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The deep sea is amongst the most food-limited habitats on Earth, as only a small fraction ( Desmophyllum pertusum (formerly known as Lophelia pertusa), are well adapted to these fluctuations in food availability. Laboratory and in situ measurements revealed their dietary flexibility, tissue reserves, and temporal variation in growth and energy allocation. Thirdly, the high structural and functional diversity of CWC reefs increases resource retention: acting as giant filters and sustaining complex food webs with diverse recycling pathways, the reefs optimise resource gains over losses. Anthropogenic pressures, including climate change and ocean acidification, threaten this fragile equilibrium through decreased resource supply, increased energy costs, and dissolution of the calcium-carbonate reef framework. Based on this review, we suggest additional criteria to judge the health of CWC reefs and their chance to persist in the future.

Coastal areas are subjected to several anthropogenic stressors with much of the world’s intertidal areas receding due to human activities, coastal erosion, and sea level rise. The Eastern Scheldt (ES) tidal bay in The Netherlands is predicted to lose around 35 % of intertidal areas by 2060. Our study investigated differences between biogeochemical fluxes of intertidal and subtidal sediments of the ES and assessed how ongoing intertidal loss may modify the sedimentary ecosystem functioning of this tidal bay in the coming decades. Monthly fluxes and porewater concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), silica, carbon and oxygen (O2) as well as organic matter characteristics were measured from intertidal and subtidal sediments from June 2016 – December 2017. O2 fluxes were 37% higher in the intertidal, and these sediments exhibited influxes of nitrates (-1.2 mmol m-2 d-1) and DIP (-0.03 mmol m-2 d-1). In contrast, subtidal sediments exhibited an average efflux of nitrates (0.28 mmol m-2 d-1) and DIP (0.09 mmol m-2 d-1). Intertidal areas removal of DIN and DIP was 34 % and 38% higher than in the subtidal suggesting stronger denitrification and phosphorus adsorption to solid particles. The potential loss of biogeochemical functionality due to intertidal area loss by 2060 was estimated. In the next 40 years, the ES tidal bay may experience a reduction of 11 % and 8 % for respective nitrogen and phosphorus removal. Given the global observations of eroding intertidal areas and rising sea levels, we suggest that the predicted habitat loss may cause significant changes for coastal biogeochemistry and should be investigated further to understand its potential consequences for coastal ecosystems.

Internal tides are known to be an important source of mixing in the oceans, especially in the bottom boundary layer. The depth of internal-tide generation therefore seems important for benthic life and the formation of cold-water coral mounds, but internal-tidal conversion is generally investigated in a depth-integrated sense. Using both idealized and realistic simulations on continental slopes, we found that the depth of internal-tide generation increases with increasing slope steepness and decreases with intensified shallow stratification. The depth of internal-tide generation also shows a typical latitudinal dependency. Using a global database of cold-water corals, we found that the depth-pattern of internal-tide generation is remarkably similar to the depth-pattern of cold-water corals globally: shallowest near the poles and deepest around the equator with a shoaling around 25 degrees South and North and shallower north of the equator than south of the equator. We further found that cold-water corals are, more than what would be expected by chance, associated to the (super)critical reflection of internal tides (i.e., situated on topography that is steeper than the internal tidal beam) and to trapped internal tides (i.e., above the critical latitude of 70 degrees for semidiurnal tides and 30 degrees for diurnal tides). The (super)critical reflection of internal tides and trapped internal tides therefore provide an interesting new angle of food supply mechanisms that has not yet been considered in cold-water coral studies. With climate change, stratification is expected to increase. Based on our results, this would cause a shoaling of internal-tide generation, possibly creating new shallower suitable habitat for cold-water corals on continental slopes.

Holothurians are the dominant megabenthic deposit feeders in the Peru Basin (South-East Pacific) and feed to various degrees of selectively on the heterogenous pool of sedimentary detritus, but diet preferences for most holothurian species are unknown. This study reconstructs the diets of 13 holothurian species of the orders Elasipodida, Holothuriida, and Synallactida, from bulk stable isotope analyses (δ13C, δ15N) of holothurian body walls and guts, gut contents, and feces that were combined with compound-specific stable isotope analyses of amino acids, phospholipid-derived fatty acids, and neutral lipid-derived fatty acids in the body wall. Fatty acid concentrations showed high levels of storage lipids, an likely adaption to limited food supply to abyssal plains. Amino acid δ15N isotope values allowed estimating trophic levels of holothurian species and calculating heterotrophic re-synthesis of amino acids. Fatty acids served as trophic markers for feeding on diatom- and dinoflagellate derived phytodetritus, bacteria, Foraminifera, and detritus containing the PUFA C22:1ω9-cis. Several holothurian species seemed to be secondary consumers of detritus, while bacteria in their guts were primary consumers of this detritus. A Sørensen–Dice coefficient based cluster analysis using data of trophic levels, levels of heterotrophic re-synthesis of amino acids, feeding selectivity, and food sources/ diet suggested three trophic groups, characterized by different trophic levels. We show that this multi-biomarker driven approach allows to deconvolve trophic niches and feeding selectivity in one of the most challenging environments on earth and to identify dependence of deep-sea species to organic matter inputs that vary with season and/or climate.

The abyssal seafloor (3500–6000m) remains largely unexplored but with deep-sea mining imminent, anthropogenic impacts may soon reach abyssal communities. Thus, there is a growing need for baseline studies of biodiversity, ecosystem functioning, and connectivity in both potential mining and no-mining areas across the Clarion-Clipperton Zone (CCZ), a key target region for polymetallic nodule mining. In this study, in situ pulse-chase lander experiments were conducted for 1.5 days in three no-mining areas (called Areas of Particular Environmental Interest, or APEIs) in the western CCZ, a region with a seafloor particulate organic carbon (POC) flux gradient. A decreasing trend was seen in mean seafloor respiration, macrofaunal abundance, and biomass from the more eutrophic APEI 7 to the more oligotrophic APEI 1, although this trend was not statistically significant (p = 0.18) most likely due to small samples sizes and high variability. In this study, most (96%) of the 13C-labeled processed phytodetritus was respired within 1.5 days. Experimental uptake of phytodetritus by macrofauna and bacteria was detected but was lower than in the previously studied and more eutrophic eastern CCZ over similar time scales (1.5 d). Bacteria dominated the short-term (∼1.5 d) uptake of organic carbon at the seafloor, yet macrofauna processed more organic carbon per unit biomass than previously found in the eastern CCZ (0.003 mg C m −2 d −1 and 0.5 × 10 −5 mg C m −2 d −1 for the western and eastern CCZ, respectively). Our study provides important information on C-uptake and respiration rates in areas set aside from mining in the western CCZ and suggests high variability may occur in the rates of benthic Corg-cycling across the CCZ. We recommend that benthic ecosystem functions be explored across gradients of POC flux which may be a major environmental factor driving ecosystem dynamics in the CCZ.

Coral reefs in the northern Red Sea experience strong seasonality. This affects reef carbon (C) cycling, but ecosystem-wide quantification of C fluxes in such reefs is limited. This study quantified seasonal reef community C fluxes with incubations. Resulting data were then incorporated into seasonal linear inverse models (LIM). For spring, additional sponge incubation results allowed for unique assessment of the contribution of sponges to C cycling. The coral reef ecosystem was heterotrophic throughout all seasons as gross community primary production (GPP; 136–200, range of seasonal means in mmol C m−2 d−1) was less than community respiration (R; 192–279), and balanced by import of organic carbon (52–100), 88‒92% of which being dissolved organic carbon (DOC). Hard coral GPP (74–110) and R (100–137), as well as pelagic bacteria DOC uptake (58–101) and R (42–86), were the largest C fluxes across seasons. The ecosystem was least heterotrophic in spring (highest irradiance) (GPP:R 0.81), but most heterotrophic in summer and fall with higher water temperatures (0.68 and 0.60, respectively). Adding the sponge community to the model increased community R (247 ± 8 without to 353 ± 13 with sponges (mean ± SD)). Sponges balanced this demand primarily with DOC uptake (105 ± 6, 97% by cryptic sponges). This rate is comparable to the uptake of DOC by pelagic bacteria (104 ± 5) placing the cryptic sponges among the dominant C cycling groups in the reef.

Coastal areas are subjected to several anthropogenic stressors with much of the world's intertidal areas receding due to human activities, coastal erosion and sea level rise. The Dutch Eastern Scheldt (ES) has been predicted to lose around 35% of intertidal areas by 2060. This study investigates differences between biogeochemical fluxes of intertidal and subtidal sediments of the ES and assesses how ongoing erosion may modify the sedimentary ecosystem functioning of this coastal bay in the coming decades. Monthly fluxes and porewater concentrations of dissolved inorganicnitrogen (DIN), phosphorous (DIP), silica (DSi), carbon (DIC) and oxygen (O 2) as well as organic matter (OM) characteristics were measured from intertidal and subtidal sediments from June 2016–December 2017. Compared to subtidal stations, OM was significantly more reactive in intertidal samples and exhibited 37% higher O2 fluxes, suggesting a strong influence from microphytobenthos. Subtidal sedimentsexhibited an average efflux of nitrates (0.28 mmol m−2 d −1) and phosphates (0.09 mmol m−2 d−1) into the water column, while intertidal areas displayed average influxes (nitrates = −1.2 mmol m−2 d−1, phosphates = −0.03 mmol m−2 d−1) directed into the sediment. The calculated removal of total DIN and DIP from the water column was 34–38% higher in intertidal compared to subtidal samples suggesting stronger denitrification and phosphorus adsorption to solid particles from intertidal sediments. As an upscaling exercise, we estimate potential erosion induced changes if the ES stations are representative for the system. With this assumption, we estimate 11% and 8% reductions for respective nitrogen and phosphorus removal in the entire ES by 2060. Given the global observations of eroding intertidal areas and rising sea levels, we suggest that the predicted habitat loss may cause significant changes for coastal biogeochemistry and should be investigated further to understand its potential consequences.

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

Arctic coastal ecosystems are rapidly changing due to climate warming. This makes modeling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and simplify nutrient regeneration. These simplifications may lead to underestimations of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacterial ammonium remineralization, reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. Overall, model complexity in terms of the number of parameters is comparable to the phytoplankton growth and nutrient biogeochemistry formulations in common ecosystem models used in the Arctic while improving the representation of nutrient-co-limitation-related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

Estuarine systems filter nutrients and organic matter from riverine input and lower concentrations reaching the sea. Sediments within these ecosystems play a significant role in the mineralization and retention of nutrients and organic matter within the estuary. Such processes are influenced by abiotic (e.g. salinity, temperature, etc.) and biological (e.g. fluctuations in the benthic community) parameters which may contrast remarkably between intertidal or subtidal zones. Despite their relative importance, few studies have investigated the biogeochemistry of intertidal sediments with high spatiotemporal resolution. This study reports the results of monthly biogeochemical monitoring in intertidal muddy sediments along the salinity gradient of the Western Scheldt estuary (NL). Budgets of OM mineralization and nutrient retention were calculated for the fresh, brackish, and marine water zones. Temperature controlled sediment oxygen consumption rates and nutrient fluxes. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (-1.62 mmol DIN m-2 d-1 and -2.84 mmol DIN m-2 d-1, respectively), while the freshwater area had the only net influx of phosphate (-0.07 mmol m-2 d-1). Marine sediments showed net effluxes of DIN and DIP. Despite the net influx observed in freshwater sediments, geospatial analysis showed that their contribution to the total estuarine filtering capacity was minimal due to their small area. In contrast, brackish and marine regions had a more important contribution to the estuarine filter because of their larger surface area. Overall, sediments removed 11% (1,500 t N y-1) and 15% (~200 t P y-1) of the total nitrogen and phosphorus entering the estuary from riverine input. Our findings highlight the importance of using spatially-resolving remineralization budgets to improve models and nutrient cycling estimates in estuarine systems.

Cold-water coral (CWC) reefs are numerous and widespread along the Norwegian continental shelf where oil and gas industry operate. Uncertainties exist regarding their impacts from operational discharges to drilling. Effect thresholds obtained from near-realistic exposure of suspended particle concentrations for use in coral risk modeling are particularly needed. Here, nubbins of Desmophyllum pertusum (Lophelia pertusa) were exposed shortly (5 days, 4h repeated pulses) to suspended particles (bentonite BE; barite BA, and drill cuttings DC) in the range of ~ 4 to ~ 60 mg.l-1 (actual concentration). Physiological responses (respiration rate, growth rate, mucus-related particulate organic carbon OC and particulate organic nitrogen ON) and polyp mortality were then measured 2 and 6 weeks post-exposure to assess long-term effects. Respiration and growth rates were not significantly different in any of the treatments tested compared to control. OC production was not affected in any treatment, but a significant increase of OC:ON in mucus produced by BE-exposed (23 and 48 mg.l-1) corals was revealed 2 weeks after exposure. Polyp mortality increased significantly at the two highest DC doses (19 and 49 mg.l-1) 2 and 6 weeks post-exposure but no significant difference was observed in any of the other treatments compared to the control. These findings are adding new knowledge on coral resilience to short realistic exposure of suspended drill particles and indicate overall a risk for long-term effects at a threshold of ~20 mg.l-1.

Cold-water coral (CWC) reefs create hotspots of metabolic activity in the deep sea, in spite of the limited supply of fresh organic matter from the ocean surface (i.e. phytodetritus). We propose that ‘dead’ coral framework, which harbours diverse faunal and microbial communities, boosts the metabolic activity of the reefs, through enhanced resource retention and recycling. Analysis of a video transect across a 700-540 m-deep CWC mound (Rockall Bank, North-East Atlantic) revealed a high benthic cover of dead framework (64%). Box-cored fragments of dead framework were incubatedon-board and showed oxygen consumption rates of 0.078–0.182 μmol O 2 (mmol OC)−1 h−1, indicating a substantial contribution to the total metabolic activity of the CWC reef. During the incubations, it was shown that the framework degradation stage influences nitrogen (re)cycling, corresponding to differences in community composition. New (less-degraded) framework released ammonium(0.005 ± 0.001 μmol NH4+ (mmol OC)−1 h −1), probably due to the activity of ammonotelic macrofauna. In contrast, old (more-degraded) framework released nitrate(0.015 ± 0.008 μmol NO3− (mmol OC)−1 h −1), indicating that nitrifying microorganisms recycled fauna-excreted ammonium to nitrate. Furthermore, the framework community removed natural dissolved organic matter (DOM) from the incubation water (0.005–0.122 μmol C (mmol OC)−1 h−1). Additional feeding experiments showed that all functional groups and macrofauna taxa of the framework community incorporated 13C-enriched (‘labelled’) DOM, indicating widespread DOM uptake and recycling. Finally, the framework effectively retained 13C-enriched phytodetritus, (a) by physical retention on the biofilm-covered surface and (b) by biological filtration through suspension-feeding fauna. We therefore suggest that the dead framework acts as a ‘filtration-recycling factory’ that enhances the metabolic activity of CWC reefs. The exposed framework, however, is particularly vulnerable to ocean acidification, jeopardizing this important aspect of CWC reef functioning.

Nitrogen (N) is often the key nutrient limiting primary production in coastal waters. Quantifying sources and sinks of N is therefore critical to understanding the factors that underpin the productivity of coastal ecosystems. Constraining nitrogen inputs can be difficult for some terms such as N fixation and marine exchange as a consequence of uncertainties associated with scaling and stochasticity. To help overcome these issues, we undertook a N budget incorporating an isotope and mass balance to constrain N sources in a large oligotrophic coastal embayment (Western Port, Australia). The total N input to Western Port was calculated to be 1400Mg Nyear−1, which is remarkably consistent with previous estimates of sedimentation rates within the system. Catchment inputs, N fixation, marine sources and atmospheric deposition comprised 44, 28, 28 and 13% of N inputs respectively. Retention of marine-derived N equated to ~3 and ~10% of total N and NOx flushed through the system from the marine end-member. The relatively high contribution of N fixation compared with previous studies was most likely to be due to the high proportion of nutrient-limited intertidal sediments where N is mediated by seagrasses and sediment cyanobacteria.

Highlights • Total modeled carbon cycling at disturbed sites is lower than at reference sites. • Projected microbial loop functioning is reduced 26 years after sediment disturbance. • Estimated faunal respiration has recovered from sediment disturbance. • Estimated microbial respiration has not recovered from the sediment disturbance. Abstract Due to the predicted future demand for critical metals, abyssal plains covered with polymetallic nodules are currently being prospected for deep-seabed mining. Deep-seabed mining will lead to significant sediment disturbance over large spatial scales and for extended periods of time. The environmental impact of a small-scale sediment disturbance was studied during the ‘DISturbance and reCOLonization’ (DISCOL) experiment in the Peru Basin in 1989 when 10.8 km2 of seafloor were ploughed with a plough harrow. Here, we present a detailed description of carbon-based food-web models constructed from various datasets collected in 2015, 26 years after the experiment. Detailed observations of the benthic food web were made at three distinct sites: inside 26-year old plough tracks (IPT, subjected to direct impact from ploughing), outside the plough tracks (OPT, exposed to settling of resuspended sediment), and at reference sites (REF, no impact). The observations were used to develop highly-resolved food-web models for each site that quantified the carbon (C) fluxes between biotic (ranging from prokaryotes to various functional groups in meio-, macro-, and megafauna) and abiotic (e.g. detritus) compartments. The model outputs were used to estimate total system throughput, i.e., the sum of all C flows in the food web (the ‘ecological size’ of the system), and microbial loop functioning, i.e., the C-cycling through the prokaryotic compartment for each site. Both the estimated total system throughput and the microbial loop cycling were significantly reduced (by 16% and 35%, respectively) inside the plough tracks compared to the other two sites. Site differences in modelled faunal respiration varied among the different faunal compartments. Overall, modelled faunal respiration appeared to have recovered to, or exceeded reference values after 26-years. The model results indicate that food-web functioning, and especially the microbial loop, have not recovered from the disturbance that was inflicted on the abyssal site 26 years ago.

Arctic coastal ecosystems are rapidly changing due to climate warming, which makes modelling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and neglect bacterial driven regeneration, leading to an underestimation of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacteria ammonium remineralisation, and reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. The model also highlights the importance of organic matter excretion, and post bloom ammonium accumulation. Overall, model complexity is comparable to other ecosystem models used in the Arctic while improving the representation of nutrient co-limitation related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

The abyssal seafloor is at some locations covered with polymetallic nodules that provide hard substrate for sessile organisms. Extraction of these mineral-rich nodules will likely severely modify the trophic and non-trophic interactions within the abyssal food web, but the importance of nodules and their associated sessile fauna in supporting this food web remains unclear. Here, we present highly resolved interaction webs with ~200 (Peru Basin) and ~450 (Clarion-Clipperton Zone, CCZ) food-web compartments based on an extensive literature research. Compartments were connected with ~3,100 (Peru Basin) and ~8,500 (CCZ) trophic and non-trophic (e.g. substrate-providing nodules) links. The webs were used to assess how nodule extraction would modify the number of network compartments, number of links, link density and web connectance. We showed that nodule removal would reduce the number of food-web compartments and links by ~25% and ~35%, respectively, in the Peru Basin and by 21% and 20%, respectively, in the CCZ. Subsequent analysis identified stalked sponges, living attached to the nodules, as key structural species that support a high diversity of commensal and mutualistic fauna. We conclude that nodules are critical for food-web integrity and suggest the deployment of artificial sponge stalks as a potential mitigation strategy for deep-sea mining.

Cold-water coral (CWC) reefs are one of the most diverse and productive ecosystems in the deep sea. Especially in periods of seasonally-reduced phytodetritus food supply, their high productivity may depend on the recycling of resources produced on the reef, such as dissolved organic matter (DOM) and bacteria. Here, we demonstrate that abundant suspension feeders Geodia barretti (high-microbial-abundance sponge), Mycale lingua (low-microbial-abundance sponge) and Acesta excavata (bivalve) are able to utilize 13C-enriched (diatom-derived) DOM and bacteria for tissue growth and respiration. While DOM was an important potential resource for all taxa, utilization of bacteria was higher for the sponges as compared to the bivalve, indicating a particle-size differentiation among the investigated suspension feeders. Interestingly, all taxa released 13C-enriched particulate organic carbon, which in turn may feed the detritus pathway on the reef. Especially A. excavata produced abundant (pseudo-)fecal droppings. A second stable-isotope tracer experiment revealed that detritivorous ophiuroids utilized these droppings. The high resource flexibility of dominant reef suspension feeders, and the efficient recycling of their waste products by the detritivore community, may provide important pathways to maintain the high productivity on cold-water coral reefs, especially in periods of low external food supply.

The deep sea is the largest biome on Earth but the least explored. Our knowledge of it comes from scattered sources spanning different spatial and temporal scales. Implementation of marine policies like the European Union’s Marine Strategy Framework Directive (MSFD) and support for Blue Growth in the deep sea are therefore hindered by lack of data. Integrated assessments of environmental status require tools to work with different and disaggregated datasets (e.g. density of deep-sea habitat-forming species, body-size distribution of commercial fishes, intensity of bottom trawling) across spatial and temporal scales. A feasibility study was conducted as part of the four-year ATLAS project to assess the effectiveness of the open-access Nested Environmental status Assessment Tool (NEAT) to assess deep-sea environmental status. We worked at nine selected study areas in the North Atlantic focusing on five MSFD descriptors (D1-Biodiversity, D3-Commercial fish and shellfish, D4-Food webs, D6-Seafloor integrity, D10-Marine litter). The objectives of the present study were to i) explore and propose indicators that could be used in the assessment of deep-sea environmental status, ii) evaluate the performance of NEAT in the deep sea, and iii) identify challenges and opportunities for the assessment of deep-sea status. Based on data availability, data quality and expert judgement, in total 24 indicators (one for D1, one for D3, seven for D4, 13 for D6, two for D10) were used in the assessment of the nine study areas, their habitats and ecosystem components. NEAT analyses revealed differences among the study areas for their environmental status ranging from “poor” to “high”. Overall, the NEAT results were in moderate to complete agreement with expert judgement, previous assessments, scientific literature on human-pressure gradients and expected management outcomes. We suggest that the assessment of deep-sea environmental status should take place at habitat and ecosystem level (rather than at species level) and at relatively large spatial scales, in comparison to shallow-water areas. Limited knowledge across space (e.g. distribution of habitat-forming species) and the scarcity of long-term data sets limit our knowledge about natural variability and human impacts in the deep sea preventing a more systematic assessment of habitat and ecosystem components in the deep sea. However, stronger cross-sectoral collaborations, the use of novel technologies and open data-sharing platforms will be critical for establishing environmental baseline indicator values in the deep sea that will contribute to the science base supporting the implementation of marine policies and stimulating Blue Growth., Sí

The deep sea plays a critical role in global climate regulation through uptake and storage of heat and carbon dioxide. However, this regulating service causes warming, acidification and deoxygenation of deep waters, leading to decreased food availability at the seafloor. These changes and their projections are likely to affect productivity, biodiversity and distributions of deep-sea fauna, thereby compromising key ecosystem services. Understanding how climate change can lead to shifts in deep-sea species distributions is critically important in developing management measures. We used environmental niche modelling along with the best available species occurrence data and environmental parameters to model habitat suitability for key cold-water coral and commercially important deep-sea fish species under present-day (1951–2000) environmental conditions and to project changes under severe, high emissions future (2081–2100) climate projections (RCP8.5 scenario) for the North Atlantic Ocean. Our models projected a decrease of 28%–100% in suitable habitat for cold-water corals and a shift in suitable habitat for deep-sea fishes of 2.0°–9.9° towards higher latitudes. The largest reductions in suitable habitat were projected for the scleractinian coral Lophelia pertusa and the octocoral Paragorgia arborea, with declines of at least 79% and 99% respectively. We projected the expansion of suitable habitat by 2100 only for the fishes Helicolenus dactylopterus and Sebastes mentella (20%–30%), mostly through northern latitudinal range expansion. Our results projected limited climate refugia locations in the North Atlantic by 2100 for scleractinian corals (30%–42% of present-day suitable habitat), even smaller refugia locations for the octocorals Acanella arbuscula and Acanthogorgia armata (6%–14%), and almost no refugia for P. arborea. Our results emphasize the need to understand how anticipated climate change will affect the distribution of deep-sea species including commercially important fishes and foundation species, and highlight the importance of identifying and preserving climate refugia for a range of area-based planning and management tools., ATLAS, A Trans-AtLantic Assessment and deep-water ecosystem-based Spatial management plan for Europe, SI

Cold-water corals (CWC) are known to tolerate a relatively wide range of environmental conditions. However, along the basin margins of the Rockall Trough (NE Atlantic), the habitat of CWC is confined to a narrow range of 525–1200 m water depth, and the reason for that is not fully understood. To investigate the distribution of CWCs in this area, current velocities and water mass distribution in the Logachev Mound Province were measured with two long-term (1 year) moorings equipped with an acoustic doppler current profiler and fluorescence+turbidity sensors. Additional ship-based water column profiles and discrete water samples, covering a full diurnal tidal cycle, were taken for chemical parameters. The results indicate the presence of a boundary current along the eastern flank of the Rockall Bank, that transports a nutrient-rich water mass southwards and governs the direction of particle transport at the depth band of the CWC mounds. Based on literature results, this northern water mass is identified as Wyville Thomson Ridge Overflow Water (WTOW). The density envelope and depth distribution of WTOW match the reported occurrence of CWC mounds in the Rockall Trough. Hence, the presence of WTOW may be a necessary condition for coral growth and therefore mound formation in the Rockall Trough, e.g. by forming a conduit for particles and coral larvae.

Microphytobenthos forms an important part of the diet of macrofauna (macrozoobenthos) in many intertidal ecosystems. It is unclear, however, whether the dependence of macrofauna on microphytobenthos varies spatially within and among tidal systems. We aim (1) to assess the spatial variability in the importance of microphytobenthos in the diet of macrofauna (i.e., between and within two tidal basins and as function of elevation), (2) to quantify grazing pressure of the macrofaunal community on different potential food sources (microphytobenthos, phytoplankton and terrestrial organic material) for several sites in two tidal basins and (3) to compare microphytobenthic production and summer/autumn grazing of the total macrofaunal community and grazing pressure per feeding type, with potential microphytobenthic production estimated from rates in early spring, when grazing was low. Using a natural stable isotope approach, we identified microphytobenthos as a more important food source for macrofauna than phytoplankton and terrestrial organic material. Microphytobenthos dependency differed between tidal basins for the genera Bathyporeia (sand digger shrimp), Macoma (Baltic tellin), and Peringia (mudsnail) and for sampled individuals of all genera combined, and did not vary as function of elevation. We showed that macrofaunal grazing on microphytobenthos is quantitatively important and, in some cases, approached microphytobenthic production rates in early spring. No positive relation between microphytobenthic production in early spring and macrofaunal grazing in summer/autumn was observed. This suggests that the studied consumer‐resource interactions are coupled on a larger spatial scale (i.e., mesoscale, ≈10 to 100 km), rather than the fine (mm to m) scale.

Benthic suspension feeders have developed a variety of feeding strategies and food availability has often proven to be a key factor explaining their occurrence and distribution. The feeding biology of coral species has been the target of an increasing number of studies, however most of them focus on Scleractinia and Octocorallia, while information for Antipatharia is very scarce. The present study focused onAntipathella wollastoni, a common habitat-forming antipatharian in the Azores Archipelago, forming dense black coral forests between 20 and 150m. The objective of the study was to investigate the food preferences of the target species upon availability of different isotopically enriched food substrates and determine its ability to capture zooplankton prey under different flow speeds. The species was able to utilize different food sources including live phytoplankton, live zooplankton and dissolved organic matter (DOM), indicating the ability to exploit seasonally available food sources. However, ingestion of zooplankton enhanced carbon (C) and nitrogen (N) incorporation in coral tissue and metabolic activity, highlighting the importance of zooplankton prey for vital physiological processes such as growth and reproduction. Maximum zooplankton capture rates occurred under 4cm−1, however the species displayed high capacity to capture zooplankton prey over different flow rates highlighting the ability ofA. wollastonito exploit high quantities of shortly available prey.

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 (similar to 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 algal-derived 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.

Sponges are the oldest known extant animal-microbe symbiosis. These ubiquitous benthic animals play an important role in marine ecosystems in the cycling of dissolved organic matter (DOM), the largest source of organic matter on Earth. The conventional view on DOM cycling through microbial processing has been challenged by the interaction between this efficient filter-feeding host and its diverse and abundant microbiome. Here we quantify, for the first time, the role of host cells and microbial symbionts in sponge heterotrophy. We combined stable isotope probing and nanoscale secondary ion mass spectrometry to compare the processing of different sources of DOM (glucose, amino acids, algal-produced) and particulate organic matter (POM) by a high-microbial abundance (HMA) and low-microbial abundance (LMA) sponge with single-cell resolution. Contrary to common notion, we found that both microbial symbionts and host choanocyte (i.e. filter) cells and were active in DOM uptake. Although all DOM sources were assimilated by both sponges, higher microbial biomass in the HMA sponge corresponded to an increased capacity to process a greater variety of dissolved compounds. Nevertheless, in situ feeding data demonstrated that DOM was the primary carbon source for both the LMA and HMA sponge, accounting for ~90% of their heterotrophic diets. Microbes accounted for the majority (65–87%) of DOM assimilated by the HMA sponge (and ~60% of its total heterotrophic diet) but

Mining polymetallic nodules on abyssal plains will have adverse impacts on deep‐sea ecosystems, but it is largely unknown whether the impacted ecosystem will recover, and if so at what rate. In 1989 the “DISturbance and reCOLonization” (DISCOL) experiment was conducted in the Peru Basin where the seafloor was disturbed with a plough harrow construction to explore the effect of small‐scale sediment disturbance from deep‐sea mining. Densities of Holothuroidea in the region were last investigated 7 yr post‐disturbance, before 19 yr later, the DISCOL site was re‐visited in 2015. An “ocean floor observatory system” was used to photograph the seabed across ploughed and unploughed seafloor and at reference sites. The images were analyzed to determine the Holothuroidea population density and community composition, which were combined with in situ respiration measurements of individual Holothuroidea to generate a respiration budget of the study area. For the first time since the experimental disturbance, similar Holothuroidea densities were observed at the DISCOL site and at reference sites. The Holothuroidea assemblage was dominated by Amperima sp., Mesothuria sp., and Benthodytes typica, together contributing 46% to the Holothuroidea population density. Biomass and respiration were similar among sites, with a Holothuroidea community respiration of 5.84 10−4 ± 8.74 10−5 mmol C m−2 d−1 at reference sites. Although these results indicate recovery of Holothuroidea, extrapolations regarding recovery from deep‐sea mining activities must be made with caution: results presented here are based on a relatively small‐scale disturbance experiment as compared to industrial‐scale nodule mining, and also only represent one taxonomic class of the megafauna.