Contaminants in eggs of some waterbird species from the Scheldt estuary, SW Netherlands

The contaminated Scheldt estuary is a wetland of international importance for waterbirds. In eggs of Common Terns (Sterna hirundo) from the brackish zone, concentrations of Hg, Cr, Cu, Cd, Zn, and PAHs did not exceed no-observed effect levels reported in the literature. Eggshell thickness did not differ from the pre-DDT period. However, concentrations of PCB-118 (0.1–0.7 µg g-1 WW) may indicate adverse effects on reproductive success. In terns' eggs from the marine zone, PCB-118 did not exceed the no-observed effect concentration.In eggs of Black-headed Gulls (Larus ridibundus) concentrations of HCB, lindane, DDT and dieldrin are low and showed no spatial gradient. PCB levels, on the other hand, are high in the brackish zone where adults forage on intertidal flats and eight times lower in eggs from the North Sea coast where adults feed mainly inland. No adverse effect on breeding success of Black-headed Gulls was observed in the field. The estimated biomagnification factor for PCB-153 for Oystercatcher (Haematopus ostralegus) is higher than for terns and gulls

Waterbird communities along the estuarine salinity gradient of the Schelde estuary, NW-Europe

The zonation of non-breeding waterbirds along the Schelde estuary (The Netherlands-Belgium), one of the longest estuaries in NW-Europe with still a complete salinity gradient, including a large freshwater tidal area, was described. Numbers of birds were counted monthly over the period October 1991 to June 1997. Highest numbers of waterbirds were observed in late autumn and winter, with annual peak numbers ranging between 150,000 and 235,000 individuals for the whole estuary. Based on a multivariate analysis different waterbird communities were observed along the salinity gradient. The polyhaline areas of the estuary were numerically dominated by the waders Oystercatcher and Dunlin. Due to the presence of a large brackish marsh in the mesohaline zone, the waterbird community in this area was dominated by the herbivores Wigeon and Greylag Goose. In the oligohaline and freshwater tidal areas, the waterbird community was dominated by duck species, with Teal and Mallard being the most important. The international importance of the Schelde estuary for waterbirds was evidenced by the fact that for 21 waterbird species the 1% level criterion, according to the Ramsar convention, was exceeded. The relation of the observed diversity and community patterns with the functional and habitat diversity of the Schelde estuary as well as the effect of recent conservation measures to preserve this habitat were discussed