Plastic pollution monitoring in the wrack line: baseline and seasonality trends along several coastlines from Brittany (Erquy, France)

Plastic pollution is acknowledged across all environmental compartments, ranging from high mountain ranges to the deepest abyssal plains. It has been identified in the lithosphere (sediment), hydrosphere (water bodies), atmosphere (air), and biosphere (living organisms). In this context of ubiquitous pollution, beaches, and in particular the wrack line, are commonly used as monitoring sites for plastic pollution. There are established monitoring programs to track plastic pollution at different scales along beaches, such as the OSPAR beach litter monitoring program at the North-East Atlantic scale or the French monitoring program for meso- and large microplastics on beaches.

This study aims to build upon the expertise and experience gained from existing monitoring programs to provide a comprehensive approach for understanding the processes of plastic influx, accumulation, and impregnation on beaches. Four types of coasts were selected in Brittany (Erquy, France) to represent various configurations: (i) Accreting sandy beach, (ii) Eroding sandy beach, (iii) Protected cliff (iv) Exposed cliff. The study covers a period from August 2022 to August 2023, where bimonthly statements were conducted, resulting in seven dataset collection points (308 measurements). Each of the four sectors, measuring 100 meters along the wrack line, was studied using eleven 40x40 cm quadrats spaced every 10 meters. The top centimeter of sand was collected using a trowel and filtered through a 1mm mesh sieve. Seawater flotation was employed to separate and recover plastics.

Plastics were then classified into three categories: large microplastic (1-5mm, LMP), mesoplastic (5-25mm) and macroplastics (>25mm). Plastics were counted and weighed within each category. Four indicators were quantified to monitor potential sources of plastic pollution: (i) "Pellet" indicator of chronic or accidental losses along the plastic production chain, (ii) "Port" indicator for port and related recreational activities, (iii) "WWTP" indicator for water network management issues, (iv) "Butt" indicator for activities linked to the improper disposal of cigarette butts.

Results are presented in the form of box plots providing rich information illustrating variability, outliers, and the overall distribution of quadrat measurements. The maximum value by quadrat reaches 706 items/m² of wrack line. The annual survey provides baseline values for different coast types of 106, 39, 39 and 3 items/m² of wrack line for accreting sandy beach, eroding sandy beach, protected cliff, and exposed cliff, respectively. Out of the total 308 measurements, 82 of them have the smallest value possible, which is 0. Principal Component Analysis (PCA) was finally carried out to understand the importance of various environmental factors (e.g., wind, wave, tidal range) on the influx, accumulation, and distribution of plastics along the wrack line.

By combining surveys across different coastal types in a specific region, this work enhances the understanding of the dynamics of plastic pollution, especially to implement effective environmental monitoring strategies.