Risks to Aquatic Organisms from Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 153

from exposures to CPY. These strengths have helped reduce uncertainty in the ERA
since these cosms included a number of taxa for which there were few toxicity data
from laboratory studies. These cosms provide data on responses of aquatic organ-
isms to CPY under realistic conditions.
Just as there were strengths in the ERA there were several areas of uncertainty,
some more relevant than others. There were few usable toxicity data from amphib-
ians, but evaluations of the relative sensitivity of fi sh and amphibians to several
classes of toxicants (Weltje et al. 2013 ) suggest that toxicity data from fi sh can
provide equivalency for amphibians. There were few data on recovery of AChE, the
target enzyme for CPY, in aquatic organisms, and this is an uncertainty in the analy-
sis of the relevance of the duration between exposure-events. Because of this, longer
and more conservative durations were used in the assessment. In addition, this is an
uncertainty that is relevant to all organophosphorus insecticides, as they share the
same target enzyme and toxicodynamics of recovery.
There was uncertainty with regard to the demonstrated effects of CPY on behav-
ior and the relevance of these to survival, growth, development, and reproduction
(SGDR). Pesticides that target the nervous system are expected to cause effects on
behavior, but it is diffi cult to determine the relevance of these responses to SGDR.
For invertebrates in cosms, all responses, including those mediated by behavioral
effects, are subsumed into the responses and recovery of exposed populations and
communities and are refl ected in the NOAEC eco of 0.1 g L −1 . Data to extrapolate

behavioral responses to SGDR for fi sh and other vertebrates are not available for
CPY or, for that matter, all other pesticides that target the nervous system. This is a
general uncertainty that has still to be addressed in the science of ERA.
When this ERA was initiated, there was uncertainty about the relevance of the
formation of CPYO from the parent, CPY, and how this might infl uence risks. While
still somewhat uncertain, this issue is judged to be of lesser relevance than that of
CPY itself. There are several lines of evidence to support this conclusion. The oxon
is an integral component of the toxicodynamics of CPY and is formed in vivo .
Toxicity of the oxon in aquatic organisms is not vastly or consistently different from
that of the parent CPY, and, to some degree is included in the toxicity studies with
CPY. The oxon is more rapidly hydrolyzed in the environment, partitions more into
water, and is less likely to bioconcentrate into organisms than CPY (see discussions
in the companion paper, Mackay et al. 2014 ).

μ

6 Summary

The risk of chlorpyrifos (CPY) to aquatic organisms in surface water of North
America was assessed using measured concentrations in surface waters and model-
ing of exposures to provide daily concentrations that better characterize peak expo-
sures. Ecological effects were compared with results of standard laboratory toxicity
tests with single species as well as microcosm and mesocosm studies comprised
of complex aquatic communities. The upper 90th centile 96-h concentrations