CONTAMINANTS IN EGGS OF SOME WATERBIRD SPECIES FROM THE SCHELDT ESTUARY, SW NETHERLANDS

The contaminated Scheldt estuary is a wetland of international importa nce 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, concentrati ons of PCB-118 (0.1-0.7 mug g-1 WW) may indicate adverse effects on re productive success. In terns' eggs from the marine zone, PCB-118 did n ot exceed the no-observed effect concentration. In eggs of Black-heade d Gulls (Larus ridibundus) concentrations of HCB, lindane, DDT and die ldrin 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 ad ults feed mainly inland. No adverse effect on breeding success of Blac k-headed Gulls was observed in the field. The estimated biomagnificati on factor for PCB-153 for Oyster catcher (Haematopus ostralegus) is hi gher than for terns and gulls.