From Watershed to Whales: Tracking the source and transport of microplastics in the greater Monterey Bay region

Plastic pollution is one of the defining environmental challenges of the 21st century. Microplastics are now pervasive throughout marine ecosystems, yet the pathways by which they move from land into coastal food webs remain poorly understood. From 2023-2025, our team conducted a comprehensive regional assessment of microplastics across the river-ocean continuum of central California. We sampled four major rivers, beach sands, coastal seawater, and key marine organisms including forage fish, krill, and blue whales. Processing more than 15,000 liters of water and 1,500 liters of sand yielded 25,000 microplastic particles as confirmed by uFTIR. While uFTIR analysis confirmed over two dozen polymer types in these particles, 94% were, polystyrene, polyethylene, polyamides, and polyester. Microplastic concentrations in river water declined across sampling events spanning before, during, and after high-flow periods, consistent with rapid downstream export and deposition of microplastics in the coastal ocean. Beaches emerged as major accumulation zones, with concentrations roughly thirty times higher than adjacent waters. Fibers, often blue and consistent with textile-derived microfibers, dominated larger particle classes, while polystyrene foam fragments suggest local consumer sources. Early biological analyses indicate that microplastics are reaching upper levels of the marine food web, including blue whales. Together, these findings provide the first source-to-sea baseline for microplastics in central California and reveal how human activity and environmental transport processes structure exposure across ecosystems, from rivers to the largest animals on Earth. 

Speaker: Matthew Samboca, Stanford University