| Polycyclic aromatic hydrocarbons (PAHs) comprise a large class of widespread environmental contaminants. These compounds are widely distributed in the environment, notably in sediments of estuaries and coastal marine areas throughout the world. Due to their carcinogenic potential, there is great interest in their fate in the environment in recent years.Due to the range of hydrodynamic and chemical conditions, estuaries have high biodiversity and intense biological activity. The feeding, burrowing, and reworking of benthic invertebrates, collectively termed bioturbation, are expected to modify the physical, chemical, and biological properties of the sediment. As a result, the fate of the contaminants in sediment should be modified. The purpose of the present study was to determine whether PAH fate would be influenced by bioturbation in the sediment through both field investigation and laboratory microcosm experiments.Firstly, the bioturbation of Helice tientsinensis, Nereis diversicolor, Bullacta exarata, and Cyclina sinensis on sediment from the Beitang Estuary (Tianjin, China) was studied quantitatively by the microcosm tracer technique. The results showed that the distribution of sediment was significantly affected by bioturbation. The bioturbation capacity of different macrofauna varied a lot, with diffusion coefficient (10-3 cm2/d) in an order of N. diversicolor (2.95)> H. tientsinensis (1.00)> C. sinensis (0.78)> B. exarata. (0.35)> control (0.05) in surface sediment, while in deep sediment, the order was H. tientsinensis (3.10)> N. diversicolor (2.33)> C. sinensis (0.28)> B. exarata. (0.15)> control (0.05). These differences were mainly caused by that the macrofauna belonged to different functional groups. Correlation analysis revealed that the biovolume was significant correlated to the diffusion coefficient in surface sediment (p< 0.05), indicating that the biovolume of macrofauna could be used to predict their bioturbation intensity in sediment.Bioturbation by the burrowing crab H. tiensinensis was investigated to determine its impact on PAH-contaminated estuarine sediments in a field investigation at Beitang Estuary. The concentrations of 16 USEPA priority PAHs in sediment and porewater from a crab bed (including surface and burrow samples) and a control area, as well as in crabs were measured. The total concentration of the 16 USEPA priority PAHs in surface sediment of the crab bed (average 2772 ng/g dry weight) was significantly higher than in the control area (1173 ng/g dry weight). In the crab bed, the total concentration of PAHs in burrow sediment (1239 ng/g dry weight) was lower than in surface sediment, and similar trend was found for most of individual PAHs except for InP and BgP. In order to further clarify the fate of PAHs in burrow, the mass of PAHs in the burrow as compared to that in the surface sediment was examined. The results shown that the mass difference was calculated to be< 0 to 73% for individual PAHs and 50% for the total PAHs. The enhanced PAH desorption in the burrow, which could be attributed to the increase in dissolved organic matter in porewater as well as the mechanic mixing by the crab, is expected to increase PAH flux to sea. Furthermore, the stimulated microbial degradation was proposed as an ignorable factor for the lower the PAH concentration in burrow sediment because crab bioturbation could increase the abundance and activity of microorganisms by several ways. In addition, H. tientsinensis incorporated a total PAHs concentration of 8816 ng/g lipid in their body, this may be another reason for the lower PAH concentration in burrow sediment. This field investigation indicated that crab bioturbation influences PAH fate in estuarine sediments.In order to verify the conclusion of the field study, the effects of crab bioturbation on PAHs release from the sediment were studied in laboratory microcosms. The results shown that sediment resuspension was enhanced by the bioturbation. Due to sediment resuspension, both particulate and dissolved PAHs increased into the water column. Moreover, dissolved organic carbon (DOC) in overlying water was elevated by the bioturbation of crab. The DOC has been confirmed to enhance the distribution of PAHs from sediment to water. As a consequence, obvious desorption of PAHs from suspended particles and sediment to water occurred. This supports the conclusion of the field study.Once the PAHs is released from sediment to overlying water, they will be uptaken by aquatic organisms, then pose potential risks to fish and to humans and wildlife that consume aquatic organisms. The impact on PAHs bioavailability in the water column of bioturbation was studied by semipermeable membrane device (SPMD) through laboratory experiments. During 119 d test, all the individual PAHs in SPMD in the microcosm with crab showed higher concentration than that without crab. However, the availability of the individual PAHs was different between small-intermediate and large ringed PAHs based on the SPMD-sediment accumulation factor. For the small-intermediate ringed PAHs, the SSAF value was higher in treatment with crab than without crab, while there was an inverse relationship for the large ringed PAHs. It indicates that the bioavailability was enhanced for small-intermediate ringed PAHs by crab bioturbation, and the large ringed PAHs were not significantly affected. This is because large ringed PAHs associated to DOC was not available.The effects of crab bioturbation on PAH biodegradation from sediment were studied in microcosm. The results showed that the crab bioturbation enhanced the organic matter and fine particles in the sediment by the egesta, stimulating microbial activity. Thus, PAH biodegradation was enhanced with biodegradation rate of some PAhs increasing by 3.3-4 times by the crab bioturbation. Biodegradation enhancement occurred mainly for small-intermediate ringed PAHs. Additionally, some of large ringed PAHs were degraded by comebolism. The experiment in microcosm verified the conclusion of the field that biotubance enhanced biodegradation. Furthermore, the result showed the the most important impact of crab bioturbation on the environmental behavior of the PAHs in sediment is the PAHs were enhaned release fome sediment to water.This dissertation could provide some useful information on bioturbation influences on the PAH fate in estuarine surface sediments. It is very important to the evaluation the ecological risk of the PAHs transfer by the bioturbation. |
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