A deep-sea (4500 m) trial of a pre-prototype polymetallic nodule collector with independent scientific monitoring revealed that a gravity current formed behind the collector channeled through steeper seafloor sections and traveled 500 m downslope. The prevailing bottom currents dominated sediment dispersion up to the end of the monitoring area at 4.5 km distance. The maximum suspended particle concentration recorded 50 m from mining lanes was up to four orders of magnitude higher than ambient values but decreased rapidly with increasing time, distance, and altitude. Most of the plume remained close to the seafloor, with the highest concentrations at 1 m monitoring altitude and reaching background concentrations at 50 m. Rapid particle flocculation was followed by fast and substantial sediment redeposition. A mm-scale photogrammetric seafloor reconstruction allowed quantitative estimates of the thickness of redeposited sediment next to mining lanes of ≈ 3 cm and a minimum erosional depth of 5 cm.

Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants transported by sediments from continental areas to the sea is crucial for marine resources protection. However, the complexity of estuarine dynamics makes it difficult to quantify sediment fluxes from field measurements alone and requires numerical modeling. Thus, using a realistic 3‐D hydrodynamic and sediment transport model, this study aims at evaluating the influence of model empirical parameters on sediment fluxes and estimating uncertainties on mud and sand transfers at a macrotidal estuary mouth. A sensitivity analysis, considering changes in sediment transport parameters, revealed that the system is sensitive not only to settling and erosion parameterizations but also to the spin‐up period and the sediment sliding process. Both estuarine circulation and tidal pumping induce a residual up‐estuary transport, which is balanced by seaward export during spring tides. Although more fine sediments are exported within the surface turbid plume during high river discharge, the net mud transport is directed up‐estuary due to increased baroclinic circulation. Besides, model results highlighted a strong seasonal variability in sediment fluxes with a short and high import during high river flow and a long and weak export during low river flow. Uncertainties associated with the simulated fluxes were about 93% for mud and 51% for noncohesive classes, based on the best performing parameter sets for surface suspended sediment concentrations. These results can be reliably extrapolated to similar macrotidal estuarine systems.

The spatial distributions of (1) surface sediment characteristics (D_0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ¹³C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.

Where fine-grained marine sediments are being brought in suspension and become subject to transport by currents, aggregation of cohesive primary particles (flocculation) can occur. By changing the inherent particle properties, flocculation processes play an important role in speeding up settling and redeposition of sediment particles. However, while numerous laboratory experiments have been conducted to understand properties and behavior of flocs, so far, there is not yet an appropriate method to monitor flocculation in-situ. Gaining a better understanding of the flocculation process and how it will affect the dispersion of man-made sediment plumes is important to assess the impact of human activities on the environment, for example in the context of deep-sea mining or offshore dredging.In this study, the inherent acoustical and optical properties of sediment particles are studied using in-situ plume monitoring data collected by the MiningImpact2 project consortium during the first deep-sea mining trial of a pre-prototype polymetallic nodule collector vehicle. The trial was conducted in April 2021 at 4500 m water depth in the Clarion-Clipperton Zone (eastern equatorial Pacific Ocean) by the Belgian contractor DEME-GSR. During this trial, one of the main goals was to monitor the spatiotemporal evolution of the sediment plume generated by the mining vehicle. For this purpose, numerous sensors were deployed around the test area including ADCPs of different frequencies, OBSs and deep-sea particle camera. In this study, the main interest is to use this dataset to gain knowledge on the variability of particle properties and to monitor flocculation in the generated plume.The monitoring array of sensors proved successful in measuring the dispersion of the plume around the mining site. In the data recorded in the plume, a gradient in optical and acoustic response was found, suggesting a change in inherent particle properties such as their size and shape induced by flocculation. The evolution of particle size as inferred by the particle camera recordings (PartiCam) corroborated this finding. In combination with currents and environmental measurements, this dataset provided valuable information to better understand the flocculation process.