A method for assessing chronic effects of toxic substances on the midge,Paratanytarsus parthenogeneticus—Effects of copper

A method was developed for the assessment of the chronic effects of copper on midge larva. Growth and reproduction of the midge was related to copper concentrations greater than 0.08 mg/L; 50% reproductive impairment was apparent at a copper level of 0.37 mg/L. Growth and amount of food uptake by larvae were inversely related to copper concentration. Accumulation of copper in green algae was proportional to copper concentration in the water.     

Sublethal Effects of Three Pesticides on Larval Survivorship, Growth, and Macromolecule Production in the Aquatic Midge, Chironomus tentans (Diptera: Chironomidae)

Effects of long-term exposure to each of three pesticides including atrazine, DDT, and chlorpyrifos on larval survivorship, growth, and macromolecule (total body protein and RNA) production were evaluated in the aquatic midge, Chironomus tentans, under laboratory conditions. Newly hatched larvae were exposed to atrazine at 30 and 150 μg/L, DDT at 0.01 and 0.05 μg/L, or chlorpyrifos at 0.02 and 0.10 μg/L throughout one life cycle. Larval survivorship was evaluated at 20 d and the end of the test, and larval growth at 20 d. Chlorpyrifos at 0.1 μg/L reduced the midge survivorship by 67% after 20-d exposure. However, neither atrazine nor DDT affected larval survivorship. The ash-free dry weight of midge larvae exposed to chlorpyrifos at 0.1 μg/L was 1.5-fold greater than that of the control whereas neither atrazine nor DDT showed a significant effect on the ash-free dry weight. In addition, exposures of midges to chlorpyrifos at 0.1 μg/L increased the adult emergence rate by 81% as compared to the control although the actual number of adults that emerged from chlorpyrifos-treated larvae was significantly decreased. Both the increased ash-free dry weight of larvae and increased adult emergence rate were likely caused by reduced competition for both food and space among the survivors due to increased larval mortality. Although neither total protein nor total RNA production was significantly affected in larvae exposed to each pesticide for 20 d, a significantly higher number of males over females (ratio = 4) emerged from midges exposed to DDT at 0.05 μg/L. Our study indicates that chronic exposure to low concentrations of chlorpyrifos and DDT results in significant mortality of midge larvae and alteration of the sex ratio of adult emergence, respectively. Because midges are important components of the food web, our results suggest that effects elicited directly or indirectly by long-term pesticide exposures may potentially disrupt both food chains and community structure in aquatic environments.     

Bioactive compounds in the aquatic environment: Studies on the mode of uptake of DDE by the aquatic midge,Chironomus tentans (diptera: Chironomidae)

The mode of uptake of DDE-¹⁴C byChironomus tentans larvae was investigated. There was no difference in the amount of DDE-¹⁴C accumulated by live and dead fourth instar larvae as a function of exposure time. Dead and live larvae were also exposed to an aqueous and substrate source of DDE-¹⁴C contamination and again no differences were found in the amount of DDE-¹⁴C accumulated indicating passive accumulation. Cuticle surface area and DDE-¹⁴C uptake relationships were found to have a high degree of correlation. The amount of DDE-¹⁴C concentrated by the larvae was increased by manipulation of water hardness. Calcium and magnesium ion concentrations in the water were increased and a subsequent increase in DDE-¹⁴C accumulation by the larvae resulted. An adsorption-diffusion mechanism is proposed to account for the mode of uptake and biological concentration capabilities of the midge.     

Chronic toxicity of copper to a partial life cycle of the midge,Chironomus decorus

The development and hatchability ofChironomus decorus eggs were not affected by 0.1 to 5 mg/L of copper in water. The embryos developed normally and hatched at about the same time (after 55 hr of incubation). All larvae survived the duration of the test (72 hr) except those subjected to 5 mg/L of copper in water, which died after only partial emergence from the egg shell. Apparently, eggs were protected by their shell from copper. Growth ofC. decorus larvae was reduced significantly when they were reared in copper-spiked food-substrate (bound copper) from the age 1 to 15 days old (900–4,500 mg/kg of copper). The substrate copper concentration at which larval growth was reduced to 50% (EC50) was 1,602 mg/kg. Substrate copper caused deformities in the epipharyngeal plate of larval mouthparts and copper concentration higher than 1,800 mg/kg delayed adult emergence. The copper concentration in pupal exuviae and adults were positively correlated to copper concentration in the substrate in which they had been reared as larvae. Larval stage appeared to be the most sensitive to copper toxicity, while eggs were the least sensitive. Larval growth was the best indicator in detecting copper pollution, since it could detect copper at relatively low concentrations. The time to adult emergence was not considered a very good indicator, while larval deformities offered a quick tool to evaluate copper pollution.     

The bioavailability of sediment-sorbed chlorobenzenes to larvae of the midge, Chironomus decorus

Larval stages of the midge, Chironomus decorus, were used to define the bioaccumulation of sediment-sorbed mono-, di-, tri-, and hexachlorobenzene. Larvae were exposed to high- and low-organic-content sediments that had been equilibrated with individual radiolabeled chlorobenzenes prior to testing. Equilibrium or nonequilibrium aqueous concentrations of the volatile test chemicals were flowed through sealed chambers in a sediment-water exposure system. The uptake of chlorobenzenes by midge larvae was rapid for all compounds tested, and apparent steady-state conditions were reached within 48 hr of exposure. Bioconcentration factors for the accumulation of chlorobenzenes from sediments and from interstitial and overlying waters were related to the octanol/water partition coefficients of the compounds. Because the diffusion of chlorobenzenes to overlying water during nonequilibrium flow through conditions was very slow, bioaccumulation was dependent on the concentration of the chemicals in interstitial water. These results show how benthic organisms may be able to accumulate significant levels of chlorinated aromatic compounds from ecosystems where their concentrations in the water column are relatively low.     

Chronic effects of Cd on the reproduction of the guppy (Poecilia reticulata) through Cd-accumulated midge larvae (Chironomus yoshimatsui)

Chronic effects of Cd on the growth and reproduction of the guppy (Poecilia reticulata) were studied using a food chain model, midge larvae as prey and guppy as predator. The transfer rate of Cd from the midge to the guppy was between 0.5 and 1% during the 30-day experiment. Growth rate of the guppy fed Cd-accumulated midge larvae (270 micrograms/g dry wt) for 30 days was not impaired. Cumulative numbers of fry produced by the guppy fed Cd-accumulated midge larvae (210 micrograms/g) for 2 months decreased to ca. 80% of the control. Guppies had been fed the Cd-accumulated midges from 30 days old for 7 months. Cumulative numbers of fry produced by the guppy fed midge larvae-accumulated 500, 800, and 1,300 micrograms Cd/g for 6 months decreased to 79, 65, and 55% of the control, respectively. Similar effects of Cd on the reproduction of guppy were shown between the guppies fed the Cd-accumulated midge larvae (500 micrograms Cd/g) and exposed to 10 or 20 micrograms Cd/liter for the 6 months. The Cd concentrations of the digestive tract, liver, and kidney increased strongly, indicating that such Cd accumulation was brought on mainly through the Cd-accumulated midges. Mortality of the female guppies fed larvae-accumulated 1,300 micrograms Cd/g increased abruptly from the 6 months of the experiment, whereas no male guppy died during the experiment.     

Life-cycle effects of sediment-associated 2,4,5-trichlorophenol on two groups of the midge Chironomus riparius with different exposure histories

Effects of 2,4,5-trichlorophenol (TCP) on life-cycle traits of the midge Chironomus riparius and the ability of the midge to evolve tolerance to TCP were assessed using a reference group and a group preexposed to TCP during three generations, both originating from the same laboratory culture. F1 larvae of these groups were then exposed to nominal TCP concentrations of 51, 177, 355, and 532 micromol TCP/kg dry sediment and a control sediment in a life-cycle experiment. Most studied life-cycle traits (mortality, egg production, life span, male dry wt) were fairly insensitive to TCP, and significant effects were observed only at high (> or = 355 micromol/kg) concentrations. Larval development rate was variable, and in some cases it responded more readily to low TCP concentrations than other life-cycle parameters. Some of the observed responses were attributed to changes in food availability. No clear evidence of tolerance to 2,4,5-trichlorophenol was found, but the preexposed midges produced more eggs than the reference group. They also emerged earlier and at a smaller size than the reference midges. These differences between the midge groups suggest that some changes toward tolerance induction had occurred during the preexposure.     

Screening of oomycete fungi for their potential role in reducing the biting midge (Diptera: Ceratopogonidae) larval populations in Hervey Bay, Queensland, Australia

Biting midges are globally distributed pests causing significant economic losses and transmitting arbovirus diseases to both animals and humans. Current biological and chemical control strategies for biting midge target destruction of adult forms, but strategies directed at immature stages of the insect have yet to be explored in Australia. In the present study, coastal waters of Hervey Bay region in Queensland, Australia were screened to detect the habitats of biting midge at immature stages. These results were then correlated to local environmental conditions and naturally occurring entomopathogenic fungal flora, in particular the Oomycete fungi, to determine their reducing effect on insect immature stages in the search for biological control agents in the region. The dominant species of biting midge found within this study was Culicoides subimmaculatus occuring between mean high water neaps and mean high water spring tide levels. Within this intertidal zone, the presence of C. subimmaculatus larvae was found to be influenced by both sediment size and distance from shore. Halophytophthora isolates colonized both dead and alive pupae. However, the association was found to be surface colonization rather than invasion causing the death of the host. Lack of aggressive oomycete fungal antagonists towards midge larvae might correlate with increased incidences of biting midge infestations in the region.     

Assessing the toxicity of dodecylbenzene sulfonate to the midge Chironomus riparius using body residues as the dose metric

Dodecylbenzene sulfonate (DBS) is a component of linear alkylbenzene sulfonate (LAS), an anionic surfactant, mainly used in household detergents. Due to the large quantity of DBS in use, there is concern over adverse environmental effects. This work examined the toxicokinetics and toxicity of the 2-phenyl isomer of dodecylbenzene sulfonate in 4-d, 10-d, and partial life-cycle tests on the midge, Chironomus riparius, exposed to aqueous solutions. Toxicokinetics were determined in 10-d uptake and 5-d elimination tests. The toxicokinetics were based on parent compound concentration in water and yielded an uptake coefficient (ku) of 17.5 (14.87-20.20) ml/g/h, an elimination rate constant (ke) of 0.073 (0.062-0.085) per h, a bioconcentration factor (BCF) of 56 to 240, and a half-life (t 1/2) of 9.5 (8.0-11.0) h. Biotransformation measurements did not reveal evidence for DBS metabolism. Thus, body residues, determined in the toxicity study, represent parent compound. In toxicity tests, 4- and 10-d LR50s (the body residue required to cause 50% mortality) in live midges were 0.72 (0.65-0.79) and 0.18 (0.08-0.42) mmol/kg, respectively. Thirty-day LR50s were 0.18 (0.09-1.64) and 0.21 (0.15-0.39) mmol/kg in duplicate studies. Of the sublethal endpoints, only developmental time increase was significant, with the lowest-observed-effect residues of 0.085 (0.067-0.105) and 0.100 (0.087-0.114) mmol/kg for male and female midges, respectively. Deformities in surviving larvae were also observed as chronic responses for body residues exceeding the 30-d LR50. The body residues required for mortality suggest that DBS acts like a polar narcotic in the midge.     

Comparison of the Toxicity Using Body Residues of DDE and Select PCB Congeners to the Midge, <Emphasis Type="Italic">Chironomus riparius,</Emphasis> in Partial-Life Cycle Tests

Due to the long time course required to achieve steady state with highly lipophilic contaminants such as PCBs (polychlorinated biphenyls), data derived from short-term toxicity tests may lead to an erroneous interpretation of hazard. In addition, PCBs bioaccumulated over time can cause sublethal impairments in organisms at concentrations much lower than required for mortality. Here, the body residues of 1,1-dichloro-2,2-bis-p-chlorophenyl ethane (DDE) and select PCB congeners associated with a spectrum of chronic effects in the midge, Chironomus riparius, were evaluated. The route of exposure was ingestion of the PCB-contaminated alga, Chlorella vulgarus, and trout chow loaded with the selected test compound. Two separate exposures of midges were performed. In the first experiment, midges were exposed from the second instar to the pupal stage. In the second exposure, midges were exposed from the second instar to the adult stage. A variety of sublethal endpoints was monitored, including developmental time within a stadium, body weight, and fecundity for the female adult. The dose was assessed as the whole body residue concentration of the contaminant. Overall, the midge concentration increased with increasing exposure concentration in algae and trout chow. Body weight at the end of each stadium was the assessment parameter that was least significantly affected among the test endpoints monitored. In contrast, a significant increase in development time was the endpoint that was most frequently observed in response to contaminant exposure. Reduction in fecundity was found only for DDE-exposed midges. These data, in which chronic endpoints are related to body residues, suggest that body residues will be useful in defining sublethal hazards of DDE and some PCB congeners.     

Effect of temperature on cadmium and zinc uptake by the midge larvae Chironomus riparius

The effect of temperature (5 to 25°C) was studied on the uptake of cadmium and zinc by larvae of the midge Chironomus riparius, using artificial chemically defined solutions. The influence of prior acclimation of midge larvae at five temperatures on metal uptake was examined. At all acclimation temperatures metal uptake in organisms increased with increasing exposure temperature. Among the different temperature exposure groups the effect of temperature acclimation on metal uptake is rather variable and a general pattern can not be distinguished. The integration of the different temperature effects explains 69% of the total variation in cadmium uptake and 68% of the total variation in zinc uptake by midge larvae. The factor that accounts for respiration explained 59% of the cadmium uptake and 60% of the zinc uptake by midge larvae. The apparent activation energies for larval metal uptake are much higher than the activation energy for the free diffusion of the cadmium or zinc ion but close to the activation energy for respiration, showing that metal uptake by midge larvae is largely controlled by active physiological rather than by passive diffusional processes.     

Chronic toxicity of cadmium to Chironomus riparius (Diptera: Chironomidae) at different food levels

The interacting effects of cadmium toxicity and food limitation on the midge, Chironomus riparius, were studied during chronic exposure in laboratory experiments. If the food was supplied ad libitum, both larval developmental time and mortality of the larvae were negatively affected by cadmium concentrations of 2.0–16.2 g/L. The number of eggs deposited per female and the mean life span of the imagines were not affected by cadmium. Integration of these separate effects into a population growth rate showed a clear reduction with increasing cadmium concentrations. Food limitation of unexposed larvae at high population density reduced fitness, judged on all parameters studied and consequently reduced the population growth rate (up to 85%).The effects on larvae, which were exposed to both cadmium and food limitation, differed considerably from the response to the individual stress factors. Exposure to cadmium increased mortality among food-limited first and second instar larvae. Consequently, the amount of food available for each surviving larva increased. At the two lowest concentrations studied (2.0 and 5.6 g Cd/L), these indirect positive effects of cadmium overruled the direct negative effects and caused an increase of the fitness of the food-limited exposed larvae compared to the food-limited, unexposed controls. At a concentration of 16.2 g Cd/L, the negative effects of cadmium on food-limited midges balanced the positive effects of reduced food limitation. At this concentration, the population growth rate did not differ significantly from the food-limited control any more. It is concluded that the indirect positive effects of cadmium on food limitation could eliminate negative, direct effects of low cadmium concentrations on food-limited chironomid populations.     

Biomimetic solid-phase microextraction to predict body residues and toxicity of chemicals that act by narcosis

A biomimetic extraction technique using solid-phase microextraction (SPME) fibers has been developed for the risk assessment of contaminants with a narcotic mode of action. Our goal is to apply this technique in the future for the prediction of total baseline toxicity of environmental water and effluent samples. Validation of this method requires establishing the relationship between contaminant accumulation and toxicity in biota and accumulation in the surrogate solid phase (the SPME fiber coating). For this purpose, we determined the median lethal concentration (LC50) values for Chironomus riparius midge larvae exposed to two halogenated aromatic compounds separately and measured body residues in the exposed larvae. Solid-phase microextraction fibers with an 85-microm polyacrylate (PA) coating served as the surrogate hydrophobic phase, mimicking the uptake of the compounds by midge larvae. The toxicant concentrations in SPME fibers measured directly by gas chromatography/mass spectrometry (GC-MS) or calculated from the SPME fiber-water partition coefficient, K(SPME) were related to the toxicant concentrations found in midge larvae. Our results demonstrated that the biomimetic SPME method enables the estimation of body residues in biota and prediction of the degree of baseline toxicity of a water medium.     

Effects of pH on the acute toxicity and uptake of [14C]pentachlorophenol in the midge, Chironomus riparius

The acute toxicity of pentachlorophenol (PCP) was determined at pH levels 4, 6, 9 to the midge, Chironomus riparius, with the findings that PCP is of greatest toxicity at pH 4 and of least toxicity at pH 9. This differential toxicity is attributable to variations in uptake levels at the respective pH levels. At pH 4, PCP is fully protonated and therefore highly lipophilic. The amount of [14C]PCP present in the midges at 24 hr is thus highest at pH 4. Conversely, at pH 9, the compound is completely ionized. The reduction in lipophilicity at pH 9 decreases the ability of the compound to penetrate into the midge, thereby decreasing the observed toxicity of the compound.     

Bioaccumulation and Elimination of Waterborne Mercury in the Midge Larvae, <Emphasis Type="Italic">Chironomus riparius</Emphasis> Meigen (Diptera: Chironomidae)

Here, mercury kinetics and behavioural effects in the midge larvae under a water-only exposure were assessed. Uptake and elimination of waterborne mercury were described by using a one-compartment kinetic model. Results show that midges were able to readily accumulate the heavy metal (BCF = 450), presenting a fast uptake, up to 13.1 μg Hg g of animal⁻¹ at the end of the exposure period. Elimination was slow, with c.a. 39 % of the mercury in larvae being depurated after 48 h in clean medium. Behaviour did not present differences upon exposure or elimination, but a trend to increase ventilation was noticed during the exposure period.     

Relative Contributions of Aqueous and Dietary Uptake of Hydrophobic Chemicals to the Body Burden in Juvenile Rainbow Trout

This study assessed the relative contributions of aqueous versus dietary uptake of three hydrophobic chemicals, 1,2,4-trichlorobenzene (1,2,4-TCB), 1,2,3,4,5-pentachlorobenzene (PeCB), and 2,2′,4,4′,6,6′-hexachlorobiphenyl (HCBP). Juvenile rainbow trout (Oncorhynchus mykiss) were exposed separately to chemically spiked water and food for 4 days and 12 days, respectively. Chemical concentrations were measured in the food, water, and tissues, and this allowed calculation of uptake rate constants (k₁ from water exposure, k_d from food exposure). The k₁ values for the three test chemicals were approximately five orders of magnitude greater than the k_d values. Using these measured uptake rate constants, a simulation model was used to predict the relative aqueous versus dietary uptake when fish were exposed simultaneously to water and food contaminated with these hydrophobic chemicals. The model predicted for all three test chemicals that the two uptake routes would contribute equally to the chemical body burden in fish whenever the food:water chemical concentration ratio was near 10⁵. However, using food:water chemical concentration ratios that might be expected in nature, the model predicted that gill uptake could account for over 98% of fish body burden for both 1,2,4-TCB and PeCB uptake (log K_ow values of 3.98 and 5.03, respectively). For HCBP (log K_ow of 7.55), the model predicted that the dietary uptake could contribute over 85% of the body burden. Thus, depending on the actual food:water chemical concentration ratio, aqueous uptake via the gills can predominate even when the chemicals have a log K_ow value greater than 5.0. In addition, we confirmed that dietary uptake of hydrophobic xenobiotics increases with increasing log K_ow. Received: 30 November 1999/Accepted: 3 May 2000     

Interacting Effects of Toxicants and Organic Matter on the Midge Chironomus riparius in Polluted River Water

Toxicants and organic matter in river water have contrasting impacts on macrofauna. Through manipulations of both factors, their interactive effects on organisms were evaluated. This way, an attempt was made to clarify the presence or absence of pollution-“tolerant” and -“sensitive” species in rivers affected by mixed sources of pollution. Under controlled conditions, larval growth of the “tolerant” midge Chironomus riparius was measured in different types of river water containing varying levels of particles (obtained by selective filtration) and toxicants (either complex mixtures or metals). Exposure of first-instar larvae to water from the polluted rivers Meuse and Dommel revealed that growth was less inhibited by toxicant levels in river water than expected based on laboratory toxicity tests. Factors present in polluted river water stimulated growth of midges to such an extent that inhibiting effects of high toxicant concentrations were neutralized, and at low toxicant levels, were overcompensated for. It was found that particulate matter has great potential to reduce inhibiting effects of toxicants on C. riparius, not (only) by reducing the bioavailability of toxicants, but by serving as a supplementary, superior food source. The success of the “pollution-tolerant” midge was not explained by tolerance of this species to toxicants, but by its ability to take advantage of coinciding organic enrichment. It is hypothesized that the extent to which beneficial effects of organic compounds on organisms occur is species specific.     

Experimental Induction of Morphological Deformities in Chironomus riparius Larvae by Chronic Exposure to Copper and Lead

Five consecutive generations of Chironomus riparius Meigen larvae were chronically exposed from egg to fourth instar to four sublethal concentrations of copper (0, 1, 10, 100 μg L⁻¹) and lead (0, 5, 50, 500 μg L⁻¹) in artificially spiked water (static with renewal), with diatomaceous earth as substrate and tetraphyl? as food, in order to test the induction of morphological deformities by these metals. The use of diatomaceous earth was suboptimal because it caused high mortalities (>60%), independent of metal stress. The higher copper concentrations had a positive effect on the survivals relative to the control. Split medial mentum teeth were recorded in more than 10% of the larvae, but could not be related to metal stress. Deformities of the mentum and the mandibles were recorded in second, third, and fourth instars exposed to both metals. Concentration and generation effects were noted for unusual number of mentum teeth (0–5.3%, lead), unusual number of mandible inner teeth (0–10.4%, copper and lead), and small open mentum gap (0–6.5%, copper). These experiments demonstrated the potential of both an essential and a non-essential metal to induce weak deformities in a small proportion of a C. riparius population as well as the induction of deformities which are independent of metal stress or fluctuating over the generations. The study shows the potential of midge deformities as a biomonitoring tool, but at the same time warns for a careful interpretation of deformity scores because of the influence of population dynamics on the final outcome of deformity frequencies and of the existence of deformities not related to pollution. Received: 25 August 1997/Accepted: 17 January 1998     

The effect of food rations on tissue-specific copper accumulation patterns of sublethal waterborne exposure in Cyprinus carpio

Common carp (Cyprinus carpio) were fed to two different food rations, 0.5% body weight (low ration [LR]) and 5% body weight (high ration [HR]), and were exposed to sublethal (1 microM) copper levels for 28 d in softened Antwerp (Belgium) city tap water (Ca2+, 79.3 mg/L; Mg2+, 7.4 mg/L; Na+, 27.8 mg/L; pH 7.5-8.0). Copper accumulations in the liver, gills, kidney, anterior intestine, posterior intestine, and muscle were determined. Copper accumulation in the gills, liver, and kidney of LR fish was significantly higher than in HR fish. The only time copper uptake in HR fish was significantly higher than in LR fish was in the posterior intestine after two weeks of exposure. No difference was found between the two rations in the anterior intestine. Copper accumulation in the liver of both feeding treatments occurred in a time-dependent manner and did not reach steady state in any treatment. On the contrary, copper concentration in the gills reached a steady state for both HR and LR fish within the first week of exposure. No copper accumulation was found in muscle tissues of either treatment. Copper concentration dropped to control levels in all tissues, except liver tissue, two weeks after the exposure ended. Our studies indicated that copper uptake was influenced by the food ration in carp. The difference in copper accumulation probably is related to the amount of dietary NaCl and different rates of metallothionein synthesis. Low food availability provides less Na+ influx and leads to increased brachial uptake of Na+ and copper. In addition, it has been shown that starved animals show increased levels of metallothionein, possibly causing higher copper accumulation.     

Caged midge larvae (Chironomus riparius) for the assessment of metal bioaccumulation from sediments in situ

First-stage larvae of the midge Chironomus riparius were exposed in small enclosures at 19 sites located in three different river basins in Flanders (Belgium). Sediments were sampled and sieved at 200 microm at all exposure sites. A layer of approximately 2 cm of sediment was placed in each cage and 100 midge larvae were added. Cages were placed in watercourses where resident midge larvae were present. Accumulation of Cd, Cr, Cu, Pb, Ni, and Zn was determined after four weeks of exposure when larvae had reached the fourth stage. Comparing metal levels between caged and resident larvae revealed no significant differences. A significant correlation between metal levels in caged and resident larvae was found when all sites were considered. However, such correlation was low (r2 = 0.28) for Pb. The highly significant r2 values found for Cu and Ni probably were due to only one site. Metal levels in tissue were related to levels in water and sediment, taking into account some sediment characteristics (particle size distribution and organic carbon) and oxygen level in the water. To determine the relative importance of these different sediment factors contributing to the variation in metal accumulation by the chironomids, nonlinear regression models were constructed. With the models used, 56.1, 32.2, and 57.4% of the variation for Cd, Pb, and Zn, respectively, could be described. None and 26.9% of the variation could be described for Cu and Ni, respectively. Among the environmental factors, organic carbon and oxygen levels in water were important in describing the accumulation of metals.