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The abyssal seafloor in the Clarion-Clipperton Fracture Zone (CCZ) in the central Pacific is covered with large densities of polymetallic nodules, i.e., metal concretions containing iron, manganese, nickel, cobalt, and copper. Nodules are of economic importance for these metals, but they also host a variety of deep-sea fauna. In a recent study it was estimated that the removal of these nodules would lead to a loss of up to 18% of all taxa in the CCZ. Here, I assess the impact of removing these nodule-dependent taxa on carbon cycling at two sites (B4S03, B6S02) of the Belgian exploration license area in the eastern CCZ. For this purpose, I developed two highly resolved carbon-based food web models with 71 (B6S02) to 75 (B4S03) food-web compartments consisting of different detritus pools, bacteria, metazoan meiobenthos, macrobenthic isopods, polychaetes and other macrobenthos, megabenthic cnidarians, crustaceans, poriferans, holothurians and other invertebrate megabenthos, and fish. These compartments were connected with 303 (B6S02) to 336 (B4S03) links which were reduced by 5–9% when nodule-dependent faunal compartments were removed. The models estimated the “total system throughput” T .. i.e., the sum of all carbon flows in the food webs, in intact food webs as 1.18 mmol C m -2 d -1 and 1.20 mmol C m -2 d -1 at B4S03 and B6S02, respectively, whereby 69.8% (B6S02) to 71.2% (B4S03) of T. . flowed through the microbial loop. A removal of the nodule-dependent fauna did not affect this microbial loop but reduced the scavenger loop by 56.5% (B6S02) to 71.6% (B4S03). Overall, nodule-dependent fauna is responsible for only a small fraction of total carbon cycling at the eastern CCZ. Therefore, when the effect of prospective deep-seabed mining on carbon cycling is investigated, its impact on benthic prokaryotes and the microbial loop should be addressed specifically.

Polymetallic nodule fields provide hard substrate for sessile organisms on the abyssal seafloor between 3000 and 6000 m water depth. Deep-seabed mining targets these mineral-rich nodules and will likely modify the consumer-resource (trophic) and substrate-providing (non-trophic) interactions within the abyssal food web. However, the importance of nodules and their associated sessile fauna in supporting food-web integrity remains unclear. Here, we use seafloor imagery and published literature to develop highly-resolved trophic and non-trophic interaction webs for the Clarion-Clipperton Fracture Zone (CCZ, central Pacific Ocean) and the Peru Basin (PB, South-East Pacific Ocean) and to assess how nodule removal may modify these networks. The CCZ interaction web included 1028 compartments connected with 59,793 links and the PB interaction web consisted of 342 compartments and 8044 links. We show that knock-down effects of nodule removal resulted in a 17.9% (CCZ) to 20.8% (PB) loss of all taxa and 22.8% (PB) to 30.6% (CCZ) loss of network links. Subsequent analysis identified stalked glass sponges living attached to the nodules as key structural species that supported a high diversity of associated fauna. We conclude that polymetallic nodules are critical for food-web integrity and that their absence will likely result in reduced local benthic biodiversity.

Benthic prokaryotes include Bacteria and Archaea and dominate densities of marine benthos. They play major roles in element cycles and heterotrophic, chemoautotrophic, and phototrophic carbon production. To understand how anthropogenic disturbances and climate change might affect these processes, better estimates of prokaryotic biomasses and densities are required. Hence, I developed the ProkaBioDen database, the largest open-access database of benthic prokaryotic biomasses and densities in marine surface sediments. In total, the database comprises 1,089 georeferenced benthic prokaryotic biomass and 1,875 density records extracted from 85 and 112 studies, respectively. I identified all references applying the procedures for systematic reviews and meta analyses and report prokaryotic biomasses as g C cm −3 sediment, g C g −1 sediment, and g C m −2 . Density records are presented as cell cm −3 sediment, cell g −1 sediment/ sulfide/ vent precipitate, and cell m −2 . This database should serve as reference to close sampling gaps in the future. Measurement(s) prokaryotic benthic biomass • prokaryotic benthic density Technology Type(s) PLFA • microscopy • ATP Sample Characteristic - Organism unclassified Bacteria Sample Characteristic - Environment marine sediment • deep marine sediment • shallow marine sediment Sample Characteristic - Location Pacific Ocean • Atlantic Ocean • Indian Ocean • Southern Ocean • Arctic Ocean

The cycling of carbon (C) by benthic organisms is a key ecosystem function in the deep sea. Pulse-chase experiments are designed to quantify this process, yet few studies have been carried out using abyssal (3500–6000 m) sediments and only a handful of studies have been undertaken in situ. We undertook eight in situ pulse-chase experiments in three abyssal strata (4050–4200 m water depth) separated by tens to hundreds of kilometers in the eastern Clarion-Clipperton Fracture Zone (CCFZ). These experiments demonstrated that benthic bacteria dominated the consumption of phytodetritus over short (∼ 1.5 d) time scales, with metazoan macrofauna playing a minor role. These results contrast with the only other comparable in situ abyssal study, where macrofauna dominated phytodetritus assimilation over short (2.5 d) time scales in the eutrophic NE Atlantic. We also demonstrated that benthic bacteria were capable of converting dissolved inorganic C into biomass and showed that this process can occur at rates that are as high as the bacterial assimilation of algalderived organic C. This demonstrates the potential importance of inorganic C uptake to abyssal ecosystems in this region. It also alludes to the possibility that some of the C incorporation by bacteria in our algal-addition studies may have resulted from the incorporation of labeled dissolved inorganic carbon initially respired by other unstudied organisms. Our findings reveal the key importance of benthic bacteria in the short-term cycling of C in abyssal habitats in the eastern CCFZ and provide important information on benthic ecosystem functioning in an area targeted for commercial-scale, deep-sea mining activities.

Seamounts are isolated underwater mountains stretching > 1000 m above the seafloor. They are identified as biodiversity hotspots of marine life, and host benthic assemblages that may vary on regional (among seamounts) and local (within seamounts) scales. Here, we collected seafloor imagery of three seamounts at the Langseth Ridge in the central Arctic Ocean to assess habitats and megabenthos community composition at the Central Mount (CM), the Karasik Seamount (KS), and the Northern Mount (NM). The majority of seafloor across these seamounts comprised bare rock, covered with a mixed layer of sponge spicule mats intermixed with detrital debris composed of polychaete tubes, and sand, gravel, and/or rocks. The megabenthos assemblages consisted of in total 15 invertebrate epibenthos taxa and 4 fish taxa, contributing to mean megabenthos densities of 55,745 ind. ha −1 at CM, 110,442 ind. ha −1 at KS, and 65,849 ind. ha −1 at NM. The faunal assemblages at all three seamounts were dominated by habitat-forming Tetractinellida sponges that contributed between 66% (KS) and 85% (CM) to all megabenthos. Interestingly, taxa richness did not differ at regional and local scale, whereas the megabenthos community composition did. Abiotic and biogenic factors shaping distinct habitat types played a major role in structuring of benthic communities in high-Arctic seamounts.

Benthic fauna refers to all fauna that live in or on the seafloor, which researchers typically divide into size classes meiobenthos (32/64 µm–0.5/1 mm), macrobenthos (250 µm–1 cm), and megabenthos (>1 cm). Benthic fauna play important roles in bioturbation activity, mineralization of organic matter, and in marine food webs. Evaluating their role in these ecosystem functions requires knowledge of their global distribution and biomass. We therefore established the BenBioDen database, the largest open-access database for marine benthic biomass and density data compiled so far. In total, it includes 11,792 georeferenced benthic biomass and 51,559 benthic density records from 384 and 600 studies, respectively. We selected all references following the procedure for systematic reviews and meta-analyses, and report biomass records as grams of wet mass, dry mass, or ash-free dry mass, or carbon per m 2 and as abundance records as individuals per m 2 . This database provides a point of reference for future studies on the distribution and biomass of benthic fauna. Measurement(s) marine benthic feature • organic material • mass density Technology Type(s) digital curation Factor Type(s) geographic location • water depth • organism size class Sample Characteristic - Organism meiobenthos • macrobenthos • megabenthos Sample Characteristic - Environment marine benthic biome • ocean Sample Characteristic - Location Atlantic Ocean • Pacific Ocean • Arctic Ocean • Southern Ocean • Indian Ocean Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.12481985

Commercial-scale mining for polymetallic nodules could have a major impact on the deep-sea environment, but the effects of these mining activities on deep-sea ecosystems are very poorly known. The first commercial test mining for polymetallic nodules was carried out in 1970. Since then a number of small-scale commercial test mining or scientific disturbance studies have been carried out. Here we evaluate changes in faunal densities and diversity of benthic communities measured in response to these 11 simulated or test nodule mining disturbances using meta-analysis techniques. We find that impacts are often severe immediately after mining, with major negative changes in density and diversity of most groups occurring. However, in some cases, the mobile fauna and small-sized fauna experienced less negative impacts over the longer term. At seven sites in the Pacific, multiple surveys assessed recovery in fauna over periods of up to 26 years. Almost all studies show some recovery in faunal density and diversity for meiofauna and mobile megafauna, often within one year. However, very few faunal groups return to baseline or control conditions after two decades. The effects of polymetallic nodule mining are likely to be long term. Our analyses show considerable negative biological effects of seafloor nodule mining, even at the small scale of test mining experiments, although there is variation in sensitivity amongst organisms of different sizes and functional groups, which have important implications for ecosystem responses. Unfortunately, many past studies have limitations that reduce their effectiveness in determining responses. We provide recommendations to improve future mining impact test studies. Further research to assess the effects of test-mining activities will inform ways to improve mining practices and guide effective environmental management of mining activities.

The potential harvest of polymetallic nodules will heavily impact the abyssal, soft sediment ecosystem by removing sediment, hard substrate and associated fauna inside mined areas. It is therefore important to know whether the ecosystem can recover from this disturbance and if so at which rate. The first objective of this study was to measure recovery of phytodetritus processing by the benthic food web from a sediment disturbance experiment in 1989. The second objective was to determine the role of holothurians in the uptake of fresh phytodetritus by the benthic food web. To meet both objectives, large benthic incubation chambers (CUBEs; 50 × 50 × 50 cm) were deployed inside plough tracks (with and without holothurian presence) and at a reference site (holothurian presence, only) at 4100 m water depth. Shortly after deployment, 13C- and 15N-labelled phytodetritus was injected in the incubation chambers and during the subsequent three-day incubation period, water samples were taken five times to measure the production of 13C-dissolved inorganic carbon over time. At the end of the incubation, holothurians and sediment samples were taken to determine biomass, densities and incorporation of 13C and 15N into bacteria, nematodes, macrofauna and holothurians. For the first objective, the results showed that biomass of bacteria, nematodes and macrofauna did not differ between reference sites and plough track sites when holothurians were present. Additionally, meiofauna and macrofauna taxonomic composition was not significantly different between the sites. In contrast, total 13C uptake by bacteria, nematodes and holothurians was significantly lower at plough track sites compared to reference sites, though the number of replicates was low. This result suggests that important ecosystem functions such as organic matter processing has not fully recovered from the disturbance that occurred in 1989. For the second objective, the analysis indicated that holothurians incorporated 2.16×10-3 mmol labile phytodetritus C m-2 d-1 into their biomass, which is one order of magnitude less as compared to bacteria, but 1.3 times higher than macrofauna and one order of magnitude higher than nematodes. Additionally, holothurians incorporated more phytodetritus carbon per unit biomass than macrofauna and meiofauna, suggesting a size-dependence in phytodetritus carbon uptake.

Abyssal plain communities rely on the overlying water column for a settling flux of organic matter. The origin and rate of this flux as well as the controls on its fine-scale spatial distribution following seafloor settlement are largely unquantified. This is particularly true across regions where anthropogenically-induced seafloor disturbance has occurred. Here, we observed, quantified and mapped a mass deposition event of gelatinous zooplankton carcasses (pyrosomes) in July-September 2015 across one such physically disturbed region in the Peru Basin polymetallic nodule province (4150 m). Seafloor in this area was disturbed with a plough harrow in 1989 (as part of the DISCOL experiment) causing troughs in the sediment. Other parts were disturbed with an epibenthic sled (EBS) during a cruise in 2015 resulting in steep-walled, U-shaped troughs. We investigated two hypotheses: a) gelatinous food falls contribute significantly to the abyssal plain carbon pump and b) physical seafloor disturbance influences abyssal distribution of organic matter. We combined optical and bathymetric seafloor observations, to analyze pyrosome distribution on seabeds with different levels of disturbance. 2954 pyrosome colonies and associated taxa were detected in > 14,000 seafloor images. The mean regional carbon (C) deposition associated with pyrosome carcasses was significant compared to the flux of particulate organic C (182 to 1543%), and the total respired benthic C flux in the DISCOL Experimental Area (39 to 184%). EBS-disturbed seafloor tracks contained 72 times more pyrosome-associated C than an undisturbed reference site, and up to 4 times more than an area disturbed in 1989. Deposited pyrosomes collected had a higher proportion of labile fatty acids compared to the sediment. We document the temporal and spatial extent of an abyssal food fall event with unprecedented detail and show that physical seafloor disturbance results in the accumulation of detrital material. Such accumulation may reduce oxygen availability and alter benthic community structure. Understanding both the relevance of large food falls and the fine scale topography of the seafloor, is necessary for impact assessment of technologies altering seafloor integrity (e.g. as a result of bottom-trawling or deep seabed mining) and may improve their management on a global scale.

Snow cover on sea ice is the most important factor controlling light availability for sea ice algae, but it is predicted by climate models to become more variable and stochastic. Here, we document effects of a sudden, complete loss of the entire snow cover on first-year sea ice at Kangerlussuaq Fjord, West Greenland, due to a natural Föhn wind event that caused a ca. 17 °C air temperature increase over 36 h. We applied Imaging-PAM fluorometry to examine effects of snow cover on algal distribution and photobiology and observed a rapid decrease in algal biomass associated with loss of the skeletal ice crystal layer on the underside of the ice that had supported most of the visible algae. Furthermore, the remaining algae were photobiologically stressed, as seen in a significant decrease in the dark-acclimated fluorescence yield (ΦPSII_max) from 0.55 before snow loss to 0.41 after. However, recovery in the dark suggested that non-photosynthetic quenching was successfully dissipating excess energy in the community and that there was little photodamage. An observed decrease in the photosynthetic efficiency α from 0.22 to 0.16 µmol é m−2 s−1 is therefore likely to be due to photoacclimation and the change in community composition. Centric diatoms and flagellates were the main taxa lost in the snow loss event, whereas the sea ice specialist Nitzschia frigida increased in numbers. These observations are similar to those seen in artificial snow-clearing experiments and consistent with snow clearing being a useful approach for investigating the complex interactions between snow cover, irradiance fluctuations, and ice algal performance.