Effects of atrazine under low-light conditions such as when suspended
solids are present:  The herbicidal mechanism of action of atrazine
involves interruption of photosynthesis.  The enclosed study tested for
the effects of atrazine on the macrophyte Elodea canadensis under 3
light intensities, concluding that under limited light conditions, the
toxicity of atrazine is reduced or eliminated.  In the Atrazine
Ecological Monitoring Program, total suspended solids (TSS) were assayed
in all relevant samples, the results showing that runoff events
containing elevated concentrations of atrazine were typically
accompanied by elevated TSS.  The enclosed study provides evidence that
high TSS, low-light conditions likely results in a reduction of the
toxicological effects of atrazine.  The  CASM_Atrazine simulation does
not account for the reduced light conditions caused by the elevated TSS
that typically accompanying the atrazine peaks, and is therefore
overestimating the toxic effects on the aquatic community under such
conditions.  This is offered as an example of the highly conservative
nature of the model, and addresses the unnecessary application of
uncertainty factors (as discussed in Charge Questions 4 and 9ii).

