Micronekton communities in the oxygen minimum zone of the eastern Clarion-Clipperton Zone, Pacific Ocean, have been largely undescribed below 300 m depth. Yet, these communities could soon be impacted by deep-sea nodule mining. Deep-sea mining activities pose potential impacts to midwater ecosystems, including sediment plumes and release of dissolved metals that may be depth-specific but carried through the water column by vertically migrating organisms. Gathering baseline data on vertical distributions and migrations is thus essential. Micronekton samples were collected in spring and fall of 2021 from 0-1500 m depth during the day and night, utilizing a 10 m2 mouth-opening Multiple Opening Closing Net and Environmental Sensing System, at both a control site (intended for monitoring against mining impacts) and mining site, in the NORI-D area licensed by Nauru Ocean Resources, Inc. We found high amounts of migrating biomass moving at night from the lower oxycline and core of the oxygen minimum zone into the upper oxycline (1.9 & times; increase for fishes) and into the epipelagic zone (8.0 & times; for fishes, 29.4 & times; for crustacea excluding euphausiids, and 45.6 & times; for euphausiids). We documented the vertical distributions and migration patterns of 52 taxa, finding 22 vertical migrators and 30 non-migrating taxa. Micronekton diel vertical migration patterns and vertical distributions corresponded to spatiotemporal changes in oxygen concentrations (and other potential factors that are not part of this article but reported in a companion study) and many seasonal shifts in distributions at both the broad taxonomic level and species level were observed. We found shifts in populations of different species of the genus Cyclothone that may be due to changes in water masses or currents. Given these results, particularly evidence of significant seasonal shifts, we considered how deep-sea mining discharge plumes may affect the mesopelagic micronekton community in the CCZ.

ABSTRACT Aim Marine species ranges are often large, contributing to relatively high community similarity over space, with decreased ranges expected among taxa that have lower dispersal and/or higher substrate requirements. We tested for a weakening of biogeographic structuring among dispersing abyssal taxa along the benthic to holoplanktonic habitat continuum by comparing geographic ranges and rates of community dissimilarity across the eastern North Pacific seafloor (spanning a total of 3700 km between sites). Location Abyssal Pacific Ocean, within the Clarion‐Clipperton Zone (CCZ). Time Period Present. Major Taxa Studied Benthic, benthopelagic and holoplanktonic invertebrates (18 phyla). Methods We sampled zooplankton in situ via dual pumps attached to a benthic lander and used bulk community metabarcoding (18S V1V2, 18S V7V8, mtCOI) to characterise a diverse suite of taxa and define habitat assemblages. We measured (i) minimum geographic range, (ii) distance‐decay rates and (iii) environmental structuring, with each measure grouped by habitat assemblage to test the effect of seafloor affinity in shaping biogeography. Patterns were also assessed within broad taxonomic groups to evaluate the degree of phylogenetic constraint. Results Distance‐decay rates increased with increasing affinity to the seafloor, and potential environmental drivers differed between benthic and pelagic assemblages. Unexpectedly, range size did not differ significantly by habitat occupancy and instead varied phylogenetically, with taxa that are sessile on polymetallic nodules (e.g., bryozoans, hydroids) having among the smallest observed ranges. Main Conclusions Our results indicate a significant role for habitat occupancy in shaping biogeographic community patterns in the abyss, though community dissimilarity is likely determined by habitat availability rather than by dispersal. In the context of deep‐sea biodiversity conservation planning, we demonstrate that dispersal ranges are insufficient to connect populations inhabiting impacted areas to protected areas for select taxonomic groups, indicative of potentially high extinction risk. We suggest that marine protected areas should capture a broad range of habitats and environmental variability to protect entire abyssal communities.

The deep sea contains a wealth of potential mineral resources, many of which are being investigated for commercial exploitation. Exploration and technical tests started in the late 1970s with an initial focus on polymetallic nodules from an abyssal region in the eastern Pacific Ocean called the Clarion-Clipperton Zone (CCZ). More recently, exploration has commenced at seafloor massive sulphides (formed by hydrothermal vents) and seamounts (for cobalt-rich crusts). Here we review the many decades of environmental research in these areas with a focus on the biodiversity baseline and the results from experimental mining or disturbance tests. Data from the CCZ have revealed that the seafloor is biodiverse, albeit at low biomass. For nodule mining, geophysical and biological impacts are persistent over at least multiple decades, although some dominant animal groups show successional recolonisation. By contrast, exploration for minerals at sulphide deposits and seamounts has been quite limited, empirical data on recovery timescales are lacking and these systems are likely to respond differently to disturbance. Deep-sea mining in any habitat could potentially generate plumes from ore dewatering which may affect pelagic ecosystems depending on the technology used. For all types of deep-sea mining, we distinguish ecological resilience and recovery from the risk of biodiversity loss (species extinction). Taxonomic data based on collections are critical to this debate; for abyssal fauna we can only currently hypothesise species ranges based on habitat availability. For vents, and to some degree seamounts, there is clear evidence that biodiversity loss from deep-sea mining is likely. If these sites were to be classified as areas of ‘high biodiversity importance' under the Convention on Biological Diversity, deep-sea mining at them would not be scientifically compatible with existing policy.

Abstract The bluntnose sixgill shark (Hexanchus griseus) is a globally distributed large‐bodied predator, primarily associated with deep‐sea environments. Due to the logistical constraints of its habitat, much remains unknown about the life history and behavior of H. griseus. While typically considered solitary, this species has occasionally been observed in aggregate; however, these events are almost exclusively associated with foraging or the presence of introduced bait. Here, we present a novel unbaited observation of a mixed‐sex aggregation of at least seven H. griseus individuals near Ulong Channel, Palau. This represents the first peer‐reviewed report of such an aggregation in Micronesia. Observed in close proximity at relatively shallow daytime depths via a remotely operated vehicle (ROV), the individuals did not exhibit foraging behavior. The absence of attractants and presence of scarring on female individuals suggest potential social or reproductive drivers for this aggregation. A subsequent resighting of two individuals at the same location indicates short‐term site fidelity. These findings provide rare in situ insight into the social structure of H. griseus and highlight the potential importance of this area as a critical habitat for this species.

Micronekton are a vital part of midwater food webs and have the potential to be impacted by deep-sea mining via the release of sediment plumes, including dissolved metals. Micronekton communities in the Clarion-Clipperton Zone (CCZ) region of the eastern Pacific Ocean have been studied very little; baseline community descriptions are needed should mining commence. Micronekton samples were collected from 0 m to 1,500 m during the day and night in spring (March-April) and fall (October-November) of 2021, prior to mining activities, utilizing a 10 m2 Multiple Opening Closing Net and Environmental Sensing System. Trawls were conducted at both a Preservation Reference Zone (intended for monitoring against mining impacts) and a designated mining site (approximately 60 nautical miles or 111 km apart) in the NORI-D license area of the eastern CCZ, licensed to Nauru Ocean Resources Inc. by the International Seabed Authority. We found higher springtime densities and biomasses of fishes, crustaceans, and cephalopods compared to fall samples, principally due to juvenile recruitment. Micronekton community composition and seasonal patterns in biodiversity levels were distinctly different between the sites. Seasonal differences were likely driven by primary productivity and the seasonal dynamics of the North Equatorial Current, North Equatorial Countercurrent, and eddies. Differences in abundances and biomass between sites suggest that the control area may not be representative of the mining site. Due to the oceanographic variability found in NORI-D, and the lack of long-term time-series studies in the CCZ, additional sampling is needed to establish a sufficient baseline from which the potential impacts of deep-sea mining could be separated from natural variation.

Spanning from the North American continent to the Hawaiian Ridge and from the Aleutian Ridge to the tip of the Baja California Peninsula, the deep Northeast Pacific (>200 m depth) hosts a high diversity of species and habitats. While relatively well studied in comparison to other regions, deep-sea ecosystems of the Northeast Pacific still hold many unknowns. This review provides an overview of the current state of knowledge of deep pelagic and benthic ecosystems in the Northeast Pacific in the context of climate change and increasing anthropogenic pressures. Regional trends in biodiversity, area-based management measures and research effort are synthesized to identify gaps in scientific knowledge, management and conservation. Based on the compilation of available literature and data extracted from open access databases, we found that scientific knowledge and research effort were unevenly distributed among habitats, with the most widespread habitats (e.g., abyssopelagic and abyssal plains, areas beyond national jurisdiction – ABNJ) being the most undersampled. Similarly, the distribution and level of protection of marine protected areas varied between countries and deep-sea habitat types, with virtually no protection in ABNJ. This review provides a comprehensive summary for scientists and managers, including recommendations on how to fill remaining scientific knowledge gaps, expand deep-sea observations to ABNJ and poorly known habitats, increase capacity for deep-sea research, improve data sharing and better support conservation and management. These recommendations aim to prioritize future research and improve sustainable management in alignment with the goals of the United Nations Decade of Ocean Science for Sustainable Development.

The Island Mass Effect (IME) is the nearshore enhancement of primary productivity around islands and atolls relative to offshore waters. Although its physical and biogeochemical drivers are well characterized, the IME’s influence on the diets and distributions of consumers remains poorly resolved. We applied amino acid compound-specific stable isotope analysis (AA-CSIA) to Hawaiian zooplankton sampled across nearshore–offshore and surface–deep gradients to test whether island-derived production alters isotopic composition and trophic structure in reef-associated assemblages relative to offshore counterparts across sites, seasons, and years. Essential amino acid δ13C values (δ13CEAA) normalized to their mean values displayed contrasting nearshore–offshore patterns: lysine and threonine δ13C values increased with distance from shore, whereas phenylalanine and valine values decreased. These patterns likely reflect shifts in zooplankton diet and the amino acid biosynthetic pathways of their primary producer prey along the coastal–oceanic gradient. Source amino acid δ15N values (δ15NSAA) declined offshore for lysine and phenylalanine but increased with depth, indicating spatial variation in nitrogen sources and greater reliance on microbially reworked organic matter at depth. Trophic position estimates based on δ15N values of glutamic acid and alanine relative to phenylalanine increased offshore and with depth, consistent with longer food webs and additional microzooplankton trophic steps in offshore waters. Multivariate analysis integrating δ13CEAA and δ15NSAA values clearly distinguished reef, offshore surface, and offshore deep zooplankton assemblages, revealing a conservative isotopic tracer of island-derived production in reef communities. These results demonstrate AA-CSIA’s utility for tracing island-derived productivity to consumers and clarifying biogeochemical connectivity between coastal and open-ocean food webs.

Background:There been increasing interest in polymetallic nodule mining within the Clarion-Clipperton Zone (CCZ). Polymetallic nodule mining within NORI-D will release a sediment plume within the water column and a previous mining collector test within the Nauru Ocean Resources Inc. (NORI-D) contract area released surface pollution from mining tailings. The mid-water plume, as well as accidental surface pollution, indicate that polymetallic nodule mining could impact surface plankton. Although the ichthyoplankton within the eastern tropical Pacific have been well-studied, recent data from within polymetallic nodule mining licence areas is lacking. Environmental Expedition C5e conducted an environmental baseline assessment of both pelagic and benthic fauna within the NORI-D region of the CCZ, which included the opportunistic collection of ichthyoplankton. New information:Ichthyoplankton were collected within NORI-D from November-December 2021 using two plankton nets and a Remotely Operated Vehicle (ROV). Here, we present a checklist of ichthyoplankton within the NORI-D licence area during this winter campaign. Eighteen samples were collected and identified through morphology, with a limited number identified through genetic sequencing. Specimens were from five orders, including Argentiniformes, Stomiiformes, Myctophiformes, Beloniformes and Scombriformes. This checklist will aid contractors and scientists conducting work within the CCZ to examine how wastewater discharge from polymetallic nodule mining could impact fish reproduction and ichthyoplankton survival.

While several software packages have been developed to solve stable isotope mixing models, none are currently equipped to trace the flow of organic matter through the lower trophic levels of planktonic food webs. To address this gap, we have developed a new Bayesian mixing model tailored for use with δ15N values of individual amino acids. This model simultaneously estimates trophic relationships between consumers and organic matter sources at the base of the food web, determines the relative contributions of these basal organic matter sources to consumers, and accounts for trophic discrimination affecting amino acid δ15N values during protozoan and metazoan trophic steps. This “Organic Matter Supply Model” is uniquely suited for applications where food web structure is unknown and trophic intermediaries, such as protozoan and metazoan grazers with distinct amino acid trophic discrimination factors, play a critical role in nutrient transfer. In this paper, we describe the model’s basic structure, outline key considerations for adapting it to specific applications, evaluate its performance using simulated zooplankton data, discuss its strengths and limitations, and offer recommendations for its further development. By testing the model on simulated zooplankton amino acid δ15N data, we demonstrate that the Organic Matter Supply Model can enhance our understanding of the roles of small particles and diel vertical migration in deep-sea organic matter supply pathways. Furthermore, it provides a new framework for exploring the foundational role of heterotrophic protists in marine ecosystems. We find specific subsets of amino acids to be most useful as markers of trophic ecology (in this case including glutamic acid and proline) and to identify supply from basal organic matter sources (phenylalanine, lysine, and threonine). Other amino acids may be more ideal source tracers in other settings, although amino acids with inconsistent or poorly constrained isotope fractionation behavior (e.g., isoleucine, valine) should be excluded to optimize model reliability.

Open-ocean pelagic habitats are inherently difficult to study due to their inaccessibility, which limits our ability to properly sample the dynamic nature of these environments. The biology of the pelagic habitat of the eastern Pacific Ocean remains poorly sampled. This region is characterized by the presence of an expanding oxygen minimum zone and an emerging deep-sea mining industry. Here, we provide an integrated assessment of micronekton assemblages across two sampling occasions (spring and fall) using Saildrone active acoustics, shipboard trawls (with a multiple opening and closing net and environmental sensing system), and remotely operated vehicle video footage. Together these surveys provide the most comprehensive, multi-method evaluation of micronekton vertical distributions and migratory behaviors in the remote eastern Pacific Ocean to date. Integrated over 1000 m, acoustic and trawl data showed similar overall patterns with greater total backscatter, abundance, and biomass during the spring compared to fall surveys. However, variability in the vertical distributions of these metrics differed between acoustic and trawl data. The main scattering layers, which accounted for most of the pelagic acoustic backscatter, occurred between 250 m and 500 m (day) and above 100 m (night), but micronekton abundance and biomass from trawl catch were often greatest below 700 m. Trawl collections and video footage collected across depths greatly expanded the observed depth range of the relatively shallow micronekton distributions suggested by surface-based acoustic profiles and provided very different perspectives on taxonomic diversity. Video observations further highlighted the considerable and diverse gelatinous community not observed from trawl collections. We identified several common gas-bearing fish taxa from dissections of trawl-caught fish as the most likely contributors to acoustic backscatter: small ridgeheads (Melamphaidae), lanternfish (Myctophidae), hatchetfish (Sternoptychidae), and lightfish (Phosichthyidae). Our study highlights the need for integrating information from multiple sampling approaches to gain a holistic understanding of pelagic ecosystems.

Descriptions of bathypelagic and abyssopelagic micronekton communities anywhere in the global ocean are rare. This study reports the first description of these communities in the eastern Clarion-Clipperton Zone, a region of great interest for polymetallic nodule mining at the deep seafloor. This research was part of an environmental baseline survey conducted prior to mining. Micronekton and macroplankton were sampled using large (10 m(2) ) depth-discrete net trawls from 1,500 m to about 4,200 m, within 100 m of the abyssal seafloor. Density and biomass were very low below 1,500 m (3.5-7.5 individuals 10,000 m(-3) and 2.6-18.4 g 10,000 m(-3) ), but biomass of caridean shrimps from 100-300 m above bottom was about 5-fold higher than observed within bathypelagic depths (<3,000 m). This near-seafloor biomass peak was predominantly the result of very large shrimps, much larger than specimens of the same species collected from mesopelagic depths, suggesting an ontogenetic migration to great depths. At all depths sampled, including the abyssopelagic (>3,000 m), the community was dominated by crustaceans, mostly shrimps, mysidacea, and hyperiid amphipods. Fishes were relatively abundant to depths of 2,000 m, and cephalopods were absent. Large nemertean and polychaete worms were very abundant macroplankton. The community differed significantly between bathypelagic (more fish, giant ostracods, and hyperiid amphipods) and abyssopelagic (more polychaetes and high biomass of shrimps) depths. Though not all taxa were identified to species level, numerous taxa were found only below 1,500 m, highlighting the unique communities within bathypelagic and abyssopelagic depths. Minimizing the effects of mining discharge plumes on deep-midwater ecology and communities would be achieved best by returning the discharge water to the seafloor near, or at, the same location of the collector plume, which would also minimize the spatial footprint of environmental risks.

Deep-sea mining is expected to cause disturbances of sufficient scale and intensity to pose a risk to biodiversity and ecosystem function.1,2,3 We assess the potential impact of deep-sea mining on sharks, rays, and chimaeras in Areas Beyond National Jurisdiction (ABNJ) and identify 30 species (of the total 1,223 marine chondrichthyan species) that overlap spatially with the anticipated mining footprint, specifically through 2 pathways: benthic impacts from physical disturbance and the collector vehicle plume2 and midwater impacts from the discharge plume.4 Most species' depth ranges (83%, 25/30, range: 3%-80%) overlapped vertically with the benthic mining footprint, while all species overlapped with discharge plume scenarios. Further, 17 of these species had >50% depth overlap with benthic impacts of at least one of the mineral types. Seven species were egg-laying, benthic, or benthopelagic, which increases their susceptibility to seabed impacts. Filter-feeding species also had high depth overlap with potential midwater discharge plumes. Nearly two-thirds (60%, 18/30) are already threatened with an elevated risk of extinction, and 64.3% are predicted to be threatened. Our analysis raises concerns that deep-sea mining would compound and worsen their extinction risk. We recommend updated risk assessments of significant adverse impacts to chondrichthyans; robust baseline monitoring prior to, during, and after mining; spatial protections near crust and sulfide mining; and that the discharge plume be set at a minimum depth below 2,000 m or at the seabed to minimize overlap with midwater species.

Understanding the movement ecology and resource use of pelagic fishes is essential for their conservation and management. Molids, as predators of gelatinous zooplankton, play important ecological roles in the gelatinous food webs and undertake long-distance migrations across the Pacific Ocean. This study used a novel combination of isotope-based approaches, including isoscapes, compound-specific isotope analysis of amino acids, and Bayesian mixing models, to investigate the migration patterns and habitat uses of molids (Mola mola and M. alexandrini) at a population level in the western Pacific Ocean. Results from isoscapes revealed that most molids in Japan, Taiwan, and New Caledonia might be local residents, with measured δ15N values aligning with predicted isoscape values. However, some larger individuals in Japan and Taiwan showed δ15N values higher than predicted, suggesting recent migrations from isotopically distinct regions. In New Zealand, the measured δ15N values of molids did not overlap with predicted values, indicating non-residency. Source amino acid δ15N values and Bayesian mixing models suggest that local residents in Japan and Taiwan primarily consumed local prey, whereas recent migrants with higher δ15N values may originate from the warm pool region, potentially linked to spawning behavior. Molids in New Caledonia and New Zealand likely migrate from regions with isotopic values distinct from the local food web base. The cross-regional movements of molids highlight the importance of understanding connectivity within the western Pacific Ocean. Identifying migratory corridors and key habitats, such as spawning and feeding grounds, is essential for developing regional and international conservation strategies that ensure the long-term sustainability of molid populations.

Ecological studies of many pelagic, deep-sea animals are often limited due to their remote habitats and sampling constraints. In the eastern tropical Pacific, numerous studies have been published about the distribution of demersal fishes across the region, yet studies for midwater taxa are lacking. Here, we utilized visual survey transects from an area targeted for polymetallic nodule mining to examine the drivers behind the distribution of two mesopelagic predators, snipe eels (family Nemichthyidae) and sawtooth eels (family Serrivomeridae). Additionally, we compared the vertical distributions of their prey and examined whether the two eels displayed different behaviors in situ. Time of day was not a statistically significant driver of eel distribution, although the shallowest observations were during the night, indicating that some in the populations of Nemichthyidae and Serrivomeridae had migrated weakly to the upper 200 m. Eel distributions were found to be affected by both depth and dissolved oxygen, with Nemichthyidae observed in shallower and lower oxygenated waters and Serrivomeridae in deeper and more oxygenated ones. Two prey taxa (families Euphausiidae and Sergestidae) were abundant at shallower transect depths, which likely drives the distribution of Nemichthyidae, as pelagic crustaceans constitute their primary prey. In terms of behavior, Serrivomeridae were more active and were oriented primarily vertically, suggesting that Nemichthyidae may be less active or under lower predation pressure due to the hypoxic environment in which they occur. This work highlights the importance of visual surveys in obtaining in situ data as a complement to traditional sampling methods, particularly for collecting data on poorly understood animals.

Particulate organic matter (POM) produced in surface waters undergoes extensive reworking and breakdown by microbial and metazoan communities as it sinks to the abyssal seafloor and serves as the base of benthic and pelagic food webs. Here, we examined how various size classes of POM in the oligotrophic North Pacific Subtropical Gyre (Station ALOHA) and in the eutrophic California Current System (Station M) undergo microbial alteration throughout the water column. Compound-specific stable isotope analysis showed that sampling method strongly impacts the type of POM quantified as export to the deep sea. Moored abyssal sediment traps captured material that matched the isotopic composition of surface POM, indicating they collected large, fastsinking particles, in contrast to the more heavily reworked particles collected with in situ filtration at the same depths. Extending delta N-15 analyses of amino acids to bathypelagic and abyssopelagic depths for the first time, we confirmed that most particle remineralization and reworking occurs within the upper similar to 400 m of the water column regardless of initial surface productivity. At Station ALOHA, we further used 16S rRNA barcoding to characterize the microbial communities associated with the POM. We found that chemolithoautotrophic ammonia-oxidizing archaea are abundant in the upper water column at Station ALOHA and that their abundance corresponded to regions of high heterotrophic reworking as indicated by amino acid isotope analysis.

The abyssal benthic boundary layer (BBL) community relies on pelagic organic matter (OM) export, yet community dynamics under changing surface productivity remain poorly understood. Here, we describe temporal variability within the BBL zooplankton community, focusing on benthopelagic adults and meroplanktonic larvae at a site with seasonal OM flux. We test the hypothesis that BBL community composition, richness, and structure are stable over time, but with higher absolute abundances occurring under greater OM flux. Zooplankton were collected from three meters above the seafloor in the Nauru Ocean Resources Inc. D (NORI-D) mining contract area of the Clarion-Clipperton Zone in spring and fall of 2021 using a benthic lander mounted with two large-volume pumps (22 deployments) and analyzed by both morphotaxonomy and metabarcoding. Abundance of benthopelagic adults was elevated in springtime, as were abundances of bryozoan, bivalve, and gastropod larvae. Community structure was distinct between sampling periods (p = 0.002, R 2 = 0.18), with high rates of species replacement between seasons. Springtime patterns co-occurred with elevated surface productivity and OM flux, suggesting that distinct assemblages occur between periods of high and low OM flux. Biodiversity loss associated with polymetallic nodule mining is unlikely to be reduced if mining occurs only during select time periods, as high species richness and distinct community structure were observed in both sampling periods. Understanding the potential impacts of polymetallic nodule mining will require greater knowledge of abyssal BBL taxonomy and life history, and the nature of temporal variability in BBL communities, including climate-related shifts in OM fluxes to the seafloor.

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.

Deposit-feeding echinoderms are dominant megafauna on abyssal plains, where they consume organic detrital material at the base of the benthic food web. However, the strategies they use to survive on irregular pulses of poor-quality detritus remain poorly understood in many regions. Using compound-specific stable isotope analysis of amino acids, we found that deposit-feeding holothurians and echinoids in the oligotrophic North Pacific Subtropical Gyre are secondary rather than primary consumers of detritus, consistent with earlier findings from the productive California Current Ecosystem suggesting they consume gut microbial biomass or its products. At both sites, gut microbial communities were dominated by Actinobacteria, Proteobacteria, and Planctomycetes, and in some species, by the ammonia-oxidizing archaea Nitrosopumilales. Many of the more mobile species, including swimming holothurians, also contained high proportions of Cyanobacteria in their guts during high-flux seasons, demonstrating that fast-moving taxa can consume fresher phytodetritus. Using a mixing model based on delta 15N values of source amino acids, we found that deposit feeders capable of swimming consume a higher proportion of larger and fresher particles than obligate benthic species, directly linking swimming behavior to feeding selectivity for the first time. Differences in the gut microbiota of deposit feeders on abyssal plain ecosystems, characterized by both high- and low-flux regimes, correspond to niche partitioning based on detritus of differing nutritional qualities. We show that abyssal deposit-feeding echinoderms use multiple adaptations, including swimming behavior and gut microbial communities, to consume a variety of detrital food sources.

Mercury (Hg) is a potent neurotoxin that enters the food web and may contaminate commercial, recreational, subsistence, and ceremonial fish stocks. Understanding the pathways by which this contamination occurs in marine systems is thus an essential component of minimizing consumer health risk. Our knowledge of the biogeochemical cycling of mercury, however, is relatively limited. Temporal changes in sinking particulate mercury (PHg) fluxes throughout the upper 400 m were examined at Station ALOHA (22 degrees N, 158 degrees W) in the North Pacific Subtropical Gyre (NPSG) and spatially along a north-south transect to the Equator (17.5 degrees N to 5 degrees N x 155 degrees W) using a combination of in situ pumps and Uranium-238/Thorium-234 disequilibria as a tracer of particle export. Our results indicate that Station ALOHA is characterized by seasonally variable export fluxes of PHg, with highest fluxes occurring in May (175 m, 346 pmol m(-2) day(-1)), with the advent of summer zooplankton growth, and in September (400 m, 356 pmol m(-2) day(-1)), coinciding with a diazotroph mediated summer export pulse. PHg fluxes in May and September were higher than those previously measured in the equatorial Pacific at 150 m and continued to be high (> 100 pmol Hg m(2) d(-1)) down to 400 m, thereby providing a significant source of Hg to the mesopelagic food web. In contrast to Station ALOHA, at 8 and 5 degrees N, PHg fluxes attenuated rapidly with depth, and fluxes were generally lower, with a maximum flux of 86 pmol m(-2) d(-1) (5 degrees N). Depth profiles at 8 and 5 degrees N were significantly different from one another, with PHg fluxes higher throughout the water column at 5 degrees N and characterized by a subsurface peak in Hg flux 3 times higher than at 8 degrees N (86 vs. 29 pmol Hg m(-2) d(-1)). Monomethylmercury (MeHg) fluxes (max = 1.09 +/- 0.57 pmol m(-2) d(-1)) and concentrations (max = 0.14 fmol L-1) comprised only a small percentage of the total PHg pool. These results suggest that PHg cycling significantly differed between the NPSG and near the equator at least during an El Nino year. At Station ALOHA, microbial reworking of small particles below the deep chlorophyll maximum coupled with changes in zooplankton grazing drive seasonal export variability. In contrast near the equator, low fluxes associated with low biological productivity result in significantly lower PHg transport to depth during an El Nino year.