Environmental Engineering Reference
In-Depth Information
Oreochomis niloticus
, exposed to 10
g CPY L
−1
for 24 h declined to 47% of control
values but, after transfer to clean water, recovered to 55% after 7 d and 63% after 14 d
(Chandrasekera and Pathiratne
2005
). After 14-d exposures of the guppy (
Poecilia
reticulata
) to 0.325
μ
g CPY L
−1
, activity of whole-body AChE was 22% of that in
control fi sh (van der Wel and Welling
1989
). Following removal to clean water for a
further 14 d, activity of AChE had recovered to 40% of that in control fi sh. Similar
observations have been reported for other organophosphorus insecticides. For exam-
ple, activity of brain-AChE in Atlantic salmon parr (
Salmo salar
) exposed to formu-
lated fenitrothion (50% inhibition at initiation of recovery) and transferred to fresh
water, recovered to 66% of control values in 7 d and 93% in 42 d (Morgan et al.
1990
). It is not known if AChE recovery rates in fi sh differ among organophosphorus
insecticides having
O
-methyl (fenitrothion) or
O
-ethyl (CPY) substituents, and
whether there are models, with which recovery rates can be extrapolated.
If, as is generally suggested (Morgan et al.
1990
), recovery of phosphorylated
AChE in fi sh requires synthesis of new enzyme, rates of recovery would be slow and
dependent on rates of metabolism and the physiological and biochemical character-
istics of fi shes, which appear to be unknown. In the absence of having a model for
predicting recovery periods, empirical observations suggest that inter-exposure
intervals of the order of 4-8 wk might be required for complete recovery of AChE
in fi sh. This period was incorporated into the ERA (Sect.
4.2
).
μ
2.3
Protection Goals and Assessment Endpoints
Protection goals and assessment endpoints are strongly linked and do not change as
higher tiers or refi nements are applied in the ERA. The protection goals applied in
this assessment were to protect populations and communities of most aquatic organ-
isms most of the time and at most locations. Specifi cally, Species Sensitivity
Distributions (SSDs) were used (Posthuma et al.
2002
) for crustaceans, insects, and
fi sh to calculate the 5th centile (also referred to as the HC5) as a community-focused
endpoint. Because of functional redundancy and resiliency, some effects on a small
proportion of species can be tolerated in an ecosystem and the 5th centile of these
distributions has been shown to be generally protective of ecosystems and the ser-
vices that they provide (Brock et al.
2006
; Maltby et al.
2005
). Furthermore, based
on results of studies in the fi eld and in cosms, exposures equivalent to the 5th centile
appear to not cause adverse effects on populations or communities. This is due in
part to reduced bioavailability compared to exposures of organisms under labora-
tory conditions, and to more rapid dissipation of CPY under fi eld conditions.
2.4
Conceptual Models of Effects
Based on the likely effects of CPY on aquatic animals, a conceptual model for
effects was constructed to serve as a guide for developing risk questions and
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