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Long-term toxicity to aquatic invertebrates

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A 21-day Daphnia magna reproduction test under flow-through conditions with bronopol was performed according to OECD guideline 202 [Boots Pharmaceuticals 1992]. Test parameters were mortality, immobility and presence of eggs in the brood pouch of parental daphnia, as well as number and condition of newborn, unhatched eggs and incidence of immobility in the F1 generation.

At the highest concentration (1.7 mg/L), 100 % mortality was observed, whereas in all other treatments, mortality was < 20 % and therefore within the control range. The 1.7 mg/L treatment has therefore subsequently been deleted from the concentration range.

For all tested concentrations (except for the highest test concentration of 1.7 mg/L where 100 % parent mortality occured), no adverse effects on reproduction could be detected (number and condition of newborn, unhatched eggs, immobility of newborn). In fact, from day 9 of exposure a statistically significant stimulation of reproduction was noted. This increase in reproductive capacity was considered not to be an adverse effect.

Analytical monitoring of the test substance showed a recovery rate ranging from 35 to 52 % of nominal concentrations. At the end of the test, the recovery rate was about 13 %. This accelerated degradation of bronopol was probably due to a combination of factors including adsorption to algal cells, the presence of bacterial growth and the pH level of the test medium (ranging from 7.8 - 8.3). Hence, bronopol is known to hydrolyse rapidly at pH values above 5. This hydrolysis might mainly be responsible for the low analytical recovery reates. Even an increase of the flow rate did not succeed in maintaining test concentrations of bronopol.

The NOEC was determined to be 0.53 mg/L (based on nominal concentrations) and 0.27 mg/L (based on measured concentrations), respectively.