The dynamics and interactions of plastic debris on the Isle of Cumbrae, Scotland

Marine plastic debris is a considerable environmental threat, contaminating habitats worldwide. This study investigated factors influencing plastic accumulation between two shores on the Isle of Cumbrae, an island in the Firth of Clyde, Scotland, and its ingestion by local biota. Between November 2018 and August 2019, sediment, water and infaunal species were collected from the sheltered Ballochmartin Bay (BMB) and the exposed Kames Bay (KB). The first data chapter assessed the sediment plastic load between the two shores across four shore heights and multiple months. Plastic debris was prevalent at both sites, but significantly higher at KB, peaking in November at the high and mid-high shore. A decline in concentrations was observed between November and February. In the second data chapter, surface water samples assessed the plastic load of the two bays and found KB in February to have significantly greater quantities than any other month and site. Extreme weather events, such as Storm Erik in February 2019, as well as shore exposure and coastline topography, were likely influencing drivers in the distribution and accumulation of plastic debris in the sediments and water. In the third data chapter, plastic ingestion was assessed temporally between four key sediment-dwelling organisms and compared between feeding modes and sites. Plastic ingestion was evident across all species, including two previously undocumented. Feeding mode significantly influenced its ingestion. Suspension feeders (Cerastoderma edule and Macomangulus tenuis) ingested the highest levels, followed by the predator Nephtys spp. and the deposit feeder Arenicola marina. The predator ingested significantly smaller particles compared to the other feeding modes. Plastic exposure had little effect on the quantity ingested, suggesting feeding mode and particle selectivity have a stronger influence. These results contribute to understanding plastic debris dynamics in intertidal habitats and provide a baseline for assessing the impact of recent flood defence work in KB on its distribution and accumulation.