Effect Of The Hyporheic Water Exchange On The Macroinvertebrate Community Structure In The Weihe River Basin

Exchange of surface water and groundwater occurs in the hyporheic zone is called hyporheic water exchange.It plays an important role in river ecosystems health.Patterns and magnitudes of hyporheic water exchange always show the spatial-temporal variability.Additionally,effect of hyporheic water exchange on macroinvertebrate communities is also a key subject in ecological hydraulics.However,current researches mainly focused on the response of macroinvertebrates to the water chemical factors.Effect and influence mechanism of the hyporheic water exchange on macroinvertebrate community structure is still unclear.The study conducted field surveys of hyporheic water exchange used temperature gradient.Macroinvertebrates collected by Surber sampler on the Weihe River and Beiluo River in the different seasons from May 2017 to April 2018 and conduct investigation and identification in the laboratory.Sediment samples and river water samples were collected for particle size analysis and water quality measurement,respectively.The results show that:1)Magnitude of hyporheic water exchange in the Weihe River Basin is obviously different in the middle and upper reaches of the river,and pattern of hyporheic water exchange in the Weihe River Basin also has seasonal variations.Magnitude of hyporheic water exchange in the middle reaches of the Weihe River was significantly larger than that in the upstream?P=0.01?.While the magnitude of hyporheic water exchange in the upstream reaches of the Beiluo River was significantly larger than that in the middle reaches?P=0.021?.Hyporheic water exchange in winter and spring was mainly downwelling and upwelling,respectively.However,there was no consistent way of hyporheic water exchange in summer and autumn.Order of the magnitude of hyporheic water exchange flux was winter>autumn>spring>summer,yet the difference is not significant?p=0.130?.2)Pattern of hyporheic water exchange in the Weihe River Basin affects sediment particle transport and river water quality,which in turn causes differences in the macroinvertebrate community structure.downwelling is more likely to produce a rich macroinvertebrate community than upwelling.Abundance?232?and diversity?H=1.56,J=0.72,D=2.20?of macroinvertebrate community in the sites of downwelling is greater than that in the upwelling sites?Abundance=138,H=1.17,J=0.56,D=1.53?.Upwelling is more likely to cause fine sediments in the river bed to become suspended?Z_R=0.82?,which increases the proportion of swimmers?such as Baetidae?in community?43.41%?.Higher degree of the suspension,the greater number of macroinvertebrates drifting away from the riverbed,resulting in the reduction of the abundance and diversity of the community.Concentrations of NH₄-N and HCO₃^-are significantly different in river water with different patterns of hyporheic water exchange?p=0.027;p=0.049?.Upwelling controls the transportation of nitrogen nutrients from groundwater to surface water.Downwelling controls the transportation of dissolved oxygen?DO?and dissolved organic carbon?DOC?from surface water into the sediment,and conducive to the growth of Chironomidae.Additionally,both upwelling and downwelling regulate the river temperature.These all affect the growth and metabolism of macroinvertebrates.3)Magnitude of hyporheic water exchange in the Weihe River Basin determines the concentration of solutes required for the growth of macroinvertebrates in river water.Therefore,macroinvertebrate communities would be limited under the greater or lower magnitude of hyporheic flux.The optimal magnitude of hyporheic water exchange for macroinvertebrate communities in the Weihe River Basin was 150–200 mm/d.Above this rate,hyporheic water exchange provide low concentrations of oxygen and organic matter required for the respiration and metabolism of macroinvertebrates.Above this rate,high concentrations of nitrogen nutrients result in abundance and diversity of the community decline.4)In both winter and spring,magnitude of hyporheic water exchange in the Weihe River Basin has greater influence on the abundance and diversity of macroinvertebrate communities than the pattern of hyporheic water exchange.The optimal magnitude of hyporheic water exchange in winter is close to that in summer?150-200 mm/d?,which is higher than that in spring.In autumn,Upwelling could also produce abundant macroinvertebrate communities,but the optimal magnitude based on community diversity is lower than that based on community abundance.