Environmental Engineering Reference
In-Depth Information
1970 (Switzer et al.
1973
) found higher productivity and lower levels of DDE.
Mean DDE levels were 4.52 ppm with a range of 0.13-26.2 ppm. A weak negative
correlation was found between eggshell thickness and DDE level.
Fox (
1976
) reported DDE levels in eggs from common terns nesting in Alberta,
Canada. Thirty-nine intact eggs had a mean concentration of 3.42 ppm DDE. A pooled
sample of fi ve eggs with dented shells had a mean concentration of 6.67 ppm DDE.
The average shell thickness of intact eggs was not different from common tern eggs
collected prior to the DDT era; average shell thickness in the dented eggs was 12.5%
thinner. One could conclude, based on these fi ndings, that the threshold for egg shell
thinning and hatching failure for common terns is greater than 3.42 ppm.
Nisbet and Reynolds (
1984
) also reported high levels of DDE in a small sample of
common tern eggs that failed to hatch during the 1975 breeding season. DDE levels in
the three highest eggs ranged from 1.8 to 4.6 ppm. However, in 1971-1972, when the
highest levels of DDE were measured in their study, productivity was very high.
Ohlendorf et al. (
1985
) concluded from the work of Fox (
1976
) and Switzer et al.
(
1973
) that: “In Common Terns, DDE contamination above 4 ppm was thought
responsible for reduced eggshell thickness and quality and lowered hatching suc-
cess.” Since DDE levels range widely above and below the mean of 4 ppm and
hatching failure only occurs in a fraction of the eggs, the threshold for hatching
failure may be well above 4 ppm.
Weseloh et al. (
1989
) concluded that DDE was no longer an important factor in
the population dynamics of common terns in the Great Lakes. Geometric mean
DDE levels ranged from 0.95 to 2.46 ppm at four locations. Eggshell thickness did
not correlate with DDE level.
Hoffman et al. (
1993
) measured geometric mean levels of 1.7-2.9 ppm DDE in
eggs from four colonies of common terns in the Great Lakes. The DDE levels
ranged from 0.60 to 5.0 ppm. Embryotoxicity observed in the eggs was attributed to
PCBs and dioxins. DDE in the eggs was not considered to be suffi ciently high to
singly account for the observed embryotoxicity.
King et al. (
1978
) reported mean DDT levels in Caspian tern (
Sterna caspia
)
eggs of 15.1 ppm, but no correlation between DDT level and eggshell thickness.
Struger and Weseloh (
1985
) reported mean DDE levels of 4.6-8.8 ppm in Caspian
tern eggs collected in 1980 at various locations in Lake Michigan. Eggshells were at
or above the thickness measured in eggs collected prior to the DDT era. The authors
concluded that: “Organochlorine levels exhibited in Lake Michigan in 1980 do not
appear to have had a detrimental effect on reproductive success in Caspian Terns.”
Ohlendorf et al. (
1985
) reported a geometric mean level of 9.3 ppm DDE
(ranging from 2.1 to 56 ppm) in Caspian tern eggs collected in San Diego Bay in
1981. More than one-third of the eggs were lost before hatching, mostly from
failure to hatch. On average, eggshells were 7.8% thinner. The authors stated that:
“We suspect that higher DDE concentrations in eggs from some nests were, at least
in part, responsible for reduced hatching success.” Since shell thinning and hatching
failure are correlated with DDE levels in eggs, one would expect that the more than
one-third hatching failure occurred in eggs with DDE levels above the geometric
mean of 9.3 ppm.
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