| The freshwater ecosystems in China have experienced severe deterioration in recent years. Sediment is a major component of aquatic environment, so an effective assessment of sediment quality is critical for aquatic risk assessment and management. Nevertheless, research on sediment risk assessment was far lagged behind compared to the assessment of water quality in China, especially for the assessment of chronic toxicity in sediment. The main objectives of the present dissertation were to evaluate the capability of using a weight of evidence(Wo E) approach to assess the risk of sediments which were not acutely lethal to benthic midges and to develop an effect-directed analysis(EDA) method to incorporate bioavailability of sediment-bound contaminants and in vivo bioassays for identifying organic toxicants in sediment.To improve the sensitivity and accuracy of assessing chronic toxicity in sediment, a Wo E framework was applied to assess sediment quality in the north end of Tai Lake, where the sediments were contaminated by various contaminants but showed no acute lethality to the benthic invertebrate, Chironomus dilutus. Whole life cycle bioassays with C. dilutus were performed and adverse effects were observed in most sites, including decreased survival, impaired growth, delayed emergence, and low fecundity. In addition, the change in biomarkers of the midges was also evident after 20-d exposure to the sediments. The biomarkers being studied included three enzymes related to biotransformation processes(cytochrome P450(P450), glutathione S-transferase(GST), and carboxylesterase(Car E)), a specific enzyme acetylcholinesterase(ACh E) indicating neurotoxicity, and two antioxidant defense enzymes(catalase(CAT) and lipid peroxidation(LPO)). The changes of the biomarkers were summarized for individual sampling sites by computing the i ntegrated biomarker response(IBR) indices to assess sediment risk.Meanwhile, the potential contaminants in the north end of Tai Lake were analyzed, including persistent organic pollutants(polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polybrominated diphenyl ethers, and organochlorine pesticides), current-use pesticides(organophosphate and pyrethroid pesticides) and metals. To infer pupative toxicants to the observed chronic toxicity in the midges, the contaminants in sediment were expressed as two dose metrics, namely a traditional toxic unit(TU) and bioavailable TU. Firstly, the contaminants were screened by traditional TUs approach for the potential contributions to chronic toxicity in C. dilutus. Subsequently, the relationship between chronic toxicity and contaminant-derived TUs was evaluated using a canonical correlation analysis. And the results suggested that two pesticides(hexachlorocyclohexane and chlorpyrifos) and two metals(Cr and Ni) were potential contributors to chronic effects in C. dilutus. Furthermore, the bioavailability of sediment-bound contaminants was estimated by Tenax extractable concentrations(bioavailable TU), which corroborated that two pesticides were main contributors to chronic toxicity to C. dilutus. To incorporate the bioavailability measurements into sediment toxicity assessement improved the accuracy for identifying the causative agents to the observed toxicity. Lastly, within a Wo E framework, three lines of evidence, including chronic toxicity testing, biomarker measurements, and chemical analysis, were integrated using a semi-qualitative scoring method. The contaminated sites were clearly distinguished and ranked by the levels of contamination. Complementary information was also confirmed by the interrelationship of the lines of evidence. The significant inhibition of ACh E in C. dilutus was corroborated by the detection of chlorpyrifos in sediment, an organophosphate pesticide, and the IBR indices correlated well with the impairments of emergence to C. dilutus. In conclusion, when the lethality of benthic organisms was not obseved in acute toxicity tests, the Wo E method to integrate the measurements of biomarkers in vivo and bioavailability of sediment-bound contaminants would help to improve the accuracy in the ecological risk of sediment.The EDA was mainly applied to identify organic toxicants in sediment, but the difference in bioavailability of sediment-bound contaminants was not considered in traditional use of EDA. In present study, passive dosing technique was incorporated into the EDA procedure to take bioavailability into consideration. The contaminants were extracted from field-collected sediments in Guangzhou reach of the Pearl River. The extract was then cleaned with gel permeation chromatography and fractionated on a high performance liquid chromatography with C18 reversed phase column. The original extract and the fractions were individually loaded to poly-dimethylsiloxane(PDMS) membranes. After PDMS membranes were put into reconstitute water, contaminants in PDMS membranes were released into the aqueous phase, and the partition of chemicals in the PDMS–water system simulated the equilibrium of contaminants in the sediment–water system. In this passive dosing system, aquatic toxicity tests were performed with third instar C. dilutus.After 4-d exposure, abnormal behaviors of the midges and the changes of biomarkers in vivo, including GST, Car E, ACh E, CAT, and LPO, were determinated for identifying the toxic fractions. The fraction showing sublethal toxicity to C. dilutus in the bioassays was then screened for potential toxicants by gas chromatography-mass spectrometry. The potential toxicants of 15 compounds were further confirmed with neat compound standards. The sediment concentrations were also analyzed for all potential toxicants, and the relationship between the toxicity of C. dilutus in probit form and the logarithm of concentrations were also performed as an additional piece of evidence. Results suggested that dimethomorph and triadimenol were pupative toxicants to the observed sublethal toxicity in C. dilutus caused by the sediments from the Guangzhou reach of the Pearl River.In addition to the identified toxicants which have log Kow value of approximately 3 through the EDA procedure, other contaminants in urban waterways in Guangzhou were also reported to contribute to acute lethality of the midges in previous studies, e.g. pyrethroid insecticides(log Kow > 6). Conversely, the fractions with relatively high hydrophobicity did not exhibit sublethal toxicity to the midges in the current study. The discrepancy between the EDA investigation in current study and the toxicity assessment in previous studies may be probably explained from the two aspects. On one hand, the two toxicants identified by the current EDA were not included in the list of the target compounds in previous studies. On the other hand, the contribution of contaminants with high hydrophobicity to sediment toxicity may be underestimated by the EDA procedure. Due to their high hydrophobicity, these contaminants tend to binding to PDMS membranes. As a result, their release from PDMS were much slower than those of the contaminants with moderate hydrophobicity in passive dosing, subsequently the possible underestimation of their toxicity contribution. Therefore, the concentrations of pyrethroids were quantified in the sediments in Guangzhou reach of the Pearl River. The relationship between chronic mortality and predicted TUs of cypermethrin to the midges were analyzed, and results showed that the chronic mortality of C. dilutus in probit form were correlated with the logarithm of chronic TUs of cypermethrin(r2 = 0.37, p < 0.01). These results suggested that the EDA incorporating passive dosing can better reflect the variation of bioavailbility of different contaminants, but more studies are required to improve the efficiency of using this technique for chemicals with high hydrophobicity. |
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