Effects Of Tubificid Bioturbation On The Physical And Chemical Characteristics Of Water-sediment Interface

Sediment is an important part of natural water body. The physical and chemical changes of water-sediment system have significant influences on water quality, nutritional composition, transformation of contaminants, etc. There are a large amount of water benthoses at the water-sediment interface. They affect the physical and chemical characteristics of water-sediment system directly or indirectly. And their bioturbation, including feeding, excreting, digging, breathing, etc., significantly affects the behaviors of contaminants in aquatic environments. Therefore, bioturbation is one of key factors of water quality and behaviors of contaminants in aquatic environments. So, bioturbation has received widespread attention. Bioturbation is considered as an important content in the research of water pollution, nutritional composition of water, sediment biogeochemical characteristics, aquatic ecosystems, etc.In a natural aquatic environment, the bioturbation of the benthic oligochaeta, tubificid, has a representation. Tubificids are widely distributed bioturbators in freshwaters. Tubificids have a high density, although the individual organism is small. The bioturbation of tubificid will significantly affects the physical and chemical characteristics of water-sediment system and behaviors of contaminants in aquatic environments. At present, the effects of tubificid bioturbation on the migration and transformation of some contaminants have been clear basically. But the mechanisms are largely unknown yet. In this thesis, the tubificids are selected as the typical bioturbator in freshwaters. The research includes following aspects.Effects of tubificid bioturbation on burrows and macropore structure in sediment were studied to understand the influences of tubificid bioturbation on internal physical structures of sediment. A microcosm simulation was built in the lab. The distribution and variation of the tubificid burrows and the macropores in the sediment were investigated by X-ray computed tomography(CT) and digital image collecting, without sampling or disturbing the sediment. The the effects of tubificid bioturbation on the distribution and variation of burrows and macropore structure in heavy metal contaminated sediment were investigated by adding artificially contaminated sediment to the experimental units. The results indicate that after the addition of the tubificids, both the burrows and the macropores in the sediments increased with time, and the rate of increase slowed gradually. With the increased worm density, the burrows and the pore structures also increased. The in-depth distribution of the burrows and macropores was determined by the settlement time of the worms: with the settlement time increasing from 3 d to 19 d, the depth of the zone with the highest density of burrows and macropores increased from 0-30 mm to 30-50 mm and from 0-10 mm to 30-60 mm, respectively. The distribution of the burrows and macropores is closely related to the distribution of the tubificids. Thickening of the oxidized zones in the superficial sediments in the presence of tubificid bioturbation was also observed. The macropores increased in the heavy metal contaminated sediment. Among the individually contaminated sediment with Cu, Cd, and Pb, Cu had the strongest influence on bioturbation of tubificid. And the effect of joint action of three heavy metals was more significant than individually contaminated.To investigate the effect of tubificid on the exchange interaction in water-sediment system, the migration of the sediment particles was also determined using CT by adding BaSO4 to the sediment as a tracer. And the effects of tubificid bioturbation on the distribution and migration of contaminants in the sediment were verified by adding contaminated sediment layers to the sediment. The results indicate that the main action of tubificids on the sediment particles was the transport of particles from the inner sediment(especially in the range of 30-50 mm in depth) to the water-sediment interface. The migration of Pb and Cd in the contaminated sediment with tubificid bioturbation could be interpreted by the migration of sediment particles. When the top layer of the sediment was contaminated, the transport of uncontaminated sediment particles from lower layers to the water-sediment interface led to the downward migration of the contaminated layer. When the contaminated layer was added to a medium depth(e.g., 20-30 or 40-50 mm), the contaminated particles were transported to the water-sediment interface and increased the heavy metals concentration in the new top layer. In unit time, the migration of heavy metal contaminants increased with density of tubificid. But it was disproportionate between them.The variation of some chemical characteristics of microenvironment near the tubificids burrows was investigated to understand the effects of tubificid on chemical composition of water-sediment system. High-resolution distribution and variation of pH, DO and redox potential in the microenvironment of sediment near the tubificids burrows were investigated using microelectrodes by laboratory simulation experiments. The results indicate that the pH of overlying water and sediment increased with time at first, and then decreased, and stabilized at last. The bioturbation of tubificids had an inhibitory effect on this decrease of pH value. At the same bioturbation time, surface pH of the sediment around the tubificid burrows declined(up to 0.3 pH units) in the horizontal direction. This effect of declining could affect a range of 1 mm radius around the burrows. The change of pH in tubificids’ burrows was smaller in the vertical direction at the same bioturbation time. Without tubificid, DO level and redox potential of overlying water were stable. But DO level and redox potential of the microenvironment near water-sediment interface decreased with time. The DO level and redox potential of overlying water decreased with the presence of tubificids. The bioturbation of tubificids had an inhibitory effect on this decrease of DO level and redox potential of sediment surface near burrows. Compared with the system without tubificid, the DO level near the water-sediment interface around the tubificid burrows increased(increased by 9.2-17.0 μmol/L) in the horizontal direction. The penetration depth of DO also increased, and the maximum penetration depth increased from 3 mm to 5 mm. The variation of redox potential was similar to DO level. Therefore, the bioturbation of tubificids would affect the behaviors of contaminants in the water-sediment interface by changing the distribution and variation pattern of pH, DO and redox potential of the sediment microenvironment.The effects of DOM solution produced by tubificids on the adsorption of heavy metals in sediment were evaluated to investigate the biochemical action of tubificids on water-sediment system. The tubificid samples were cultured in laboratory to obtain the DOM solution which was produced by tubificids. Measure and analyze DOM solution at different culture time with UV-visible absorption spectroscopy, three-dimensional fluorescence and synchronous fluorescence. And adsorption kinetics and thermodynamic tests were carried out with the presence of DOM solution. The results indicate that DOM solution might contain aromatic amino acids. Tryptophan and tyrosine might be the main kinds of aromatic amino acids. The concentration of DOM in the solution increased with settlement time of the tubificids. The DOM solution produced by tubificids had an inhibitory effect on adsorption of Pb and Cd onto sediment particles. And it was more obvious for the inhibition of Pb adsorption.