Abstract
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.