The exploitation of biofilm by migrant western sandpipers (Calidrismauri)

Assessing the quality of migratory shorebird stopover sites requires good measures of food availability. We developed simple methods to measure biofilm grazing by migrant western sandpipers (Calidris mauri), a species for which biofilm is an important dietary component. We used a field-portable chlorofluorometer to measure the density of chlorophyll-a (Chl-a) in surficial biofilms on Roberts Bank, a large intertidal mudflat in British Columbia, Canada, during northward migration. Chl-a density begins at a low level during each diurnal emersion period, and increases steadily during emersion at 4.1 mg m^-2 h^-1 for a total of ∼24.6 mg m^-2 over a typical 6 h emersion period and ∼41 mg m^-2 over a 10 h emersion period. Western sandpipers grazed at 1.35-1.45 mg Chl-a m^-2 min^-1, thus biofilm production supports 17.6 min m^-2 of grazing time during a 6 h low tide period and 29.3 min m^-2 during a 10 h period. During peak northward migration, the average grazing intensity of western sandpipers over an intertidal emersion period was 3.3-6.4 min m^-2, suggesting that biofilm accumulation was 2.7-8.8 fold greater than the amount consumed. We found Chl-a density was highest (∼65 mg per m²) within 40 m of the shoreline. Grazing intensity was lowest close to shore, where predation risk from falcon attacks is highest. Grazing intensity peaked at 240 m and then declined, lowering Chl-a density at greater distances to a uniform level of ∼54 mg m^-2. These results indicate that interactions between biofilm production and sandpiper grazing underlie spatio-temporal patterns in biofilm abundance on Roberts Bank.