Open Literature Review Summary 

Chemical Name: Atrazine

CAS No: 1912-24-9

PC Code: 080803

ECOTOX Record Number and Citation: 

Baxter, L.R., P.K. Sibley, K.R. Solomon, and M.L. Hanson. 2013. Interactions between atrazine 

and phosphorus in aquatic systems: Effects on phytoplankton and periphyton. Chemosphere 

90: 1069-1076.

Date of Review: February 23, 2016

Abstract 

“It has been proposed that the herbicide atrazine may increase rates of parasitic trematode 

infection in amphibians. This effect may occur indirectly as a result of increased biomass of 

periphyton and augmented populations of aquatic snails, which are the trematode’s primary 

larval host. Evidence has also shown that nutrients alone may induce the same indirect 

responses. Since both atrazine and nutrients commonly enter surface waters from agricultural 

run-off, their spatial and temporal co-occurrence are highly probable. In light of recent widespread declines in amphibian populations, a better understanding of the role of atrazine in the 

proposed ecological mechanism is necessary. A microcosm study was conducted to quantify 

biomass of phytoplankton and periphyton over a range of atrazine and phosphorus 

concentrations (from 0 to 200 lg L_1 each) using a central composite rotatable design. Over 10 

weeks, biomass and water chemistry were monitored using standard methods. Regression and 

canonical analyses of the response surfaces for each parameter were conducted. We found 

significant effects of atrazine and phosphorus on dissolved oxygen, pH, and conductivity 

throughout the study. Additions of phosphorus mitigated the apparent inhibition of these 

photosynthetic indicators caused by atrazine. Despite these changes, no consistent treatmentrelated differences in algal biomass were observed. These results indicate that the indirect 

impacts of atrazine on total growth of periphyton and likely, subsequent effects on aquatic 

snails, are not expected to be ecologically significant at the concentrations of atrazine tested 

(up to 200 lg L_1) and over a range of nutrient conditions commonly occurring in 

agroecosystems.” 

Study Description: 

This study conducted a 10 week microcosm study with different combinations of atrazine and 

phosphorus. Microcosms were colonized naturally by taxa occurring in an adjacent irrigation 

pond by circulating water for three weeks (circulation ceased 2 days prior to study initiation). 

The study was designed around a central composite rotatable design with a core of 4 cosms 
with similar atrazine and phosphorous concentrations (14.1 and 100 ug/L respectively) then 

two or less cosms with different combinations of atrazine and phosphorus (Table 1). 

Table 1: Nominal and Measured Concentrations of Atrazine and Phosphorus in each microcosm. 

Shaded cells indicate central cosms for comparative basis, changes in atrazine and phosphorus 

concentrations were made to the other cosms with maximum replication of 2 cosms. 

Microcosm Nominal atrazine concentration 

(measured day-1, d-63) ug/L 

Nominal phosphorus 

concentration (ug/L) 

1 0 (0,1) 100 

2 0 (0, 0.25) 100 

3 2.2 (2.2, 2.0) 30 

4 2.2 (2.2, 1.9) 170 

5 14.1 (14.9, 11.4) 0 

6 14.1 (16.1, 12.9) 0 

7 14.1 (13.35, 13.4) 100 

8 14.1 (13.6, 13.6) 100 

9 14.1 (13.4, 11.5) 100 

10 14.1 (12.9, 13.7) 100 

11 14.1 (13.8, 13.4) 200 

12 14.1 (14.4, 14.3) 200 

13 92 (91, 68) 30 

14 92 (91, 80) 170 

15 200 (221, 154) 100 

16 200 (196, 166) 100 

Reviewer description of results. 

Water Chemistry: Dissolved Oxygen 

Clear differences between the tested -0, 14 ug and 200 ug/L exposures when phosphorous 

concentrations were 100 ug/L or less. There appears to be a mitigation of the atrazine effects 

on dissolved oxygen when phosphorus concentrations are 200 ug/L. Figure 1 below illustrates 

this however only presents a subset of the test cosms, and fails to show the results for the core 

cosms which had 14 ug/L 
Figure 1. mean concentrations of dissolved oxygen in microcosms over the 63-d exposures to 

atrazine and phosphorus for the start point treatment concentrations in a CCRD (n=2 

microcosms/treatment) 

Figure 2 provides more complete comparison of the DO concentrations at day 7 and day 21. 

The day 7 effects of atrazine are apparent at 2.1, 14.2, 92, and 200 ug/L, however by day 21 

most of these effects have been reduced. There are several complications that arise from the 

comparisons. First, in the absence of added phosphorus, the effects of atrazine appear to 

remain at day 21, due to the cosm maintaining a reduced DO compared to the cosms with no 

added atrazine. The comparison is complicated because the cosms with no atrazine added had 

100 ug/L of phosphorus added. The impact of phosphorus on the cosms without atrazine 

added isn’t explored well with this study design, however looking across cosms that had 2.1 

ug/L atrazine or less there are negligible reductions in DO at day 7 however by day 21 there is a 

clear difference in DO that is responding in a dose response manner. Similar patterns of 

reduced DO in the cosms without atrazine could be expected if there was rampant 

consumption of the available phosphorus resulting in an algal or bacterial decay bloom that 

caused the reduction, similar to results seen in eutrophic conditions. Another comparison that 

can be made is across the cosms with 100 ug/L of phosphorus. These include 0, 14.2, and 200 

ug/L atrazine. The DO levels across the cosms were significantly different at day 7 however had 

recovered to similar levels by day 21. This can be interpreted as the phosphorus mitigation of 

atrazine effects, or a balancing of effects of atrazine and effects of phosphorus across these 

cosms (with low atrazine cosms being more influenced by phosphorus than in high atrazine 

concentrations). In low concentrations phosphorus effects are expected to be less on the 

cosms without atrazine, so comparison of those cosms with the lowest phosphorus 

concentrations results in 21-day reduction of DO in an atrazine dose response manner. 
-

Figure 2. Observed values (black circles) and predicted response surfaces for dissolved oxygen 

on d 7 (left; R2 = 0.94) and 21 (right; R2 = 0.73) in microcosms after treatment with atrazine and 

phosphorus on d 0. 

Water Chemistry: pH 

The pH in all ponds increased with time with the exception of an initial decrease in the 200 ug/L 

atrazine cosms. Trends in the shape of response surfaces for pH were similar to those of 

dissolved oxygen (figure 2) and the description provided above. However, the surface of the 

response at day 21 is steeper than that predicted in day 21 of figure 2. There appears to be a 

poor fit of the surface with regard to the cosms with low atrazine and high phosphorus at day 

21 in figure 4, suggesting an increasing trend, however the data shows a reduction in pH as 

compared to adjacent cosms in the CCRD (Figure 3 does not illustrate test cosm #4). 

Fig. 3. Mean pH in microcosms over 63-d exposures to atrazine (ATZ) and 

phosphorus (P) for the star-point treatment combinations in a CCRD (n = 2 microcosms per treatment).
Figure 4. Observed values (black circles) and predicted response surfaces for pH on d 7 (left; R2 

= 0.97) and d 21 (right; R2 = 0.86) in microcosms after treatment with atrazine and phosphorus 

on d 0. 

Algal Response: Chlorophyll-a 

Influence of phosphorus is consistent with results of DO, cosms without added atrazine and 100 

ug/L of phosphorus show significant increases in chlorophyll a in both phytoplankton and 

periphyton between days 0 and 7 as compared to all other cosms displayed in figure 5 (note the 

central cosms are not included in the figure). Between day 7 and day 21 significant decreases in 

chlorophyll for both periphyton and phytoplankton are observed. The influence of atrazine is 

evident by the significant difference between the cosms with 100 ug/L phosphorus. Cosms with 

200 ug/L atrazine had phytoplankton communities with significantly less chlorophyll than those 

without atrazine exposure. Comparative reduction of chlorophyll a in the phytoplankton 

community are also seen with the two displayed test groups with 14 ug/L atrazine added. 

Convergence in the cosm phytoplankton chlorophyll a was seen by day 21 of the study. The 

periphyton response to atrazine exposure is also clear at day 7, with cosms exposed only to 

atrazine having the highest reduction compared to the cosms without atrazine. The potential 

mitigation of atrazine effects by increased phosphorus at day 7 may explain the higher 

chlorophyll a concentration s in the other two reported atrazine exposure concentrations 

having higher concentrations of phosphorus and less atrazine leading to higher chlorophyll a 

measurements. Convergence of the cosm periphyton in terms of chlorophyll a concentration 

occurred by day 28. 
Figure 5. Mean chlorophyll-a of phytoplankton (top) and periphyton from tiles at the microcosm base (bottom) 

over the 63-d exposures to atrazine (ATZ) and phosphorus (P) for the star-point treatment combinations in a CCRD. 

Symbols are the mean of n = 2 microcosms per treatment ± SD. 

Algal Response: Ash Free Dry Mass (AFDM) 

AFDM results are less clear than the other measured effects (Figure 6). Phytoplankton AFDM 

shows some separation of the 14.1/0 exposure cosms at day 7 of the exposure, however these 

do not appear to be significant compared to their relative change between days 0 and 7, nor 

the day 7 direct measured AFDW. Periphyton results show the same rampant growth 

suspected by DO, and chlorophyll a. 
Figure 6. Mean ash-free dry weight (AFDW) of phytoplankton (top) and periphyton from tiles at the microcosm 

base (bottom) over 63-d exposures to atrazine (ATZ) and phosphorus (P) for the star-point treatment combinations 

in a CCRD. Symbols are the mean of n = 2 microcosms per treatment ± SD. 

Description of Use in Document (QUAL, QUAN, INV): QUAL 

Rationale for Use: The study contributes to the understanding of aquatic plant community 

responses to atrazine and phosphorus under a laboratory testing setting. Because of the listed 

study limitations this study will not be included in the methodology for developing the CELOC. 

Limitations: The primary limitation in this study is the lack of a set of comparable negative 

controls, and the poor reporting of the central cosms tested in the study. Therefore, it is 

difficult to isolate either ATZ or phosphorous independent effects because the study design and 

provided analyses. In addition, the study doesn’t include saturated soil substrate as this would 

influence the amount of free phosphorous in the system. 

Primary Reviewer: Frank T. Farruggia Ph.D., EPA/OCSPP/OPP/EFED/ERB3 

Digitally signed by FRANK 

FARRUGGIA 

DN: c=US, o=U.S. Government, 

ou=USEPA, ou=Staff, cn=FRANK 

FARRUGGIA, 

dnQualifier=0000048865 

Date: 2016.03.24 09:54:47 -04'00'