Bottom trawling grounds have been expanding to deeper areas of the oceans since the mid‐XXth century, and mitigating strategies aimed to protect fish stocks, such as temporal trawling closures, have recently been implemented. Here we investigated the biogeochemical properties of sediment from a deep‐sea trawling ground in Palamós Canyon (NW Mediterranean) to assess the effects of a 2‐months trawling closure in the recovery of sedimentary organic matter. In comparison to untrawled areas, the continuous erosion and sediment mixing in trawling grounds led to coarser reworked sediments impoverished in organic carbon (∼30% loss) and promoted the degradation of labile compounds (52–70% loss). These impacts persisted after the temporal trawling closure, highlighting that this management strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling grounds, this activity could have significant and irreversible biogeochemical impacts on ocean margins at a global scale, hampering their carbon burial capacity.

Increases of water turbidity and suspended sediment transport in submarine canyons have been associated with high-energy events such as storms, river floods and dense shelf water cascading (DSWC), and occasionally with bottom trawling along canyon flanks and rims. To assess the variations on the water column turbidity and sediment transport in the Palamós Canyon linked to both natural and trawling-induced processes, an autonomous hydrographic profiler, as well as a near-bottom current meter and a turbidimeter were deployed in the canyon axis (929 m depth) from February to June 2017, covering a trawling closure (February) and trawling activities (March-June). Periods of enhanced water turbidity during the trawling closure were mostly associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such events, the water column displayed low suspended sediment concentrations (~ 0.3 mg L^− 1) until the trawling season began, when particulate matter detachments, ranging between > 1 mg L^− 1and 3.8 mg L^− 1, were observed at the water depths where the trawling grounds are found. During the trawling closure, high near-bottom suspended sediment fluxes (35–44 g m^− 2 s^− 1) were sporadically registered at ~ 920 m depth associated with a major storm and DSWC event. Smaller but more frequent increases of near-bottom suspended sediment fluxes (0.1–1.4 g m^− 2 s^− 1) were recorded during trawling activities. Despite these smaller trawling-induced suspended sediment fluxes, 30 days of continuous bottom trawling activity transported a total amount of 40 kg m^− 2, of similar magnitude to that generated by a major DSWC event (50 kg m^− 2). Since bottom trawling in Palamós Canyon is practiced on a daily basis throughout the year, a much larger contribution of anthropogenically derived water turbidity and suspended sediment transport can be expected.

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.

Continental margins are transitional areas between the land and the deep ocean where large amounts of particulate matter are supplied, transported, and ultimately deposited in the deep sea. High-energy hydrodynamic processes such as storms, ocean currents, or internal waves and tides, as well as bottom trawling activities contribute to the resuspension and remobilization of these particles. These mechanisms favor their transference from the continental shelf to the continental slope and maintain high concentrations of particulate matter in the water column forming nepheloid layers. The temporal evolution of the hydrographic and nepheloid structure in the upper slope off Vancouver Island (British Columbia, Canada, NE Pacific) was assessed by analyzing 4 months (from mid-August to late-November 2018) of sensor data from Ocean Networks Canada (ONC) NEPTUNE cabled seafloor observatory. The distribution of particulate matter during the study period illustrated a well-defined turbidity structure consisting of surface nepheloid layers (SNLs) at 400 m depth. Moderate storm events occurred during fall when the more intense INLs were recorded at shelf-break depths. However, not all the INLs recorded during this period occurred in coincidence with these storms, indicating that these INL detachments were modulated by a different sediment resuspension mechanism. Analyses of fishing vessel activity during the study period revealed that trawlers operated over the same depth range as these INLs. Our results suggest that, in combination with the regional currents, the presence of continuous fishing along the continental slope off Vancouver Island contributes to the advection of suspended sediment particles, playing a major role in their transfer as nepheloid layers.

Dense shelf water cascading (DSWC) is a key oceanographic process in transferring energy and matter from continental shelves to deep ocean areas. Although intense DSWC (IDSWC) events have received most attention due to their large impacts, mild DSWC (MDSWC) events are the most frequent in the northwestern Mediterranean and are expected to become more common under climate change. However, their dynamics, particularly in the Cap de Creus Canyon, have been less comprehensively described and compared to strong-winter events. This study investigates MDSWC in the Cap de Creus Canyon during the mild winter of 2021–2022, examining shelf-canyon transports of both dense shelf waters and suspended particulate matter (SPM). Observations from the FARDWO-CCC1 multiplatform cruise in March 2022 revealed the presence of cold, dense, and turbid shelf waters, enriched in dissolved oxygen and chlorophyll a, on the continental shelf adjacent to the canyon. These waters cascaded into the canyon head and progressed further into the canyon along its southern flank to ∼ 390 m depth. Estimated water and SPM transports during this event were 0.7 Sv and 10⁵ metric tons (t), respectively, at the continental shelf. Within the canyon, transports were 0.3 Sv and 10⁵ metric tons in the upper section, while mid-canyon transports were lower (0.05 Sv and 10⁴, respectively), indicating that most dense shelf waters likely remained confined to the shelf area and upper canyon. During this event, dense shelf waters were transported ∼ 30 km from the shelf into the canyon. Our results show that significant transport of dense shelf waters (260 km³) and suspended sediment can occur in the Cap de Creus Canyon during MDSWC events under mild winters, also contributing significantly to the formation of Western Intermediate Water (WIW) in the canyon. The Mediterranean Sea Physics reanalysis data indicate that the cascading season lasted from late-January to mid-March 2022, with several shallow cascading pulses throughout this period. Peak transport occurred in mid-March associated with an eastern storm, which likely intensified MDSWC in the canyon. Our study reinforces the idea that dense shelf water transports exhibit marked interannual variability, even under mild winters.

This study examines dense shelf water cascading (DSWC) and estimates the dense shelf-water and associated sediment transport in the Cap de Creus Canyon (northwestern Mediterranean) during the mild winter of 2021–2022. The FARDWO-CCC1 multiplatform survey in March 2022 revealed dense shelf waters on the continental shelf, which were transported to the canyon head. These cold, dense, and turbid waters, rich in dissolved oxygen and chlorophyll-a, downwelled along the canyon’s southern flank to depths around 350 m. During the observed downwelling event, estimated water and suspended sediment transport within the dense water vein were 0.3 Sv and 10⁵ metric tons, respectively, mainly confined to upper canyon reaches. These transports were low compared to extreme winters, likely due to the influence of freshwater inputs and moderate meteorological winter conditions. Transport magnitudes were higher in the upper canyon section than in the mid-canyon section, where transport was estimated at 0.05 Sv, including around 10⁴ metric tons of sediment. This observation suggests that during mild winters, while most of the dense water either remains on the shelf or the shelf-edge area, or flows southward along the coast, the Cap de Creus Canyon acts only as a partial sink for cascading waters. Mediterranean Sea Physics reanalysis data showed that the cascading season lasted approximately three months, from January to early April 2022, with several cascading pulses within the canyon. The highest dense shelf water transport occurred in mid-March, associated with easterly/south-easterly windstorms. This study confirms that remarkable dense shelf water and sediment transport occurs in the Cap de Creus Canyon, particularly along its southern flank, even during mild winters in absence of deep cascading and limited external forcing. Nevertheless, this phenomenon appears to make a significant contribution to the formation of Western Intermediate Water (WIW) in the region.