Effects Of Tubificid Bioturbation On Sediment Structure And Solute Transport

The hyporheic zone is the spatial fluctuation cross zone between river water and shallow groundwater.It is an important area for the two-way exchange of material and energy.It is an important component of river ecosystem.Water and solute exchange through the hyporheic zone.The structure and permeability of sediments in the hyporheic zone significantly affect the transmission of solute,energy and information.The hyporheic zone plays an important role in maintaining the health of river ecosystem and ensuring river ecological function.There are abundant species and quantities of macroinvertebrates living in the hyporheic zone.Bioturbation can change the sediment structure and particle size composition.Bioturbation plays an important role in the exchange of solute,water and energy in the hyporheic zone exchange.In this paper,the Tubificid worm is taken as the experimental object,and the microcosm model of hyporheic zone is constructed indoors.The relationship between the transformation degree of Tubificid worm on sediment and biological density is studied,and the variation characteristics of biological survival state with time are observed.Through the model simulation of solute penetration experimental data,the influence of disturbance action on solute transport and sediment permeability is revealed,the vertical water content of sediment is measured,and the transformation effect of different density on each depth of sediment is studied.The results show that:（1）The effects of Tubificid disturbance with different density on sediment structure were clarified.Through burrowing,feeding,transportation and excretion,Tubificid worm have a significant impact on the sediment structure.The sediment height in each biological experimental group decreased,and the higher the biological density,the greater the decline.Tubificid worm feed on organic particles deep in the sediment,which are biologically broken,digested and decomposed and transported to the surface of the sediment through biological transport.The accumulation area of flocculent particles on the surface increases with the increase of biological density.The upper particles of the sediment are fully absorbed by organisms to form an oxide layer,and the thickness of this layer also increases with the increase of biological density.（2）The vertical quantitative distribution of Tubificid with different densities was analyzed.The pressure of survival competition,the acquisition of food and the uneven vertical distribution of dissolved oxygen lead to different quantitative distribution laws of Tubificid worm with different densities in sediments.From the mud water interface down,the number of organisms first increased and then decreased,and the maximum survival depth deepened with the increase of density（p<0.01）.The biomass increased during the experiment,but the increase was small.（3）The two-region-model can well fit the solute penetration curve of sodium chloride.R² is greater than 0.99 and MSE is close to 0.By analyzing the variation trend of dispersion coefficient and retardation factor with density and time,it is found that when the biological density is 2 ind·cm^-2～3 ind·cm^-2,the permeability of sediment can be increased in a short time,and density 4 reduces the permeability.This is because of the biological blocking effect.For a long time,the biological density will form a fine particle blocking layer in the sediment regardless of the high or low biological density,which reduces the permeability of sediment.（4）The effect of Tubificid disturbance with different density on the vertical water content of sediments was studied.Bioturbation affects the water content of sediments at different depths by changing the particle size of sediments and the movement of fine particles in sediments by interstitial water.Some biological broken fine particles migrate to the lower part of the sediment with the interstitial water until a layer of fine particle plugging layer is formed.The position of this layer is deeper with the increase of density.