Open Literature Review Summary

Chemical Name: Atrazine CAS No: 1912-24-9

ECOTOX Record Number and Citation: Gruessner, B. and M.e. Watzin. 1996. Response ofaquatic communities from a Vermont streamto environmentallyrealistic atrazine exposure in laboratory microcosms. Environ. Toxico!. Chern. 15:410-419 (MRID47543613).

Purpose ofReview: Determine the potential effects ofatrazine on an aquatic community. Endpoints from this study contribute to the weight-of-evidence regarding effects ofatrazine on aquatic plant communities and associated determination ofaLevel ofConcern (LOC). (

Date ofReview: November24, 2009

Purpose ofStudy: This study examines the potential effects ofatrazine to periphyton andbenthic invertebrates from a Vermont stream and established inrecirculating 120-L laboratory microcosms. Measured endpoints includedbiomass ofthe attached algal community and the functional and taxonomic composition ofthe benthic invertebrate community.

Summary ofStudy Findings:

Methods: 

### Collection Stream organisms were collected from PatrickBrook in Hinesburg, Vermont, about 15 km south ofBurlington in the LaPlatte River watershed. The area is predominantly rural (20% ofthe LaPlatte watershedis urbanlresidential, 40% is agricultural, and 40% is forested). PatrickBrookdrains a small lake. The collection site was about 5 km northeast ofthe lake and about 0.5 km east ofthe confluencewith the LaPlatte River. The brook atthe collection site averaged 1-2 min width and was amix ofshallow (5-15 em) riffles and 20-75 em pools. Thecollection site did notreceive any runofffrom land where atrazine was in use. Collection occurred over 3 weeks in July 1993.



# 1111••••11 2087839Artificial substrates were used to collect benthic invertebrates and periphyton. Forthe benthic invertebrates, to reduce the high natural variability ofbenthic samples, collection boxes with homogenized substrate were created and anchored to the stream bottom for 3 weeks to attractcolonization. The substrate was anatural coarse (1-4 em diameter) substrate, similarto that found at thesite, which had been washed and air-dried to remove organisms. The substrate had then been incubated for 2 weeks in the sun in a large, shallow tank containing waterfrom the stream site, to allow regrowth ofthe microbial community. High density polyethylene 10 cm by 10 em by 7 cm boxes with I em diameterholes cut into the sides were filled with the substrate. Each box had 13 holes on 2 ofthe sides, and 9 holes on the sides facing perpendicularto the streamflow, which
were wedged into the anchoring platform. The anchoring system was such that only the top I to 2 em ofthe boxes were exposed above the surrounding streambed substrate. After leaving the boxes anchored for 3 weeks arranged "randomly in acheckerboard pattern", the boxes were collected by nesting each into an empty, intactbox while still submerged and fitting with a lid. The nested boxes were transported to the laboratory in coolers.

Forperiphyton, glass microscope slides (2.5 em by 7.5 em) were anchored a few em above the stream bottom with long axes vertical and faces perpendicularto the flow. Afterbeing colonized for the same 3 weeks as the boxes, they were placed into plastic slide boxes and kept moist during transport to the laboratory in coolers. The authors note thatthe smooth surface may not have supported all periphyton species found at the site.

Laboratory microcosms Six laboratory lotic microcosms were created, each consisting ofa three-tank system with
ahead tank above, amicrocosm tank in the middle receiving gravitational flow from the head tank, and a receiving tank below that drained the microcosm tank, with a submersible pump (Little GiantPump Co., Oklahoma City, Oklahoma) in thereceiving tank to circulate waterback up to the head tank. See Figure 1 for a diagram. The specially designed microcosm tanks were made ofclear acrylic plastic (made by Vermont Plastic Specialties ofWilliston, Vermont). The head and receiving tanks were 60-L tanks made by Nalge Co. (Rochester, New York). The tanks were connected by polyurethane tubing (Nalge Co.), which was chosen for stability in water and chemical resistance. The flow rate was regulated by valves on the tankdrains to maintain a water depth of 12 em in the microcosms. A total of 120L ofwater circulated through the tanks about 250 times per day. The microcosm tanks were covered with 500jlm mesh netting to prevent the escape ofemerging adultinsects. Water draining into the receiving tank was filtered through a 500jlmmesh funnel to capture any organisms in the flow.

All waterused in the experimentwas collected atthe same site where the organisms were collected, and passed through a 250 jlm sieve to remove macroorganisms and debris before being added to.the tanks. Two full spectrum bulbs (Vita Lite, Duro Test Corporation, Fairfield, New Jersey) in fluorescent lightunits were suspended above each

2microcosm and set on a 14 hours on, 10 hours darkcycle (approximating the natural light conditions atthattime ofyear).

Aftertransportto the laboratory, four "randomly selected" invertebrate collection boxes
were removed from theirtransport container and placed into each microcosm. One set of fourboxes was preserved in 10% formalin solution containing Rose Bengal to aid in organism identification, and the composition ofthe benthic community from these four boxes were used as "initial condition" estimates. Nine "randomly selected" periphyton slides were also placed into each microcosm with the same orientation to the flow as they had been placed in the field. Three slides were scraped, washed on glass fiber filters (Type AlE, Gelman Sciences, Ann Arbor, Michigan) and frozen until chlorophyll 

### a analysis (describedbelow). The algal biomass on these slides was used as "initial condition" estimates.

The microcosms were allowed to rest and equilibrate for 24 hours, andthen atrazine was added to 3 ofthe 6 microcosms. Atrazine commercial product (AAtrex 90, Ciba-Giegy, Greensboro, North Carolina) was dissolved into 1 L ofstreamwater and added to each headtankto deliver atotal concentration of5 Ilg/L in the treatment systems. One liter of uncontaminated streamwaterwas added to the control tanks. The day ofinitial treatment was considered Day 

### 1. 1 On Day 3, about 15% ofthe waterwas removed from each ofthe treatmentand control microcosms and replaced with fresh streamwaterto ensure adequate nutrient supply andwaste removal from the systems, andto lowerthe atrazine concentrations in the treatmentmicrocosms. Then, on Days 6 and 7 about 50% ofthe water in each system was replaced, and about 15% on Day 11. The exposure pattern was designed to mimic the observed atrazine concentration dynamics in Vermont streams receiving atrazine runoffoccasionally.**Samplingandanarys~** Throughoutthe 2 week study, temperature and conductivity was measured daily (Model 33 SCTMeter, YSI Yellow Spring, Ohio). On Days 1,5, 10, and 14, three replicate water samples were collected from each microcosm and analyzed for atrazine contentby enzyme immunoassay (Atrazine RaPID Assays, Ohmicron, Inc., Newtown, Pennsylvania). Water samples from the collection sitewere also analyzedto verify that the streamwaterwas free ofatrazine. On Days 1,3,5, 7, 9, 12, and 14, emerged adult insects were collected from the mesh covers and water surface. On Days 1,5,9, and 14, any material outside that collectionboxes in the microcosm tankwas collected on a 500 11m sieve and combined with the material from the receiving tank's filter. These samples
were called emigrant samples. (Day 1emigrantand emerged insect samples were collectedpriorto atrazine addition.) On Days 3, 7, and 14, periphyton slides were removed from correspondingpositions in each microcosm for determination ofalgal biomass (seebelow). Finally, on Day 14, the invertebrate collection boxes were removed

I Throughout the review text, we will follow the study's definition ofatrazine addition on Day I. However, in the Data Summary Table, the day ofatrazine addition will be considered Day O.



### 3and all material from them preserved in 10% buffered formalin solution containing Rose Bengal.

Attached algal biomass on the periphyton slides was estimated by analyzing the scraped and washed (using glass fiber filters, as described above) material for chlorophyll 

### a. The filters were frozen for 1 to 3 weeks, depending on the time ofsampling, and extracted overnight in 90% acetone at4°C. Extracts were centrifuged at 500 rpm for 20 minutes, decanted, and brought up to 5 mL. The chlorophyll a concentration ofthe extracts, corrected"for the presence ofpheophytin

### a, were determined with spectrophotometry as described in StandardMethod<;for the Examination ofWaterand Wastewater, American Public Health Association, 1989.

Preserved invertebrate samples (from collectionboxes, emerged insects, and emigrant samples) were washed over a 500 /lm mesh sieve to remove fine particles. Organisms
were picked out under adissecting microscope. Organisms otherthan insects were assigned to general taxonomical groups. Adult emerged insects were counted butnot further identified. Insects from the collectionboxes and emigrant samples were identified to genus when possible and categorized into functional groups based on feeding behavior. The functional groups were collectors, predators, scrapers, and shredders. The chironomid tribe Tanytarsini was counted as 10% scrapers and 90% collectors based on the distribution in one box from each ofthe microcosms that were identifiedto lowertaxonomical levels. Since the functional classifications were not highly accurate due to complex feeding habits ofmany insects, insects were also classified as predators ornonpredators (consisting ofcollector, scraper, and shredder groups).

Statistical analyses ofthe collection box data consistedprimarily oftwo-level nested ANOVA to test for differences between subsamples (collection boxes within in each microcosm) and to separate that variability from the variation between treatment and control microcosms. When within-microcosm differences were significant, ANOVA and Tukey multiple comparisons were used to determine which microcosms were different from each other. Forthe emerged and emigrant insect samples, two-way ANOVA was used to test for effects oftreatment and ofcollection day on the numberoforganisms in functional and majortaxonomical groups. Similaranalysis was appliedto chlorophyll 

### a concentrations. When co1lection day effects were significant, Tukey multiple comparison tests were applied to determine which days were significantly different. Treatment tank atrazine concentrations on each sampling day were compared using one-way ANOVA and Tukey multiple comparison. A log (n) orlog (n+ I) transformation was neededto achieve homogeneity ofvariances in some ofthe data sets (specific datasets that were so transformed are not stated). Ifthe variances could not be equalizedthis way, a Wilcoxon
or Kruskal-Wallis test was used for comparisons.

Results:

4After 3 weeks ofcolonization in PatrickBrook, all microscope slides were coveredwith a dense layer ofperiphyton. Although the slides visually appeared to have variable amounts ofperiphyton gro\\-1h, the coefficientofvariation in the chlorophyll 

### a concentrations ofthe three initial periphyton samples was less than 1%. In the collection boxes, fine sediments had deposited within the coarse substrate and many organisms
were identified in the initial samples. In total during the experiment, 44 insect genera were found, 7 insecttribes or subfamilies not further identified, and other organisms were also found (gastropods, nematodes, oligochaetes, mollusks, and unknown pupae). The study lists all invertebrates groups and their functional feeding group in an appendix (not provided in this review). The functional group distribution ofthe initial benthic samples is shown in Table 

### 1. The coefficients ofvariation (CVs) are generally around 30-35%; however, the shredders had CVs near 1 due to small numbers and were not included in further analyses. Organisms not classified functionally, primarily oligochaete worms and insectpupae, were about 7% ofthe total abundance. Forthe taxonomical classification of all organisms representing more than 1% ofthe total abundance, the mean CV was about 60% (no data table provided).

Atrazine levels in the treatment microcosms were nearthe nominal level of5 J.!g/L after application and then dropped as expected (Table 2). Concentrations were not different
across the microcosms at each treatment level (no statistical testresults provided), except on Day 14 when microcosm 3 had significantly different andhigher atrazine concentrations than the othertwo (one-way ANOVA followed by Tukey test, p-value < 0.05). Temperature and conductivity readings were "consistent" (no statistics provided)
across all microcosms. Mean temperature ranged from 24.6 to 25.0°C and mean conductivities ranged from 220.8 to 226.5 mho. The conductivities in the microcosms throughout the study were similarto those recorded in PatrickBrook (no data or statistics provided). The temperatures in the microcosms were approximately 5°C higherthan that in the stream.

Mean chlorophyll 

### a concentrations did not differ significantly due to treatment (p-value 0.26), butthe variability in the treatment microcosms appeared higherthan in the controls, especially later in the experiment (Figure 1). Chlorophyll 

### a concentrations differed significantly by collection day (p-value < 0.01; see Figure 1, where decreases
over time are clear, although there was no significantdifference between Day 7 and Day 14). Using ANOVA with slides as replicates followed by Tukey multiple comparison tests, chlorophyll 

### a concentrations in microcosm 3 were found to be significantly different from and lowerthan those in the othertwo treatment microcosms on Day 7 (pvalue < 0.03).

For organisms from the boxes collected at the end ofthe experiment, there was no significant difference between treatments and controls in total number oforganisms orin the number ofcollectors, scrapers, predators, ornon-predators (Table 3). The nested ANOVA showed no differences in the numbers oforganisms in each functional group within the sets oftreatment and control microcosms, with one exception. The collectors



### 5had significantdifferences within the treatment microcosms (p-value = 0.02). Microcosm 3 was found to be significantly different than the othertwo treatment microcosms (Tukey, p-value < 0.05) and was observed to contain a highernumber of collectors. Similarly, one genus 

### (Paraleptophlebia spp., a collector) had significant differences within the treatmentgroup (ANOYA, p 0.0I) and itwas found that microcosm 3 was significantly different from the othertwo (Tukey, p-value < 0.05). There were no significantdifferences between treatments and controls in the numbers of organisms categorized into taxonomical groups (Table 4).

Foremigrant samples, there were no significantdifferences due to atrazine treatment on the total or functional groups numbers ofemigrants. There was, however, a significant collection day effect. See Table 5 for statistics and Figure 3 for graphical illustrations. Overtime, predators appeared to become more abundant while nonpredators became less abundant. Breaking abundances down by taxa (looking atthe ten most abundant insect taxa), again there Was no·significanttreatment effect on abundances, but in many cases there were significant collection day differences (see Table 6 for statistics and Figure 4 for graphical illustrations). The authors note the appearance ofgreat variability in the treatment microcosms (see Figure 4); they combined the emigrant samples with the collection box samples to look at mean CYs across all functional and taxonomic groups and determined that the mean CYs were about twice as high in the treatmentmicrocosms
as the control microcosms (no data provided).

There were significanteffects from treatment as well as collection day on the number of emerged insects from the microcosms, but no significant interaction between the two factors (see Table 7 for statistics). FromTable 7, it is clearthatby the end ofthe experimentthere were no longer significanttreatment effects on emerged insect numbers. Collection day results and significantdifferences are depicted in Figure 5.

Table 1. Mean number and percentage oforganisms in the initial benthic invertebrate samples (N=4) within each ofthe functional groups. (study Table 1)

Numberoforganisms Percent oftotal Mean SD 1 Cy2 Mean SD CY Scrapers 54.3 19.8 35.13 ]4.13 7.39 52.29 Collectors 248.7 82.07 33 63.75 28.74 45.09 Shredders 3.75 3.59 95.84 0.85 0.64 75.63 Predators 55 20.02 36.39 13.97 6.36 45.54 Nonpredators 306.75 93.94 30.62 78.72 35.24 44.77 Other 27.25 7.5 27.52 7.31 389 115.23 29.62 SD = Standard deviation 2CY Coefficientofvariation

6Table 2. Atrazine concentrations**(~g/L)** in microcosms, three samples from each microcosm each day. (study Table 2)

Day I. Mean SE1 Day 5 Mean SE DaylJ! Mean SE Daill Mean SE Treatment

Microcosm 1 Microcosm 2 .Microcosm 3 Control

Microcosm 1 Microcosm 2 4.73 4.82 4.68


### ND2 

### ND ND 0.084 0.084 0.19 3.54 3.65 3.48


### ND ND 

### ND 0.115 0.121 0.127 0.98 1.5 1.12

ND 

### ND ND 0.023 1.11 0.046 0.121 1.12 0.Q35 0.075 1.35 0.046

NA3 NA NA

1 SE Standard error
2 ND =Not detected at 0.05 Ilg/L

3 NA Not available samples contaminated after collection

Table 3. Mean number oforganisms remaining in the collection boxes at the end ofthe experiment and results ofthe statistical comparisons. (study Table 3)

-value 0.60 1 0.37 2 0.29 1 0.36 1 0.28 1 Control Mean SD 86.7 9.53 283.3 17.92 141.33 12.72 372.33 14.9 Treatment Mean SD 77.3 13.63 269.03 68.86

122 9.45 348.33 67.36

1Nested ANOVA

2 Wilcoxon rank test Scrapers Collectors Predators Nonpredators Total

7Table 4. Mean number ofcollection box organisms in major insect taxa and other groups and the results ofstatistical comparisons. (study Table 4)

Treatment Control Mean SD Mean SD Insect taxa Stenelmis spp. ]4.33 2.85 13.67 1.76 0.59 Microtendipes spp. 8.67 4.18 11.67 2.76 0.58 Tanypodinae 76.33 6.33 92.33 10.84 0.27 Tanytarsini 109.67 22.17 120.33 3.95 0.66 Heptageniidae 33.33 9.94 38.33 6.84 0.81 Paraleptophlebia spp. 145.33 43.8 154.33 16.18 0.73 Helicopsyche spp. 16.33 5.81 19.67 1.2 0.32 Cernotina spp. 30.33 1.86 30.33 3.93 1.00 Othergroups Oligochaeta 29.67 lOA 19.67 4.67 0.43 Nematoda 0 0 2.33 1.2 0.12 Sphaeriidae 1.33 0.67 4 0.58 0.25 Gastropoda 18.33 9.7 9.33 3.76 0.73 Pupae 7.67 1.76 8 1.15 0.80 7.33 IA5 9 1.53 OA7

Table 5. Results oftwo-way ANOVA testing the effects ofatrazine on the numbers ofemigrants in functional groups and on the total number ofemigrants. (study Table 5)

T Treatment Collection 

# '" p-values p-values p-values Collectors 0.65 <0.001 0.52 Scrapers 0.12 0.04 0.57 Predators 0.32 0.01 0.88 Nonpredators 0.9 <0.001 0.52 Total 0.09 <0.001 0.47

8Table 6. Results ofthe statistical tests ofthe effects ofatrazine on the numbers oforganisms in major taxa emigrating from the invertebrate collection boxes. (study Table 6)

0.43 0.67 0.75 0.85 0.75 0.17 0.68 0.29 0.9 0.98 Treatmentx Collection p-values3

0.41 0.09 0.02 0.01 <0.01 <0.01 0.24 0.03 <0.01 <0.01 Collection p-values2

0.21 0.39 0.79 0.91 0.67 0.59 0.38 0.46 0.56 0.64 Treatment J:values l Insecttaxa Stenelmis spp. Microtendipes spp. Polypedilum spp. Orthocladius spp. Tanypodinae Tanytarsini Baetidae Paraleptophlebia spp. Helicopsyche spp.**Nectop~yche** spp. Othergroups Planoboridae 0.33 0.79 NO Physidae 0.57 <0.01 0.94 Pupae 0.93 0,01 0.53 I Two-way ANOYA, except for Wilcoxon ranktestfor Planoboridae 2 Two-way ANOYA, exceptfor Kruskall-Wallis test for Planoboridae 3 Two-way ANOVA. ND Not determined

Table 7. Results ofthe statistical tests ofthe effects ofatrazine on the number ofadult emergent insects in the microcosms. (study Table 7)

Two-wa ANOVA ANOVA Treatment Collection day Treatmentx collection (interaction) <0.01 <0.01 0.35
Day 1 0.17 Day 3 0.02 Day 5 0.02 Day 7 0.04 Day 9 0.09 Day 12 I

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Totals Day 1 Day5 Day 9 Day 14 Collection Day Nonpredators Figure 3. Mean counts and standard errors for emigrantorganisms in functional groups and the total numberofemigrants in the treatment and control microcosms (N=3). Treatment and control
means were no significantly different. Significantdifferences at p-value < 0.05 between collection days are indicated by different letters overthe pairs ofboxes.

1]Day 1 Day 5 Day 9 Day 14 Collection Day po/ypedi/um Spp. a

Day 1 Day 5 Day 9 Day 14 Collection Day OrthocJadinae spp, Day 1 Day 5 Day 9 Day 14 Collection Day Microtendipes Spp.



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# .. 10 .J:l § E 10 a ::l z 0 z 0 z 0 Day1 DayS Day 9 Day 14 Day 1 Day 5 Day 9 Day 14 Day 1 Day S Day 9 Day 14 Collection Day Collection Day Collection Day Planoboridae Physidae Pupae Figure 4. Mean counts and standard errors for emigrant organisms in major taxa (>1% oftotal abundance) in the treatment and control microcosms (N=3) on different days. Treatment and control means were not significantly different. Significantdifferences at p-value < 0.05 between collection days are indicated by different letters over the pairs ofbars. Missing error bars are too small to be depicted.

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CollectiOn Day Figure 5. Mean counts and standard errors for emerged insect samples collected on differentdays from treatment and control microcosms (N=3). Letters with * indiCatewhere treatment means differed significantly from controls (p-value < 0.05). Significantdifferences atp-value < 0.05 between collection days are indicated by ditIerent letters overthe pairs ofbars.



### Data Summary Table:
Comments Day of Study Effect Occurred Nominal Atrazine Concentrat ion (Jig/L) Magnitude of Effect relative to controls Effect Time Did Weighted Recovery Average Occur if Atrazine so, when Concentration (see i

.from Day () to comment)?

I 

### I When Effect

# ~ri_~_**1:~~~:-~-n-ill-~-a-n-t~-5-~-~---~:-~-:-~-:-:-tu-t~·-:-o-~-:-:-:-:-c-~-l-e~-n-o-t---~s-e-e-~-U-d-y-t-cr-t~** ( 026) 14-day applicable p.414. p=
. experiment

not applicable not applicable no effect throughout 14-day experiment 5/lg/L no significant effect (p=0.28 in boxes, p=0.09 for total emigrants) Benthic invertebrates No significant effects for collection box
or emigrant invertebrates,
or for subdivisions by functional ,L-_

.. . -+- -+

## ~----_tI------__if__---__if_g><.lr_:_o-u~p~sand taxa +100% Effect (doubled) 2.6 !lg/L magnitude and (average for Yes (Day (difference 5/lg/L Day 0 timing significant at Day 0 Day II) estimated from 0<0.01) I 10) 

### I Figure5 

### I Increase in emerged adult insects I Bolded values are those data pOints specIfied In the "CosmData POints and Exposure Profiles" spreadsheet.

13Time weighted average calculations were complicated by the exposure pattern in the microcosms, which involved replacing significant fractions ofthe total water with fresh (uncontaminated) streamwateron specific days. On Day 3 (Day 2 ifatrazine application
occurs onDay 0) and Day 11 (Day 10), 15% ofthe water was replaced; on Days 6 and 7 (5 and 6), 50% ofthe water was replaced. Because these replacements were substantial and the time periods between replacements were short, it was assumed that concentrations were constant in between each replacement, and reduced in a stair-step fashion on the day ofeach replacement for calculation oftime weighted averages. Calculations are shown in the table below, linked to an.Excel spreadsheet.

Cumulative Atrazine Time Cone. Weighted Day (ug/L) Average 0 4.7 4.7 1 4.7 4.7 2 3.6 4.3 3 3.6 4.2 4 3.6 4.0 5 2.4 3.8 6 1.2 3.4 7 1.2 3.1 8 1.2 2.9 9 1.2 2.7 10 1.2 2.6

Reviewer Comments: Overall, the study was well presented. Considering the space constraints, many data were provided and methods and results were described in good detaiL The statistics were almost totally comprehensive, with only a few minorgaps where no p-values were given.

The main limitation ofthe study is that there appears to be a problem with Treatment Microcosm 3. Comparedto the othertreatment microcosms, this microcosm had significantly higher atrazine concentrations on Day 14, significantly lowerchlorophyll a concentrations on Day 7, and more collectororganisms inthe boxes, particularly Paraleptophlebia spp. This effect highlights some potential problems in the experimental design.

Despite the authors' efforts to create homogenous initial conditions in thebenthic community collection boxes, there could have been significantvariability in some collection boxes due to stream microconditions during the 3-weekcolonization period. The authors' reliance on "random selection" and "random placement" in the box placement, both in the stream bed during colonization and in the distribution ofthe boxes in the microcosm tanks, fails to address the potential for these effects. CVs appeared to

14

# £(l3/l{ be substantial between the four boxes analyzed for initial conditions, and insufficientdata
are provided to ensure that the study design is trulyrandom variation or ifthere is some pattern or categorical differences between boxes. Forthese reasons, it is possible that TreatmentMicrocosm 3 may have received differentinitial benthic communities than the othertreatment and/or control microcosms.

Another possibility is thatthe experimental area was not well controlled. No details are provided on the surroundings ofthe microcosm setup. In terms ofinterpreting the results, the changes in TreatmentMicrocosm 3 wouldtendto exaggerate any effects ofatrazine addition, since it had higher atrazine concentrations.

Otherpotential limitations ofthe study are due to the collection system in the stream bed. As the authors note, the microscope slides maybe too smooth for all types ofperiphyton to colonize. The types ofbenthic invertebrates thatcan colonize the boxes may also be limited. Overall, the communities collected and used in the microcosms are likelyto be much more simplifiedthan anatural lotic system. The benthic invertebrate community may have been unsustainable overtime, predators became more abundant and nonpredators less so, regardless oftreatment.

Description ofUse in Document (QUAL, QUAN, INV): Qualitative

Rationale for Use: The results ofthis laboratory microcosm study show that atrazine concentrations of:::5 Ilg/L do not impactperiphyton and benthic invertebrates communities collected from streams in Vermont.

Limitations ofStudy: Because one ofthe three treatment microcosms had several significantly differentresults from the othertwo, it is likely that there were issues associated with the experimental setup (dissimilar initial conditions oruncontrolled laboratory environment). This issue resulted in higher atrazine levels in the dissimilar treatment microcosm, which shouldhave resulted in more pronounced average effects of atrazine in the small sample; however, 2 ofthe 3 endpoints showed no effect. Thenoeffectresults, though from a simplified laboratory systemratherthan areal ecosystem, are likely to be reliablepredictors oflotic systems and for the timeperiod and exposure pattern used in the study.

Primary Reviewer: SaraHendrix, M.S., StaffScientist, Cambridge Environmental Inc

Secondary Reviewer: Anita Pease, M.S., SeniorBiologist, Environmental Fate and Effects Division, U.S. Environmental Protection Agency.

15