Lead and cadmium in eggs of colonially nesting waterbirds of different position in the food chain of Greek wetlands of international importance
Introduction
Lead and cadmium, both non-essential heavy metals, occur widely in the terrestrial and aquatic environment mainly due to inputs from human activities. In biota, these metals enter the food chains and find their way through mechanisms, that have been partly understood (Bryan and Langston, 1992, Schuhmacher et al., 1995, Furness, 1996a, Furness, 1996b, Stewart et al., 1996). Both metals are very toxic to humans and animals (Goyer et al., 1995). Long-term field and laboratory research of the effects of lead on the behavioural development of gulls indicated lower survival rates, significant lead-induced impairments in righting response, locomotion, thermo-regulatory behaviour, begging and feeding behaviour and disrupted sibling recognition of chicks (Burger and Gochfeld, 1994, Burger and Gochfeld, 1995a, Burger and Gochfeld, 1996a, Burger and Gochfeld, 1997, Burger and Gochfeld, 1998). Cadmium also results in a variety of physiological disturbances in birds. Effects may include altered behaviour, disturbance in the metabolism of some essential elements, suppression of egg production, kidney damage, testicular damage and other impairments (Furness, 1996a).
Mediterranean wetlands are ecosystems of great importance as human resources, in regard to fisheries, aquaculture and grazing that have developed long ago in wetlands. Besides, Mediterranean wetlands are valuable for providing suitable habitat for numerous species of wildlife, especially birds, of both economic and conservation importance (Goutner, 1994b, Pearce and Crivelli, 1994, Zalidis and Mantzavelas, 1994).
Use of bird organs (liver, kidney, etc.) and tissues (eggs and feathers) have extensively been used to monitor metals in the environment. Transfer of lead and cadmium in bird eggs has been demonstrated in various cases (Burger and Gochfeld, 1991, Burger and Gochfeld, 1993, Burger and Gochfeld, 1995b, Burger and Gochfeld, 1996b). As eggs at least partly reflect metal uptake from local foraging, waterbird eggs have been used in monitoring pollution by these metals, among others (Furness, 1993, Burger and Gochfeld, 1995b). Such studies are scarce in the eastern Mediterranean.
The purposes of this study are: (a) to investigate the levels of lead and cadmium in the eggs of a number of colonial waterbird species from Greek wetlands of international importance where such information was lacking; (b) to compare lead and cadmium levels in some waterbird species’ eggs in relation to their position in the food chain of two coastal wetlands; and (c) to decide whether these waterbirds’ eggs could be useful for biomonitoring of these metals in the wetlands studied.
Section snippets
Study areas
Five northern Greek wetlands were investigated in this study. The Evros Delta, at the Greek–Turkish border (Fig. 1), is the eastern-most Greek wetland (40°47′N, 26°05′E) extending over 11 000 ha, including a variety of habitats such as temporary and permanent fresh water marshes, saltmarshes, lagoons, brakish lakes, coastal sandy islets and beaches with ammophilus and nitrophilous vegetation, bush and forest stripes along the river and cultivated land (Britton and Hafner, 1978, Babalonas, 1980).
Results
Lead levels in waterbird eggs from the study area varied from below detection limits to 372 ppb in Evros Delta avocets (Table 1). In the Evros Delta the lowest median levels were found in the cormorant (7 ppb) and maximum in the yellow-legged gull (47 ppb). Lead levels were significantly different between species (Kruskal–Wallis−2=19.326, d.f.=3, P=0.0002) and more specifically were significantly higher in yellow-legged gull and in avocet than in cormorant (P=0.0019 and P<0.0001 respectively,
Discussion
The differences found between species in the levels of both lead and cadmium in the Evros and Axios Deltas are attributable to their different diets. Dietary data are lacking for the study years but have been available for most species and areas through previous studies. In the Evros Delta the avocet feeds upon nereid worms (Hediste diversicolor), amphipods such as Gamarus aequicauda and Corophium orientale and aquatic insects (Goutner, 1985). The yellow-legged gull is an omnivorous species,
Conclusions
- 1.
Birds such as the avocet and yellow-legged gull are positioned at a lower trophic level than the cormorant, and exhibit higher egg Pb levels.
- 2.
Prey sources are probably determinative for Cd and Pb egg burden in birds since water-based bird food is less polluted than the sediment-based type.
- 3.
Lack of significant differences in Pb levels among areas sampled for cormorant and in Cd levels sampled for Mediterranean gull, imply species-specific accumulation patterns.
- 4.
A significant correlation between Pb
Acknowledgements
We are indebted to Dr S. Kazantzidis, S. Goutner, D. Skartsi, T. Nazirides and M. Malakou, for aid in the field. Chemical analyses were supported by the University of Ioannina.
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