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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

The effects of benoxacor on algal growth, green algae, blue-green algae and diatoms were tested in four guideline compliant studies. An EC50 based on growth is available for Scenedesmus subspicatus (Hertl, 1997). As an extreme worst case, the lowest EC10/NOEC endpoint for algae reported in the available dataset (EC10 of 0.22 mg/L for Scenedesmus subspicatus reported by Rufli, 1989e) has been selected as the chronic endpoint.

Key value for chemical safety assessment

EC50 for freshwater algae:
13.5 mg/L
EC10 or NOEC for freshwater algae:
0.22 mg/L

Additional information

Four studies have been carried out on benoxacor to determine its effect on algal growth for three types of organisms. The most sensitive species was determined to be green algae (Scenedesmus subspicatus).

The first study (Rufli, 1989) investigated the toxicity of benoxacor to green algae (Scenedesmus subspicatus). This study was GLP compliant, was conducted to OECD guideline 201 and is considered to be reliable without restrictions (Klimisch 1). Five concentrations of benoxacor ranging from 0.41-22 mg/L were tested. Mean cell density decreased at all treatment levels during the exposure time of 72 hours in comparison to control values. The NOEC value was beneath the lowest concentration tested (<0.41 mg/L) and the calculated EC10 was 0.22 mg/L, based on biomass.

A second study (Grade, 1993b) was carried out to assess the toxicity of benoxacor to diatoms (Navicula Pelliculosa). This GLP compliant study was conducted to ASTM guideline and is considered to be reliable without restrictions (Klimisch 1). The 96-hour EbC50 was calculated to be 15.7 mg/L and the corresponding NOEC was 2.5 mg/L.

In a third study (Grade, 1993a) the influence of benoxacor on the growth of a blue algae (Microcystis aeruginosa) was investigated according to ASTM guideline. This GLP compliant study provides an EC50 of 39 mg/L and a NOEC value for algae growth of 3.1 mg/L. This study is considered to be reliable without restrictions (Klimisch 1).

An additional study (Hertl, 1997) was carried out on green algae (Scenesdesmus subspicatus) according to OECD guideline 201. This study was GLP compliant and is considered to be reliable without restrictions (Klimisch 1). The obtained EC50 values were 6.5 mg/L and 13.5 mg/L based on biomass and growth rates, respectively. The 72-hour NOEC was determined to be 0.9 mg test substance/L (mean measured concentration 1.0 mg/L), since up to this test concentration the mean biomass of the algae was not statistically lower than the control.

For this endpoint, the lowest EC50 value was obtained for green algae (0.63 mg/L) (Rufli, 1989). However, this value was determined on the basis of biomass. The ECHA Guidance R.7b indicates that endpoints based on growth should be used. Therefore, it is appropriate to select the ErC50 of 13.5 mg/L based on growth reported by Hertl (1997) as the acute endpoint. It should be noted that whilst an EC10 based on growth is available (4 mg/L), the NOEC of 0.9 mg/L reported by Hertl (1997) is actually based on biomass. It is likely that a NOEC for growth would be at least 0.9 mg/L, but the relevant value is not reported. Therefore, due to this uncertainty, it is proposed, as an extreme worst case, to use the lowest EC10/NOEC endpoint for algae reported in the available dataset (EC10 of 0.22 mg/L for green algae reported by Rufli, 1989e).