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Short-term toxicity to aquatic invertebrates

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Reference
Endpoint:
short-term toxicity to aquatic invertebrates
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
First-instar daphnids were used (the age was not reported).
25 first-instar daphnids were added to a 100 mL glass beaker filled with 50 mL dilution water (incl. 1 ppm Tween 20 as surfactant).
A series of concentrations was set up (no details provided).
The substance Monuron was added as 50 µL stock solution to the dilution medium. It is not specified if the monuron stock solution was prepared in water or acetone. The test solution was swirled for thorough mixing.
The evaluation of immobilization was performed after 26 hours.
The IC50 (based on immobilization) was determined by plotting the percentage of immobility versus log concentration on log-probit paper to provide a median immobilization concentration (IC50). The immpobization reading was performed three times.
GLP compliance:
no
Analytical monitoring:
not specified
Vehicle:
not specified
Remarks:
it is not clear if the stock solution has been prepared in water or acetone
Test organisms (species):
Daphnia magna
Details on test organisms:
TEST ORGANISM
- Common name: Daphnia magna
- Strain/clone: pathogenetic stock cultured for more than 50 generations under constant conditions
- Source: not provided
- Age of parental stock (mean and range, SD): not provided
- Feeding during test: no
- Food type for daphnia stock culture: crushed baker's yeast cake.
- Amount:1.5 ml offreshly prepared 3.3 percent suspension (weight to volume)
- Frequency: Culture animals were fed twice daily, by pipette

ACCLIMATION
- Acclimation period: not needed, the first instar daphnids were held for 2.5 hours without food before beeing used for testing.
- Acclimation conditions (same as test or not): yes (except that the test daphnids were not fed)
- Health during acclimation (any mortality observed): not reported, daphnids that have moulted during the acclimation period were removed one hour prior to starting the test.

METHOD FOR PREPARATION AND COLLECTION OF EARLY INSTARS OR OTHER LIFE STAGES:
In order to obtain test organisms, nylon net was cemented across a flexible circle of plastic tubing so that the resulting disc fitted tightly into each culture jar at a distance of approximately 8 cm above the bottom. Within 15 minutes the normally photopositive first-instar daphnids ascended through the 30-mesh net which excluded almost all larger individuals, and culture water and animals above each net were siphoned into a single reservoir and randomized by agitation. Most of the water was then siphoned back into the original jar through a glass-tipped, 25-mm-diameter tube closed by the 40- to 50-mesh net which excluded all Daphnia. Young daphnids were washed several times with boiled deep-well water, held without food for about 21/2 hours, and, about 1 hour prior to tests, were passed again through 30-mesh net to remove any that had moulted during the preparation.
Test type:
static
Water media type:
freshwater
Limit test:
no
Total exposure duration:
26 h
Hardness:
deep-well tap water having the following constituents, in milligrams per liter: Ca (as CaCO3), 40; Mg (as MgCO3), 80;
Test temperature:
21.1 ± 0.05°C
pH:
8.12
Conductivity:
conductivity (,umhos at 25°C): 493
Nominal and measured concentrations:
not reported
Details on test conditions:
TEST SYSTEM
- Test vessel: 100 mL glass beakers
- Material, size, headspace, fill volume: 50 mL fill volume
- Aeration: not reported
- No. of organisms per vessel: 25
- No. of vessels per concentration (replicates): since no further informationwas provided it is assumed that for each concentration one vessel (replicate) has been set up.
- No. of vessels per control (replicates): since no further informationwas provided it is assumed that one control vessel (replicate) has been set up.
- No. of vessels per vehicle control (replicates): No information
- Biomass loading rate: not reported

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: deep-well tap water having the following constituents, in milligrams per liter:
Ca (as CaCO3), 40;
Mg (as MgCO3), 80;
Cl, 24;
S04, 37;
B, 0.64;
SiO2, 28;
Na, 81;
K, 2.2; and
Fe 0.0;
- Conductivity: 493 umhos at 25°C
- Salinity:
- Culture medium different from test medium: Yes , Tests were performed in boiled deep-well tap water to which commercial nonionic surfactant (Tween 20) was added to provide a concentration of 1 part per million (ppm).

OTHER TEST CONDITIONS
- Adjustment of pH: not reported
- Photoperiod: constant illumination
- Light intensity: approximately 1100 lu/m² from daylight-type fluorescent lamps

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
Immobilization after 26 hours

After 26 hours, mobility was observed under strong illumination from above and then by illumination from below. Unaffected Daphnia swam vigorously toward the light; those that were unable to swim vertically 1 cm or more were considered immobile. Measurement at each concentration was replicated at least three times.

VEHICLE CONTROL PERFORMED: unclear (except the the controls contained also Tween20. It is not clear if a acetone control has been set up as well - if acetone should have been used for the stock solution preparation. if water should have been used, no further vehicle control was needed.

Key result
Duration:
26 h
Dose descriptor:
IC50
Effect conc.:
106 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
other: Immobilization
Validity criteria fulfilled:
not specified
Conclusions:
26-hour IC50: 106 mg/L
Executive summary:

To determine the acute toxicity of Monuron to Daphnia magna, Crosby and Tucker (l966) exposed first-instar daphnids for 26 hours to various concentration of monuron. Tween 20 was added to deep-well water to a final concentration of 1 ppm. This dilution water was used for controls and test solutions. 25 first instar daphnids were added to 50 mL test solution. After 26 hours the activity of the daphnids after intensive light expsosure was determined. Daphnids not able to swim more than 1 cm were considered immobile. based on that information, the median immobilization concentration for 50% of the Daphnia magna (IC50) was calculated. For monuron the IC50 for daphnids was reported to be 106 ppm (=106 mg/L).

The study is not following all aspects of the modern test design for acute daphnia studies. As major deviations, no dose verification was performed and the observation period was only 26 hours instead of 48 hours. Minor deviation were that the daphnids were not selected by having an age of < 24 hours (but the daphnids were considered to be first instar, which is considered acceptable), the test vessels were not replicated (but the number of daphnids exposed per level were higher than prescribed in modern guidelines) and the need of a solvent control is not clear,

Especially the lack of dose verification and the shorter exposure duration are considered to be relevant for the question if the study can be used for the risk assessment. Since monuron was used at concentrations well below the water solubility and monuron is biologically and hydrologically relative stable the lack of dose verification analysis is considered not to invalidate the results from this study. It is scientifically very unklikely that the prolongation of the exposure from 26 to 48 hours will change the IC50 considerably which means in this context by at least an order of magnitude. Considering the high toxicity to algae (orders of magnitudes more sensitive than daphnids), the toxicity of monuron to daphnids is of less relevance for the environmental risk assesment of monuron. Therefore, the results reported by Crosby and Tuckler are considered to be reliable with restriction.

To conclude, the IC50 (=EC50) of 106 mg/L is considered reliable with restrictions for the further assessment.

Description of key information

26 -hour EC50: 106 mg/L

Key value for chemical safety assessment

EC50/LC50 for freshwater invertebrates:
106 mg/L

Additional information

To determine the acute toxicity of Monuron to Daphnia magna, Crosby and Tucker (l966) exposed first-instar daphnids for 26 hours to various concentration of monuron. Tween 20 was added to deep-well water to a final concentration of 1 ppm. This dilution water was used for controls and test solutions. 25 first instar daphnids were added to 50 mL test solution. After 26 hours the activity of the daphnids after intensive light expsosure was determined. Daphnids not able to swim more than 1 cm were considered immobile. based on that information, the median immobilization concentration for 50% of the Daphnia magna (IC50) was calculated. For monuron the IC50 for daphnids was reported to be 106 ppm (=106 mg/L).

The study is not following all aspects of the modern test design for acute daphnia studies. As major deviations, no dose verification was performed and the observation period was only 26 hours instead of 48 hours. Minor deviation were that the daphnids were not selected by having an age of < 24 hours (but the daphnids were considered to be first instar, which is considered acceptable), the test vessels were not replicated (but the number of daphnids exposed per level were higher than prescribed in modern guidelines) and the need of a solvent control is not clear,

Especially the lack of dose verification and the shorter exposure duration are considered to be relevant for the question if the study can be used for the risk assessment. Since monuron was used at concentrations well below the water solubility and monuron is biologically and hydrologically relative stable the lack of dose verification analysis is considered not to invalidate the results from this study. It is scientifically very unklikely that the prolongation of the exposure from 26 to 48 hours will change the IC50 considerably which means in this context by at least an order of magnitude. Considering the high toxicity to algae (orders of magnitudes more sensitive than daphnids), the toxicity of monuron to daphnids is of less relevance for the environmental risk assesment of monuron. Therefore, the results reported by Crosby and Tuckler are considered to be reliable with restriction.

To conclude, the IC50 of 106 mg/L is considered reliable with restrictions for the further assessment.