Modeling Plastic Litter Sources, Transport, and Accumulation in the Strait of Sicily: Linking Hydrodynamics and Field Observations

Socio-economic and human activities represent the major impacts on marine pollution affecting the marine ecosystems, raising increasing scientific, societal and policy interest due to the long-term consequences for the environment and the coastal economies.

Especially in semi-enclosed basins such as the Mediterranean Sea, the accumulation of marine litter, and particularly plastic items, represents a serious issue, intensified by intense anthropogenic pressure, high coastal population density and circulation patterns.

The identification of plastic litter sources and their tracking on the sea surface represents a first step to identify the most vulnerable areas and accumulation hotspots.

To achieve this goal, the hydrodynamic Lagrangian model Track Marine Plastic Debris (TrackMPD) was applied in the strait of Sicily considering rivers as input sources of plastic litter. The area is characterized by the presence of important hydrodynamic processes, including sea currents, mesoscale eddies, upwelling events etc. The quantities of plastic items to be released by rivers into the marine environment were estimated considering the data provided by “The Ocean Cleanup” website.

The model’s results were compared with the in-situ data collected during the monitoring campaign conducted by “Agenzia Regionale per la Protezione ambientale” (ARPA) and “Consiglio Nazionale delle Ricerche” (CNR) in August 2018 and 2019, where microplastic items were sampled using a Manta net along fixed transects.

Since the actual discharging point and period of the debris recorded by ARPA-CNR were unknown, the comparison was performed by considering 105 scenarios, each characterized by a different starting day of the daily particle release. The scenarios span a temporal interval from May up to the sampling day; for each subsequent scenario, the discharging time window progressively decreases as the release start date is shifted forward by one day.

The accumulation and density maps were realized for each scenario. A buffer area was considered around each transect and the sum of the simulated particles within it was calculated, comparing the results with the microplastics sampled. The comparison between the simulations and the in-situ campaigns was performed by computing the coefficient of determination, R².

The results highlight the difficulties validating the hydrodynamic model by using the in-situ data, indeed very low R² values were performed for each scenario.

Further developments of this work include the determination of realistic scenarios of plastic discharge into the marine environment through a detailed study of the territory, considering local productive activities and the resulting pollutant generation, coupled with targeted sampling campaigns to provide data for model implementation and/or validation.