Effects Of Tubificid Bioturbation On Distribution Of PH And DO In Freshwater Sediment

To determine the distribution and variation of pH and DO （Dissolved Oxygen） inmicro-environment of fresh water/sediment systems with the presence of bioturbationvisually, and furthermore to investigate the effects of bioturbation on transfer andtransformation of heavy metals in aquatic environments, in this study, a real-timetwo-dimensional pH and DO monitoring system based on planar optode and a methodused to acquire the pH and DO isograms were developed. The monitoring system andthe method were applied to a simulated freshwater/sediment system, to monitor andinvestigate the spatial and temporal distribution of pH and DO in the system withtubificids used as bioturbators.Two pH-sensitive excitation bands, at404and453nm, were observed for theoptode, and the fluorescence intensity at511nm （the maximum emission） decreasedwith the increasing of pH when the optode was excited at404nm while increasedwith pH when excited at453nm. The optode has such advantages as widepH-sensitive range （6.2⁸.6）, rapid response （<30s）, high precision, and goodreversibility and stability. Both the monitoring system based on the optode and themethod used to obtain pH isograms, by calculating the differences of fluorescenceimages obtained at the two excitation wavelengths, could be used to monitor thespatial and temporal distribution of pH in the freshwater/sediment systemsuccessfully.According to the monitoring, the spatial and temporal variation of pH in thebioturbation and control sediment was small because of the ability of acid-base buffer,but the sediment had significant effects on overlying water pH. Comparative analysisof bioturbation and control freshwater/sediment systems, overlying water pH ofbioturbation had significant and dynamic conversions. This change was the combinedresult of a variety of mechanisms, and bioturbation affectd these mechanisms.Ethyl cellulose polymer was dissolved by toluene/ethanol (V_toluene: V_ethanol=4:1),which was suitable for the physical embedded tris （4,7-diphenyl-1,10-phenanthroline）ruthenium dichloride, superior to the polymer of polystyrene and toluene.Experimental results showed that emission fluorescence intensity was decreased withthe increase of the concentration of dissolved oxygen according to the Stern-Volmerequation of the fluorescence quenching mechanism. The results indicated that it had such advantages as wide DO-sensitive range （0～20mg/L）, rapid response （<60s）,high precision, and good reversibility and stability, and it could be applied in thefreshwater/sediment microcosm to show the spatial and temporal distribution of DOvisually.According to the monitoring, the similarity of bioturbation and controlfreshwater/sediment systems: obvious gradient of DO in the overlying water andsediment interface was found. Different point: about a week of bioturbation, DOconcentration gradient in the overlying water/sediment interface was slowed.Bioturbation had a greater impact on DO of ground floor of the overlying water. Therole of bioirrigation reconstructed the sediment surface and increased the oxygencontent of the deep sediments. And increase of DO concentration in sedimentmicroenvironment affected the release and shackles process of reducing the heavymetal pollutants in the sediments. Resuspension of sediment particles in theunderlying overlying water caused reoxidation and physical chemical adsorptionprocess, resulting that underlying overlying water DO of bioturbation was lower thanthe same period of control component. Significant and dynamic changes of DOdistribution pattern in overlying water were observed, which was the combined effectsof several actions, and these actions were all affected by the presence of bioturbation.