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The physical and chemical properties of microplastics and their environmental distributions may provide clues about their sources and inform their fate. We demonstrate the value of extensive monitoring of microplastics in an urban bay, San Francisco Bay. Surface water, fish, sediment, stormwater runoff, and treated wastewater were sampled across the bay and adjacent national marine sanctuaries (NMS). We found microplastics and other anthropogenic microdebris ("microdebris") in all sample types. Concentrations were higher in the bay than in the NMS, and within the bay, concentrations were higher during the wet season than during the dry season. The fate of microdebris varied depending on their morphologies and densities: fibers were dominant in fish, black rubbery fragments were common in sediment, as were fibers, while buoyant fragments and fibers were widely observed in surface waters. Notably, we found large amounts of black rubbery fragments, an emerging contaminant, in stormwater. Moreover, stormwater was a significant pathway of microdebris, with concentrations roughly 140 times greater than those found in wastewater, which was dominated by fibers. Overall, we demonstrate the value of multimatrix regional monitoring to evaluate the sources and fate of microplastics, which can inform effective mitigation for other urban bays around the world.

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