Addendum to Proposed Methodology for Specifying Atrazine Level s of Concern for Protect ion of Plant Communities in Freshwater Eco s ystems

Use of Cumulative Rather Than Time

- Averaged PATI

Russell Erickson Mid

- Continent Ecology Division National Health and Eco logical Effects Laboratory Office of Research and Development U.S. Environmental Protection Agency Duluth, MN

August 18 , 2010

In the report, Proposed Methodology for Specifying Atrazine Levels of Concernfor Protection of Plant Communitiesin Freshwater Eco system s (Erickson 2010), the Plant Assemblage Toxicity Index (PATI) was used as part of a methodology to extrapolate atrazine effects on aquatic plant communities observed in experimental ecosystem studies to other exposure scenarios, and thus support risk assessments of atrazine exposures in natural systems. PATI is the average growth rate reduction occurring across a defined set (assemblage) of plant species at a specified atrazine concentration, based on effects of atrazine on growth rates of the indivi dual species in laboratory toxicity tests. In Erickson (2010)

, the va lues of PATI for each day in an experimental ecosystem exposure were added up over a specified duration (assessment period) a nd then divided by this duration to provide a time

- averaged P ATI for that exposure, using the period with the greatest average effect if the exposure was longer than the assessment period . Th is time

- averaged PATI serves simply as a relative measure of the total toxic severity of the exposure , taking into account ex posure duration and concentration magnitude/variability

. A level of concern (LOC PATI ) for the time

- average d PATI was derived based on its relationship to the occurrence of adverse effects in a large set of experimental ecosystem exposure s. The

same time

- averaged PATI calculation was then conducted for exposure time series (chemographs) of interest in natural systems to determine if the se exposures were severe enough to exceed LOC PATI

This use of a time

- averaged PATI can be a source of confusion. It's c omputation requires that an exp osure shorter than the assessment period be averaged over the longer duration (the rest of the assessment period having no exposure). This might appear to be somehow extrapolating the short exposure to a longer one , although this is not the case . R ather

, this is just a calculation method that supports comparison of the total toxic severity of different exposures on a common basis. Another reason for confusion is that the averaging produces low values for PATI that suggest t he concern is just for low level of effects over an extended period, when in fact the primary concern is of exposures of exposures involving higher concentrations for shorter periods

. .

The manner in which PATI is used should be better linked to its purpose , which is to describe the relative amount of total toxicity occurring in an exposure and to compare different
exposu res based on this total toxicity. As such, an averaging step is both unnecessary and counterproductive. A better statistic would be the c umulative (summed) PATI v alues over the2 assessment period, without dividing by the duration of this period. This is a more simple and direct measure of the total toxic stress that occurs in an exposure time

- series , regardless of its duration and regardles s of its concentration time dependence

. For example, in Figure 10 in Erickson (2010), the LOC PATI for a time

- averaged PATI

over a 90

- d assessment period is 2.0 . In other words, a 2% reduction in PATI that persists for 90 d wou ld be considered of concer n. Although this is not unreasonable ( given the "compounded interest" accumulat ion of small effects ), the exposures of concern for field assessments are shorter and more intense. For these, the cumulative PATI value would be more informative and appropri ate . For this example, the cumulative LOC PATI

For exposures with durations less than the assessment period, the summation would stop at the exposure duration, avoiding th e aforementioned problem of averaging over a longer period . For exposures with durations greater than the assessment period, the summat ion would encompass the worst part of the exposure. This limit on the summation imposed by the assessment period reflects how long toxic stress would be considered cumulative. For example, if two short atrazine exposures were separated by 90 days, a 120 day assessment period would consider them cumulative whereas a 60 day assessment period would not, but rather consider that sufficient time had passed that the second exposure should be assessed independently of the first. Thus, using a cumulative PATI re sults in the assessment period being more clearly and appropriate ly associated with risk management decis ions about cumulative toxicity

, compared to using a time

- averaged PATI. The averaging step is unnecessary and not a fundamental aspect of defining ris k. would be 180 (= 2 .0 times 90) . This cumulative value is more simply and directly relevant to shorter exposures ; e.g., a 90% reduction in plant assemblage growth rate over 2 days, 20% over 9 days, etc.

It must be made clear, however, that whether a cumulative or time

- averaged PATI is used, the same risk assessments are made. This is because the time

- averaged PATI is simply the cumulative PATI divided by a constant

– the assessment period duration

. Thus, switching to a

cumu lative PATI simply increases the values for all the experimental ecosystem exposures , the LOC PATI , and all the natural system chemographs by the exact same factor. F or the above example from Figure 10 of Erickson (2010), the cum ulative PATI values are 90

- fold higher for all the experimental ecosystem exposures and for the LOC PAT I , but are also all 90

- fold higher for any f i eld exposure . Thus any risk characterizations (e.g., Figures 12

- 14 of Erickson 2010) would remain unchanged. This reflects the fact that PATI is used here as a relative m easure of toxic
stress that is then calibrated to the experimental ecosystem data in the same fashion whether an average or cumulative PATI is used.