Assessing the relationship between urbanization and plastic litter on sandy beaches in California, USA

Urbanization can increase the threat plastic litter poses to beaches around the world. However, many oceanic beaches lack baseline data on how urbanization may be altering plastic litter abundance. Here, I documented the abundance and type of plastic litter at 23 oceanic beaches that spanned an urbanization gradient (as measured by population density) along the California, USA coast in October 2023. Across all 23 sites, I found that urbanization significantly increased abundance for all types of plastic litter including (large microplastic litter 2-5 mm; mesoplastic 5-25 mm; macroplastic >25 mm). I also found that urban beaches had significantly more plastic litter (132.7 pieces/m2 ± 84.2 S.D.) than non-urban beaches (67.4 pieces/m2 ± 30.9 S.D.). In addition, I found that urban beaches had significantly higher abundances of cigarette butts, hard plastic, and foamed plastic than non-urban beaches. Collectively, my findings suggest that plastic litter pollution poses an important threat to California beaches, that oceanic beaches in more urban areas may be more impacted by this threat over time, and that plastic waste from recreation (both on and off-shore) could be an important source of pollution.