Seasonal changes in fatty acid composition of estuarine intertidal biofilm : implications for western sandpiper migration

Abstract

For shorebirds, long distance migration is an energy-demanding activity, and lipids (largely comprised of fatty acids) with their high energy density are an ideal fuel. Diatoms in intertidal biofilms provide a rich source of fatty acids for fuel and for critical physiological functions. We compared the composition of intertidal biofilm on mudflats at Roberts Bank, a major stopover site for shorebirds in the Fraser River estuary, between two seasons: spring, during the northward breeding migration of Western Sandpipers (Calidris mauri), and winter, when no migrating shorebirds are present. Mass fractions of fatty acids in biofilm (μg fatty acids/g sample in the upper 2 mm of biofilm-containing sediment) in April were 3-7× higher than in winter (January and February). This difference included total saturated, monounsaturated, polyunsaturated, omega-3 (n-3), and omega-6 (n-6) fatty acids, as well as individual fatty acids such as palmitoleic acid (16:1n-7), eicosapentaenoic acid (EPA; 20:5n-3), and docosahexaenoic acid (DHA; 22:6n-3). In addition, organic content was ~25% higher in spring compared to winter. The microphytobenthos in spring biofilm was dominated by marine-influenced diatoms (primarily from the genera Nitzschia and Navicula) which made up >50% (μg/ml) of total biofilm biomass. Higher fatty acid and organic content in biofilm during spring provide shorebirds with both energy and physiologically important fatty acids to support their migration. These findings are consistent with the ‘green wave’ hypothesis, whereby bird migration broadly coincides temporally with the availability of energy and essential nutrients. The role of diatoms as purveyors of important fatty acids to shorebirds underscores the need for new conservation policies that protect the abundance of organic and fatty acid content of intertidal biofilm at estuarine stopover sites.