Seafloor Ecosystems Research Articles

Partial recovery of macrozoobenthos on the northwestern shelf of the Black Sea

The northwestern shelf of the Black Sea has been affected by eutrophication and bottom hypoxia since the sixties. Consequently, the macrozoobenthos has suffered a well-established decline in biodiversity. However, the nature of the current benthic communities remains questionable. From 1995 to 2017, we compiled species and abiotic data for 138 sites over the shelf. Through an appropriate multivariate analytical approach, we identified benthic community changes solely due to organic pollution variations. Our results show signs of recovery with an increase in biodiversity and proportion of species vulnerable to organic enrichment. These changes were related to a decrease in riverine loads and subsequent eutrophication. However, some long-lived species typical of the area still did not exhibit noticeable recovery, which suggests that either the recovery process has not yet been achieved or some environmental conditions are still not met to warrant a sea floor ecosystem state substantially healthy.

Optimized 3D reconstruction in nearshore underwater environments: A cost-effective pre-processing strategy for Neural Radiance Fields implementation

Nearshore oceans, teeming with diverse benthic ecosystems, continue to be a focal point for marine research. While 2D visual representations have been the mainstay in this area, the intricate, multi-dimensional nature of the seafloor ecosystems underscores the need for 3D modeling to capture their full essence. This study unveils a novel approach tailored for static image processing and 3D modeling through Neural Radiance Fields (NeRF), with a particular emphasis on the refined instantNGP variant. A meticulously crafted pipeline is employed, centered on neutralizing the visual impediments brought about by the interplay of light and seawater in underwater imaging. This refined pre-processing strategy ensures that images are primed for a seamless transition to NeRF-based 3D reconstruction, all the while conserving computational resources. The refined image processing techniques rectify underwater color discrepancies, notably the prevalent blue-green hue resulting from unique lighting conditions. Moreover, the system's ability to identify and excise seawater boundaries guarantees that the 3D models remain singularly focused on the richness of the seafloor ecosystems. Remarkably, achieving this does not demand vast datasets or exorbitant computational prowess, positioning it as an ideal fit for processing images from nearshore regions. As a more resource-friendly and efficient counterpart to existing methodologies, this study furnishes marine ecologists with a powerful instrument for RGB-centric 3D renderings of nearshore terrains. Nonetheless, for broader applicability in diverse marine settings, fusing this approach with neural networks could prove invaluable.

Pore-water nutrient concentrations variability under different oxygen regimes: A case study in Elefsis Bay, Greece

Anthropogenic pressures considerably affect coastal areas, increasing nitrogen and phosphorous loads that lead to eutrophication. Eutrophication sometimes results in hypoxic and/or anoxic conditions near the bottom water. Dissolved oxygen (DO) concentrations influence redox-sensitive nutrients, which can alter the benthic flux of nutrients. We retrieved sediment cores from two sites in the eastern and western parts of Elefsis Bay, a semi-enclosed area of the Eastern Mediterranean, Greece, during winter and summer. In the western part, seasonally hypoxic or anoxic conditions occurred. We analysed pore-water samples under normoxic, hypoxic and anoxic bottom water conditions to study the pore-water nutrient concentrations variability under different oxygen regimes. Ex situ incubation experiments were conducted at the site experiencing oxygen deficiency by manipulating the DO concentrations. The pore-water nutrient concentrations showed higher variability at the site experiencing oxygen deficiency. Notably, elevated ammonium concentrations were observed in the pore water during anoxic conditions, in the 2–20-cm sediment layer. However, the benthic fluxes of ammonium and phosphate at the 0–2-cm sediment layer were comparable under hypoxic and anoxic conditions. The results of the incubation experiments demonstrate a direct decrease in nitrate concentrations as the DO concentrations diminished in the overlying water. The incubations after re-oxygenating the overlying water show that phosphate was more efficiently scavenged when anoxic conditions prevailed in the bottom water. The incubation experiments indicate the rapid response of the seafloor to oxygen availability, particularly concerning processes that influence nitrate and phosphate concentrations. These observations highlight the dynamic nature of nutrient cycling in shallow, seasonally anoxic environments, such as Elefsis Bay, and emphasise the sensitivity of the seafloor ecosystem to changes in bottom water oxygen availability.

Bycatch Mitigation Strategies in the Gulf of Alaska

Commercial fishing is a crucial industry in Alaska’s economy, but unsustainable fishing practices, especially bottom trawling, lead to excessive bycatch and economic discards. This poses a major threat to marine ecosystems and their biodiversity, which puts the resources within Alaska’s fisheries at risk of diminishing to a point where they cannot meet human demand. Through extensive research and evaluation of this issue and existing legislation that governs Alaska’s fisheries, we recommend the full implementation of two specific measures across all of Alaska’s fisheries. One is electronic monitoring on all vessels in the form of deep learning cameras, and the other is a “freeze the footprint” approach on bottom trawling, which protects key seafloor habitats within the existing area that has been damaged by previous trawling activity. These efficient methods are a safer alternative to on-site observation, and they will lead to a healthier seafloor ecosystem, which is essential for marine life. Alaskan communities rely on their fisheries, and maintaining healthy marine ecosystems is critical to economic stability.

Modelled broad-scale shifts on seafloor ecosystem functioning due to microplastic impacts on bioturbation

Bioturbating species play an essential role in regulating nutrient cycling in marine sediments, but their interaction with microplastics (MP) remains poorly understood. Here we investigated the linkage between MP and ecosystem functioning using experimental observations of luminophore distribution in the sediment to parametrize bioturbation coefficients (Db). this information as fed into a simplified transport-reaction model, allowing us to upscale our experimental results. We found that the composition of bioturbators modulated shifts in the ecosystem functioning under microplastic stress. Maldanid worms (Macroclymenella stewartensis), functionally deep burrowing and upward-conveyor belt feeders, became less active. The Db of M. stewartensis reduced by 25% with the addition of 0.002 g MP cm−2 at surface sediment, causing accumulation of organic matter in the oxic sediment zone and stimulating aerobic respiration by 18%. In contract, the tellinid bivalve Macomona liliana, functionally a surface -deposit feeder that excretes at depth, maintained particle mixing behaviour in MP-contaminated systems. This study provides a mechanistic insight into the impacts of MP and indicates that the functional role of bioturbating species should be involved in assessing the global impact of MP. The model allowed us to understand the broad-scale impact of MP on seafloor habitat.

Diving neoichnology: underwater fieldwork focusing on organism and seafloor ecosystem interactions

Because almost all marine trace fossils were produced in underwater settings such as shoreface, offshore, continental shelf, slope, trench, and abyssal plain, investigating their modern analogues in these settings is important. In this discussion, I delve into a range of recent developments in the field of underwater neoichnology, concentrating on both my own contributions and those of fellow researchers. These advancements have been achieved through investigations conducted in various underwater settings, spanning from shallow coastal waters to the depths of the ocean floor. Our research methods have involved SCUBA diving as well as participation in submersible scientific expeditions.

Marine bioturbation collapse during Early Jurassic deoxygenation: implications for post-extinction marine ecosystem functioning

Abstract Climate change is undermining the health and integrity of seafloor ecosystems, with declines in bioturbation expected to impact future ecosystem functioning. We explored changes in the nature and degree of bioturbation during Early Jurassic global warming and ocean deoxygenation. Understanding how these communities responded can help anticipate how bioturbation and ecosystem functioning might change over large spatial and temporal scales. Trace and body fossils from outcrop and core in the Cleveland Basin, UK show how healthy seafloor communities deteriorated through the Pliensbachian spinatum Zone, and macroinfaunal behaviour fluctuated across the Pliensbachian–Toarcian boundary coincident with mass extinction. Deoxygenation began above the stage boundary, and conditions deteriorated until bioturbation ceased completely (upper tenuicostatum Zone) for 0.6–2.5 Ma, longer than anywhere else in NW Tethys. The macroinfaunal record revealed new details on the progression and timing of deoxygenation, benthic recovery and fluctuations in the palaeoredox boundary. After the oceanic anoxic event infauna were fewer, smaller and did not mix sediments to depth, and while the depth and diversity of bioturbation had increased by the fibulatum Subzone ( bifrons Zone), the benthos had not recovered to late Pliensbachian pre-oceanic anoxic event state. Bioturbation collapse over large parts of the Northern Hemisphere probably contributed to regional-scale changes in ecosystem functioning.

Real-world impacts of microplastic pollution on seafloor ecosystem function

Emerging research shows that microplastic pollution could be impacting seafloor ecosystem function, but this has been primarily demonstrated without environmental and ecological context. This causes uncertainty in the real-world effects of microplastic pollution and leaves out essential information guiding policy and mitigation. In this study, we take a well-supported sampling design and statistical approach commonly employed in benthic ecology to evaluate real-world effects of microplastic pollution on coastal, benthic ecosystem function. We utilised environmental gradients in the Waitemata Harbour of Auckland, New Zealand to evaluate the importance of commonly assessed biological, chemical, and geological sediment variables and the characteristics of microplastic contaminants in driving essential ecosystem functions. Our results showed that models including microplastic terms were more accurate and explained more variability than those without microplastic terms, highlighting that microplastics impact real-world seafloor ecosystem function. Specifically, microplastic fibers significantly influenced oxygen flux (p < 0.03) and the diverse forms of microplastics (i.e., richness) significantly influenced ammonium flux (p < 0.02). Additionally, interactions between microplastic fiber concentrations and mollusc abundances significantly contributed to oxygen flux (p < 0.02). These results provide the first evaluation of in situ relationships between microplastics and ecosystem function. Even more importantly, this study suggests the value of environmental and ecological context for addressing microplastic impacts on benthic ecosystems and argues for further field examination.

Nearshore bathymetry and seafloor property studies from Space lidars: CALIPSO and ICESat-2.

In shallow nearshore waters, seafloor heights and properties can be accurately measured by the current generation of space-based elastic backscatter lidars: CALIOP, flying aboard the CALIPSO satellite and ATLAS aboard ICESat-2. CALIOP's 532 nm volume depolarization ratios, together with the ratios of the attenuated backscatter coefficients measured at 532 nm and 1064 nm, can efficiently distinguish optically shallow waters from nearby land surfaces and deep oceans. ATLAS's high vertical resolution photon measurements can accurately determine seafloor depths in shallow water bodies, characterize seafloor reflectance, and provide assessments of ocean biomass concentrations in the intervening water column. By adding bathymetry, seafloor optical properties (e.g., reflectance, depolarization ratio and attenuated backscatter), and nighttime observations, space lidar measurements obtained in nearshore waters can provide a wealth of unique information to complement existing satellite-based ocean color remote sensing capabilities. The results reported here demonstrate the feasibility of using satellite lidars for nearshore seafloor ecosystem analyses, which in turn provide critical insights for studies of coastal navigation and seabed topography changes due to disasters, as well as the temporal and spatial morphological evolution of coastal systems.

Investigation of Deep-Sea Ecosystems Using Marker Fatty Acids: Sources of Essential Polyunsaturated Fatty Acids in Abyssal Megafauna.

Abyssal seafloor ecosystems cover more than 50% of the Earth’s surface. Being formed by mainly heterotrophic organisms, they depend on the flux of particulate organic matter (POM) photosynthetically produced in the surface layer of the ocean. As dead phytoplankton sinks from the euphotic to the abyssal zone, the trophic value of POM and the concentration of essential polyunsaturated fatty acids (PUFA) decrease. This results in pronounced food periodicity and limitations for bottom dwellers. Deep-sea invertebrate seston eaters and surface deposit feeders consume the sinking POM. Other invertebrates utilize different food items that have undergone a trophic upgrade, with PUFA synthesized from saturated and monounsaturated FA. Foraminifera and nematodes can synthesize arachidonic acid (AA), eicosapentaenoic acid (EPA), while some barophylic bacteria produce EPA and/or docosahexaenoic acid. FA analysis of deep-sea invertebrates has shown high levels of PUFA including, in particular, arachidonic acid, bacterial FA, and a vast number of new and uncommon fatty acids such as 21:4(n-7), 22:4(n-8), 23:4(n-9), and 22:5(n-5) characteristic of foraminifera. We suppose that bacteria growing on detritus having a low trophic value provide the first trophic upgrading of organic matter for foraminifera and nematodes. In turn, these metazoans perform the second-stage upgrading for megafauna invertebrates. Deep-sea megafauna, including major members of Echinodermata, Mollusca, and Polychaeta display FA markers characteristic of bacteria, foraminifera, and nematodes and reveal new markers in the food chain.

Climate cascades affect coastal Antarctic seafloor ecosystem functioning.

Polar seafloor ecosystems are changing rapidly and dramatically, challenging previously held paradigms of extreme dynamical stability. Warming-related declines in polar sea ice are expected to alter fluxes of phytoplankton and under-ice algae to the seafloor. Yet, how changes in food flux cascade through to seafloor communities and functions remains unclear. We leveraged natural spatial and temporal gradients in summertime sea ice extent to better understand the trajectories and implications of climate-related change in McMurdo Sound, Antarctica. McMurdo Sound was expected to be one of the last coastal marine environments on Earth to be affected by planetary warming, but the situation may be changing. Comparing satellite observations of selected coastal sites in McMurdo Sound between 2010-2017 and 2002-2009 revealed more ice-free days per year, and shorter distances to open water during the warmest months each year, in the more recent period. Interdecadal Pacific Oscillation (IPO), Oceanic Niño Index (ONI) and Antarctic Oscillation (AAO) climate indices peaked concurrently between 2014 and 2017 when sea ice breakouts in McMurdo Sound were most spatially and temporally extensive. Increases in sediment chlorophyll a and phaeophytin content (indicating increased deposition of detrital algal food material) were recorded during 2014-2017 at three coastal study sites in McMurdo Sound following the major sea ice breakouts. Soft-sediment seafloor ecosystem metabolism (measured in benthic incubation chambers as dissolved oxygen and inorganic nutrient fluxes) was correlated with sediment algal pigment concentration. Epifaunal invertebrate density, particularly opportunistic sessile suspension feeders, and infaunal community composition also shifted with increased food supply. The ecological characteristics and functions measured at the food-poor sites shifted towards those observed at richer sites at a surprisingly fast pace. These results indicate the sensitivity of the benthos and shed light on Antarctic marine trophic cascades and trajectories of response of iconic high-latitude seafloor habitats to a warming climate.

Bacteria and Archaea Synergistically Convert Glycine Betaine to Biogenic Methane in the Formosa Cold Seep of the South China Sea.

ABSTRACTCold seeps are globally widespread seafloor ecosystems that feature abundant methane production and flourishing chemotrophic benthic communities. Chemical evidence indicates that cold seep methane is largely biogenic; however, the primary methane-producing organisms and associated pathways involved in methanogenesis remain elusive. This work detected methane production when glycine betaine (GBT) or trimethylamine (TMA) was added to the sediment microcosms of the Formosa cold seep, South China Sea. The methane production was suppressed by antibiotic inhibition of bacteria, while GBT was accumulated. This suggests that the widely used osmoprotectant GBT could be converted to cold seep biogenic methane via the synergistic activity of bacteria and methanogenic archaea because archaea are not sensitive to antibiotics and no bacteria are known to produce ample methane (mM). 16S rRNA gene diversity analyses revealed that the predominant bacterial and archaeal genera in the GBT-amended methanogenic microcosms included Oceanirhabdus and Methanococcoides. Moreover, metagenomic analyses detected the presence of grdH and mtgB genes that are involved in GBT reduction and demethylation, respectively. Two novel species were obtained, including bacterium Oceanirhabdus seepicola, which reduces GBT to TMA, and a methanogenic archaeon, Methanococcoides seepicolus, which produces methane from TMA and GBT. The two strains reconstituted coculture efficiently converted GBT to methane at 18°C; however, at 4°C addition of dimethylglycine (DMG), the GBT demethylation product, was necessary. Therefore, this work demonstrated that GBT is the precursor not only of the biogenic methane but also of the cryoprotectant DMG to the microorganisms at the Formosa cold seep.IMPORTANCE Numerous cold seeps have been found in global continental margins where methane is enriched in pore waters that are forced upward from sediments. Therefore, high concerns have been focused on the methane-producing organisms and the metabolic pathways in these environments because methane is a potent greenhouse gas. In this study, GBT was identified as the main precursor for methane in the Formosa cold seep of the South China Sea. Further, synergism of bacteria and methanogenic archaea was identified in GBT conversion to methane via the GBT reduction pathway, while methanogen-mediated GBT demethylation to methane was also observed. In addition, GBT-demethylated product dimethyl glycine acted as a cryoprotectant that promoted the cold seep microorganisms at cold temperatures. GBT is an osmoprotectant that is widely used by marine organisms, and therefore, the GBT-derived methanogenic pathway reported here could be widely distributed among global cold seep environments.

Underwater Hyperspectral Imaging (UHI): A Review of Systems and Applications for Proximal Seafloor Ecosystem Studies

Marine ecosystem monitoring requires observations of its attributes at different spatial and temporal scales that traditional sampling methods (e.g., RGB imaging, sediment cores) struggle to efficiently provide. Proximal optical sensing methods can fill this observational gap by providing observations of, and tracking changes in, the functional features of marine ecosystems non-invasively. Underwater hyperspectral imaging (UHI) employed in proximity to the seafloor has shown a further potential to monitor pigmentation in benthic and sympagic phototrophic organisms at small spatial scales (mm–cm) and for the identification of minerals and taxa through their finely resolved spectral signatures. Despite the increasing number of studies applying UHI, a review of its applications, capabilities, and challenges for seafloor ecosystem research is overdue. In this review, we first detail how the limited band availability inherent to standard underwater cameras has led to a data analysis “bottleneck” in seafloor ecosystem research, in part due to the widespread implementation of underwater imaging platforms (e.g., remotely operated vehicles, time-lapse stations, towed cameras) that can acquire large image datasets. We discuss how hyperspectral technology brings unique opportunities to address the known limitations of RGB cameras for surveying marine environments. The review concludes by comparing how different studies harness the capacities of hyperspectral imaging, the types of methods required to validate observations, and the current challenges for accurate and replicable UHI research.

Identifying "vital attributes" for assessing disturbance-recovery potential of seafloor communities.

Despite a long history of disturbance–recovery research, we still lack a generalizable understanding of the attributes that drive community recovery potential in seafloor ecosystems. Marine soft‐sediment ecosystems encompass a range of heterogeneity from simple low‐diversity habitats with limited biogenic structure, to species‐rich systems with complex biogenic habitat structure. These differences in biological heterogeneity are a product of natural conditions and disturbance regimes. To search for unifying attributes, we explore whether a set of simple traits can characterize community disturbance–recovery potential using seafloor patch‐disturbance experiments conducted in two different soft‐sediment landscapes. The two landscapes represent two ends of a spectrum of landscape biotic heterogeneity in order to consider multi‐scale disturbance–recovery processes. We consider traits at different levels of biological organization, from the biological traits of individual species, to the traits of species at the landscape scale associated with their occurrence across the landscape and their ability to be dominant. We show that in a biotically heterogeneous landscape (Kawau Bay, New Zealand), seafloor community recovery is stochastic, there is high species turnover, and the landscape‐scale traits are good predictors of recovery. In contrast, in a biotically homogeneous landscape (Baltic Sea), the options for recovery are constrained, the recovery pathway is thus more deterministic and the scale of recovery traits important for determining recovery switches to the individual species biological traits within the disturbed patch. Our results imply that these simple, yet sophisticated, traits can be effectively used to characterize community recovery potential and highlight the role of landscapes in providing resilience to patch‐scale disturbances.

Environmental Heterogeneity Throughout the Clarion-Clipperton Zone and the Potential Representativity of the APEI Network

Environmental variables such as food supply, nodule abundance, sediment characteristics, and water chemistry may influence abyssal seafloor communities and ecosystem functions at scales from meters to thousands of kilometers. Thus, knowledge of environmental variables is necessary to understand drivers of organismal distributions and community structure, and for selection of proxies for regional variations in community structure, biodiversity, and ecosystem functions. In October 2019, the Deep CCZ Biodiversity Synthesis Workshop was conducted to (i) compile recent seafloor ecosystem data from the Clarion-Clipperton Zone (CCZ), (ii) synthesize patterns of seafloor biodiversity, ecosystem functions, and potential environmental drivers across the CCZ, and (iii) assess the representativity of no-mining areas (Areas of Particular Environmental Interest, APEIs) for subregions and areas in the CCZ targeted for polymetallic nodule mining. Here we provide a compilation and summary of water column and seafloor environmental data throughout the CCZ used in the Synthesis Workshop and in many of the papers in this special volume. Bottom-water variables were relatively homogenous throughout the region while nodule abundance, sediment characteristics, seafloor topography, and particulate organic carbon flux varied across CCZ subregions and between some individual subregions and their corresponding APEIs. This suggests that additional APEIs may be needed to protect the full range of habitats and biodiversity within the CCZ.

High-concentration sediment plumes, Horseshoe Island, western Antarctic Peninsula

The variability in sediment concentration and spatial distribution of meltwater discharges from tidewater glaciers can be used to elucidate climatic evolution and glacier behaviour due to the association between sediment yield and glacier retreat (e.g. Domack &amp; McClennen 1996). In an accelerated deglaciation environment, higher sediment concentrations in the water column can change the glacimarine costal dynamics and affect productivity and sea floor ecosystems (e.g. Marín et al. 2013). In the Antarctic Peninsula Region, meltwater or turbid plumes were previously believed to be rare or without an important role in the sedimentary glacimarine environment (e.g. Griffith &amp; Anderson 1989), but recent studies have shown that this is a common phenomenon in subpolar and transition polar climates (Yoo et al. 2015, Rodrigo et al. 2016). In the current climate change scenario, accelerated glacier retreats and mass losses can produce an increasing input of glacial meltwater into the fjord regions, a situation that is not yet well evaluated in the Antarctic Peninsula. In this short note, after in situ observation of an unusual waterfall from the southern side of the main western tidewater glacier (Shoesmith Glacier) of Horseshoe Island (Lystad Bay), Marguerite Bay (Fig. 1), we report high turbidity values associated with plumes from the glacier, whose values were higher than reported data from subpolar/transition polar Antarctic climates.

Anatomy of subcritical submarine flows with a lutocline and an intermediate destruction layer

Turbidity currents are sediment-laden flows that travel over a sloping bed under a stagnant ambient fluid, driven by the density difference between the current and the ambient. Turbidity currents transport large amounts of carbon, nutrients and fresh water through oceans and play an important role in global geochemical cycling and seafloor ecosystems. Supercritical currents are observed in steeper slopes. Subcritical currents are observed in milder slopes, where the near-bed and interface layers are prevented from interacting across the velocity maximum. Past works show the existence of such a barrier to vertical momentum transfer is essential for the body of the subcritical current to extend over hundreds of kilometers in length without much increase in height. Here we observe the body of subcritical currents to have a three layer structure, where the turbulent near-bed layer and the non-turbulent interface layer are separated by an intermediate layer of negative turbulence production. We explain the mechanism by which this layer prevents the near-bed turbulent structures from penetrating into the interface layer by transferring energy back from turbulence to the mean flow.

Highlights from a review of microplastics in marine sediments

The ubiquity of microplastics (MPs) has now been demonstrated throughout Earth's geographic areas in both terrestrial and marine environments. Marine ecosystems are the end point of the plastic waste cycle, and marine sediments are increasingly considered to be a sink for plastic debris, with possible deleterious effects on seafloor ecosystems. However, the low concentration of MPs in a complex matrix such as marine sediment makes their reliable analysis difficult. MP concentration in marine sediments is usually determined by various extraction procedures followed by optical, spectroscopy or mass spectrometry techniques, and are therefore hard to compare. Therefore, reliable determination of MPs in sediment is a challenging task. Here we present a short review on studies dealing with analytical determination protocols and MP detection in marine sediments and discuss the advantages of the different techniques used. This analysis of the literature reveals that most of the 70 studies were carried out in European and Asian coastal environments. The use of NaCl saturated solution, hydrogen peroxide (H2O2) and Fourier transform infrared spectroscopy (FTIR) are by far the most-used techniques for the different steps of separation, digestion and identification, respectively. Based on this body of literature, we present recommendations and suggestions for future research in order to increase the reliability of results and to facilitate inter-comparison. The use of NaCl saturated and H2O2 solution is strongly recommended for the separation and digestion steps, respectively, whereas during the filtration step it is necessary to choose filter type based on the identification technique. More thorough investigation is needed to establish a systematic protocol for MP identification. A combination of techniques would permit to identify all items, and personal bias could be avoided if automatic identification was implemented. Nanoplastics (NP) occurrence in marine sediments is also discussed, although no data are available to date.

Glacial melt disturbance shifts community metabolism of an Antarctic seafloor ecosystem from net autotrophy to heterotrophy

Climate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

Development of a life cycle impact assessment framework accounting for biodiversity in deep seafloor ecosystems: A case study on the Clarion Clipperton Fracture Zone

The transformation of ecosystems is known to be a major driver of biodiversity loss. Consequently, supporting tools such as life cycle assessment methods (LCA) include this aspect in the evaluation of a product's environmental performance. Such methods consist of quantifying input and output flows to assess their specific contributions to impact categories. Therefore, land occupation and transformation are considered as inputs to assess biodiversity impacts amongst others. However, the modelling of biodiversity impact in deep seafloor ecosystems is still lacking in LCA. Most of the LCA methods focus on terrestrial biodiversity and none of them can be transposed to benthic deep sea because of knowledge gaps. This manuscript proposes a LCA framework to assess biodiversity impacts in deep seafloor ecosystems. The framework builds upon the existing methods accounting for biodiversity impacts in terrestrial and coastal habitats. A two-step approach is proposed, assessing impacts on regional and on global biodiversity. While the evaluation of regional biodiversity impacts relies only on the benthic communities' response to disturbance, the global perspective considers ecosystem vulnerability and scarcity. Those provide additional perspective for the comparison of impacts occurring in different ecosystems. The framework is operationalised to a case study for deep-sea mining in the Clarion Clipperton Fractures Zone (CCZ). Through the large variety of data sources needed to run the impact evaluation modelling, the framework shows consistency and manages the existing limitations in the understanding of deep seafloor ecosystems, although limitations for its application in the CCZ were observed mainly due to the lack of finer scaled habitat maps and data on connectivity. With growing interest for commercial activities in the deep sea and hence, increased environmental research, this work is a first attempt for the implementation of LCA methods to deep-sea products.