Contaminant profiles of two species of shorebirds foraging together at two neighboring sites in South San Francisco Bay, California.

The San Francisco Bay estuary is used by over one million shorebirds during spring migration and is home to several hundred thousand during the winter. Most shorebird use occurs in the southern reach of the estuary (South Bay). The reduced water circulation and discharge from industrial sources in the South Bay are responsible for the highest levels of some trace elements in the estuary. Wintering shorebirds have been found to have strong site fidelity to areas as small as a few kilometers in the South Bay, which may increase their exposure to contaminants near local point sources. In addition, different shorebird species foraging at the same site have been shown to have different contaminant burdens. Thus, our objectives were to test whether contaminant burdens differed by species, or whether contaminant burdens differed in shorebirds collected at adjacent sites. We examined the contaminant profiles of two species of shorebirds, long-billed dowitchers (Limnodromus scolopaceus) and western sandpipers (Calidris mauri) that forage together at two sites, Hayward and Newark, separated by 8 km in the South Bay. We used multivariate analysis of variance tests to compare the composition of 14 elemental analytes in their liver tissues and estimated their molar ratios of Hg and Se. Composite samples were used for contaminant analyses because of the small body size of the shorebirds. Seven elemental analytes (Ag, Ba, Be, Cr, Ni, Pb, V) were below detection limits in a majority of the samples so statistical analyses were precluded. In the measurable analytes (As, Cd, Cu, Hg, Mn, Se, Zn), we found no significant intra-site differences of contaminant profiles for the two species. We pooled the samples to examine inter-site differences and found significant differences in contaminant profiles between shorebirds at the neighboring sites (P = 0.03). Shorebirds at Newark had higher (P < 0.05) concentrations of As, Cd, and Se than those at Hayward. Dowitchers at Newark had concentrations of Hg and Se which were highly correlated (P < 0.003) in a mean molar ratio of 1:19, similar to that reported in other birds. In the larger dowitcher species, we also examined exposure to 20 organochlorine compounds. Organic analyses showed that the dowitchers had been exposed to DDE, PCBs, dieldrin and trans-nonachlor, but with no significant differences in concentrations between Hayward and Newark (P > 0.05).