or on seamounts that are flat-topped (guyots). There will clearly be limits to the conditions under which mining machines can operate. It was not clear what those conditions may be, how many seamounts will be mineable, or how many of these are in the prime crust zone. General depths considered for seamount crust extraction are 800-2000 m. Currently there are 3 exploration claims in the NW Pacific and another in the SW Atlantic.

Next, the group evaluated the potential physical impacts of mining. Mining will convert hard substratum into a soft substratum of ground up material – rubble rather than clean rock surfaces. Contractors have different machines for different types of bathymetry or bottom type. Machines operating on seamounts are likely going to grind off the top ~5-50 cm of the cobalt crusts. In contrast, for massive sulphide deposits at vents, mining will create pits many meters deep targeting layered deposits that are often subsurface. Also, in some cases the substrates are a mosaic of hard seafloor, crushed rubble or even a top layer (overburden) of sediment that may generate a significant plume during removal. In both cases however, there is some assumption that the mining technology used will be similar, as grinding of hard substrates is required.

Depth is an important environmental characteristic of these habitats as it covaries with many other environmental variables (pressure, light levels, temperature etc). Pelagic communities are very depth-stratified. Thus seamounts or mid ocean ridges with depths near or above the daytime depth of sound scattering layers will affect these biotic vertical migrations and may enhance delivery of food to pelagic and demersal predators. Further, shallow features may alter flow such that nutrients are injected into the euphotic zone enhancing local primary production. This is unlikely to be the case for seamounts currently targeted for mining because they are deeper, but injection could occur if mining targets the deep flanks of shallower features. Similarly, ISA contracts for SMS deposits are mostly below 2000 m depth.

It was unanimously agreed that site-specific bathymetry is a key environmental characteristic of seamounts and vents. These habitats are highly complex and heterogeneous. Fortunately, bathymetry will likely be known in claim areas, often from detailed AUV surveys, due to its importance in mining activities. Bathymetry partly determines whether seamounts have an effect on local primary production, the flux of particulate organics to bottom feeders, the impingement of pelagic fauna on the seafloor and hence top predator aggregations, and the flow environments, which will be important for larval connectivity and plume dispersal. Due to complex interactions, it is difficult to predict the flow field from bathymetry alone, however.

Clearly related to complex and elevated bathymetry (and also the background flow field) is enhanced current flow, mixing and turbulence. These enhanced physical processes will occur on deep to shallow features to varying degrees. Flow and turbulence enhancements around seamounts, ridges and axial valley bathymetry will increase the dispersal and mixing of mining induced sediment plumes in the benthic boundary layer and in the water column above. Vent and seamount habitats are extremely complex bathymetrically so site-specific characterization of the flow field will be very important. In general, nearly symmetrical seamounts will have more flow enhancement around flanks and trapped circulation features at their summits. It was noted that seamounts rarely have Taylor columns, rotating columnar circulation features above the seamount peak, but there are some Taylor caps (conical circulation features (Goldner and Chapman 1997, Kunze and Toole 1997) that persist for long enough periods of time to enhance primary production, if they are shallow enough, or temporarily retain some pelagic animals in deeper waters (Rowden et al. 2010). However, the levels of endemism in seamount communities do not suggest very long durations of physical retention on seamounts/ridges, as was hypothesized many decades ago (Rowden et al. 2010). Elongate ridges or seamounts will have flow enhanced over their summits and these features will generate internal waves. On large mid ocean ridge features, abyssal currents are trapped on the flanks, resulting in complex dynamics both along and over the crest as topographic flow rectification (Lavelle 2012). Within axial valleys flow can proceed along the axis near bottom (Garcia Berdeal et al. 2006) which may act to retain plumes. In addition, habitats with elevated bathymetry will alter flow downstream of the mean background flow analogous to island wakes, which will act to modify dispersal of plumes from seamount mining. Despite our general knowledge of flow in these habitats, due to their great diversity and site-specific heterogeneity, everyone agreed that site-specific characterization of the flow field would be required for each mining claim. There are existing HYCOM or ROMS models with resolution to 2000 m depth available, which would be good as an initial starting point but would not obviate the need for empirical measurements.

Regional flow and stratification are also environmental characteristics that affect the site-specific flow fields. Water column stratification is important because it influences flow effects and mixing and it varies latitudinally and seasonally. The background flow field in these habitats is variable from gyre circulation to equatorial jets and from the east to west of basins, which influences the velocity of general circulation. Background flow provides the initial conditions from which to understand local flow around the seamount or ridge. Further, background flows can be seasonal as has been shown at vent sites on the Mid-Atlantic Ridge.

A key environmental characteristic of hydrothermal vent systems is the presence of natural particulate plumes. They have been characterized in some regions and generally extend to ~200 m vertically and have high concentrations of metals, organics, reduced compounds and microbes (e.g. Cowen et al. 2001). Some natural plumes have been characterized and the influence of natural plumes on water chemistry is a regional phenomenon (e.g. regionally higher metal concentrations; e.g. Geotraces). However, more localized, feature-specific data are often lacking. It was noted that these plumes occur at active venting sites but we expect that sites with vigorous venting will not be targeted for mining. No such natural plumes exist at inactive vent sites, which can be between zero and 100’s km away from active sites and their associated plumes. However, even if natural plumes are some distance away from mining activities this background natural activity will make it difficult to separate the water column effects of natural and mining activities. Nonetheless, the behavior of natural plumes intersecting with the regional flowfield can provide insight into dynamics of mining-generated plumes. It was noted that some seamounts can have sediment plumes naturally from “storms” or enhanced turbulence events over their flanks and summits, but the extent of such plumes was thought to be small and transient.

Overlying primary production is important as it affects food supply to deep-sea ecosystems – both benthic and pelagic. Seamount-specific local alterations can occur and they are currently being documented. Enhancements of primary production can stimulate increased fisheries production. Local primary production will also be affected by general latitudinal and basin scale variation that can be characterized with satellites. Furthermore, productivity in vent or seamount habitats may have clear seasonality or a lack of it based on regional scale dynamics (e.g. above 36º N the Mid-Atlantic Ridge exhibits clear seasonality in production).

Suspended sediment concentrations and organic particle fluxes are key variables in the context of mining because mining induced plumes could greatly alter these environmental characteristics. Natural levels are related to productivity providing some ability to predict values though there are few direct measurements on seamounts. Particle concentrations and vertical flux are higher in axial valleys than over seamounts due to venting at the former. Organic particle concentrations in vent plumes derive both from the benthos and from carbon fixation in the plume itself (Bennett et al. 2011), thus vent plumes have marked input of carbon into the deep-sea ecosystem (Levin et al. 2016). A recent review of global particle concentrations does not include much data on mid-ocean ridges or seamounts (Gardner et al. 2018). More study into these environmental characteristics is needed.

A number of water chemistry parameters were discussed because mining induced sediment plumes could alter oxygen and metal concentrations among other variables. The regional and depth distribution of parameters such as oxygen concentration and pH have been investigated. They vary by basin location and age of water mass. Regional maps can be found in the literature (e.g. Levin 2002, Sabine et al. 2004). Global programs such as GEOTRACES also provide regional-scale context for trace metals and suspended particles but their resolution is low and not capable of resolving seamount or vent habitats in detail. At the local scale, vent plumes clearly increase metal concentration, reduce pH, alter redox chemistry and reduce oxygen concentrations as mentioned above. Indeed vent plumes are detectable somewhat beyond local scales in broad sampling programs such as CLIVAR (but this doesn’t measure metals) and GEOTRACES. The local chemistry above crust rich seamounts and potential alterations is not known but the expectation was that there would be little contrast to the background water chemistry of the region.

The group discussed how climate change will affect many of the above environmental characteristics, including ocean acidification, deoxygenation, changes to large scale primary productivity patterns, circulation and water column stratification. Thus, these long-term changes must be taken into account as multiple stressors given the long duration of many proposed deep-sea mining activities.

The group evaluated the scales at which environmental characteristics in the pelagic should be known. In the context of mining, it is the mesoscale (10’s of meters in the vertical, 100’s of meters to kilometers in the horizontal) that is the smallest scale that matters because sediment plume effects will be larger than this. For the temporal, the scale that matters is years as mining will be ongoing at a site for ~10-30 years. For the purposes of monitoring and observing the scale of anthropogenic changes to the environment sampling would have to occur at these spatial and temporal scales, possibly finer scales and frequencies (and ongoing), to capture changes to environmental features.

Correlated to many of the environmental characteristics above is the geographic location of any vent or seamount feature. Thus, the location of a feature could be used to predict general features of a mining area. For instance, latitude affects food web interactions via seasonality of primary production, which results in variation of the vertical detrital flux (as seen on the mid Atlantic ridge). Latitude also affects the physical environment such as proximity to equatorial jets. Furthermore, the Atlantic, Indian and Pacific Oceans are very different from one another in terms of vertical profiles of temperature, oxygen, surface primary productivity, and surface or deep storm activity: all characteristics that could affect sediment plume dispersal or settling. These differences stem in part from global deep-water mass circulation and age as well as meteorological patterns. Biogeography of organisms was also discussed briefly in this context because different regions have different faunas. Each regions fauna could respond differently to impacts due to their evolutionary history and environmental differences (see answers to question 2 below) which lead to differences in community composition and the relative importance of taxa that might be more susceptible to mining effects. In the context of geographical locations of seamounts, it was pointed out that what we know about them largely comes from studies in the Atlantic and NE Pacific. We know very little about seamounts in the prime crust zone in central West Pacific.

In the context of this discussion, the depth of discharge of dewatering plumes was evaluated. Given the strong and heterogeneous flow environments of seamounts and vents it was suggested that the plume should be dispersed below the level of most turbulence to minimize dispersal and localize effects to the pelagic. Further, most agreed that the discharge plume should be as deep as possible – below the mesopelagic and below strong turbulence and mixing regions. Some thought that the BBL layer was preferred in terms of targeting plume return because this layer would already be the most disturbed part of the water column from benthic mining activities.

Seamounts and ridges/vents have many similarities in terms of environmental characteristics but do differ mostly due to the presence of natural sediment and chemical plumes at vents. This means that vents in axial valleys have higher suspended particle concentrations compared to seamounts. This affects biological communities with axial valley organisms likely more adapted to suspended sediment loads. It should be noted however, that much of the suspended particles in natural plumes include organic materials, whereas mining plumes will be dominated by the inorganic fraction. Also, as a consequence of their natural plumes, vent and ridge systems have higher background metal concentrations compared to seamounts, which again may translate to sensitivities to some extent. However, it was noted that the MIDAS program performed toxicity studies on vent organisms and found that while they may have had lower sensitivities they were just as sensitive to changes in concentrations as other organisms (Hauton et al. 2017).

In the context of this discussion, the group strongly felt that not all seamounts, ridges and associated vents were the same. For these features generalizations of environmental parameters was very difficult.

However, these features had much more in common with one another than they did with abyssal plain habitats. The largest differences were the high variability of setting, including depth (800-4000 m vs 4000-5500 m on the plains), and the flow environment (very slow with low turbulence on the plains). Further, the mining induced sediment plumes generated at vents and seamounts will be different compared to those from nodule mining because the ore will be crushed, creating more angular and sharp fines compared to material harvested from nodule fields.

To begin the discussion, the group first identified abyssal habitats for which the midwater ecology/biology was well defined. There has been substantial study of the oxygen minimum zone (OMZ) principally to depths of ~1000 m in the Eastern Pacific and off Baja California. The OMZ is strongest and shallowest in the eastern Pacific and to the north of the Equator (the southern CCZ) diminishing in intensity moving west and north and increasing in intensity over the last several decades (Stramma et al. 2008). Studies show reduced diversity and biomass in the lowest oxygen zones and also alterations to vertical distributions within biotic species, higher taxa and functional groups (Maas et al. 2014, Wishner et al. 2013). They have also documented a fauna that is specific to the OMZ-dominated eastern central Pacific region. These results are useful and partially generalizable for the eastern CCZ and possibly to other low-oxygen regions of the CCZ. There is also some research on the benthic boundary layer off California at Station M, a multi-decadal time series study site which largely has focused on benthic communities. In addition, long time-series data are available from the HOT program at Station ALOHA north of Hawaii; the HOT program has focused on the epipelagic (Valencia et al. 2016), but some mesopelagic studies likely relevant to the northwestern CCZ have been conducted in the same location (Gloeckler et al. 2018, Hannides et al. 2013, Sommer et al. 2017, Steinberg et al. 2008). Pelagic research has also been conducted along the equator to characterize epipelagic, and to a lesser extent, mesopelagic zooplankton and micronekton biogeographies, fisheries resources, and ecology (Barnett 1984, Clarke 1987, McGowan 1974). In addition to region specific information, it is also well known that on a global scale, food availability (satellite-derived surface water productivity as a broad proxy) controls pelagic community biodiversity, biomass and abundance. Though we thus have some knowledge of epi and mesopelagic community structure and biogeography within the eastern North Pacific, it was agreed that that the CCZ mesopelagic to bathypelagic remain virtually unstudied and we can extrapolate only very general relationships from other regions. Detailed mesopelagic and bathypelagic studies are needed.

The group then evaluated which ecological/biological characteristics are pertinent to understanding the impacts of deep-sea mining. For each depth zone of the pelagic (epipelagic, mesopelagic, OMZ, bathypelagic, abyssopelagic to BBL, especially bathypelagic to BBL) the following were characteristics considered important.

Basic characteristics to be assessed included depth-stratified biomass, abundance, community structure, diversity (species or operational taxonomic units and functional diversity) and the diel cycle of these parameters as a consequence of animal vertical migration. These data could be acquired with depth-stratified net samples for hard-bodied animals such as fishes and crustaceans, and ROV/AUV visual transects targeting gelatinous zooplankton and fragile taxa. Such approaches require standardized imaging techniques. Net tows in the BBL are logistically challenging so study could also include plankton pumps and baited free-vehicle camera deployments for some taxa. Acoustic surveys could also contribute substantially to biomass/abundance estimates and vertical migration dynamics over large regions, but this approach lacks taxonomic resolution. The physical taxa collected by nets and other approaches would need to be identified morphologically and genetically for all size classes (e.g. barcoding). It was also emphasized that samples and data should be archived in a centralized collection, including physical specimens, images/video, and genetic data.

Ecosystem function variables also are needed but generally lacking. For instance, food web structure and function is typically known only for commercially important species and their predators and prey. However, food webs vertically connect pelagic strata and thus could translocate the impacts of deep-sea mining plumes. Thus, studies of food webs are warranted and could utilize physical specimens for stable-isotope and gut-content analyses. For instance, isotopic analysis of particles and animals in the deep mesopelagic and bathypelagic has shown that food webs there may be adapted to take up very small particles (Choy et al. 2015, Gloeckler et al. 2018, Hannides et al. 2013).

The group also identified the depths where ecological/biological characteristics should be characterized in relation to deep-sea mining. It was agreed that for baseline studies, full depth, high-resolution studies of microbes, primary producers, zooplankton, gelatinous zooplankton, and nekton communities from the sea surface to the seafloor are needed. This baseline information could then be used to identify the preferred depth of plume deposition based on the vertical profiles of ecological characteristics.

At the regional scale, it was recognized that pelagic communities are relatively broadly distributed and more homogenous than at the seafloor, with similar assemblages across 100’s of kilometers in the horizontal. Thus, the group suggested that baseline characterization of pelagic communities at 3-4 target reference regions (with multiple replicate sites within each) across the CCZ region, capturing the mesopelagic biogeographic provinces of Sutton et al. (2017), might be sufficient. These sites could include the eastern Pacific, equatorial upwelling, central gyre and western gyre. Sampling might be best placed in the APEIs to allow monitoring of a regional baseline after mining begins. Such an approach would need to be ISA-mandated, perhaps through a fund paid by contractors and administered by the ISA. This approach would save each contractor time and money by pooling their resources, and would provide several regional baselines. For monitoring of mining activities, additional sampling (as suggested above) would be needed in each claim area for comparison to the baseline sites.

As for question 1, discussion of vents and seamount habitats was combined into one group.

The group outlined which ecological/biological characteristics are pertinent to understand in the context of mining effects on pelagic processes. Biodiversity was considered important, but due to concerns about the absence of species-level data in many cases (due to varying taxonomic expertise, cryptic species and undescribed species) functional diversity was suggested as a very important alternative. Microbial diversity was also discussed but again more from the perspective of functional diversity to evaluate variations in microbial loop and food web interactions.

Community composition and biomass in the pelagic are also important to consider. Both are generally uniform over large spatial scales but can differ over seamounts and vents/ridges. For the latter, the influence of vent plumes can extend into the overlying water column by 200 m or more. It was noted that there was a need to consider deep-scattering layers (biomass and vertical migration) not only in the mesopelagic but to depths of 2000 m or more based on some studies which show migration to such great depths (Cook et al. 2013, Marsh et al. 2018).

In terms of functional characteristics, food webs were identified as very important to understand. In particular, filter feeders (e.g. appendicularians, salps, pteropods) are important components of pelagic food webs. They may increase sediment packaging and downward transport, and may be particularly sensitive to sediment plumes. There was some discussion about how microbes might be important to the mesopelagic in terms of mining especially in response to release of reduced compounds. It was not clear how (or for how long) microbial assemblages might change in response to mining plumes. The microbial community can be quite resilient and due to its phylogenetic diversity and physiological versatility will likely respond quickly to environmental change and also recover quickly. However, it was considered important to understand what microbially mediated processes are occurring that may be important indicators of mining impacts.

Connectivity was also considered very pertinent to understand as both seamounts and vents are “island” habitats that rely on successful dispersal and recruitment. Larval transport will be different for seamounts compared to ridges due to more constrained flow on the latter. For seamounts, proximity to continental margins is important for larval supply while vent systems are more like island archipelagos in which distance between sites and size of sites is likely very important (Baker et al. 2016). Vertical connectivity is very important for many species whose eggs are buoyant and whose larvae travel vertically, often from the surface to the adult depth. This is a common strategy for many midwater fishes and crustaceans. Many benthic species also utilize the water column for larval dispersal and population connectivity. For instance, some vent shrimp and mussel larvae rise to the surface whereas others remain in the axial valley, and others have crawl away benthic larva.

The group evaluated what could be extrapolated based on studies from elsewhere. For this purpose, biogeographic provinces for the mesopelagic may be the best available information (Sutton et al. 2017; Fig. 4). They are general but provide a context for understanding biodiversity and community structure. They are based on environmental characteristics (temperature, oxygen concentration, salinity, etc) then adjusted where biological community patterns are known. For instance, primary production is very important and at broad scales relates to the mesopelagic fauna (Proud et al. 2017). Both the level of primary production and its seasonality is important to predict biomass and faunal composition, as noted in answer to question 1 above. With regards to seasonality, the timing of mining activities may be important due to seasonal spawning or migrations. Overall, the group decided that regional environmental management needs to take known mesopelagic biogeographies into account. All agreed that we know little about the bathypelagic, quantitatively at least, but proceeding from the general assumption that vertical zones of the ocean are linked, using mesopelagic provinces was a starting point.

Figure 4.  

The mesopelagic ecoregions or biogeographic provinces of the world’s oceans proposed by Sutton et al. (2017), available under a CC BY 4.0 license. The numbers are simply for reference, and relate to the geographical names referenced in the paper and in the workshop discussion below. Areas with depths less than 200 m shaded in black.

The group then discussed the pelagic communities of seamounts in more detail. Some seamounts are known to have endemic pelagic species not found in the neighboring open ocean (Boehlert and Mundy 1993, Wilson and Boehlert 2004). Additionally, there is a clear “seamount effect” for upper food web predators such as tunas that have enhanced abundances and biomass over some seamounts. These mobile species are visitors to the seamount. For instance bigeye tunas have high residence times over shallow seamounts and fuller stomachs there (Holland and Grubbs 2007). The mechanisms facilitating this enhanced feeding is unclear but shallow low latitude seamounts can have planktonic production peaks stimulating bottom up effects. Additionally, shallow topography may trap plankton and micronekton advected over it during the night when the animals try to migrate back to depth in the morning. Finally, flow or upwelling over and around seamounts may concentrate the background prey field. Studies have also demonstrated that due to local faunas and visitors seamount mesopelagic and epipelagic communities have enhanced biodiversity. Also some commercially important mesopelagic species such as black scabbardfish and pelagic armorheads use seamounts as sites for reproductive aggregation. Though much of what is known about seamounts is from the mesopelagic, studies have found concentrations of biomass at 2000 m off Tasmania and near the Azores at 2500 m using acoustics (Sutton et al. 2008). Therefore, the midwater influences of seamounts and future studies need to extend to these bathypelagic depths.

Ridges associated with vent systems were discussed as well. These have been fairly well studied in the north Atlantic as part of the MARECO and EcoMar projects focused on the Mid-Atlantic Ridge (MAR). Ridge habitats have an enhanced pelagic fauna in terms of biomass and diversity, which may be due to a resident community. It was also noted that diel vertical migration was occurring to at least 2500 m along the MAR. Ridges may also harbor a large fraction of the large reproductive individuals in some populations, at least for fishes (Sutton et al. 2008). Further ridge structures are very large and the MAR has a channeling effect associated with topographic fracture zones. Seamounts differ in this regard due to their smaller size.

Within axial valleys and in the benthic boundary layer highly elevated zooplankton levels have been recorded up to 50 m above bottom (Skebo et al. 2006). Studies have shown that the vent plumes attract or enhance the abundance of some animals such as shrimp and copepods. For instance, along the East Pacific Rise plumes enhanced zooplankton biomass including copepods that feed in the vent plume. In contrast, fishes avoided it. Plume-influenced pelagic communities have high diversity and high endemism suggesting that at least some species are adapted to these conditions. The sphere of influence of vent plumes needs to be considered in more detail but may be relatively small (100’s m) in terms of immediate chemical alterations but large in terms of organic inputs and longer term elemental cycling in the oceans (Levin et al. 2016). It appears that natural plume effects on the pelagic are a mixture of vent plume induced changes from the seafloor-up and normal migrations of fauna from above it.

The mesopelagic provinces where existing seamount exploration claims occur were evaluated specifically. Seamount exploration contracts are now concentrated in the northwestern Pacific lying within the North Pacific Gyre biogeographic province (#4 in Fig. 4). This province is bounded by the North Equatorial and Kuroshio Currents. It is oligotrophic and has a fauna distinct from that of central Equatorial Pacific. The exploration area in the south Atlantic Rio Grande seamount area is located in province #27. This is the tropical and west Equatorial Atlantic province dominated by easterly winds that cause divergence and upwelling. The region is generally oligotrophic except for regions of upwelling and it is noted for a distinct cephalopod fauna.

The state of biological/ecological knowledge of pelagic communities below the mesopelagic was discussed as a major knowledge gap. Both faunal inventories and biomass levels are very poorly known. Based on general depth-related declines in biomass and diversity, we assume lower levels of these variables compared to the mesopelagic. Despite the poor characterization of this zone, it is likely to be influenced by the discharge of dewatering plumes and much more data is needed.

The group discussed additional knowledge needs. It was agreed that depth-stratified biomass from the mesopelagic to the seafloor categorized by functional diversity at least, was the most important need. Where OMZs are present it would be critical to sample in relation to oxygen concentrations. Sampling above the benthos in the BBL would also be important in relation to benthic collector plumes. Active acoustics could define layers of enhanced biomass and net tows could then determine each layers taxonomic and functional composition. Standardization across studies is important and samples could be archived in publicly available locations from which key species to focus on could be determined. Finally, it was agreed that for any mining area a time series of these parameters would be needed starting before mining and continued afterwards to monitor impacts.

The biological/ecological characteristics were then compared between the three main mining ecosystems – abyssal plains, seamounts, and vent regions. All three were considered to be quite different biologically. There is often high biomass and abundance in the pelagic over ridges and vents (e.g. the east Pacific Rise, MAR). This may be true to some extent over seamounts but likely due to different mechanisms such as physical compaction/compression/concentration of pelagic community and or spawning aggregations. There is higher diversity at seamounts and ridge/vent systems compared to neighboring waters but the mechanisms causing this vary. At seamounts, diversity is typically high due to visitors such as pelagic fishes, mammals, and turtles, rather than endemism at vent/ridge systems. Connectivity patterns are also likely different between habitats which should be considered. For instance, axial valley transport plays a role in ridge/vent systems but not elsewhere. These connectivity patterns are most likely different between benthic species using the pelagic for larval dispersal and for habitat specific pelagic endemics. In contrast to both, the mesopelagic fauna is generally widespread. It was also pointed out that seamount systems are very different from one another, requiring more investigation into the drivers of variation in seamount pelagic communities.

The group considered impacts of mining on different taxonomic and functional groups. Microbes were thought likely to exhibit changes in community composition due to the additional surfaces in the water column (from particles) and due to alteration of water chemistry from plume solutes. Suspension feeders are likely to have degraded diets (gelatinous and crustacean suspension feeders) from an abundance of inorganic particles. This will incur increased metabolic costs for processing which could lead to a variety of sublethal effects. Gelatinous taxa would also experience suffocation due to clogging of respiratory surfaces by particles. Fishes and marine mammals would experience altered prey distribution fields, and fish could experience respiratory distress due to particles interfering with their gills and reduced oxygen content in plumes. All of these taxa, with the exception of microbes and non-visual invertebrates, would also suffer from the degraded light field in plumes. The relative probabilities, severities and scales of impacts along with mechanisms behind impacts were tabulated in the table below (Table 1).

Table 1. Download as CSV 

The direct effects of sediment plumes on deep midwater taxa. The impacts of light and noise are not included here.

	Microbes	Gelatinous suspension feeders	Crustacean suspension feeders	Predatory jellies	Fishes	Mammals
Mechanisms of impact	Unknown. Extra surfaces for attachment and new solutes from plume. New microbial community introduced from sediments. Microbes may be scavenged. Community structure and function will be impacted but difficult to predict specifics.	Interference with feeding; salps and appendicularians are mucous feeders, so this would foul their feeding structures; suffocation/ smothering likely.	Altered ingestion behavior because of selective feeding and loss of energy through parsing more useless particles (related to particle size); smothering unlikely; will olfactory sensors be clogged?	Food web perturbation; stickiness of tentacles may be impacted by sediment sticking to it – capture efficiency would be reduced.	Food web perturbation - their prey will be redistributed; suffocation possible at high sediment concentration and in OMZs.	Food web dynamics are unlikely to affect them substantially as their prey is higher in the water column. However recent evidence points to deeper feeding than previously believed (Marsh et al. 2018).
Scale of perturbation	Range of microbes is greater than the plume and connectivity is high.	Mortality may extend beyond the plume because of avoidance and migratory behavior.	Escape depends on scale of plume in relation to range of organism – vertically and horizontally.	May extend beyond the plume.	Beyond the scale of the plume.	Travel scales are large in comparison to plume. They are likely to avoid it.
Probability/ sensitivity	Very high; sensitivity is high.	Certain; sensitivity is higher than microbes (lethal impacts).	High, but less sensitive than for gelatinous suspension feeders.	Not as high as suspension feeders.	Less sensitive if they can migrate away from the plumes.	Low.
Recovery	Recovery will be relatively quick once plume is gone.	Dependent on advection from outside plume; distributions are broad, so probability is high at time scale of flow.	Generation times and reproductive rates are unknown (months-years), advection will be more important to repopulation.	Unknown.	Very long timescale; slow growth and reproduction times.	High potential recovery from migration back into region after plumes have settled.

The group discussed the effects of a degraded light field on various bioluminescent modalities, which are extremely important in deep-sea ecosystems, particularly deep midwater ecosystems (Haddock et al. 2010, Martini and Haddock 2017). Some taxa use bioluminescence for prey detection, others for mate location and intraspecific communication, predator confusion and for camouflage in the form of counterillumination. In short, the midwater fauna use bioluminescence to feed, hide and reproduce. All of these uses of bioluminescence are potentially affected dramatically because of a) the very low baseline particle concentrations in these ecosystems, b) small clay sized particles which will dominate the abyssal sediments result in high light attenuation and c) suspended sediments absorb blue light, the most common wavelength for bioluminescence (Warrant and Locket 2004). Thus, sediment plumes from midwater discharges and seafloor collections will reduce encounter distances between organisms that using bioluminescence, resulting in reductions in mating and/or feeding success.

The varying effects of particle size and concentration were also discussed. The group assumed that there are threshold effects, but the thresholds are unknown. Further they would be difficult or perhaps impossible to determine with laboratory studies because of the challenges of keeping midwater animals alive in captivity. In situ studies along concentration gradients created by test mining are a possibility to develop thresholds for acute effects in lieu of controlled laboratory study. It was also concluded that the very small particles (< 10 µm) are particularly important because suspension feeders feed in this size range and there is an increase in reliance on small particles with depth in midwater food webs (Gloeckler et al. 2018, Hannides et al. 2013). Further, it is presumed that there will be particle size dependence for clogging and suffocation. Sediment concentrations resulting in lethal effects are likely to vary widely across taxa, and stress thresholds for chronic exposure (over days to months) will be much lower than for acute (short-term) exposure.

Ecosystem effects were also evaluated and considered very likely but highly dependent upon the scales of environmental perturbation. Effects would likely include alterations to ecosystem function, oxygen consumption, and carbon dynamics. However, it was unclear what the scale of these impacts would be in space and time. This is dependent upon how rapidly and widespread operations are and their resulting plumes. An increase from the 100’s of meters scale to the 1000 km diameter scale elevates impacts from the mesoscale to the regional scale. To address these issues, it was concluded that simple models will be useful for first order approximations. The group also concluded that management should ensure that a persistent regional-scale haze in pelagic mid-waters is not created.

Studies from other ecosystems that could provide useful data to address the effects of sediment plumes on midwater organisms were discussed. The group considered whether there may be analogous regulatory regimes and strategies from the arenas of fisheries and dredging (and others?) that might inform different ways of approaching monitoring and management. Another area of likely importance is the effect of suspended particulate matter (sizes and concentrations) on suspension feeding, including the developing microplastics literature, which may also evaluate toxicity. However, extrapolations from other ecosystems must be done with care because deep-sea particle concentrations are extremely low compared to other aquatic ecosystems and many deep-sea animals have longer generation times compared to related shallow water taxa. Indeed, specific stress thresholds from other regulatory frameworks are probably going to be too high for the CCZ, where baseline particle concentrations are very low (Gardner et al. 2018) and thus the thresholds of midwater organisms are probably low too. It was also suggested that thresholds may not be the best management tool, but industry is used to this approach.

The abyssal ecosystem group generated some important considerations and conclusions in the context of this workshop question. First, the oligotrophic meso/bathy/abysso-pelagic waters (e.g. over the CCZ, the most likely region for nodule mining) are the clearest on Earth (Gardner et al. 2018). Its inhabitants are adapted to live in these particle-poor ecosystems and thus, the impacts of introducing particles to these ecosystems are likely to be larger than for any other ecosystems. The mesopelagic ecosystem affects carbon cycling and connects to fisheries, so plumes should be discharged below the mesopelagic. Planktonic animals travel with plumes, so they will be affected over their life cycle and across generations. Despite this fact, water column impacts are likely to be more transient than benthic impacts because the sediment clouds will eventually settle to the seafloor though this may take more than a decade (Rolinski et al. 2001).

The vent group first evaluated how mining effects might vary by taxonomic and trophic groups. Bacteria are known to colonize plumes, which then carry the products of chemosynthetic production. Above some hydrothermal vents higher concentrations of plankton are associated with the top of the natural plume, likely due to feeding on the enhanced microbial production. In contrast, there is also evidence that fish will avoid natural plumes, possibly due to chemical or particulate avoidance. This information helps to inform potential impacts of mining plumes, though they are not perfect analogs.

In general, the ‘background’ midwater fauna was considered likely to avoid sediment and chemical plumes if they were nektonic. There is some potential for bacterial colonization of mining discharge plumes resulting in increased microbial loop activity if there is enough resuspended organic material, though most deeper sediments have low organic content. This could affect food webs, particularly suspension feeding taxa. Suspension feeders were considered particularly sensitive and important in the community response to sediment plumes. Small particles (few µm) will be the most damaging due their greater persistence and dispersal and the greatest effects will likely occur for suspension feeding species. These species will be size selective, but in the size range of the small (1-10 µm) released material which could clog filters or dilute organic particles. Furthermore, many suspension feeders are gelatinous (e.g. salps, appendicularians) with finely tuned neutral buoyancy that could be affected by consumption of heavier sediment (rather than organic) particles or settling of particles on their surfaces (Robison 2009). Organismal interactions other than suspension feeding could be altered by the plumes. For instance, scavengers could be attracted if there were dead or dying animals. Also, water column turbidity will impact bioluminescent interactions, such as mate detection or luring of prey.

Varying responses to the plumes across taxa and life history stages (would larvae be most susceptible as is often the case for shallow water taxa ?) would most likely lead to reduced abundance and biomass as well as sub-lethal effects. In combination, these alterations would cause a shift in pelagic community structure. Additionally, the group considered that in the long term there was strong potential for bioaccumulation and biomagnification of toxic metals through the food web to fishes. With regard to time scales of these effects, the group concluded that effects at lower trophic levels would be acute and that higher trophic levels would be affected on longer time scales and that chronic effects (10+ y) would determine the altered community state.

The vent ecosystem group then evaluated the question “How will these effects vary with plume particle size, concentration and/or extent, which then can be used to evaluate the temporal and spatial extents of perturbations?” Small particles have high surface area to volume ratios, high pelagic persistence and dispersal and as a result greater potential to obstruct biological structures (filtering apparatus, gas exchange surfaces). Intermediate sized particles could be rapidly ingested promoting repackaging and flux to the sea floor. Larger particles will rapidly sink minimizing pelagic impacts but smothering benthic fauna.

Discussion then centered on where the dewatering plumes should be discharged to minimize impacts. Discharge at the seafloor would reduce the spatial extent of impact in the water column as there would already be a collector plume there and it would reduce the effects of plumes in much of the pelagic environment. This would minimize damage in the mesopelagic, which provides the ecosystem services of carbon flux and provisioning of commercially important species. For vent systems, benthic dewatering plume placement could mean into the main mining pit. Discharge of the dewatering plume into the vent system’s axial valley will restrict the spatial extent of the impact but this could adversely impact connectivity of vent communities between locations because some species rely on larval connections through axial valleys. The Mid-Atlantic Ridge (and other ridges) produces more mixing over elevated topography – within ~400-500 m the valley’s ridges. Thus, discharge into turbulent waters over the ridge will keep the plume suspended for a longer interval, exacerbating horizontal dispersal and possibly interfering with dispersal of larvae within this zone. There was then debate about the pros and cons of a midwater discharge or a benthic (axial valley) discharge depth. An intermediate option would be to discharge below the depth of the mesopelagic (~1000 m) but above the depth of turbulent mixing above the axial valley or ridge. Such a deep midwater discharge was considered the least worst option.

Finally, the group evaluated “How will the effects on organisms alter ecosystem function?” Following from the depth discussion, the group agreed that discharge of the dewatering plume in the axial valley would be catastrophic for the benthopelagic community – including the important larval community which mediates vent connectivity. Discharge in the mesopelagic would also be damaging for the reasons identified above. An intermediate depth of discharge could reduce alterations to major ecosystem functions. Additionally, the group suggested that lethal effects will lead to species/group removal from the food web or could lead to species replacement in diverse food webs with trophic or functional redundancy. Sub-lethal or chronic effects (including behavioral avoidance and migration) will potentially impact the rates of ecosystem functions. In addition, it is important to note that rates of discharge may determine rates of faunal removal or replacement but rates of species dispersal/advection may control the rate of and potential for recovery.

Several insights were shared by comparing potential mining impacts to the FAO criteria defining significance of impact from bottom fisheries. The FAO fisheries criteria define intensity and severity but in the case of pelagic ecosystems, these criteria may require experimentation to constrain their quantitative meaning. The FAO criteria further suggest the importance of ‘the spatial extent of the impact relative to the availability of the habitat type affected’ Some of vent mineral resources are relatively small in spatial extent, so the spatial extent of the associated communities could also be considered relatively small on or near the seafloor. However, the scale of the associated ‘bathypelagic habitat’ is uncertain. If it is viewed as ‘large’ then the potential available habitat is also large potentially minimizing the issue. However, in this habitat some species are long-lived (decadal generation times for some species in the bathypelagic). Furthermore, this approach to the broad-scale effects does not consider local hotspots such as the Northern MAR, where large, reproductive individuals may aggregate for reproduction (Sutton et al. 2008). So, even though the habitat may be viewed as large generally, smaller scale/localized disturbance or displacement may lead to loss of reproductive potential with long-term effects on the community.

The seamount group first evaluated effects across groups (taxonomic and functional) and by major environmental change. Regarding changes in light levels, the main issue is that sediment plumes will reduce light in the mesopelagic with consequences for vertical migration (taxa seek isolumes). Noise will be generated from hydraulic pumps along the riser pipes in midwater and also from near bottom grinding of seamount crusts. This could have metabolic costs for marine mammals, turtles, sharks, and tunas that then might avoid the seamount and associated foraging opportunities. The bathypelagic is very quiet and it was thought that the animals living there would be the most sensitive to both vibration and noise in the water column. The noise perturbation might reduce predation success or predator avoidance if sensory systems were overwhelmed (‘deafening effect’).

From the sediment plume itself, effects will also likely vary across taxa and functional groups. Microbes could react quickly to the new plume conditions, perhaps benefitting from increases in microbial habitat (particle surfaces) or even a food source (e.g. resuspended organics) but specific effects remain untested. Fishes and other visual predators (e.g. cephalopods, some crustaceans) were hypothesized to be strongly affected. Many have image forming eyes and are visual predators such that predation success would be reduced. Indeed myctophids, a very abundant, speciose and ecologically important mesopelagic family of fishes (Brodeur and Yamamura 2005), are not naturally found in turbid waters. They may be especially sensitive in terms of gill clogging, an effect that could be species dependent. This effect could reduce respiratory efficiency, which would be a sublethal effect. In contrast to fishes, gelatinous taxa may have their filtering apparatus clogged very quickly, leading to lethal or sublethal effects, which is a major concern because these taxa are central in midwater food webs as prey for other taxa and to carbon cycling and packaging processes (Conley et al. 2018). For selective filter feeders, plumes will decrease food quality by introducing large volumes of inorganic particles. Thus, these animals will have to increase effort to acquire beneficial organic particles. Overall, it was concluded that suspension feeders would be the group most affected by sediment plumes.

The group also considered how these effects might vary with plume particle size, concentration and/or extent. Plumes from cobalt crust/seamount mining may not have as many very fine particles, at least for the collector plume. However, it was unclear what the particle sizes would be in the dewatering plume because engineering specifications and the nature of the ground ore were not clear. For metals and toxins, seamount mining would grind material and dissolution chemistry will be important at the seafloor and at the depth of the dewatering plume. Seamounts are sites of enhanced benthopelagic coupling (Clark et al. 2010, Preciado et al. 2017) suggesting that effects could be translocated via food webs across the water column depending upon the depths of coupling. Thus the group had a strong recommendation to discharge below the bottom extent of the mesopelagic migrators (at least 1000 m) ideally right back to where the material was extracted (if below 1000 m). The bathypelagic has lower animal density and possibly diversity (to the best of our knowledge).

The group also considered the potential for seamount mining to alter ecosystem function, which was mostly an effect of the sediment plume. Vertical carbon transport is an essential ecosystem service provided by the water column community. Roughly 80% of active carbon transport occurs through planktivores, in the middle of the food web, rather than larger top predators. Disruption of these active migrators could have serious implications for carbon transport and food webs. Bathypelagic food webs are arguably the most tightly coupled and are dominated by detrital feeders and species with generalist diets (Drazen and Sutton 2017, Gloeckler et al. 2018). There was concern that trophic cascades were possible in this system. For instance, if the basal detritivores/ suspension feeders were eliminated the whole ecosystem could be affected. Thus, the sediment concentration (or dilution factor) at depth of the plume becomes a critical variable and reduction of particulate discharges is of great importance. Further, the hydrographic regime at the seamount controls the potential for mixing and dispersal of the plume and thus the associated scale of impact in the large bathypelagic habitat. Finally, mining sediment plumes have the potential to alter overall water column metabolism or oxygen demand, depending on chemistry, microbial response, and increased stress across diverse animal taxa.

In terms of the temporal and spatial scales of the midwater effects from seamount mining, the group had several comments and concerns. Seamount mining will be patchy in time or space, not 30 years of continuous mining on one feature and thus somewhat different to nodule mining. The spatial extent could be a small halo around the seamount and not as widespread as other forms of mining. The temporal extent may be only a few years on a single seamount but it is likely that multiple seamounts in a region would be exploited over time. In was concluded that for seamount pelagic communities there was the potential for fairly rapid recovery from impacts, at least locally because epi- and mesopelagic animals would be advected into the area after mining ceased. In addition, the epipelagic would recover faster than the mesopelagic, and the mesopelagic faster than the bathypelagic, due to the relative productivities and generation times of the fauna in these zones. Relatively fast pelagic community recovery is in direct contrast to seamount benthic communities which contain centenarian species and from our knowledge of past fisheries damage will take decades to centuries to recover (Clark et al. 2016).

The intensity or severity of effects on groups or on ecosystem function will likely be dependent on the depth of plume discharge. Discharge in the epipelagic could cause rapid advection and mixing away from the seamount itself thus having a lower localized impact but one spread over a larger area (also affecting primary production). Mesopelagic discharge depths were considered as the worse zone from an environmental (high biomass and diversity) and ecosystem services perspective. Discharging sediments into the bathypelagic was considered to have potentially the lowest environmental impact but we still know so little about this zone that this conclusion could be driven by an absence of knowledge.

An overarching knowledge gap that was identified, incorporating physical and engineering terms, was the absence of dynamic plume models. Plume models require baseline information such as background vertical particle flux (Lutz et al. 2007) and suspended particle loads (particle size, shape and mass concentration; some information available in Gardner et al. 2018), input from general particle-tracking models, as well as data on plume discharge parameters and mining vehicle information. The latter is critical for understanding exactly where and how the plumes will be discharged. These models need to include predicted changes to optical properties, namely light attenuation and backscatter, which can inform changes to animal’s perceptional distances. With such models and realistic mining scenarios some bounds on the scale of ecosystem effects can be generated.

Physical oceanographic knowledge gaps exist, some of which are high priority to fill. Many of these gaps are considered technologically feasible to complete, and therefore considered low-hanging fruit. The first is the need for nested (global/regional/site) particle tracking models. Such models would incorporate the regional and site-specific observational data. The second knowledge gap concerns ocean observations to assimilate into physical oceanographic models, including basic advection structure, background vertical diffusion and variation in circulation over time. These were all considered a very high priority. The third physical knowledge gap identified is the lack of detailed bathymetry. These likely will be collected by contractors and should be made publicly available. However, currently there is no mandate to collect detailed bathymetry in the APEIs of the CCZ, which is required to evaluate their representivity as reserves of biodiversity and ecosystem function in the region.

Chemical oceanographic knowledge gaps include background concentrations of some nodule relevant metals. Concentrations of some elements and metals have already been determined by the GEOTRACES program. A synthesis of this chemical information for the CCZ should be assembled. This chemical information is a priority to inform toxicological studies and for assigning thresholds for mining operations.

There were a number of knowledge gaps in primary biological knowledge for the midwaters over the CCZ. There is very little information for pelagic communities below the epipelagic, and none from the bathypelagic. Thus, a high priority is to determine the abundance, biomass, diversity, and community structure of deep pelagic communities (gelatinous and crustacean zooplankton, micronekton and nekton) and the spatial and temporal variability of these parameters, particularly in the bathypelagic and benthic boundary layer. These data could be acquired using multi-frequency acoustics (for micronekton and crustaceans), optical survey approaches for gelatinous taxa (e.g. AUV or ROV video transects), and net sampling for hard-bodied forms. It would take several years of study to quantify variability in these parameters across seasonal and interannual time scales. Another high priority is characterization of vital rates and life history parameters such as feeding rates, reproductive rates, generation times, maturation time, size at maturity, and growth rates for a broad suite of pelagic taxa. These rates and parameters are very important for understanding organisms’ sensitivities to disturbance and potential recovery rates. Some of these data can be acquired through conventional analysis of physical specimens (feeding, age and growth, size at maturity, fecundity) but others (e,g. rates of feeding, annual reproduction) are very challenging to ascertain.

Major biological data gaps for which measurement was deemed technologically very difficult or perhaps impossible were also identified. One such gap was to determine the effects of particles from mining plumes on suspension feeders. What are the exposure thresholds for sublethal and lethal effects (dose-response effects)? It will be important to evaluate both acute effects (e.g. visible signs of clogging from short-term exposure) and chronic effects (e.g. increased metabolic costs of foraging from repeated exposure). The former would be easier to ascertain, likely deriving from in situ midwater plume dispersal experiments (which were recognized to be very difficult) and ROV observations of suspension feeders. The latter would be extremely difficult and perhaps impossible, as it would require monitoring individuals over days to months. Given the challenges to understanding how plumes will affect suspension feeders, it was concluded that perhaps the only viable strategy would be monitoring changes in abundance of the fauna over time in response to pilot and full-scale mining activities. Because pilot mining very likely will occur over smaller space and time scales than full-scale mining, chronic effects may not be addressable until full scale mining occurs.

Another biological data gap identified was the sensitivity of pelagic animals to diminished light. This is very difficult to measure and our current knowledge is based on estimates of visual sensitivity from experiments on animal eyes in the laboratory, and from water-column properties (Warrant and Locket 2004). Some abyssal fishes have large eyes indicating adaptations for greater visual sensitivity. Thus, studies of visual sensitivity should be extended to the animals near the abyssal seafloor.

The first knowledge gap that the vents breakout group identified was the spatial and temporal extent (or distribution) of sediment plumes (both near seafloor collector and midwater dewatering plumes). This information is required to evaluate the true magnitude of pelagic ecosystem consequences for mining in vent (or any) regions. The first step for filling this gap is modeling the nearfield sediment plume properties and generation. This step couples naturally with contractor engineering efforts and could result in iterative designs to minimize sediment plume extent. The group also felt that such models needed to be validated with empirical plume tracking studies, including measurements from AUVs, CTD casts, and chemical tracers. Further, in conjunction with sediment plume modeling and empirical tracking there is a clear need to evaluate the nature of the dissolved chemical plumes. As for the sediment phase of plumes, the chemical phase will depend on mining techniques including the grinding and transport of deposits and shipboard separation of ore.

The background chemistry in vent regions has been characterized, but there is a chemical knowledge gap, namely how the ground ore and disturbed sediment will leach and weather metals and other compounds into the water column. This information is important to evaluate potential mining impacts from a toxicological point of view. Lab experiments using sulphide deposits could measure leaching and weathering rates.

Three important biological knowledge gaps were identified. First, the sensitivities and thresholds to (dissolved) contaminants and suspended sediments/particle loads are poorly known. Initial experiments with vent animals by the MIDAS project provided some insights (Hauton et al. 2017). These studies suggested both synergistic and antagonistic effects of multiple metal exposure. Thus, the group considered that the best approach for future studies would be to evaluate the toxicity of the bulk resource (not individual metals) for a particular mining location. This would involve lab exposure experiments on diverse community components (microbes, phytoplankton, larger organisms). These data could be combined in the form of a “weight of evidence approach” to quantify the relative bulk toxicity of the resource. The group also discussed how, based on laboratory toxicology studies, contractors might work in real time to assess impacts. There are rapid phytoplankton-based assays available that could be used to test the toxicity of discharged waters and their bulk ore leachates, but these tests (e.g. MICROTOX assays) on phytoplankton or microbes may not be relevant to deep-sea taxa.

Another biological knowledge gap identified was the in situ determination of microbial activity and how mining will affect it. We have estimates of in situ activities such as oxygen consumption etc. Therefore, there are some pre-existing data in some regions but more is needed and particularly in response to mining plumes. It was thought that the best approach to begin filling this knowledge gap would be incubation experiments coupled with metagenomics.

Finally, the group considered the biology of the bathypelagic and concluded that our understanding of this zone is a critical knowledge gap. This system likely will be affected by both near bottom collector plumes and potentially midwater dewatering plumes. Thus, we need to focus research in this region as a very high priority. Some information exists for the Mid-Atlantic Ridge (Cook et al. 2013, Sutton et al. 2013) that is pertinent to mining claims in that region but other regions are wholly unstudied. Contractors may place more baseline and monitoring attention in the bathypelagic if this becomes the zone of discharge but currently contractors are not studying this region. Important parameters include diversity, community structure, food web structure, biogeography (and thus habitat size), and the regions contribution to ecosystem services. The best approach to addressing this knowledge gap was discussed. The group agreed that the ideal combination of censusing approaches would be depth-stratified trawling for hard-bodied zooplankton and micronekton, combined with visual transects to quantify gelatinous and fragile taxa (e.g. ROV, AUV, or towed visual surveys).

The seamount group identified characterization of bathypelagic flow around seamounts as an important knowledge gap with relevance to understanding sediment and chemical plume dispersal. Empirical measurements are not technologically limited and could characterize the variability, strength, and seasonality of flow.

From the chemical oceanographic perspective, a baseline of trace metal concentrations is a knowledge gap that can be filled. This information is lacking on seamounts though regional values may be available from previous tracer studies. In association with this data gap was a need to have more information on trace metal concentrations of seamount-associated pelagic fauna, particularly species of interest to fisheries. Again these data can be acquired rather easily from commercial fisheries catches.

This group concluded that the tracking of plume spread and spatial extent were major information gaps preventing quantitative estimates of the scale of ecosystem damage. The group agreed that this was a universal data gap across the resource/habitat types. A key element for filling this knowledge gap was the need for more logistical information regarding the mining process. Plume tracking is still a technological challenge, particularly in complex flow environments, because the plume may generate filaments or complex shapes. Thus, comprehensive tracking with repetitive AUV or glider monitoring seemed a good approach to take.

Another knowledge gap was the ambient sound environment around seamounts. Some passive acoustic data on shallower seamounts exists (e.g. Giorli et al. 2015), but data are required on the specific features to be mined. These would be relatively easy to acquire as the passive-acoustic monitoring moorings are commercially available. Indeed, the group wondered if this instrumentation could be paired with other mooring based instruments such as current meters and active acoustic pingers to evaluate midwater biomass and vertical migration behavior over a seamount. Some sound thresholds and impacts on marine mammals are known but there is no information on deep-water fishes.

A major biological data gap identified was a quantitative description of the bathypelagic community both within the near-field of the seamount and in the surrounding background community. Such data are available for some mesopelagic seamount assemblages (Porteiro and Sutton 2007), but the bathypelagic is very poorly studied worldwide. Data on the composition and abundance of the fauna would be required and could be garnered with a combination of quantitative net sampling, imaging for gelatinous forms, and a splat camera to measure bioluminescence. It was suggested that a database should be created from trawl caught specimens, including identity and sequence data, to inform future eDNA studies. In addition, shipboard and deep towed (or AUV mounted) bioacoustics surveys need to be conducted to evaluate the characteristics of deep sound scattering layers of organisms. These community characteristics should be evaluated over at least seasonal time scales (and this was considered true across all of the habitat/resource types). When conducting such studies the group felt that for seamounts there should be some focus on the connectivity between the meso and bathypelagic zones and the seamount demersal fauna (such coupling is well recognized from past studies). Further, the seamount associated hyperbenthic or benthopelagic fauna should also be surveyed. These fauna would be part of the near-field midwater assemblages and many members would likely be resident and possibly endemic. Methods to sample these faunas were suggested to be terrain following rather than pre-defined, discrete-depth sampling of the water column, because distance from the seafloor might be the most important environmental determinant.

Understanding the sensitivity of gelatinous plankton and detritivores to plumes was considered a major knowledge gap with important implications for understanding how plumes will alter midwater communities. It was also considered a universal knowledge gap across habitats. Though important, this was considered a technologically challenging arena to gain knowledge due to the difficulty of keeping these fragile animals in captivity or conducting controlled in situ experiments. Some headway might be possible through synthesis of existing related studies from riverine, estuarine and coastal habitats. However, it was noted that these fauna and their systems are quite different to those in the deep ocean.

The group further concluded that animal sensitivities to environmental disturbance and hence the actual impact from mining was generally unknown and a gap for all taxonomic categories. The group thought that measuring acute impacts would be difficult but possible through monitoring of avoidance reactions, and changes in abundance, for instance. Assessing chronic impacts would be impossible at this stage because such studies would require long term monitoring in situ or require animals in controlled laboratory settings to evaluate effects on vital processes such as age and growth, productivity and metabolism.

Finally, the group felt that ongoing efforts by the ISA and scientists to bring together contractor data (e.g. topography, physics, biology and other non-proprietary information from mining claims) and making it publicly available was a key way to further expand our knowledge and refine knowledge gaps.