Aims: Coastal wetlands provide a diversity of prey to birds and thus variable diet-mediated mercury budgets. The main purpose of the study was to test the effect of dietary specialization and trophic status on mercury levels of birds using common coastal wetlands in a different manner, as revealed by stable isotopes signatures.
Location: Seven saltpans and a large area of ricefields in Portugal.
Methods: Total mercury concentration and stable isotope ratios were measured in chick feathers of black-winged stilt Himantopus himantopus and little tern Sterna albifrons,from Portuguese coastal wetlands during breeding seasons.
Results and Conclusions: Variation in trophic status explained the inter-specific variation of mercury levels, the intra-specific variation of mercury levels in H. himantopus, but not the intra-specific variation of mercury levels in S. albifrons. A different diet specialization and a mixture of prey groups, teleosts and invertebrates from different saltpan ponds and trophic status are suggested as being the basis for intra-specific variation of mercury levels not being related with the trophic status in S. albifrons. H. himantopus only uses invertebrates, but a wide range of stable isotope signatures and mercury levels were observed. Because chicks are precocial, the feeding area may be associated with a confined area in the ricefield, in the saltpan or in the saltpan pond. Differences of d13C in chick feathers of H. himantopus between ricefields and saltpans, and also between saltpan ponds suggested that different contributions from marine versus freshwater sources exist among preys. Prey exhibited differences in d15N and mercury levels, and significant correlation between mercury concentration and d15N revealing that prey occupy different trophic status. Differences between ponds for d13C in prey also support the idea that variation in food resources may occur within saltpans in the basis of the trophic chain.