San Francisco Bay microplastics study finds particles 10 times smaller than before, threatening carbon sinks and marine food chain

A new pilot study from the San Francisco Estuary Institute has detected microplastic particles in the San Francisco Bay as small as 20 microns, roughly the width of a human hair. That is 10 times smaller than previous measurements. Researchers say these tiny particles may make up the vast majority of microplastics in the water and come from sources like tire wear, synthetic clothing fibers, and household dust. Urban stormwater runoff carries them into the Bay, where they enter the food chain. The study has implications beyond pollution tracking. Microplastics are being found in krill, anchovies, and juvenile rockfish, which are key parts of the marine food web. Research shows microplastics can harm krill reproduction and development. They can also attach to carbon-capturing phytoplankton, slowing their descent to the sea floor. Since the ocean stores 20% to 35% of atmospheric carbon, any disruption to this process could weaken a major natural carbon sink. The state of California is already taking action. A statewide plastics monitoring project starts this summer under the Ocean Protection Council. Separately, Attorney General Rob Bonta sued Exxon Mobil in 2024 over alleged deception about plastic pollution. This study could help standardize how smaller microplastics are tracked across California and inform both environmental regulation and climate policy.