The streaming of plastic in the Mediterranean Sea

Abstract

Plastic debris is a ubiquitous pollutant on the sea surface. To date, substantial research efforts focused on the detection of plastic accumulation zones. Here, a different paradigm is proposed: looking for crossroad regions through which large amounts of plastic debris flow. This approach is applied to the Mediterranean Sea, massively polluted but lacking in zones of high plastic concentration. The most extensive dataset of plastic measurements in this region to date is combined with an advanced numerical plastic-tracking model. Around 20% of Mediterranean plastic debris released every year passed through about 1% of the basin surface. The most important crossroads intercepted plastic debris from multiple sources, which had often traveled long distances. The detection of these spots could foster understanding of plastic transport and help mitigation strategies. Moreover, the general applicability and the soundness of the crossroad approach can promote its application to the study of other pollutants.

An Integrative Assessment of the Plastic Debris Load in the Mediterranean Sea

“…Even though plastic debris generated in cities may reach oceans via rivers, information about how to express city and river contribution together is lacking. Therefore, we consider them separately, in agreement with previous studies (Baudena et al, 2022;Liubartseva et al, 2018). Usually, city and river relative contribution to the total flux of plastic entering the oceans is estimated to be p c = 40% and p r = 40% respectively (Lebreton et al, 2012;Li et al, 2016) while the rest (20%) is expected to originate from ships.…”

“…These two fields were then added together, giving the final velocity field, which therefore has a spatial resolution of 1/24°and a temporal resolution of 1 h. This combination allowed us to consider the geostrophic component, the Ekman effect, and the Stokes drift, which implicitly considers the windage effect. This choice is supported by recent Lagrangian models built specifically for the Mediterranean basin considering the importance of the effect of Stokes drift in plastic debris transport (Baudena et al, 2022;Liubartseva et al, 2018). Backward advection was computed with a 4th order Runge-Kutta scheme, with a time step of 0.5 h.…”

“…Usually, city and river relative contribution to the total flux of plastic entering the oceans is estimated to be p c = 40% and p r = 40% respectively (Lebreton et al, 2012;Li et al, 2016) while the rest (20%) is expected to originate from ships. Based on recent studies (Baudena et al, 2022;Liubartseva et al, 2018;Soto-Navarro et al, 2020) we changed these values to p c = 50% and p r = 30%.…”

“…where PEP Ci is Plastic Emission Probability of the i-th city, pop i is the population of the i-th city, and MPW (country) is the mismanaged plastic waste rate of the country where the city is found. This formulation has been used in a recent model validated with in situ observations (Baudena et al, 2022) to quantify the number of plastic particles released by cities.…”

“…S9). However, in the Mediterranean, the residence time of plastics in surface waters is estimated to be relatively short, between 7 and 90 days (Baudena et al, 2022;Liubartseva et al, 2018), so such a degraded state is difficult to explain by a long residence time in the water. Given the limited area of the basin and the lack of long-term accumulation (Cózar et al, 2015;Mansui et al, 2015), an alternative explanation is an input of MPs in a more advanced state of fragmentation from nearshore waters.…”