Zinc diricinoleate

Toxicity to aquatic algae and cyanobacteria

Description of key information

For assessing the aquatic risk for freshwater algae the  growth rate EL50 of 7.28 mg/L, based on the nominal loading of the zinc ricinoleate, a poor water soluble substance, from the OECD 201 guideline study is used.

Key value for chemical safety assessment

EC50 for freshwater algae:

7.28 mg/L

EC10 or NOEC for freshwater algae:

3.86 mg/L

Additional information

The effects of water accommodated fractions (WAFs) of the test item zinc ricinoleate on the growth of the unicellular fresh water green alga Pseudokirchneriella subcapitata exposed for a test period of 72 hours under static conditions was determined according to OECD 201. Since the test item is of very low water solubility and effects on algal growth were expected at low concentrations, direct addition of a suitable amount of the poorly soluble test chemical to the test medium was not possible (below 1 mg/L). Therefore, according to the OECD guidance No. 23, serial dilution of a Water Accommodated Fraction (WAF) of a properly prepared stock solution is recommended as applicable method for such chemicals.

The E_rL₅₀and E_rL₁₀for growth rate were calculated in terms of the nominal loading to be 7.28 mg/L and 3.86 mg/L, respectively. The NOEL values for growth rate was calculated to be 2.50 mg/L.

Based on the measured concentrations of dissolved zinc the effect values of the test item were calculated based on the zinc content of 9.73 %: E_rC₅₀ and E_rC₁₀ values for growth rate were 935.25 and 184.58 µg test item/L, respectively. The NOEC was 128.5 µg test item/L, respectively.

However, at the LOEL of 2.50 mg/L (LOEC measured concentration of 256.9 µg test item /L) for effects on growth rate, the inhibition was 5.51 % compared to the controls. This effect was statistically significant different from controls due to the low variability of the replicates. Since effects below 10 % are generally not considered to be ecotoxicologically relevant, it is recommended to use the growth rate E_rL₁₀ of 3.86 mg/L (E_rC₁₀ of 184.6 µg test item/ measured) as relevant endpoint for the long-term aquatic risk assessment.