Effects Of Bioturbation By Urechis Unicinctus On Key Process Of Sediment Nitrogen Cycle In Coastal Aquaculture Area

As one of the important pillar industries in coastal areas,marine aquaculture,playing an important role in marine fishery,has developed rapidly in recent years.However,disordered and high-density farming activities have also caused some adverse impacts on the offshore ecological environment,such as accumulation of organic matter in sediments and changes in the structure of biological communities.Integrated multi-trophic aquaculture（IMTA）,through the scientific integration of different trophic organisms,not only achieves better economic benefits,but also maintains a good water ecological environment.IMTA provides an effective solution for realizing the synergy between large-scale mariculture activities and ecological environmental protection.However,the efficiency of existing IMTA system still limited by low cascading efficiency,especially lack of suitable species for benthic functional groups.Therefore,there is an urgent need to explore new benthic species to further enhance the function of the system.In this study,Urechis unicinctus,which has high economic and nutritional value and strong adaptability to the environment,was used as a model organism to study the impact of bioturbation on the environment,in order to clarify the impact and mechanism of U.unicinctus bioturbation on the process of sediment nitrogen cycling,and explore its ecological effects on benthic environments.The research results will provide important theoretical basis and data support for combing the U.unicinctus as a benthic functional group species into the IMTA system.The main findings are as following:1.The effects of slow and sudden temperature changes on the physiological metabolism of U.unicinctus.Using the temperature control experiment of circulating water tank in the laboratory,four temperature gradients（5°C,10°C,15°C,20°C）were set up to study the effects of different temperature treatments on the oxygen consumption rate and ammonia excretion rate of U.unicinctus.At the same time,the changes of superoxide dismutase（SOD）,catalase activities（CAT）and malondialdehyde（MDA）content of U.unicinctus under sudden temperature change were analyzed.At 5℃,10℃,15℃,and 20℃,the ammonia excretion rates of U.unicinctus were 0.17,0.05,0.06,and 0.14 umol/h/g,respectively.The results of related enzyme activities showed that the activity of CAT in the temperature sudden change group reached the highest at 3h～6h,and the enzyme activity was negatively correlated with the increase of time.The MDA content of the 5℃sudden change group reached the highest at 3h,and the related enzyme activities were generally lower.The abrupt changes in temperature did not significantly affect the physiological metabolism and biochemical activities of U.unicinctus.2.The effect of U.unicinctus bioturbation on the vertical distribution of sediment particles.Using glass beads with a diameter of 0.5～0.6 mm as the tracer particles,studying the disturbance of sediment particles by the U.unicinctus.The U.unicinctus is divided into two sizes,small size（10～15 mg/tail）and large size（1.5～1.8 mg/tail）.The results of the distribution of tracer particles in the surface sediments at different depths showed that the migration depth was significantly corelated with the size of U.unicinctus,and the number of tracer particles decreased significantly with the increase of the migration depth.The results of tracer particles at a depth of 7 cm of the sediment showed that,after perturbation,upward and downward migration occurred in the vertical direction.There was a good correlation between the size of the U.unicinctus and the depth of the downward migration,and a negative correlation with the upward migration.3.The effect of U.unicinctus bioturbation on nutrient diffusion flux at different temperatures.The effect of bioturbation on the nutrient diffusion flux at different temperatures was studied.The results showed that compared with the control group,with the increase of temperature,the NH₄+diffusion flux in the treatment group increased by 97%,47%,and55%,respectively;the total dissolved inorganic nitrogen（DIN）diffusion flux in the treatment group increased,respectively.40%,46%,55%,DIN at different temperatures is positively correlated with increasing temperature.The NH₄+diffusion flux in all treatment groups was manifested as diffusion from the sediment to the overlying water as the source of the sediment-water interface.Ammonia excretion of U.unicinctus accounts for 2%,1%,and 5%of the NH₄+diffusion flux at 5,10,and 15℃,respectively.The promotion of the NH₄+diffusion flux is mainly due to the Biodisturbance of U.unicinctus.4.The effect of bioturbation U.unicinctus of different densities on the nutrient diffusion flux.The effect and persistence of the bioturbation of U.unicinctus on the diffusion flux of nitrogen and phosphorus from the sediment-water interface were investigated by laboratory ecological methods.The results showed that the diffusion flux of dissolved inorganic nitrogen（DIN）in the treatment group varied from 10.6 to 765.3μmol/（m²·d）.Report to the control group,DIN flux in the low,medium and high treatment groups They have increased by 57%,76%,and 88%,respectively.NH₄+was the main contributor to DIN,and its contribution to DIN accounted for 55%,65%,and 80%in the low,medium,and high treatment groups,respectively.Compared with the control group,the average NH₄+fluxes in the low,medium and high-density groups were increased by 39%,111%and 257%,respectively.The treatment groups increased by 43.7%and 23.6%,respectively.From day 2 to day 10,the NH₄+flux was in a continuous increasing trend.The PO₄^3-flux in the treatment group varied from-7.85 to 6.42μmol/（m²·d）,and the PO₄^3-flux in the treatment group continued to grow from day 2 to 6.The bioturbation of U.unicinctus can continuously promote the diffusion of DIN from sediment into water,and there is an obvious density effect.5.The effect of U.unicinctus bioturbation on the microbial community structure of sediments.In order to explore the influence of the bioturbation of U.unicinctus on the structure of the sediment microbial community,the microbial community in the sediment was analyzed by metagenomics technology.The results showed that:Proteobacteria accounted for more than 46%of the total,and were the dominant bacteria in sediments.The low-density treatment and the control group shared 5513 species,62 unique species,an increase of 279 families,and high the density treatment group and the control group had a total of 5460 families,131 families were unique,and 302 families were added.Ammonia-oxidizing bacteria,ammonia-oxidizing archaea（Nitrosopumilus）,and nitrite-oxidizing bacteria（Nitrospina）were found in all samples.Compared with the control group,the relative abundance of ammonia-oxidizing bacteria（AOB）in the treatment group was significantly decreased（P<0.05）,the ammonia-oxidizing archaea（AOA）was significantly increased（P<0.05）,and the nitrite-oxidizing bacteria（NOB）was almost There was no change,and functional genes related to nitrogen cycling processes were most abundant in the high-density treatment group.The bioturbation of U.unicinctus makes the microbial community structure of the sediment more complex,and the research results provide basic data for in-depth understanding of the bioturbation mechanism of U.unicinctus.