Fatty Acid Profiles and Microplastic Pollution in Intertidal Biofilm at the Cowichan-Koksilah Estuary, Canada

On estuarine tidal flats, intertidal biofilms are rich in essential fatty acids that provide critical nutrients for long-distance flights of migratory shorebirds. Microplastics can promote the adsorption of dissolved organic carbon and influence the composition of microbial biofilms, and therefore studies of intertidal biofilm and microplastics inform assessments of the consequences of microplastic pollution on habitat for migratory birds. This study examined the fatty acid profiles and microplastic load in intertidal biofilms across four sections and three tidemarks of the Cowichan-Koksilah Estuary, British Columbia, Canada. The most abundant fatty acids included palmitoleic (C16:1n-7), eicosapentaenoic (EPA; C20:5n-3), palmitic (C16:0), 6Z,9Z,12Z-hexadecatrienoic (C16:3), stearidonic (SDA; C18:4n-3), and oleic (C18:1n-9), all of which had median abundance > 2.43 µg/g dry sediment. These fatty acid profiles match those found in shorebird fat reserves, linking biofilm nutritional quality and successful migration. The highest fatty acid content occurred in the central part of the estuary, and the lowest in an area surrounding a marine terminal. Notably, a clear spatial pattern was observed with fatty acid abundance decreasing from high to low tidemarks. Fatty acid content in surficial sediments was positively correlated with measures of organic content and a healthy physiological state of microalgae (chlorophyll preservation index) within surface sediments. In contrast, fatty acid content was weakly and negatively correlated with microplastic loads, although details varied by fatty acid. These results suggest that microplastic accumulation did not directly correlate with nutritional quality of intertidal biofilm, though one station near shore showed high levels of both, suggesting localized interactions worth monitoring for potential impacts on migratory bird nutrition. Future monitoring of microplastic accumulation in these critical feeding habitats will help elucidate potential trade-offs in nutritional quality for migratory birds relying on estuarine ecosystems. Intertidal biofilm in the Cowichan-Koksilah Estuary provides essential fatty acids critical for migrating shorebirds. Most abundant fatty acids were palmitoleic (C16:1n-7), EPA (C20:5n-3), palmitic (C16:0), hexadecatrienoic (C16:3), stearidonic (C18:4n-3), oleic (C18:1n-9). Fatty acid abundance varied by location, with lowest levels near industrial terminals and highest levels at elevated tidemarks. Intertidal biofilm was dominated by diatoms, evidenced by high palmitoleic acid (C16:1n-7) and EPA (C20:5n-3) levels, with distinct bacterial communities at mid-tidal zones. Microplastics showed weak negative correlations with fatty acids, indicating no strong pollution-nutrition relationship.