Accurately identifying the types and formation mechanisms of seafloor bedforms is crucial for understanding sediment transport and deposition on continental margins, and assessing seafloor stability for marine geohazard evaluation. This study, using highresolution bathymetric, 2-D/3-D seismic and sediment core data, investigates sediment waves at the heads of the Shenhu Canyon System in the northern South China Sea. These sediment waves have wavelengths up to 1.4 km and heights reaching 50 m, exhibiting continuous internal reflections and an upslope migration trend. Sediment samples reveal that these waves are predominantly composed of silt (73.21%-88.45%). Our findings suggest that internal waves, particularly those at diurnal frequencies, are the primary driver for the formation of these sediment waves. In critical and near-critical areas, the interaction of internal waves with the seafloor induces strong energy dissipation, which enhances bottom mixing and facilitates sediment resuspension and transport, leading to the formation of sediment waves. The variation in sediment wave scale is linked to differences in internal wave conditions, energy, and slope morphology. In the northeastern canyon heads (C11-C19), near-critical/ critical conditions of diurnal internal waves, higher initial internal wave energy, and longer slopes enhance sediment resuspension, forming larger sediment waves. Conversely, reflective conditions, lower wave energy, and shorter slopes in the central (C8-C10) and southwestern (C1-C7) canyon heads limit sediment resuspension and transport, resulting in smaller or no sediment waves. These results provide new insights into sediment wave formation adjacent to submarine canyon heads, highlighting the significant role of internal waves in shaping continental margin geomorphology.

Submarine canyons are ubiquitous geomorphic features found intercepting the continental margins. As such, they provide environmental conditions suitable for many suspension feeding organisms, as they settle on steep rocky canyon walls, whilst taking advantage of increased currents that bring suspended organic matter and food. Additionally, demersal fishing grounds can be found surrounding submarine canyons where it can negatively affect species inhabiting these environments, including vulnerable ecosystems such as cold-water corals (CWCs). In order to understand the impacts of demersal fisheries in CWC communities, we first need to understand their distribution, species composition and vulnerability. Blanes Canyon is an example of a submarine canyon surrounded by demersal fishing grounds, where baseline knowledge on CWCs currently lacks. This study contributes to filling these knowledge gaps by using a dense grid of ROV transects along the canyon, high resolution bathymetry data and CTD surveys, which altogether provide a quantitative description of megabenthic assemblages. Blanes Canyon hosts at least 12 CWC species within 450-1300 m depth range, mainly inhabiting the steep canyon walls. Different assemblages of CWC species were found. Desmophyllum dianthus was the most abundant species, found throughout the entire canyon. Colonial scleractinian species (Desmophyllum pertusum and Madrepora oculata) were found in the canyon head but were lacking in the eastern canyon branch, where octocorals (Muriceides lepida) and black corals (Leiopathes glaberrima) were prevailing. Detailed CTD survey indicated that nepheloid layers (bottom and intermediate) were found at the same depth range as the megabenthic communities, since they provide suspension feeders with particulate organic matter (POM). Overall, this study confirms Blanes Canyon as a CWC habitat, providing densities and spatial distribution of different megabenthic species, along with information of their environmental niches.

Sedimentary dynamics in the Palamós Canyon are influenced by river inputs and storm resuspension, as well as by bottom trawling on the canyon flanks. In this study, we estimate recent sediment deposition patterns along the canyon axis using the excess activity concentration of the short-lived radiotracer 234Th (half-life of 24.1 days). Sediment cores were obtained at various locations along the canyon axis from a depth of approximately 800 m to 2100 m in June 2023 and August 2024. Excess 234Th (234Thxs) was detected in all sampled sites with variable penetration depths (0.5–3.5 cm). 234Thxs-derived estimations of mixing rates decreased downcanyon from up to 15.6 cm2 y−1 at the canyon head (~800 m) to negligible mixing at the canyon mouth (~2100 m). 234Thxs inventories, a proxy of recent sediment deposition, were high (1800–3490 Bq m−2) at the canyon head and at the upper canyon (~1400 m) close to fishing grounds and decreased downcanyon (82–694 Bq m−2) at the lower canyon (~1800 m) and canyon mouth. Inventories varied 2-fold across years presumably attributed to enhanced riverine and bottom trawling sediment fluxes. Similar 234Th-derived sediment deposition patterns can be found in submarine canyons worldwide, highlighting the value of this radiotracer for sedimentary dynamics studies in such complex environments.

Marine sediments represent a hot spot of ecosystem services, but their integrity is increasingly put at risk by anthropogenic disturbance, most notably by demersal fisheries. The need for global action to minimize the impacts of destructive fishing techniques on the marine environment is urgent. The urgency to act, however, needs to be met with caution, as scientists are pushed for action, global predictions of trawling impacts are tempting, yet poor validation and oversimplified assumptions can lead to large uncertainties. We visit the scientific literature on trawl studies to map out current evidence from the literature and report on a global meta-analysis to quantify the effects of demersal fishing on sedimentary and biogeochemical properties. Studies examining the direct impacts of bottom fishing revealed significant reductions in total organic carbon (TOC; -10%), chlorophyll-a (Chl-a, -10%), phaeopigments (-21%) and proteins (-24%), and largest impact was detected on surficial sediment (0-2 cm). Implications of methodological biases as a result of inappropriate sampling in trawl studies and the importance of context-dependency for effect size is flagged up. Environmental parameters such as bottom current velocity and surface primary productivity significantly influenced both the direction and magnitude of fishing effects. We highlight where the lack of evidence lies that might create bias in regional and global models that require empirical data for validation. The objective is to summarize current knowledge and to direct future studies towards more robust analysis of the impacts of bottom trawling, which will provide a basis of sound advice to fisheries managers and policy makers.

We investigated changes in sediment grain size, elemental (total organic C, TOC; total N, TN), isotopic (δ13C, δ15N), biochemical composition (proteins, carbohydrates, lipids, phytopigments), nutritional quality, potential degradation of sedimentary organic matter across a river-to-lagoon continuum before-after high (HRP) and low (LRP) rainfall periods. Both rainfalls relocated the coarser sediment fraction downstream in the river, whilst only the finer fraction reached the lagoon. The high swell associated to the storm preceding the LRP contributed to the seawater overwash over the outer lagoon, which masked the effects of the freshwater discharge. After the HRP, TOC and TN in the outer lagoon increased 5–6 times, whilst after the LRP both decreased 2–8 times in the whole lagoon. Rainfalls caused a δ13C enrichment (from − 27.1 to − 22.0 ‰) of the lagoon sediments and changes in the biogeochemistry of both river and lagoon sediments. After both rainfalls, time for the degradation of the biopolymeric C increased downstream the river (5–11 times) and in the outer lagoon (3 times). We conclude that the effects of rainfall-driven river runoff and their spatial extent towards the adjacent lagoon depend on the magnitude and duration of the rainfall, which differently alter sediment biogeochemistry.

Dense shelf water cascading (DSWC) is an oceanographic process that occurs when dense shelf water overflows over the shelf edge downslope toward the deep sea. Monitored in the northwestern Mediterranean by moorings since 1993 in the Lacaze-Duthiers Canyon and since 2005 in the Cap de Creus Canyon, numerical modeling with reanalysis extends this timeline further into the past. This study investigates a regional reanalysis (1987-2021) validated against mooring observations at 750-1000 m depth. The reanalysis successfully reproduces observed intense DSWC (IDSWC) events from 1999, 2000, 2005, 2006, 2012, 2013, and 2018 while identifying one previously unreported event in 1987 and detecting no IDSWC between 1988 and 1998. The reanalysis effectively matches 84 % of observed IDSWC days within the same week and 56 % on the exact date. Instead of assimilating IDSWC events from mooring observations to resolve the cascading process, the model relies solely on the seawater density on the shelf and revealed the seawater properties along the canyon that caused IDSWC. This work highlights the importance of high-resolution reanalyses in investigating the impacts of mesoscale processes on larger scales in the deep ocean.

Submarine canyons act as conduits of terrigenous and marine organic carbon (OC) to deep-sea environments, although the contribution of each of these sources can largely vary depending on the canyon morphology and the prevailing sedimentary dynamics. The Gulf of Palermo is incised by several submarine canyons of similar dimension and depth range, but with slightly different morpho-sedimentary characteristics. Using a combination of geochemical parameters (OC, TN, delta 13C, delta 15N, and Delta 14C), as well as biomarker signatures (proteins, carbohydrates, lipids, phytopigments, glycerol dialkyl glycerol tetraethers, and n-alkyl lipids) and compound-specific delta 13C analyses of surficial sediments, we assess the sources of OC deposited on the shelf and in three major submarine canyons (Arenella, Oreto and Eleuterio canyons). The aim is to provide further insights on the role of submarine canyons in transporting terrigenous OC across continental margins. According to a dual isotopic end-member mixing model with delta 13C and Delta 14C, the contribution of terrigenous OC was highest on the shelf (80 %) and decreased offshore, with contributions that ranged between 50 % to 70 % across the studied canyons. The dispersal mechanism of terrigenous OC and its specific sources differ among canyons primarily because of local differences of hydro- and sediment dynamics. Arenella Canyon, which is up-current and farthest from any river mouth, exhibited the relatively lowest terrigenous OC contributions (50 %), Oreto Canyon in the central part of the gulf had slightly higher contributions (50 %-70 %), and Eleuterio Canyon down-current and closest to shore had the highest proportion of terrigenous OC (60 %-70 %). Besides natural sediment dispersal mechanisms acting on this continental margin, continuous sediment resuspension by bottom trawling activities inside Oreto Canyon contributes to the down-canyon displacement of terrigenous OC, while promoting the ageing and degradation of OC in the canyon axis. Compound-specific delta 13C analyses of fatty acids revealed that the sources of terrigenous OC differ across the studied submarine canyons, with Arenella and Oreto canyons receiving OC from a similar terrigenous source up-current from the gulf, whereas terrigenous OC deposited on the shelf and in Eleuterio Canyon originates from the Oreto and Eleuterio rivers that discharge into the Gulf of Palermo. This study provides further evidence that even non-river connected submarine canyons, such as Arenella, Oreto, and Eleuterio canyons in the Gulf of Palermo, are important sites of terrigenous OC sequestration and transfer to deep-sea environments, and that bottom trawling activities within submarine canyon environments can contribute to its resuspension and dispersal towards deeper regions.

Cold-water coral habitats (Madrepora oculata and Desmophyllum pertusum) were mapped in the Lacaze-Duthiers Canyon, within the "Parc naturel marin du golfe du Lion" (northwestern Mediterranean), to support the creation of a strongly protected area for these vulnerable marine ecosystems (VME). The analyses are based on ROV data, high-resolution digital terrain models (5-10 m), hydrodynamic model simulations (currents, temperature, salinity from SYMPHONIE), and fishing pressure data derived from Vessel Monitoring System (VMS) and Automatic Identification System (AIS) fishing data. Habitat models indicate coral habitats are mainly on the western canyon flank (250 to 600 m depth). Desmophyllum pertusum predominantly colonizes gentle slopes at cliff bases, while M. oculata is found on steeper slopes, and the eastern flank. Some western flank areas support both species. Direct impacts from fishing activities, including lost longlines and nets entangled with corals, were observed. In addition, bottom trawling at the continental shelf edge induces sediment resuspension, potentially smothering the underlying coral ecosystems in the canyon. This study recommends enhancing fishing activity monitoring by increasing VMS positioning transmission frequency across European countries and facilitating the combination of VMS and AIS data to derive higher-resolution fishing effort and footprint assessments. Protection should prioritize canyon flanks and the surrounding continental shelf, not just the canyon itself, to mitigate fishing pressure. Tailored management measures are needed for sensitive habitats. Creating a strongly protected area will require crossborder cooperation (France, Spain) to ensure the effective conservation of these fragile ecosystems.

The potential threat of fisheries on seabed carbon is a topic of growing concern, yet existing literature presents inconsistencies leaving experts divided on the topic. We conducted a global meta-analysis to synthesize the current knowledge and quantify how demersal fishing impacts various biogeochemical properties. Direct impact studies revealed overall reductions in chlorophyll-a (Chl-a, 17%), phaeopigments (24%), and proteins (32%). Effects on these reactive compounds were more pronounced on surface sediment (0-2 cm), where the impact on total organic carbon (TOC) also became significant, demonstrating the effect of gear penetration, and highlighting that sampling strategies combining sediment layers can mask observed effects. Current velocity and primary productivity significantly influenced the direction and magnitude of fishing impacts. Trawling-induced subsurface reductions of TOC in low-energy habitats may affect carbon sequestration due to the preferential removal of semi-reactive carbon. Intriguingly, fishing intensity gradient studies showed an average increase in TOC in chronically fished areas, possibly reflecting fishing preferences for meso-eutrophic grounds. We estimate a similar to 300-day recovery period post-fishing for Chl-a, though values for other parameters are less certain. Limited data on seasonality, gear types, and an under-representation of studies in tropical and deep-sea areas pose challenges to quantifying global scale geochemical impacts of demersal fisheries. Knowledge gaps persist in understanding the fate of disturbed organic matter including its mineralization, transport, and sequestration. Nonetheless, our insights and estimates provide foundational knowledge that can contribute to science-based approaches for spatial fisheries management while preserving natural carbon dynamics on the seabed.

Sources and pathways of the Hg accumulated in the sediments of the Gulf of Lion (GoL) and its adjacent marine areas (Northwestern Mediterranean) have been explored using sediment grab samples, sediment cores, and sediment trap samples. The main source of Hg along this margin is the Rh <^>one River, whose suspended sediments settle mainly in the prodelta area but also along the mid-shelf, then reaching the continental rise via wave resuspension and cascading of dense shelf waters. Seaward, these riverine particles are mixed with carbonated ooze conveyed to the bottom by the biological pump. The Hg is enriched in surface sediments of the GoL with decreasing concentrations westward and seaward from the Rh <^>one prodelta to the continental rise. Dated cores from the Rh <^>one prodelta give access to riverine sediments deposited over the last 400 years and show that the Hg concentration time trend resembles the evolution of coal consumption in France, peaking during in the 1960s. Similar trends were observed in sediment cores collected along the GoL slope and submarine canyons along with the preservation of traces of erosion and sediment instability events. Seaward, on the continental rise, the Hg concentration distribution suggests the deposition of fine planktonic-derived material and particles episodically advected from the shelf during deep cascading pulses. Anthropogenic Hg accumulated in GoL and continental rise sediments during the Industrialized Era is similar to 150 Mg (tons), two-thirds of which are buried in the Rh <^>one prodelta area. Significant correlations are found between Hg and organic matter in GoL sediments, but the relationships differ between areas and are disrupted by the inputs of anthropogenic Hg and by the Hg availability in the Rh <^>one River watershed during the pre-industrial period. The HgT availability in waters appears to be the limiting factor to the Hg enrichment of the particulate organic matter. Monomethyl mercury (MMHg), which represents on average 0.3 % of the total Hg, was positively correlated to total Hg. Their distributions suggest in situ MMHg formation and a more effective net Hg methylation for surface sediment or particles collected in traps compared with particles buried in the sedimentary column.

Bottom trawling in submarine canyons can affect their sedimentary dynamics, but studies addressing this topic are still scarce. In the Gulf of Palermo (NW Sicily, SW Mediterranean Sea), bottom trawling occurs on the continental slope, but principally concentrates within Oreto Canyon. Hydrographic profiles and time series data of temperature, turbidity, and currents obtained by a CTD probe and by moored instruments, respectively, revealed increased turbidity values and the presence of bottom and intermediate nepheloid layers coinciding with periods of bottom trawling activity. The delay between the onset of trawling activities along the Oreto canyon axis and the increase in water turbidity at the mooring location indicate that trawling resuspended particles are progressively advected down-canyon by hydrodynamic processes. Topographic waves and near-inertial currents seem to contribute to the sediment transport of resuspended particles as bottom and intermediate nepheloid layers. Results presented in this paper highlight the complex relationship between hydrodynamic processes and sediment resuspension by trawling in submarine canyons.

Trace metal contents and fluxes in downward particulate matter and dated sediment cores of the NW Alboran Sea are analysed in this study with the aim of assessing the role of the Atlantic inflow on their transport. Increases in Zn, Cu and Pb were detected in downward particulate matter collected by sediment traps after river flooding events and after the Aznalcollar mining spill. Their arrival coincided within the recently estimated time range for river particles discharged into the Gulf of Cádiz to reach the Alboran Sea, indicating that their transfer is enhanced during events of increased river inputs of contaminated particulate matter. This also suggests that the effects of potential tailing dam failures in the Gulf of Cádiz watersheds could reach the Alboran Sea. These trace metals also increased in the sediment cores from the continental rise since the second half of the 19th century, suggesting that contaminated particles have been continuously transferred towards the Mediterranean Sea since that time, when mining concessions and production increased in the SW Iberian Pyrite Belt.

Bottom trawling on marine environments can drastically modify seafloor geomorphology and sedimentary dynamics not only on the fishing grounds but also in adjacent downslope regions, particularly in submarine canyons environments, which are hotspots of benthic biomass and productivity in the deep sea. When this type of fishery occurs along submarine canyon flanks, it can induce sediment gravity flows that descend along tributary gullies towards the main canyon axis. However, these flows had only been clearly identified in the Palamós Canyon, where they could be recorded synchronously with the passage of the trawling fleet. In this study we also recorded trawl-induced sediment gravity flows in the Blanes Canyon, both synchronously and asynchronously with the passage of trawlers. Increases in particulate matter fluxes in other trawled submarine canyons occurring in absence of natural triggering mechanisms, were not directly associated with bottom trawling because of the lack of direct synchronicity of these events with this human activity. Here we show, however, that the practice of bottom trawling along canyon flanks can not only resuspend and directly trigger sediment gravity flows, but they can also pile up disturbed sediment on steep areas, which can become unstable and collapse afterwards, asynchronically with the passage of trawlers. Our study provides evidence that sediment gravity flows in submarine canyons affected by bottom trawling, where the causal mechanisms are presently unidentified, may potentially be linked to instabilities in sediment originating from recurrent bottom trawling, which can precondition these events.

Bottom trawling can significantly affect benthic communities, directly through immediate removal of sessile organisms and indirectly through sediment resuspension. Submarine canyons, often surrounded by fishing grounds, are important habitats for cold-water corals (CWC). Vulnerability of CWCs to increased suspended sediment concentration (SSC) is key to understanding the severity of bottom trawling effects on those communities. Here we show survival, growth, and physiological response of six CWCs from a Mediterranean submarine canyon (Dendrophyllia cornigera, Desmophyllum dianthus, Desmophyllum pertusum, Madrepora oculata, Leiopathes glaberrima and Muriceides lepida), exposed to a long-term, aquarium-based sedimentary disturbance experiment. Compared to cup coral and octocoral, which did not exhibit symptoms of distress, our data indicate that colonial scleractinian corals and black coral, which experienced substantial polyp mortality in enhanced SSC treatments, are more vulnerable. Indirect impact of bottom trawling could thus contribute to structural simplification of CWC communities posing an additional stressor alongside with global climate change.

Bottom trawling is a fishing method that involves towing of nets along the seafloor to catch demersal species. The dragging of trawling gears along the seafloor results in scraping and ploughing of the surficial sediments, leading to the formation of turbid plumes of resuspended sediments and causing measurable changes in the seabed morphology. High-resolution multibeam data, side scan sonar, sediment grain size and vessel tracking data have been used to investigate the impact of bottom trawling on the seafloor morphology and surficial sediments of the northern Catalan continental shelf (NW Mediterranean), providing new insights into the impact of this anthropogenic activity on the seafloor. Multibeam data evidenced the occurrence of large-scale erosive features as a consequence of repeated scouring by fishing gears in localized areas. They are characterized by elongated (70-300 m wide and up to 8 km long) channelized areas of high backscatter with variable incision (from 0.2 m to 1.2 m). The spatial distribution of these morphologies shows a similar pattern to that observed in the fishing intensity, with maximum values in the areas of increased trawling intensity, corresponding to the main fishing grounds. Side scan sonar data also shows higher densities of trawl marks in these areas than in the surroundings. Sediment cores collected on these features show an upward-coarsening trend in the first 4-5 cm of the core, suggesting that part of the finer fraction resuspended by trawling is winnowed, increasing the sand content of the surface sediment. The identification of such large erosive morphologies in the main fishing grounds evidences that repeated trawling over the same fishing ground during decades can result in deep excavation of the seafloor, leading to permanent large-scale morphological changes. Furthermore, the evolution of these erosive features over a 13-year interval points towards long recovery periods after the cessation of trawling activities.

Accurately predicting the flow speed is crucial for applications of coastal ocean circulation simulations such as sediment, larval or contaminant dispersal. This study aims to assess the accuracy of simulated flow speed in a coastal circulation model in comparison with field observations. Deviation between simulated and observed flow speed was assessed in four shallow, coastal locations and four deep, offshore locations in the Gulf of Lion (NW Mediterranean Sea) using six indicators (bias, relative bias, root mean square error, Hanna & Heinold index, correlation and scatter index). Statistical distributions of indicators were calculated during reference periods with low wind, no waves and no stratification. During these periods, relative bias indicated the model displayed a higher performance in predicting transport at shallow stations than at deep stations probably due to grid refinement at these stations. However, there was a low correlation between simulated and observed flow speed, indicating short term time/space mismatches, at all stations during reference periods. Indicators were then calculated during three types of events (wind, waves and stratification) when model assumptions were expected to be violated and their corresponding probability during reference periods indicated that neither wind, wave nor stratification events worsens model's performance.

A new species of sea anemone (Actiniaria, Metridioidea) is described and illustrated from cold-water coral environments at 600 m depth at the Blanes Canyon in the north-western Mediterranean Sea. Litophellia enoplosa sp. nov. is characterized by a column divisible into scapus and scapulus, the former with cuticle forming tenaculi, a mesogleal marginal sphincter muscle, more mesenteries proximally than distally arranged octomerously, and a single pair of directive mesenteries; it has strong retractor and parietobasilar muscles, and acontia with basitrichs and p-mastigophores B2a. Litophellia enoplosa sp. nov. is the second species described in the genus and it can be distinguished from the other species by the cuticle in the column, size ranges of the cnidae, and geographic and bathymetric distribution.