Abstract. With knowledge of typical hydrodynamic behavior of waste plastic material, models predicting the dispersal of benthic plastics from land sources within the ocean are possible. Here we investigated the hydrodynamic behavior (density, settling velocity and resuspension characteristics) of non-buoyant preproduction plastic pellets in the laboratory. From these results we used the MOHID modelling system to predict what would be the likely transport and deposition pathways of such material in the Nazaré Canyon (Portugal) during the spring/summer months of 2009 and the autumn/winter months of 2011.Model outputs indicated that non-buoyant plastic pellets would likely be transported up and down canyon as a function of tidal forces, with only a minor net down canyon movement resulting from tidal action. The model indicated that transport down canyon was likely greater during the autumn/winter, primarily as a result of occasional mass transport events related to storm activity and internal wave action. Transport rates within the canyon were not predicted to be regular throughout the canyon system, with stretches of the upper canyon acting more as locations of pellet deposition than conduits of pellet transport. Topography and the depths of internal wave action are hypothesized to contribute to this lack of homogeneity in predicted transport.

Given the complexity of quantitative collection, knowledge of the distribution of microplastic pollution in many regions of the world ocean is patchy, both spatially and temporally, especially for the subsurface environment. However, with knowledge of typical hydrodynamic behavior of waste plastic material, models predicting the dispersal of pelagic and benthic plastics from land sources into the ocean are possible. Here we investigate three aspects of plastic distribution and transport in European waters. Firstly, we assess patterns in the distribution of plastics found in fluvial strandlines of the North Sea and how distribution may be related to flow velocities and distance from source. Second, we model transport of non-buoyant preproduction pellets in the Nazaré Canyon of Portugal using the MOHID system after assessing the density, settling velocity, critical and depositional shear stress characteristics of such waste plastics. Thirdly, we investigate the effect of surface turbulences and high pressures on a range of marine plastic debris categories (various densities, degradation states and shapes tested) in an experimental water column simulator tank and pressure laboratory. Plastics deposited on North Sea strandlines varied greatly spatially, as a function of material composition and distance from source. Model outputs indicated that such dense production pellets are likely transported up and down canyon as a function of tidal forces, with only very minor net down canyon movement. Behaviour of plastic fragments under turbulence varied greatly, with the dimensions of the material, as well as density, playing major determining roles. Pressure was shown to affect hydrodynamic behaviours of only low density foam plastics at pressures ≥ 60 bar

In this study, a hydrodynamic model was applied to the Nazaré submarine canyon with boundary forcing provided by an operational forecast model for the west Iberian coast for the spring of 2009. After validation, a lagrangian transport model was coupled to the hydrodynamic model to study and compare the transport patterns of three different classes of organo-mineral aggregates along the Nazaré canyon. The results show that the transport in the canyon is neither constant, nor unidirectional and that there are preferential areas where deposited matter is resuspended and redistributed. The transport of the larger class size of organo-mineral aggregates (2000 μm and 4000 μm) is less pronounced, and a decrease in the phytodetrital carbon flux along the canyon is observed. During the modelled period, the Nazaré canyon acts as a depocentre of sedimentary organic matter rather than a conduit of organo-mineral aggregates to the deep sea, as has been reported by other authors. The results of this study are crucial for the understanding of the oceanic carbon sequestration at the continental margin, and therefore important for evaluating the role of submarine canyons within the global carbon cycle

Abstract. In this study, a hydrodynamic model was applied to the Nazaré submarine canyon with boundary forcing provided by an operational forecast model for the West Iberian coast. After validation, a Lagrangian transport model was coupled to the hydrodynamic model to study the transport patterns of the organo-mineral aggregates along the Nazaré canyon comparing three different classes of organo-mineral aggregates. The results showed that the transport in the canyon is neither constant, nor unidirectional and that there are preferential areas where suspended matter is resuspended, transported and deposited. The results showed that the transport of the larger size classes of organo-mineral aggregates is less pronounced, and that there is a decrease in the phytodetrital carbon flux along the canyon. The Nazaré canyon acts as depocenter of sedimentary organic matter and the canyon is not a conduit of organo-mineral aggregates to the deep sea.