Effects Of Bioturbations On Physicochemical Properties Of Streambed Sediments In The Hyporheic Zone

The streambed sediments is an important part of river ecosystems. The conductivity of streambed sediments could influence the interaction of water exchange between surface water and groundwater. At the same time, the streambedsediments act as the "source" and "sink" of pollutants in water bodies. When the external contaminates are effectively controlled, the pollutants enclosed in sediments will gradually become main threats to the river ecosystem health. In present research, the Chironomid larvae was selected as the research object, because it is characterized as the dominant species in the hypothetic zone of the Weihe River basin. Field experiments were conducted in the four testing sites. After the samples were collected and investigated, lab-scaleexperiments were carried out to simulate bioturbations. Subsequently, the effects of Chironomid larvae bioturbation on streambed hydraulic conductivity and the release of typical contamination including N and P have been determined by the Pearson correlation analysisand regression analysis. In addition, the bioturbations of Chironomid larvae were also investigated under the other water environmental factors including the changing water temperature and pH.The conclusions of the present study are as follow:(l)The Chironomid larvae bioturbation can significantly increase the hydraulic conductivity of the sediments. Bioturbation changes the grain size of sediment particles and redistribution by nesting and migration activities. Increasing the biological density, the weight ratio of coarse sands will increase at the depth between0～10cm, additionally, there is "U" curved type between biological density and conductivity within a certain range.(2)The Chironomid larvae bioturbation significantly increased the release of pollutants in sediments when its density was set asl.5ind./cm2and3.0ind./cm2, respectively. And there is an inverse relationship between the visibility of overlying water and the density of Chironomid larvae. Over a given period, TN’s average daily net increment decreased because of the enhancing oxygen levels induced by increased biological density in sediments, and displayed the inversion of "N" type with the biological density; Compared with the reference group, the content of TP in biological groups increased by2.2times and2.8times, respectively.(3)In a certain range of pH, the bioturbation caused by chironomid larvae are conductive to promote the release of pollutants in the sediments. The most suitable living pH for chironomid larvae is subject to greater acid environment. The releasing level sequence of TN in biological group is:weak acid environment> neutral environment> weak base environment. Compared with the reference group, the release content of TP in biological group exhibited the highest point in weak acid environment, and attained3.3times than that in the control group. However, the content in neutral environment and weak base environment is just increased by1.1times and0.6times, respectively.(4) The chironomid larvae is appeared relatively lower activity under the environmental condition where the temperature is below15Celsius Degree(℃), and showed a special status which is called"suspended animation". Meanwhile, there was no significant promoting effecton the release of TN in the sediments by increasing the temperature. When the temperature was beyondthe critical temperature around15℃, the total nitrogen releases were enhanced because of the increasing activities of chironomid larvae in the sediments.