Addendum to Proposed Methodology for Specifying Atrazine Levels of Concern for

Protection of Plant Communities in Freshwater Ecosystems

Use of Cumulative Rather Than Time-Averaged PATI

Russell Erickson

Mid-Continent Ecology Division

National Health and Ecological 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 Ecosystems (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 

individual species in laboratory toxicity tests. In Erickson (2010), the values of PATI for each 

day in an experimental ecosystem exposure were added up over a specified duration (assessment 

period) and then divided by this duration to provide a time-averaged PATI for that exposure, 

using the period with the greatest average effect if the exposure was longer than the assessment 

period. This time-averaged PATI serves simply as a relative measure of the total toxic severity 

of the exposure, taking into account exposure duration and concentration magnitude/variability. 

A level of concern (LOCPATI) for the time-averaged PATI was derived based on its relationship 

to the occurrence of adverse effects in a large set of experimental ecosystem exposures. The 

same time-averaged PATI calculation was then conducted for exposure time series 

(chemographs) of interest in natural systems to determine if these exposures were severe enough 

to exceed LOCPATI

This use of a time-averaged PATI can be a source of confusion. It's computation requires 

that an exposure 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. Rather, 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 the 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 

exposures based on this total toxicity. As such, an averaging step is both unnecessary and 

counterproductive. A better statistic would be the cumulative (summed) PATI values over the 
2

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 regardless of its concentration time dependence. 

For example, in Figure 10 in Erickson (2010), the LOCPATI 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 would be considered of concern. Although this is not unreasonable (given the "compounded 

interest" accumulation 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 

appropriate. For this example, the cumulative LOCPATI

For exposures with durations less than the assessment period, the summation would stop 

at the exposure duration, avoiding the aforementioned problem of averaging over a longer 

period. For exposures with durations greater than the assessment period, the summation 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 results in the assessment period being more clearly and 

appropriately associated with risk management decisions about cumulative toxicity, compared to 

using a time-averaged PATI. The averaging step is unnecessary and not a fundamental aspect of 

defining risk. 

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 

cumulative PATI simply increases the values for all the experimental ecosystem exposures, the 

LOCPATI, and all the natural system chemographs by the exact same factor. For the above 

example from Figure 10 of Erickson (2010), the cumulative PATI values are 90-fold higher for 

all the experimental ecosystem exposures and for the LOCPATI, but are also all 90-fold higher for 

any field 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 measure of toxic 

stress that is then calibrated to the experimental ecosystem data in the same fashion whether an 

average or cumulative PATI is used.