Phytotoxicity and antioxidative enzymes of green microalga (Desmodesmus subspicatus) and duckweed (Lemna minor) exposed to herbicides MCPA, chloridazon and their mixtures

In this study, we evaluate the toxicity of MCPA (auxin-like growth inhibitor), chloridazon (CHD) (PSII-inhibitor) and their mixtures to floating plants and planktonic algae. Toxicity of MCPA (4-chloro-2-methylphenoxyacetic acid) and CHD (5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone) was first assessed in two growth inhibition tests with Lemna minor (ISO/DIS 20079) and Desmodesmus subspicatus (ISO 8692). Next, herbicide mixtures at concentrations corresponding to the EC values were used to assess their interactive effects, and the biomarkers were: for duckweed fresh weight, frond area, chlorophyll content and number of fronds, and for algae cell count and cell volume. The 3d EC?? and EC?? values using cell counts of D. subspicatus were 142.7 and 529.1 mg/L for MCPA and 1.7 and 5.1 mg/L for CHD. The 7d EC?? and EC?? values using frond number of L. minor amounted to 0.8 and 5.4 mg/L for MCPA and 0.7 and 10.4 mg/L for CHD. Higher sensitivity of reproductive (number of cells/fronds) than growth processes (cell volume/frond area) to herbicides applied individually and in mixtures was especially pronounced in the responses of Desmodesmus. Herbicide interactions were assessed by the two-way ANOVA and Abbott's formula. Generally, an antagonistic interaction with Lemna was revealed by MCPA and chloridazon, whereas additive effect of both herbicides was observed for Desmodesmus. A significant stimulation of SOD and APX activity by binary mixtures was noted in algal cells mainly after 24 and 48 hours of exposure. The extremely high stimulation of the activity of both enzymes was induced by the combination EC??CHD + EC??MCPA (48 h). Presumably due to oxidative stress, the treatment with CHD at concentration EC?? after 72 h was lethal for algae grown in aerated cultures, in contrast to standardized test conditions. Taking into account the consequences of risk assessment for herbicide mixtures we can state that a relatively low toxicity, as well as the lack of significant synergy between MCPA and CHD to non-target plants appears to be the most important result.     

Heavy metal toxicity to Lemna minor: studies on the time dependence of growth inhibition and the recovery after exposure

Environmental concentrations of toxic substances are not necessarily constant but fluctuate over time. Periods of intense exposure might be followed by episodes with a relatively low or no exposure, in principle allowing exposed organisms to recover from toxic injury. The growth reproduction assay with the limnic vascular plant Lemna minor allows for convenient studies on the time dependence of the aquatic toxicity of chemicals. Here we report on a study with four priority metals (Zn, Cu, Ni, Cd). Aims of the study were to determine the impact of the exposure duration on the observed toxicities and to determine the potential for recovery. The bioconcentrations of the test metals were recorded during the exposure in order to analyse, whether changes in the internal concentrations are a governing factor for the dynamics of toxicity. After an exposure of 7 days, Cd and Cu showed the highest toxicity to Lemna (EC50's of 1.9 and 9.7 microM respectively), while Ni and Zn had a slightly lower toxicity (EC50's of 56.3 and 46.1 microM respectively). Additionally, Zn showed a severely delayed toxicity and the exposed plants did not recover even 7 days after the exposure had ended. This is in sharp contrast to the other test metals, for which a considerable recovery was observed. These results indicate the necessity to more thoroughly consider the dynamics of toxicity, instead of recording toxic effects only after a constant exposure over a fixed time.     

Initial risk assessment for three beta-blockers found in the aquatic environment

Daphnia magna, Desmodesmus subspicatus and Lemna minor were used to determine the ecotoxicity of beta-blockers. Propranolol was the most toxic substance, with EC50s of 7.7 mg l-1 in the Daphnia test and 0.73 mg l-1 in the algal test. The toxicity of atenolol and metoprolol in the Daphnia test was very low, with EC50s of 313 and 438 mg l-1, respectively. In the algal test, metoprolol (EC50: 7.9 mg l-1) was more toxic than atenolol (EC50: 620 mg l-1). Lemna minor was the least sensitive species. All substances showed PEC/PNEC ratios <1, whereas propranolol with a ratio of 0.81 seems to be the most relevant substance and I recommend to perform chronic biotests to refine the estimate of the environmental risk. Taking bioconcentration factors (BCF) into account, resulting internal effect concentrations (IEC) differ only slightly, which means that the differences in the EC50 depend mainly on the diverse logP levels causing narcosis via disruption of membrane integrity. Predictions of toxicity using ECOSAR were close to the empirically measured data. Mixture toxicity of the three compounds could be predicted accurately using the concept of concentration addition.     

Survival, growth and reproduction of Daphnia carinata (Crustacea: Cladocera) exposed to chronic cadmium stress at different food (Chlorella) levels

In nature, organisms have to respond to a diversity of factors acting simultaneously. The present investigation was conducted to study whether changes in food (Chlorella) levels could modify the chronic toxicity of cadmium on the various life-history parameters, such as survivorship, longevity, life expectancy, fecundity, age at first reproduction, R(0), T, r and growth rates of the cladoceran Daphnia carinata. The study indicated that at low food levels (0.5 x 10(6) cells ml(-1) Chlorella), cadmium concentrations in the range of 27-162 microg litre(-1) reduced these life-history parameters by 50% (EC(50)). At medium food levels (1.5 x 10(6) ml(-1) Chlorella) the EC(50) of cadmium was in the range of 51-127 microg litre(-1). At high food levels (4.5 x 10(6) cells ml(-1) Chlorella), the toxic effect of cadmium was greatly reduced. The decreases in survival, growth and reproduction of D. carinata at high cadmium-low food levels affected the fitness parameter 'r'. The study emphasises the need to include reproductive parameters other than mere survival in toxicity bioassays. The study also stresses the need to incorporate in laboratory tests other relevant factors that might modify pollutant toxicity.     

Acceleration of catalase and peroxidase activities in Lemna minor L. and Allium cepa L. in response to low levels of aquatic mercury

The purpose of this study was to assess certain physiological responses of Lemna minor L. (duckweed) and Allium cepa L. (onion) to aquatic mercury at low concentrations. Following a 96-h exposure of plants to nutrient medium contaminated with known levels of mercuric chloride (HgCl(2)), 0.001 to 4 mg litre(-1) (0.0007 to 2.95 mg Hg litre(-1)) or methyl mercuric chloride (MeHgCl(2)), 0.0001 to 0.1 mg litre(-1) (0.0007 to 0.07 mg Hg litre(-1)), the physiological endpoints measured were the growth of fronds (Lemna minor) or roots (Allium cepa), and catalase and peroxidase activities in both plant assays. The EC(50) for HgCl(2) on the basis of the growth curve of Lemna minor was found to be 2.1 mg litre(-1). HgCl(2) and MeHgCl(2) were lethal to L. minor at concentrations of 4 and 0.01 mg litre(-1), respectively. The range of low concentrations that accelerated growth as well as enzymic activities in L. minor was 0.004 to 0.04 mg litre(-1) for HgCl(2) and 0.001 mg litre(-1) for MeHgCl(2). HgCl(2) and MeHgCl(2) induced maximum enzymic activity in Lemna fronds at concentrations of 0.008 and 0.0005 mg litre(-1), respectively. In Allium roots, catalase activity was accelerated at all the concentrations of HgCl(2) (0.001-2 mg litre(-1)) and MeHgCl(2) (0.0001-0.1 mg litre(-1)) tested. The activity of peroxidase was, however, accelerated by HgCl(2) at concentration range 0.01-1.0 mg litre(-1), or by MeHgCl(2) at 0.001 mg litre(-1). The concentrations of HgCl(2) and MeHgCl(2) that induced the highest enzymic activity in Allium roots were 0.05 mg litre(-1) and 0.001 mg litre(-1), respectively.     

Phytotoxicity and antioxidative enzymes of green microalga (Desmodesmus subspicatus) and duckweed (Lemna minor) exposed to herbicides MCPA,chloridazon and their mixtures

In this study, we evaluate the toxicity of MCPA (auxin-like growth inhibitor), chloridazon (CHD) (PSII-inhibitor) and their mixtures to floating plants and planktonic algae. Toxicity of MCPA (4-chloro-2-methylphenoxyacetic acid) and CHD (5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone) was first assessed in two growth inhibition tests with Lemna minor (ISO/DIS 20079) and Desmodesmus subspicatus (ISO 8692). Next, herbicide mixtures at concentrations corresponding to the EC values were used to assess their interactive effects, and the biomarkers were: for duckweed fresh weight, frond area, chlorophyll content and number of fronds, and for algae cell count and cell volume. The 3d EC₁₀ and EC₅₀ values using cell counts of D. subspicatus were 142.7 and 529.1 mg/L for MCPA and 1.7 and 5.1 mg/L for CHD. The 7d EC₁₀ and EC₅₀ values using frond number of L. minor amounted to 0.8 and 5.4 mg/L for MCPA and 0.7 and 10.4 mg/L for CHD. Higher sensitivity of reproductive (number of cells/fronds) than growth processes (cell volume/frond area) to herbicides applied individually and in mixtures was especially pronounced in the responses of Desmodesmus. Herbicide interactions were assessed by the two-way ANOVA and Abbott's formula. Generally, an antagonistic interaction with Lemna was revealed by MCPA and chloridazon, whereas additive effect of both herbicides was observed for Desmodesmus. A significant stimulation of SOD and APX activity by binary mixtures was noted in algal cells mainly after 24 and 48 hours of exposure. The extremely high stimulation of the activity of both enzymes was induced by the combination EC₁₀CHD + EC₅₀MCPA (48 h). Presumably due to oxidative stress, the treatment with CHD at concentration EC₅₀ after 72h was lethal for algae grown in aerated cultures, in contrast to standardized test conditions. Taking into account the consequences of risk assessment for herbicide mixtures we can state that a relatively low toxicity, as well as the lack of significant synergy between MCPA and CHD to non-target plants appears to be the most important result.     

Toxicity of uranium and copper individually, and in combination, to a tropical freshwater macrophyte (Lemna aequinoctialis)

Copper (Cu) and uranium (U) are of potential ecotoxicological concern to tropical freshwater biota in northern Australia, as a result of mining activities. Few data are available on the toxicity of U, and no data are available on the toxic interaction of Cu and U, to freshwater biota. This study determined the toxicity of Cu and U individually, and in combination, to a tropical freshwater macrophyte, Lemna aequinoctialis (duckweed), in a synthetic soft water (27 degrees C; pH, 6.5; hardness, 40 mg CaCO3 l-1, alkalinity, 16 mg CaCO3 l-1), typical of many fresh surface waters in coastal northern Australia. The growth rate of L. aequinoctialis decreased with increasing Cu or U concentrations, with the concentration of Cu inhibiting growth by 50% (EC50) being 16+/-1.0 microg l-1, with a minimum detectable effect concentration (MDEC) of 3.2 microg l-1. The concentration of U inhibiting growth by 50% (EC50) was 758+/-35 microg l-1 with a MDEC of 112 microg l-1. The EC50 value for the exposure of L. aequinoctialis to equitoxic mixtures of Cu and U was significantly (P0.05) higher than one toxic unit (1.35; 95% confidence interval, 1.18-1.52), indicating that the combined effects of Cu and U are less than additive (antagonistic). Therefore, inhibition of the growth rate of L. aequinoctialis was reduced when Cu and U were present in equitoxic mixtures, relative to individual metal exposures. Since non-additive (e.g. antagonistic) interactions of metal mixtures cannot be predicted using current mixture models, these results have important potential implications for the protection of freshwater ecosystems through the derivation of national water quality guidelines.     

Effects of low dissolved oxygen on organisms used in freshwater sediment toxicity tests

Minimum dissolved oxygen requirements are part of standard guidelines for toxicity testing of freshwater sediments with several benthic invertebrates, but the data underlying these requirements are somewhat sparse. We exposed three common test organisms to ranges of dissolved oxygen concentrations to determine their responses in 10-d exposures, relative to published guidelines for sediment toxicity tests. The oligochaete, Lumbriculus variegatus, showed 100% survival in all exposures down to the lowest concentration tested, 0.7 mg O(2)l(-1). Midge (Chironomus dilutus) larva showed a more pronounced response; while survival was less than 90% only below 1.0mg O(2)l(-1), the biomass endpoint showed EC(50), EC(20), and EC(10) values of 1.00 (0.91-1.10), 1.41 (1.16-1.71), and 1.67 (1.25-2.24) mg O(2)l(-1). The amphipod, Hyalella azteca, showed no adverse effects at concentrations as low as 2.12 mg O(2)l(-1). The combination of these data with other literature data suggest that DO minima in current North American 10-d sediment test guidelines are reasonable.     

Phytotoxicity of coloured substances: is Lemna duckweed an alternative to the algal growth inhibition test?

Coloured substances cause problems when interpreting algal tests, because effects due to light absorption can interact with potential toxicity. The Lemna Duckweed growth inhibition test can complement the algal test, on condition that the test is performed on a black, not reflecting surface. On white surfaces, test solution colour can strongly impact Lemna growth. For example, average control sample growth rate of is much higher on white surfaces (0.362 d(-1)) than on black surfaces (0.284 d(-1)). We found that 10 mg l(-1) of the dyestuff "Brilliant Blue R spezial" inhibited average Lemna growth rate about 22% on white surfaces but did not inhibit growth on black surfaces. The reason for this difference stems from the difference in amount of light reflected from below the test beakers. With Brilliant Blue on white surfaces, the test solution colour reduces utilizable light and causes a deterioration of light conditions, whereas on a black surfaces, reflected light is absent a priori, and thus no inhibiting effect was measured. Of particular importance is the choice of test parameter. With Brilliant Blue, a LOEC for average growth rate, based on frond numbers, of 320 mg l(-1) was determined. However, when average growth rate was calculated using dry weights of the plants, the LOEC decreased clearly to 1.0 mg l(-1). In this study, the Lemna test was much more sensitive than the algal test. We recommend Lemna tests be used in addition to algal tests, because doing so may significantly improve the assessment of phytotoxicity of chemicals and sewage.     

Duckweed (Lemna gibba) growth inhibition bioassay for evaluating the toxicity of olive mill wastes before and during composting

Two-phase olive mill waste (TPOMW) is considered the main problem confronting the modern oil extraction and processing industry. Composting has been recently proposed as a suitable method to treat TPOMW so that it is suitable for use in agriculture. In the work reported here, the Lemna gibba bioassay was tested to assess the toxicity of TPOMW before and during the composting process. The method was compared with the Lepidium sativum bioassay and with other chemical maturity indices traditionally reported in the literature. The L. gibba test proved to be a simple, sensitive, and accurate method to evaluate toxicity before and during the composting of TPOMW. Plant growth response was measured by two methods: counting the number of fronds (leaves) and measuring total frond area (TFA) with image analysis software. Compared to the counting of fronds (L. gibba) or seeds (L. sativum), the use of area-measuring software permitted a very rapid, unbiased and easy way of analysing the toxicity of TPOMW before and during composting. Although the accuracy of the frond count method was similar to the traditional cress seed test, data analysis showed that the TFA measurement method was statistically more accurate (significantly lower variance) than the frond count approach. Highly significant correlations were found between TFA and some important maturation indices commonly reported in literature indicating that the L. gibba bioassay can be a useful tool to determine the degree of maturity of TPOMW composts.     

Selenium oxyanion inhibition of hydrogenotrophic and acetoclastic methanogenesis

Inhibitory effects of selenite and selenate towards hydrogenotrophic and acetoclastic methanogenesis were evaluated in anaerobic toxicity assays. The 50% inhibitory concentration (IC(50)) of both selenium oxyanions was below 6.1x10(-5)M in hydrogenotrophic assays, whereas acetoclastic methanogens were less inhibited: IC(50)=8.3x10(-5)M and 5.5x10(-4)M for selenite and selenate, respectively. Selenite completely inhibits methanogenesis from both substrates tested at concentrations 10(-3)M selenite, while only marginal methanogenic activities occur at equimolar concentrations of selenate. Selenite becomes irreversibly inhibitory upon a single exposure, whereas selenate inhibits methanogens upon repeated exposure. Consequently, methane recovery can be seriously hampered or even impossible during anaerobic treatment of highly selenium contaminated waste streams.     

Haloacetic acids in the aquatic environment. Part II: ecological risk assessment

Haloacetic acids (HAAs) are environmental contaminants found in aquatic ecosystems throughout the world as a result of both anthropogenic and natural production. The ecological risk posed by these compounds to organisms in freshwater environments, with a specific focus on aquatic macrophytes, was characterized. The plants evaluated were Lemna gibba, Myriophyllum spicatum and M. sibiricum and the HAAs screened were monochloroacetic acid (MCA), dichloroacetic acid (DCA), trichloroacetic acid (TCA), trifluoroacetic acid (TFA) and chlorodifluoroacetic acid (CDFA). Laboratory toxicity data formed the basis of the risk assessment, but field studies were also utilized. The estimated risk was calculated using hazard quotients (HQ), as well as effect measure distributions (EMD) in a modified probabilistic ecological risk assessment. EMDs were used to estimate HAA thresholds of toxicity for use in HQ assessments. This threshold was found to be a more sensitive measure of low toxicity than the no observed effect concentrations (NOEC) or the effective concentration (EC10). Using both deterministic and probabilistic methods, it was found that HAAs do not pose a significant risk to freshwater macrophytes at current environmental concentrations in Canada, Europe or Africa for both single compound and mixture exposures. Still, HAAs are generally found as mixtures and their potential interactions are not fully understood, rendering this phase of the assessment uncertain and justifying further effects characterization. TCA in some environments poses a slight risk to phytoplankton and future concentrations of TFA and CDFA are likely to increase due to their recalcitrant nature, warranting continued environmental surveillance of HAAs.     

Effects of procymidone, fludioxonil and pyrimethanil on two non-target aquatic plants.

Procymidone, fludioxonil, and pyrimethanil are widely used to control the pathogenic fungus Botrytis cinerea in Champagne's vineyards. These fungicides may end up in surface waters and present potential risks for aquatic vascular plants and algae. Therefore, their toxicity was evaluated on Lemna minor and Scenedesmus acutus in six-day or 48-h tests, respectively. Based on growth and chlorophyll (Chl) content of L. minor and S. acutus cultures, the results showed that the alga was the most sensitive to the fungicides. Among the fungicides, pyrimethanil was the most toxic for L. minor, its nominal IC50 was 46.16 mg l(-1) and that of the other two was >100 mg l(-1). In contrast, pyrimethanil appeared the least toxic for S. acutus at low concentration, nominal IC50 were 22.81, 4.85, and 4.55 mg l(-1) for pyrimethanil, fludioxonil, and procymidone, respectively. Fate of the fungicides in the media was also investigated and acute toxicity of the agrochemicals is discussed in regard to concentration in the culture media. Poor solubility of procymidone and fludioxonil appeared to be partly responsible for the low toxicity of these fungicides. Based on these toxicity data and the concentrations found in ponds collecting vineyard runoff water, these pesticides should not impair the establishment of pioneer plants.     

Copper toxicity to Lemna minor modelled using humic acid as a surrogate for the plant root

Humic acids are chemically analogous to plant root cell walls in that their surface sites are principally comprised of carboxylic and phenolic acids which bind both metals and protons. Based on this analogy, we developed a biotic-ligand type of model to predict Cu toxicity to Lemna minor, using particulate humic acid (HA(part)) of the Windermere Humic Aqueous Model (WHAM), and 7d static-renewal exposures with five surface waters and one nutrient media which varied in DOC (1-10 mg L(-1)), pH (6.9-8.7), and water hardness (35-236 mg equivalent CaCO(3)L(-1)). Although the range of waters tested resulted in a 36-fold variation in 50% inhibitory concentration (IC50) values, the calculated concentration of Cu bound to HA(part) using this framework was highly correlated with pooled percent net root elongation (%NRE) (R(2)=0.95). Ten and fifty percent IC values based on [Cu-HA(part)] were additionally within a factor of ±1.5 and ±1.4, respectively, inclusive of 95% confidence limits. This model construct, which defines the free metal ion and the first hydrolysis product (but not metal carbonate complexes) as being bioavailable, provides an alternative means of defining the binding surface in bioavailability models, whereby a heterogeneous mixture of ligands collectively influence root-metal sorption and toxicity.     

A comparative study on characterization of textile wastewaters (untreated and treated) toxicity by chemical and biological tests

Toxicity of textile wastewaters (untreated and treated) and their ingredient chemicals was quantified in terms of their chemical characteristics, fish (Gambusia affinis) mortality and end point growth responses of duckweed (Lemna aequinoctialis) in short-term bioassays. Other parameters of fish bioassay were erythrocyte morphology and its counts. Despite of a definite correlation between data of biological tests (LC/EC(50) values) with that of chemical tests, biological tests were found to be relatively more sensitive to both wastewaters and ingredient chemicals. Amongst all the examined parameters of test organisms, fish RBCs (morphology and counts) sensitivity to pollutants in the wastewaters was usually maximum and therefore, their study should be included in the routine fish bioassay. Other advantage of biological test such as on Lemna is even detection of eutrophic potential of wastewaters, as noted at their higher dilutions. The ingredient chemicals (major) contributing maximum toxicity to textile dye wastewater were, acids (HCl and H(2)SO(4)), alkali (Na(2)O SiO(2)), salt (NaNO(2)) and heavy metal (Cu), whereas dyes (4) were relatively less toxic.     

Decolorization of Orange G and Remazol Brilliant Blue R by the white rot fungus Dichomitus squalens: toxicological evaluation and morphological study

Dichomitus squalens efficiently decolorized both Orange G and Remazol Brilliant Blue R (RBBR) at concentrations of 0.5gl(-1) and 3gl(-1) in static and shaken culture and also on solid medium within 14d. The presence of the dyes in the culture medium mostly caused a decrease in biomass production and in growth rate, which was more significant in the case of RBBR. After 14d of cultivation, electron microscopy showed substantial morphological changes in mycelia of D. squalens growing in media containing dyes. The hyphae deformations were more intensively manifested in solid media than in liquid culture. In all the cases, the morphological changes were more prominent in the presence of RBBR. Higher concentrations of both dyes brought about more intensive changes. The toxicity of synthetic dyes Orange G and RBBR was tested using a bioassay based on the growth inhibition of duckweed Lemna minor. Two endpoints such as the number of fronds and their weight were studied during the bioassay. The results showed higher toxicity of RBBR than that of Orange G. The toxicity of both dyes decreased after the decolorization process.     

Quantification of bioavailable nickel in sediments and toxic thresholds to Hyalella azteca

Bioaccumulation and chronic toxicity of nickel (Ni) to Hyalella azteca in Ni-spiked sediments was strongly affected by the source of sediment used. The total range in LC50s on a sediment concentration basis ranged over 20 fold. Differences in Ni toxicity generally matched differences in Ni bioaccumulation, and toxicity expressed on a body concentration basis varied less than three fold. Body concentrations, therefore, provide a much more reliable prediction of Ni toxicity in sediments than do concentrations in the sediment. Ni in overlying water was also a reliable predictor of Ni toxicity, but only in tests conducted in Imhoff settling cones with large (67:1) water to sediment ratios. Overlying water LC50s for tests in beakers varied 18 fold. Sediment and body concentrations of Ni tolerated by Hyalella were slightly higher in cones than in beakers. Reproduction was not affected significantly by Ni at concentrations below the LC50 and 10-week EC50s for survival and biomass production (including survival, growth and reproduction) were only marginally lower than 4-week EC50s (survival and growth only).     

Comparison of different physiological parameter responses in Lemna minor and Scenedesmus obliquus exposed to herbicide flumioxazin

The sensitivity of different physiological parameters in Scenedesmus obliquus and Lemna minor exposed to herbicide (flumioxazin) was investigated to indicate the most convenient and sensitive parameter. To assess toxicity of flumioxazin, we used a panel of biomarkers: pigment contents, chlorophyll fluorescence parameters and antioxidative enzyme activities. Algae and duckweed were exposed to 48-h IC50 for growth rate. In L. minor, the sensitivity of the parameters was as follows: QN > Oxygen emmision > phiS(PSII) > QP > phi(PSII) > CAT, GR > Pigment> APO > Growth. For S. obliquus, this ranking was as follows: CAT > Oxygen emission > QP > APO > GR > Pigment > phiS(PSII) > Growth > phi(PSII) > QN (from the greatest to the least sensitive). The results demonstrated that the observed toxicity is related not only to interspecific variations but also to the selected parameter.     

Acute, chronic and sublethal effects of the herbicide propanil on Daphnia magna

Acute and chronic toxicity tests with propanil were conducted on Daphnia magna. The 24 and 48 h LC50 were 43.74 and 5.01 mg/l respectively. Chronic toxicity tests were carried out using sublethal propanil concentrations (0.07, 0.10, 0.21 and 0.55 mg/l) during 21 days. The effect of propanil on survival, reproduction and growth of D. magna organisms was monitored. The parameters used to evaluate herbicide effect on reproduction were: mean total young ones per female, mean brood size, time to first reproduction, mean number broods per female and intrinsic rate of natural increase (r). Survival and growth (body length) were also determined after 21 days of exposure to the herbicide. Reproduction was significantly reduced when propanil concentration increased in the medium. The intrinsic rate of natural increase (r) decreased with increasing concentrations of propanil especially in those animals exposed to 0.55 mg/l. However, growth as well as survival of the exposed organisms only decreased in daphnids exposed to the highest propanil concentration tested. The maximum acceptable toxicant concentration (MATC) was calculated for D. magna exposed to the herbicide using as parameter of evaluation the intrinsic rate of natural increase (r). The interpolation of these results gave MATC values of 0.08 mg/l herbicide. We have derived the EC50 values for some selected parameters on D. magna exposed to propanil. EC50 values indicated that reproductive parameters were very sensitive of the effect of propanil on daphnids. Finally, the daphnids were exposed to the same sublethal herbicide concentrations as in the chronic study and the effect of the toxicant on filtration and ingestion rates was determined. Feeding rates of D. magna declined with increasing propanil concentrations. The effective propanil concentrations at which feeding rates were reduced to 50% of that in controls (EC50) were also calculated.