Bioturbation By The Crab(Macrophthalmus Japonicus) And Clam Worm(Perinereis Aibuhitensis) Affects Dynamics Of Biogenic Elements In Estuarine Wetland

The effects of benthic macroinvertebrate on biogenic elements igration and transformation of the estuarine wetlands cannot be ignored.In this study,an indoor experiment was carried out with in-situ substrate,overlying water and living specimen from estuary,in order to investigate the bioturbation of the crab?Macrophthalmus japonicus?and clam worm?Perinereis aibuhitensis?on the migration and transformation of biogenic elements.We explored the mechanism of bioturbation on biogenic elements of wetland in the estuary by monitoring 16 chemical variables:?the sediment oxygen consumption?SOC?,fluorescence characteristics of colored dissolved organic matter?CDOM??,5 in pore water?DOC,ammonia nitrogen?NH₃-N?,nitrous nitrogen?NO₂^--N?,nitrate nitrogen?NO₃^--N?,dissolved total phosphorus?DTP??,4 in sediment?total organic carbon?TOC?,total nitrogen?T-N?,total phosphorus?T-P?and microbial activity?MBA??as well as flux of sediment-water interface?DOC,NH₃-N,NO₃^--N,NO₂^--N,DTP?.The main conclusions are as follows:The SOC rate increased with the bioturbation of two benthic macroinvertebrates.The SOC rate of the crab group was always higher than that of the clam worm group,which should be caused by the fact that the crab could promote more bottom sediment oxidation and the higher MBA content in the sediments of crab group.The change of SOC rate was an important factor for the bioturbation to affect the dynamic of biogenic factors.Benthic macroinvertebrate promoted DOM mineralization in sediments to generate CDOM into overlying water.Since the Dagu estuary was obviously affected by land sources,UV humic-like substance was the main component released.The results showed that the fluorescence peak A of crab and clam worm groups showed red shift on the excitation-emission matrix fluorescence spectroscopy,and the average content of CDOM in the overlying water of crab and clam worm groups could reach 1.32 and 1.24 times of the control group,respectively.The bioturbation changed the vertical distribution structure of DOC,NH₃-N,NO₂^--N,NO₃^--N and DTP in pore water.On the 28th day of the experiment,the DOC content in the pore water of each group was different.The DOC content of the crab group increased first,with an average content of 42.88 mg L^-1.On the 35th day of experiment,the content of DOC in the clam worm group gradually increased and exceeded the crab group,with an average content of 46.84 mg L^-1.Compared with the control group,the bioturbation increased the contents of NH₃-N,NO₂^--N,and NO₃^--N in surface pore water?0-2 cm?by up to 120%,982%,and 114%,respectively.The bioturbation and cave structure of the clam worm were more conducive to the entry and distribution of highly concentration NO₂^--N into the deep sediments and pore water.The effects of bioturbation on DTP content of different depths was significant.Bioturbation increased the DTP content in 1cm depth to 18%,but decreased the DTP content in 2-4 cm depth to 18%.The decrease of the DTP content may be related to the good adsorption of DTP by Fe?III?produced by the oxidation of Fe?II?.Bioturbation promoted the release flux of DOC,NH₃-N,NO₂^--N,NO₃^--N and DTP at the sediment-water interface.In the early stage of colonization,the contribution of clam worm to DOC release was less than that of crab,but the gap between the two decreased with time.This may be because the bioturbation of crab was concentrated in the early stage of colonization,while the scale of clam worm caves was formed in the late stage of colonization.Since the nitrification rate in the clam worm cave and the control group did not support the net release of NO₂^--N from the sediment,the flux data at the initial colonization?day 15?showed that the sediment absorbed NO₂^--N.It was proved that the bioturbation of crab was more conducive to the nitrification.For NO₃^--N,the crab and clam worm groups always showed release from sediment,while the control group always absorbed NO₃^--N.This was related to the lower nitrification rate in the control group.The clam worm and crab reduced the TOC?p<0.05?,T-P?p<0.01?and T-N?p<0.05?crab?;p=0.08?clam worm??content in surface sediments?0-2 cm?.Crab and clam worm significantly increased MBA content at 2-4 cm sediment?p<0.05?.The higher content of MBA in the sediments of animal group may be related to the increase of surface area?cave formation?,intestinal microflora and feces.In conclusion,the biogeochemical processes in the sediment were changed by the clam worm and crab,which promoted the DOC,CDOM,NH₃-N,NO₂^--N,NO₃^--N,DTP in the surface sediments to the pore water and overlying water,and reduced the contents of TOC,T-N and T-P in the surface sediment.Therefore,clam worm and crab as the key components of the estuarine ecosystem,play an important role in regulating and changing the dynamics of biogenic elements in estuarine wetland.