Effect Of Bioturbation By Macroinvertebrates On The Transport Of Heavy Metal Copper In The Hyporheic Zone Of The Weihe River

Macroinvertebrate is an important component of hyporheic zone in river ecosystem.Bioturbation has a significant influence on the ecological process and biogeochemical cycle of pollutants in the hyporheic zone.In this study,the Weihe River and its tributaries,the Jinghe River and Beiluo River in the Weihe River Basin were selected to conduct the field macroinvertebrate survey in May 2017.Based on the investigation and identification,the community structure and diversity of macroinvertebrates in the Weihe River Basin were determined.Two dominant species belong to different functional groups,the Limnodrilus hoffmeisteri and Chironomid larvae,were chosen for the 21-day microcosmic simulation experiment.The effect of the two species in single and combination on the sediment structure and property of overlying water were analyzed.The influence of bioturbation by the Limnodrilus hoffmeisteri and Chironomid larvae on Cu concentration and flux and its transport in overlying water and pore water were analyzed.And reveal the mechanism of specific bioturbation modes on the transport of heavy metal in overlying water and pore water.The conclusions were summarized in this following:1)Macroinvevrtebrates in the Weihe River Basin were abundant,and the community composition and spatial distribution was different in these tributaries.A total of 135 species were collected and identified,belonging to 57 families,17 orders,6 classes,3 phylum,including annelida,mollusca and arthropoda.Among them,the number of species in the phylum arthropoda was the largest,accounting for 88.9% of the total species,and mainly composed by aquatic insects.The Weihe River mainstream had the most species with a total of 91 species,followed by 85 species in the Jinghe River and 44 species in the Beiluo River.The alpha diversity index in the Jinghe River was the highest,followed by the Beiluo River and Weihe River mainstream.The beta diversity index(βsor)was highest in the Weihe River mainstream(0.92),followed by the Jinghe River(0.91)and Beiluo River(0.83).The total number of species in the Weihe River Basin was large;however,the number of dominant species was small.The worm Limnodrilus hoffmeisteri and Chironomid larvae were the most dominant species and widely distribute in the basin.2)The microcosmic simulation experiment reflected that the water content,pH value and DO content were influenced by bioturbation.Bioturbation increased the sediment water content.Different species and densities lead to different changes.The variation of water content was more obvious in worm treatments than larvae treatments,and highest in mixed treatments.Water content was srtongly affected by density,and higher in high density treatments than medium and low density treatments,and control treatment without organism was the lowest.The pH values in control treatment higher than bioturbation treatments.It increased then decreased and tends to stable with sampling time in the bioturbation treatments,which might relate to the bioturbaiton intensity.DO content have variation in each treatment and slightly varied in the late period in control treatment.The ventilation of burrows caused by bioturbation promotes the decrease of DO content.The pH values and DO content were significant difference in worm and larvae treatments and that related to specific bitourbation mode.3)Based on the simulation experiment,bioturbation by worm and larvae in single and combination both promoted the release of Cu from sediment to water and increased Cu concentration in overlying water and pore water.Different macroinvertebrates have different effects on Cu concentration in overlying water and in pore water with different densities and depths.The Cu concentration in overlying water in worm treatments and worm-dominant treatment had more varied trend than other treatments.It may result from the continually burrowing and conveying activity of worms that enhanced the release of Cu.The variation in physiochemical properties of overlying water can also affect the Cu concentration.High density treatments had the most obvious increasing Cu concentration under the strongest bioturbation.The Cu concentration in worm treatment was positive related to its density,while the variation in Cu concentration in pore water in larvae and mixed treatment was positively correlated with the density.It may result from the interaction of organisms that increased particle movement lead to the release of metal.The bioconveying of worms and bioirrigation of larvae increased the release of Cu,while have different effect on Cu transport.The Cu concentration in worm treatment was decreased with sediment depth,which can be related to the weak bioturbation.The intermittent of larvae bioturbation caused not significant variation in depth of Cu concentration.Also,worms and larvaes interact each other and their competition that enhance the Cu redistribution.4)The simulation experiment revealed that bioturbation increased the Cu diffusive flux and the magnitudes were positively correlated with density in worm and larvae treatments,and caused different diffusive direction.The Cu flux in worm treatments showed upward from pore water to overlying water that aggravate river contamination.While Cu flux in larvae treatments was from overlying water to pore water.The flux in mixed treatments was the same as the larva treatment,and the magnitude lower than high density treatment.It potentially results from the larvae inhabited the bioconveying of worms.The worm and larvae have different bioturbation modes that can support the heavy metal prevention in the hyporheic zone.