Deep-sea Mining Discharge Can Disrupt Midwater Food Webs

Dowd, M.H., Assad, V.E., Cazares-Nuesser, A.E. et al. Deep-sea mining discharge can disrupt midwater food webs. Nat Commun 16, 9575 (2025). https://doi.org/10.1038/s41467-025-65411-w

The Clarion-Clipperton Zone contains extensive beds of polymetallic nodules on the abyssal seafloor, with vast areas (~1.5 million km²) under license for deep-sea mining. Mining companies have proposed discharging excess waste generated during nodule extraction in the lower mesopelagic and upper bathypelagic zones, which are home to a unique faunal community including zooplankton and micronekton. Here, using compound-specific isotope analysis of amino acids, we find that natural background particles larger than 6 µm form the base of the food web, but will be diluted by the same sized, nutritionally deficient mining-associated particles. Given that 53% of zooplankton taxa are particle feeders and 60% of micronekton taxa are zooplanktivores at proposed discharge depths, there is significant potential for food-web disruption. Therefore, we show that a midwater mining plume could trigger bottom-up ecosystem impacts with potentially severe consequences for the faunal community, extending beyond zooplankton and micronekton to nekton, including large marine predators.

Introduction

The Clarion-Clipperton Zone (CCZ) in the Eastern Topical Pacific is a region of great interest for proposed deep-sea mining of polymetallic nodules. The International Seabed Authority, the UN established regulatory agency¹, has granted 19 exploration licenses for polymetallic nodules covering a vast area of ~1.5 million km². Previous research has documented environmental risks of mining, including biodiversity loss in seafloor communities^2,3. In addition to the seafloor, midwater communities may also be at risk. During the mining process, nodules are collected from the abyssal seafloor, along with seawater and sediments, and transferred through a pipe to a collection ship for separation of nodules from sediment waste. This seawater-containing sediment waste, as well as pulverized nodule particles, must be returned to the ocean. Although the release depth for this waste is currently unclear, some mining operators have proposed midwater mining discharge within the lower mesopelagic/upper bathypelagic zone^4,5. The risks to midwater communities remain poorly understood.

A midwater mining waste plume is anticipated to negatively affect mesopelagic fauna, with impacts ranging from replacing or diluting the food of filter feeders to visual impairment of active predation and bioluminescent communication^6,7. Pelagic food webs are complex, linking small primary producers in the upper ocean to top consumers at a range of depths. Diel vertical migration from deeper daytime depths to shallower nighttime foraging depths is common, vertically mixing the food web^8,9. Additionally, about 5–25% of the organic matter produced in the euphotic zone is exported to deeper depths through passive sinking and active transport^10,11. The material that reaches the mesopelagic undergoes aerobic microbial decomposition and transformation, resulting in a characteristic decrease in organic carbon and nitrogen content^12,13. The mining plume, consisting of deep abyssal sediments from <2 µm to 63 µm in size¹⁴, is predicted to have a high concentration of low-quality particles, along with fragmented inorganic nodules. These mining waste particles could clog the respiratory and olfactory surfaces of organisms across all trophic levels and obstruct filter-feeding structures^6,15.

Of importance to midwater food webs in the CCZ region is the presence of an oxygen minimum zone (OMZ), a region characterized by particularly hypoxic conditions (to <1 µM)¹⁶. Oxygen gradients influence the depth distribution of zooplankton and micronekton communities^17,18, as biological adaptations to low oxygen create species-specific habitat thresholds that determine where organisms reside and feed^19,20. In the mesopelagic, zooplankton biomass may exhibit a modest increase in the lower oxycline (LO) in comparison to the OMZ core^18,21, however, highly mobile and migratory micronekton (small fish, cephalopods, and crustacea, ~2–20 cm in size⁹ often reside at these depths^17,18.

Compound-specific isotope analysis of amino acids (CSIA-AA) can identify the particle size fraction that forms the base of the food web^22,23, and it can characterize trophic structure of the community^24,25. Understanding the contribution of different particle size fractions to the base of the food web is critical, as mining plumes will introduce particles of varying abundance, nutritional content, and quality depending on their size. This knowledge is essential for assessing the potential impacts of midwater mining plumes on the faunal community at depth.

In this study, we apply a Bayesian mixing model²⁶ to isotopically differentiate particle sources based on size and quantify their contribution to the base of the food web. The goal of this study is to characterize the trophic sources of the faunal food web at a proposed lower mesopelagic discharge depth to evaluate how the introduction of a midwater mining waste plume could disrupt it. We examine both the composition of mining discharge plume material and the nutritional quality at the base of the food web, along with the trophic modes of the animals living there. This work provides a critical step towards understanding the potential impacts of full-scale mining on the mesopelagic community.

Results and discussion

During this study, we collected 3 types of particle samples: background, discharge, and plume samples in 3 size fractions, small (0.7–6 μm), medium (6–53 μm), and large (>53 μm) (Fig. 1). Background samples were collected at various depths (800–1500 m) during 3 cruises, either when no mining activity was occurring or at locations and depths far from mining activity, ensuring that these particles represent natural midwater particles. Discharge and plume samples were collected in Fall 2022 during a small-scale test mining operation. During this operation, a collector vehicle on the abyssal seafloor collected sediments and nodules, which were brought to a surface ship. The nodules were separated from abyssal sediments onboard, and the effluent waste was released at ~1250 m. This event represented a rare opportunity to study midwater discharge associated with deep-sea mining. Discharge particle samples consisted of shipboard effluent waste collected after nodules were removed but before midwater release. Plume particle samples were collected in situ after the mining waste was discharged, using transmissometry to measure turbidity and confirm that we were sampling within the plume.

Background particle data were obtained as a proxy for the potential base of the food web from previous cruises where particles, zooplankton, and micronekton were collected. Isotopic analysis was performed to characterize these particles for use in a mixing model to identify the base of the food web. This approach allows us to evaluate how the introduction of mining waste might affect the pelagic community in the future. Because plume generation lasted a maximum of 32 hours and no animal samples were collected during the Fall 2022 cruise, we lack direct data on the incorporation of plume material into the food web.

We found that mining-associated particles were nutritionally poor. Background particle amino acid concentrations were 4.7 ± 2.7, 41.1 ± 25.3, and 46.3 ± 34.7 ngN/μgPN for small, medium, and large particles, respectively. In contrast, plume and discharge particle amino acid concentrations were substantially lower at 3.8 ± 4.4, 1.7 ± 1.5, and 4.2 ± 4.7 ngN/μgPN (Fig. 1 and Table S1). There was no significant difference in amino acid concentration of small particles between background and plume/discharge samples (p = 0.663, one-way ANOVA, Table S2). However, medium and large background particles had significantly higher amino acid concentrations compared to plume/discharge particles (p_Medium = 0.028, p_Large = 0.035, one-way ANOVA, Fig. 1 and Table S2). These findings indicate that the medium and large particles in the plume were significantly depleted in amino acid content relative to background material.

Background particle concentrations were much lower than plume particle concentrations. During the Fall 2022 plume sampling, we used a LISST (Laser In Situ Scattering and Transmissometry, LISST-DEEP, Sequoia Scientific) mounted on the CTD rosette to measure in situ particle concentration and size in 32 logarithmically spaced classes centered between 1.36 and 230.14 μm. As the medium and large plume and discharge particle pools were significantly different in amino acid content compared to background particles, samples were pooled into small (0.7–6 μm) and large (>6 μm, medium + large) size fractions for this analysis. Background particle concentrations were 0.08 and 0.23 μL/L for small and large size fractions, respectively, while plume particle concentrations were 9.80 and 2.18 μL/L for the same size fractions (Fig. 2 and Table S3). Background particle counts were 0.014 and 0.003 × 10⁸ particles/L for small and large size fractions, respectively, compared to plume particle counts of 11.15 and 0.12 × 10⁸ particles/L for small and large size fractions (Table S4).

Previous analyses of CCZ abyssal seafloor sediments suggest that sedimentary particles range from <2 µm to 63 µm in size¹⁴. The plume particle size distribution showed maxima between 2.6 and 4.3 µm (Fig. S1). Visual analysis of plume samples corroborates this pattern, showing an increase of small (0.7–6 μm) particles in the plume compared to the background. These particles were likely sedimentary in nature due to its light brown color, while dark pulverized nodule particles were observed in the 6–53 μm size range (Fig. 1). It should be noted that the “plume” is not a well-defined entity but rather an area where particle concentrations form a gradient exceeding background levels (see Supplementary Note 1). Some parts of this plume may be highly opaque, while others are only slightly turbid. Consequently, our particle concentrations represent a point along the continuum of the plume. The maximum particle concentration and count reflect the extent of our sampling but may not necessarily represent the true maximum within the plume itself.

To assess the potential impact of a plume on the faunal community, it is first necessary to understand the current contribution of particles as trophic sources to the base of the food web. Using δ¹⁵N values of source and δ¹³C values of essential amino acids (see Supplementary Note 2, Tables S5 and S6, and Fig. S2), we constructed a Bayesian mixing model to estimate plausible ranges to consumers. While diel vertical migration (DVM) is a common phenomenon and occurs at this site, an epipelagic source was not included in the mixing model, a decision based on several considerations. First, the consumer samples analyzed were collected during nighttime tows, meaning that most individuals sampled were likely non-migratory residents. Second, only ~20% of the zooplankton biomass present below the OMZ in the deep mesopelagic migrates to the upper ocean at night. Finally, the micronekton taxa included in this analysis are non-migratory, so any influence from an epipelagic DVM source from DVM should be minimal.

We found that particles >6 µm make up a significant proportion of the base of the food web in lower mesopelagic and upper bathypelagic depths below the OMZ (Fig. 3). Of the 46 animal samples collected below the OMZ (700 to 1500 m), these large particles accounted for at least 50% of the food web trophic base in 30 samples. Specifically, at the discharge depth (1000–1500 m), 16 of the 26 animal samples had large particles making up at least 50% of the food web base (Fig. 3, Source Data 3). Overall, on an individual animal basis, 65% of consumers relied on particles >6 µm for more than half of their food web base. When grouped by zooplankton size fraction/micronekton taxa, 5 of the 8 groups showed a mean contribution of particles >6 µm exceeding 50% (Fig. 3 and Table S7).

In sum, our mixing model indicates that large particles make a substantial contribution to the base of the food web below the OMZ (Fig. 3), a region expected to be impacted by mining waste plumes. Furthermore, we found that large plume and discharge particles are significantly lower in amino acid content (Fig. 1) and significantly higher in concentration compared to background particles (Fig. 2). This material would serve as the main food source for fauna, and the reduced amino acid concentration of these particles is a key indicator of their reduced nutritional value, leading to lower protein intake for the same foraging effort. Together, these findings show that a mining plume would drastically dilute the native particle supply for suspension feeders, replacing it with low nutritional quality seafloor sediments and inorganic fragmented nodules, which could inhibit the ability of organisms to meet their metabolic demands by diluting the relatively organic-rich native particles^27,28. This alteration of the in situ particle field has serious implications for the faunal community at the depth of the proposed mining plume.

The breadth of impact of the midwater plume on pelagic food webs is likely to be high based on our knowledge of the diversity and trophic ecology of zooplankton and micronekton below the OMZ. The deep (700–1500 m) zooplankton community contained 186 amplicon sequence variants (ASVs) that were classified into 79 distinct taxa (e.g., genus, family) based on whole-community metabarcoding (18S rDNA), with 53% classified as particle feeders (proportions were nearly identical for just the discharge depth from 1000 to 1500 m; Supplementary Data 1). These taxa, including calanoid copepods, ostracods, and oncaeid copepods rely on suspended particulate organic matter as a primary food source^29,30,31,32and may be particularly vulnerable to resuspended sediments from mining plumes. Euphausiids and midwater shrimp may also be impacted as they are known to opportunistically consume detrital aggregates at depth^33,34. In addition, gelatinous taxa such as siphonophores, tunicates, and scyphozoans accounted for 20% of the taxa in this deep midwater community, increasing in relative read abundance with depth. These organisms may be particularly susceptible to plume-related impacts, as suspended particles can adhere to their gelatinous bodies, clog feeding structures, disrupt buoyancy, and increase mucous production and energy demands^6,7 (zooplankton community analysis detailed further in Supplementary Note 3: Functional Ecology Analysis, Zooplankton Functional Group Results). Given the importance of particle-feeding and gelatinous taxa at midwater depths, mining-induced plumes could significantly disrupt zooplankton communities and trophic interactions, with cascading effects on micronekton and higher trophic level predators. Indeed, we found 60% of 80 unique micronekton taxa from 700 to 1500 m were zooplanktivores, 37.5% were pelagic micronektonivores (feeding on smaller zooplanktivorous micronekton and occasionally zooplankton), 1.25% were gelatinous zooplanktivores, and 1.25% were pelagic generalists (proportions were nearly identical for just the discharge depth from 1000 to 1500 m; Supplementary Data 2). Furthermore, 85% of the density of micronekton within the discharge depth is zooplanktivores (micronekton community analysis detailed further in Supplementary Note 4). Specifically, among the micronekton sampled for CSIA-AA, Cyclothone spp., Eucopia spp., and Japatella spp. are zooplanktivores^35,36, whereas Acanthephyra brevicarinata shrimps are classified as pelagic micronektonivores^34,37. Overall, the results suggest that the potential impacts on the zooplankton community due to mining plumes could significantly diminish the primary food source for micronekton, potentially leading to starvation or, for more mobile micronekton, migration from the area.

Accurately identifying the scale of these food web effects is challenging but essential to consider. Factors such as particle flow rate, particle size, and concentration thresholds all significantly influence the vertical and horizontal dispersion, and consequently, the volume of water affected by the sediment discharge^38,39. Depending on the exact metrics applied, the scale of effect of the impact is estimated to be on the order of 1000 s km³/yr^38,39. Given the potential for multiple simultaneous operations in the eastern CCZ, this represents considerable environmental risk to these communities. They provide essential ecosystem services, playing crucial roles in the biological pump^8,40, provisioning commercially harvested fishes^6,9,41, and maintaining a healthy, functioning ecosystem (Fig. 4).

There are potential options for mitigating the effects of the plume, though each comes with its own risks. Discharging at a shallower depth than the proposed discharge depth could result in similar or worse impacts, because it would affect regions of the water column with significantly higher biomass and important top predators^18,21. Releasing the mining plume in the oxyclines above and below the OMZ core, or within the OMZ core itself, could also be especially harmful. Some zooplankton species are highly sensitive to small oxygen variations and adjust their vertical position to remain within their preferred oxygen levels^17,19,21. In low oxygen conditions (<10 μM), changes in zooplankton abundance have been linked to oxygen concentration shifts of only a few micromolars²¹. This finely tuned behavior could be disrupted by introducing an oxygenated midwater mining waste plume into the oxyclines or within the OMZ core (see Supplementary Note 1, Table S8 and Fig. S3). A number of zooplankton and micronekton also vertically migrate within the top 1000 m, potentially expanding the zone of a plume’s influence^18,19. Some larger nekton also dive into these depths to forage^42,43, although it should be noted that this may be limited by the strong OMZ in this region. Discharging below the current depth, into the deeper bathypelagic, remains largely unstudied. Limited data suggest that while the bathypelagic is an area of low abundance, it harbors high biodiversity, including species that are distinct from the mesopelagic^44,45. Furthermore, this community may depend on large particles as the base of its food web, like the processes observed from 700 to 1500 m in this study. Introducing a mining plume into this zone could therefore have comparable adverse effects.

A final potential option is to return the sediment waste to the seafloor. In the case of nodule mining, a seafloor sediment plume will already be generated by the collector vehicle⁴⁶. A few studies of mining tests suggest that this plume may behave as a turbidity flow, staying within 10’s of meters of the seabed^46,47. Introduction of the mining waste material near the seabed could alter the dynamics of this plume, requiring further study and modelling to evaluate its effects on dispersion. However, this approach may reduce the overall impact footprint by releasing one mining plume instead of two. Given our results showing that a midwater release will disrupt whole food webs, including those linked to seafood sources, bringing the discharge plume back down to the seafloor may minimize environmental risks, and we urge that it be considered.

In conclusion, this study provides empirical evidence that discharge mining waste in midwaters is likely to disrupt midwater food webs. The base of the zooplankton and micronekton food webs at this site, located below the OMZ, rely heavily on large particles (65% of consumer samples). Approximately half of zooplankton taxa in this community are particle feeders, employing suspension feeding or detritivory, and their diets would be severely impacted by the introduction of a midwater mining waste plume composed of similarly-sized particles but of lower nutritional quality. As more than half of the micronekton taxa in this community are zooplanktivores, the micronekton community would also face severe disruption due to the reduction or loss of their zooplankton food source. There are confirmed stress responses to sediment exposure in gelatinous plankton⁷, and other risks such as suffocation and toxic metal exposure are also possible. This unique and diverse faunal community is at significant risk if full-scale mining proceeds (Fig. 4). Mitigation is possible through careful selection of discharge depths by mining companies, combined with regulation by national and international agencies.

Photo by Eunkwang Choi on Unsplash