Shorebird Abundance is Associated With Nutritional Quality of Intertidal Biofilm on the Fraser River Estuary

Abstract

Dense congregations of shorebirds forage on tidal flats during long-distance migration, and their abundance is presumed to mirror the underlying ecological conditions. We quantified the nutritional content of intertidal biofilm (a thin layer of microalgae, bacteria, and other micro-organisms embedded in a mucilaginous matrix) to assess whether biofilm biomass and the abundance of macronutrients (lipid, protein, and carbohydrate) provide a measure of habitat quality for migrating shorebirds. We compared shorebird use, biofilm biomass, nutritional quality, and stable isotope signatures at two mudflats on the Fraser River estuary, British Columbia, Canada, during summer 2020 (southward migration) and spring 2021 (northward migration). The abundances of Western Sandpipers (Calidris mauri) and Dunlin (Calidris alpina) were consistently higher (6-100×) at Brunswick Point relative to the Iona Foreshore site during both migration periods. Biofilm biomass (chlorophyll a) was 2× higher at Brunswick Point than Iona Foreshore during southward migration, and was similar between sites during northward migration. Macronutrient content (lipid, protein, and carbohydrate) in intertidal biofilm was 1.4× to 3.8× higher at Brunswick Point than Iona Foreshore during both migration periods. Carbohydrate content was higher during southward migration at both tidal flats, whereas protein and lipid content at Iona Foreshore did not show significant differences between years/migration periods. Carbon and nitrogen stable isotope signatures during spring 2021 did not differ between the two sites, suggesting that nutrients had similar origins from marine and terrestrial inputs. The findings tie shorebird use to nutrients available in intertidal biofilm during migration periods.