Abstract
In this study, the abundance and properties (size, shape, and polymer type) of microplastics (MPs) in sea surface water samples, collected during two sampling campaigns over 2018–2019, in four coastal areas of the Mediterranean Sea (Saronikos Gulf, LIgurian Sea, Gulf of Lion, and Gabes Gulf) were investigated. Coupled hydrodynamic/particle drift model simulations with basin-scale Mediterranean and high resolution nested models were used to provide a better understanding on the variability of the abundance/size of MPs, originating from wastewater and river runoff, in the four areas. Different size classes of MPs were considered in the model, taking into account biofouling induced sinking, as a possible mechanism of MPs removal from the surface. The Gabes Gulf showed the highest mean MPs abundance (0.073–0.310 items/m2), followed by Ligurian Sea (0.061–0.134 items/m2), Saronikos Gulf (0.047–0.080 items/m2), and Gulf of Lion (0.029–0.032 items/m2). Overall, the observed MPs abundance and size distribution was reasonably well reproduced by the model in the four different areas, except an overestimation of small size contribution in Saronikos Gulf. The basin-scale simulation revealed a strong decrease of smaller size MPs in offshore areas, due to biofouling induced sinking, with larger (floating) MPs being able to travel longer distances in the open sea. A significant impact of waves drift and advection of MPs from non-local sources was identified from model simulations, particularly in the Gulfs of Lion and Gabes, having a stronger effect on larger microplastics. In Gabes Gulf, most MPs originated from offshore areas, being mainly (floating) larger size classes, as suggested by the observed quite small contribution of <1 mm particles. The MPs observed abundance distribution in each area could be partly explained by the adopted sources distribution. The modeling tools proposed in this study provide useful insight to gain a better understanding on MPs dynamics in the marine environment and assess the current status of plastic pollution on basin and regional scale to further develop environmental management action for the mitigation of plastic pollution in the Mediterranean Sea.
Highlights
The simulated distribution reveals a strong decrease of MPs abundance from coastal areas near source inputs to offshore areas, modulated by hydrodynamic circulation patterns
Among the four key study areas, biofouling appears to have a stronger effect in Gabes Gulf. This is a quite shallow area, where sinking MPs may accumulate on the seafloor (Tsiaras et al, 2021). This may be attributed to a strong phytoplankton biomass induced by high nutrient concentrations through terrestrial or sediments inputs characteristic of this area, where the presence of biofouled MP items were frequently found in samples from Gabes Gulf (Zayen et al, 2020)
High resolution models, nested in the Mediterranean basin-scale model, were downscaled in the four areas to better resolve the MPs distribution and fine-tune existing source inputs, in order to obtain a better fit with the observations
Summary
In 2019, worldwide production of plastic was 368 million metric tons and, after a decrease in the first half of the year 2020, due to COVID-19 pandemic, it has started to increase again (PlasticEurope, 2020; Patrício Silva et al, 2021). Plastics have been found worldwide in the marine environment, with estimates suggesting that 5–10 million tonnes (accounting for 1.5–4% of the global plastic production) enter the ocean every year. These amounts are expected to increase 1 order of magnitude by 2025 (Maes et al, 2018; Kane et al, 2020; Shabaka et al, 2020). MPs can originate from different sources (Freeman et al, 2020). MPs can originate from various land- and sea-based sources, entering the marine environment via different pathways, such as sewage outlets, wind and land run-off, rivers or urban effluents (Veiga et al, 2016)
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