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Toxicity to aquatic algae and cyanobacteria

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Description of key information

Five studies investigated the toxicity of cloquintocet-mexyl to three freshwater algae species, the green algae Scenedesmus subspicatus, the blue algae Microcystis aeruginosa and the diatom Navicula pelliculosa. The selected values for this endpoint are the ErC50 of 1.4 mg/L (mean measured) base on Rufli (1988) and a NOEC of 0.09 mgL (calculated mean measured, based on nominal of 0.2 mg/L) reported by Rufli (1988).
In the case of the metabolite CGA153433, the lowest endpoint values were an EbC50 and a NOEC of 1.9 and 1.3 mg/L, respectively.

Key value for chemical safety assessment

EC50 for freshwater algae:
1.4 mg/L
EC10 or NOEC for freshwater algae:
0.09 mg/L

Additional information

A total of five studies are available for the toxicity of cloquintocet-mexyl to green algae, blue algae and diatom. Cloquintocet-mexyl was tested with three freshwater algae species, the green algae Scenedesmus subspicatus, the blue algae Microcystis aeruginosa and the diatom Navicula pelliculosa. Main results are summarised in the table below.

Table: Toxicity of cloquintocet-mexyl to algae

Species

Test duration

Basis for endpoint

EC50

EC10

NOEC

Measured or nominal?

Reference

[hours]

[-]

[mg/l]

[mg/l]

[mg/l]

[-]

 

Green algae (Scenedesmus subspicatus)

72

growth

>1.8

 

0.2

Nominal

Rufli (1988, corrected EC50 value)

biomass

0.42

 

0.2

 

Green algae (Scenedesmus subspicatus)

72

biomass

0.63

 

0.22

Nominal

Grade (1993)

 

Green algae (Scenedesmus subspicatus)

72

growth

>2.2

0.9

0.12

Mean measured

Hertl and Schreitmüller (1996)

biomass

>2.2

between 0.12 and 0.47

0.12

 

Blue-green algae (Microcystis aeruginosa)

96

biomass

2.5

 

0.611

End measured

Grade, (1993)

Freshwater diatom (Navicula pelliculosa)

96

biomass

1.7

 

0.4

Nominal

Grade, (1993)

 

In a GLP-compliant study conducted to ASTM guideline E 1218-90 (Grade, 1993), the freshwater green algae Scenedesmus subspicatus was exposed to nominal concentrations of 0.047, 0.078, 0.13, 0.22, 0.37 and 0.61 mg cloquintocet-mexyl/L, in a 96-h static test. Results are expressed as nominal concentrations. The 96-h EbC50(biomass) and NOEC values were 0.63 mg/L and 0.22 mg/L, respectively. This study was considered to be reliable without restriction. ErC50 (growth rate) has not been reported. However based on the data given in the report ErC50 is >0.61 mg/L (nominal), since the highest growth rate inhibition values calculated were only approx. 14% (96-h) and 28% (72-h), both for the highest concentration tested. The mean measured concentration (i.e. the mean of the measured values after 0h and 96h, 0.47 mg/L and 0.01 mg/L, respectively), corresponding to nominal 0.61 mg/L, was 0.24 mg/L.
Remark: This mean measured concentration was apparently the basis of the EC50 of 0.23 mg/L (based on the biomass EbC50 reported by Grade, 1993) mentioned in the clodinafop Draft Assessment Report (DAR, Directive 91/414/EEC).

In a second study Scenedesmus subspicatus was exposed to nominal concentrations of 0.022, 0.067, 0.2, 0.6 and 1.8 mg cloquintocet-mexyl/l, in a 72-h static test (Rufli, 1988). Results are expressed as nominal concentrations. The growth-based 72-h ErC50 and NOEC values were 1.1 mg/L (this value is assumed to be wrong, see below) and 0.2 mg/L, respectively. The biomass-based EbC50 was 0.42 mg/L. This study was conducted to OECD guideline 201, GLP compliant and is considered to be reliable without restriction.
Remark: The original report (Rufli, 1988) did not report growth-based EC50 values. However in a later report amendment an ErC50 value of 1.1 mg/L (based on nominal concentration) was reported. This value has been used in the past. However based on the data of the original report it is clear that this value is not correct. The highest growth rate inhibition value calculated was only approx. 30% (after 72h), for the highest concentration tested. Thus the correct ErC50 value is >1.8 mg/L. The mean measured concentration (i.e. the mean of the measured values after 0h and 72h, 1.76 mg/L and 1.13 mg/L, respectively), corresponding to nominal 1.8 mg/L, was 1.4 mg/L.

In a third study the influence of cloquintocet-mexyl on the growth of the green algae S. subspicatus was investigated in a 72-hour static test (Hertl and Schreitmüller, 1996).Due to the pH-dependent instability of the test substance, the pH of the media was adjusted to about pH 7 at the start of the experiment. Results are expressed as mean measured concentration. The growth-based ErC50 and NOEC values were determined to be > 2.2 mg/L and 0.12 mg/L, respectively. This GLP compliant study was conducted to OECD guideline 201 and is considered to be reliable without restriction.

The freshwater blue-green alga Microcystis aeruginosa sensitivity to cloquintocet-mexyl was investigated in a 96-h static test, conducted to ASTM guideline E 1218-90 (Grade, 1993). This study is considered to be reliable without restrictions. The test species was exposed to six nominal concentrations ranging from 1.1 to 13 mg/l and corresponding measured end concentrations ranged from 0.05 to 10.1 mg/L. No effects were observed at the three lowest concentrations tested. The 96-hour EbC50 (biomass) and NOEC values, based on measured concentrations at the end of the study, were 2.5 mg/L and 0.6 mg/L, respectively.

Toxicity to the freshwater diatom Navicula pelliculosa was investigated in a GLP compliant study conducted to ASTM guideline E1218-90 by Grade (1993). The test species was exposed to nominal concentrations of 0.40, 0.60, 1.1, 1.8 and 3.0 mg/L, in a 96-h static test. The values were based on nominal concentrations. Only the lowest concentration did not show significant effects. The 4-day EbC50 (biomass) and NOEC values were 1.7 mg/L and 0.4 mg/L, respectively. This study is considered to be reliable without restriction.

It is noted that both growth rate and biomass endpoints are reported for some of the studies. ECHA R.7b guidance states that “Often both acute growth rate EC50 (ErC50) and biomass (EbC50) endpoints are reported however the latter should not be used”.

Available EC50 endpoints based on growth (ErC50) are >1.8 mg/L (nominal), reported by Rufli (1988, corrected value) and >2.2 mg/L (mean measured) reported by Hertl and Schreitmüller (1996). Due to the rapid dissipation of cloquintocet-mexyl in aquatic systems, it may not be appropriate to select a nominal value for the Rufli (1988) study. The corresponding ErC50, based on mean measured concentrations, is >1.4 mg/L. From both studies the more conservative result is selected. Therefore the key acute toxicity value selected for the chemical safety assessment is the ErC50 of 1.4 mg/L (based on mean measured concentration).

The lowest available NOEC values for cloquinocet-mexyl are 0.12 mg/L (mean measured) reported by Hertl and Schreitmüller (1996) and 0.2 (nominal) reported by Rufli (1988). Considering the rapid dissipation of cloquintocet-mexyl in aquatic systems, it may be more appropriate to consider an endpoint based on an average of the measured concentrations across the testing duration. In the case of the Rufli (1988) study, the measured concentration for nominal NOEC of 0.2 mg/L was 0.153 mg/L at 0h and 0.02 mg/L at 72 hours. Taking the arithmetic average of these values gives a NOEC of 0.09 mg/L. It is proposed that this represents a worst-case, since the Hertl and Schreitmüller (1996) study reported higher mean measured NOEC (0.12 mg/L) and EC10 (0.9 mg/L) values for the same substance.

Additionally, the toxicity of CGA153433, a metabolite of cloquintocet-mexyl, has been investigated in three studies by Grade (1993, 1993, 1993). Test specied were S. subspicatus, M. aeruginosa and N. pelliculosa. The lowest endpoint values were obtained for M. aeruginosa, with a LC50 and a NOEC of 1.9 and 1.3 mg/L respectively.