Abstract
The Mediterranean is considered a hot-spot for plastic pollution, due to its semi-enclosed nature and heavily populated coastal areas. In the present study, a basin-scale coupled hydrodynamic/particle drift model was used to track the pathways and fate of plastics from major land-based sources (coastal cities and rivers), taking into account of the most important processes (advection, stokes drift, vertical and horizontal mixing, sinking, wind drag, and beaching). A hybrid ensemble Kalman filter algorithm was implemented to correct the near- surface circulation, assimilating satellite data (sea surface height, temperature) in the hydrodynamic model. Different size classes and/or types of both micro- and macroplastics were considered in the model. Biofouling induced sinking was explicitly described, as a possible mechanism of microplastics removal from the surface. A simplified parameterization of size-dependent biofilm growth has been adopted, as a function of bacterial biomass (obtained from a biogeochemical model simulation), being considered a proxy for the biofouling community. The simulated distributions for micro- and macroplastics were validated against available observations, showing reasonable agreement, both in terms of magnitude and horizontal variability. An 8-year simulation was used to identify micro- and macroplastics accumulation patterns in the surface layer, water column, seafloor and beaches. The impact of different processes (vertical mixing, biofouling, and wind/wave drift) was identified through a series of sensitivity experiments. For both micro- and macroplastics, distributions at sea surface were closely related to the adopted sources. The microplastics concentration was drastically reduced away from source areas, due to biofouling induced sinking, with their size distribution dominated by larger (>1 mm) size classes in open sea areas, in agreement with observations. High concentration patches of floating plastics were simulated in convergence areas, characterized by anticyclonic circulation. The distribution of macroplastics on beaches followed the predominant southeastward wind/wave direction. In the water column, a sub-surface maximum in microplastics abundance was simulated, with increasing contribution of smaller particles in deeper layers. Accumulation of microplastics on the seafloor was limited in relatively shallow areas (<500 m), with bottom depth below their relaxation depth due to defouling. The simulated total amount of floating plastics (∼3,760 tonnes) is comparable with estimates from observations.
Highlights
Plastic global production has continuously increased since the 1950s, reaching almost 300 million tons in recent years (Plastics Europe, 2014)
The hydrodynamic model is based on the Princeton Ocean Model (POM; Blumberg and Mellor, 1983) that is currently operational within POSEIDON forecasting system1 (Korres et al, 2007)
Based on the adopted formulation, smaller size microplastics (50, 200, 350, and 500 μm) are gradually sinking due to the buoyancy loss resulting from the attachment of heavier biofilm (∼1,500 kg/m3), while larger size particles (>1 mm) practically remain afloat
Summary
Plastic global production has continuously increased since the 1950s, reaching almost 300 million tons in recent years (Plastics Europe, 2014). Existing observations in the global ocean (Cózar et al, 2014, 2015; Eriksen et al, 2014) suggest an important removal process of smaller size MPs from the surface waters. The increase of the plastic items density from the attachment of (heavier) marine organisms, known as biofouling, is a size-specific process (Fazey and Ryan, 2016) that has been hypothesized in several studies (Cózar et al, 2014, 2015; van Sebille et al, 2015; Koelmans et al, 2017; Kaandorp et al, 2020) as a potential explanation of the observed decreasing microplastics abundance for smaller sizes in ocean surface waters. Other processes that might contribute to the removal of microplastics from surface waters include aggregation with sinking particulate organic matter (Long et al, 2015) or fecal pellets (Cole et al, 2016) and fragmentation to as-yet undetectable sizes. While the above processes have been observed in the lab or in the field, these have not been thoroughly quantified and their relative importance remains unclear (van Sebille et al, 2020)
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