Plastics are versatile and durable, but these traits have led to their widespread environmental accumulation. Plastic items degrade and fragment into particles smaller than 5 mm, called microplastics (MPs). Due to their ability to easily disperse, MPs are a pervasive environmental contaminant. In aquatic ecosystems, animals may unintentionally consume MPs, which can have harmful effects including irritation of digestive tissues, transfer of toxins or pathogens, and bioaccumulation through trophic transfer. This study explores how environmental variables influence MPs in mussels (Mytilus spp. L.) and across California estuaries. We are investigating if the abundance and characteristics of MPs are influenced by seasonality, estuary protection status, and watershed land-use and land cover (LULC). We predict there will be 1) higher MP abundance and characteristic diversity at unprotected urbanized estuaries compared to protected estuaries with forest or grassland-dominated watersheds, and 2) MP communities will be dominated by fibers across all sites and seasons. Field samples were collected from fifteen sites coastal and estuary sites that vary in protection status and LULC. Preliminary LULC analysis revealed that watershed urbanization across sites ranged from 2.06% (Drake’s Estero, CA) to 96.72% (San Diego Bay, CA) at twelve hydrologic units. Microplastic extraction of samples involved a potassium hydroxide tissue digestion for mussels. Sample MPs were fractionated to separate by size class, then vacuum-deposited onto filters, and enumerated by color and morphology. A subset of MPs are being picked for polymer identification using u-FTIR (Fourier transform infrared) and Raman spectroscopy. Our findings will provide critical insight into the susceptibility of mussels to MPs, potentially serving as an indicator for the degree of plastic pollution in California estuaries.