Changes Of Nitrogen And Phosphorus Nutrients And Their Microbial Mediated Effect In Estuarine And Offshore Sediments

Nitrogen and phosphorus pollution is the main factor to induce water quality safety problems in rivers,lakes and oceans.The damage of excessive nitrogen and phosphorus to river and marine ecosystem is directly related to the ecological stoichiometric balance of aquatic ecosystem,which leads to water eutrophication and other water environmental problems.As the sink and source of nitrogen and phosphorus pollution,sediment is closely related to the concentration and form of nitrogen and phosphorus nutrients in water body,and the microorganisms related to nitrogen and phosphorus cycle in sediment play a decisive role in the migration and transformation of nitrogen and phosphorus nutrients.Estuarine and offshore areas are the main places for pollutants to gather.To Study the migration and variation of nitrogen and phosphorus nutrients in estuarine and offshore areas and their microbial driving role is the basis for understanding the input dynamics of pollutants and conducting pollution risk assessment.However,the mediated effect of microbial of nitrogen and phosphorus pollutants in estuarine and offshore sediments is still unclear.In this study,the sediments of Duliujian River Estuary and offshore waters in Tianjin were selected as the research objects,and the distribution characteristics of nitrogen and phosphorus nutrients in the sediments were determined and analyzed.Then,the microbial driving mechanism and environmental impact factors of nitrogen and phosphorus nutrients were discussed in combination with the bacterial flora,physical and chemical properties,functional groups and enzyme activities in the sediments.The results can provide some data and theoretical basis for the management department to track the pollutants on the watershed scale and divide the pollution risk areas and control areas.The main results are as follows:(1)The basic physicochemical properties and the concentrations of nitrogen and phosphorus nutrients in estuarine and offshore sediments changed to some extent,but the difference was related to the specific location of sampling points.The contents of MC,salinity,TN,NH₃-N and silt in offshore sediments were significantly higher than those in estuaries,but the contents of TP were significantly lower than those in estuaries.Other indexes such as p H,OM,AP,NO₂^--N and NO₃^--N were not significantly different between estuaries and offshore sediments.Correlation analysis showed that MC,salinity and particle size were important factors affecting nitrogen and phosphorus nutrient concentration.(2)The Shannon index and Simpson index of microorganisms in estuarine sediments were significantly higher than those in offshore areas.The bacterial diversity in estuarine sediments was higher than that in offshore areas,but there was no significant difference in ACE and Chao1.At the level of phylum,family and genus,the composition of bacterial community structure was abundant,and the relative abundance was significantly different between estuarine and offshore areas.Proteobacteria is the dominant phylum in estuarine and offshore sediments.The high abundance of Proteobacteria may be closely related to nitrogen and phosphorus cycling.The relative abundances of AAM,LM and MOAA in the estuary were significantly higher than those in the offshore,while the relative abundances of EM,EF and NM in the offshore were significantly higher than those in the estuary.(3)Only the relative peak area of carboxyl O-H,ketone-c,carboxylate ions C=O,aromatic compound C=C and C-H,amides C-N,aliphatic C-H,phenolic O-H and C-O,CO₃^2-in the estuary was significantly higher than that in the offshore sediments.The URE activity in the estuary was significantly higher than that in the offshore sediments,but the AKP activity was not significantly different.RDA analysis showed that salinity,MC,URE and carboxylate ions C=O,aromatic compounds C=C and C-H,amides C-N,aliphatic C-H,phenols O-H and C-O,CO₃^2-were the most influential factors on the bacterial community structure at the phylum level.As a dominant phylum,Proteobacteria is highly correlated with salinity and MC.Salinity,MC,silt,URE and sand could explain the abundance of bacterial functional groups significantly.(4)NH₃-N,TP,NO₃^--N,AP and NO₂^--N play an important role in the distribution of bacterial community structure at phylum level.NH₃-N and NO₃^--N were positively correlated with Proteobacteria and Fusobacteria,NO₂^--N was positively correlated with Firmicutes,TP was positively correlated with Actinobacteria.TN,TP,NH₃-N,NO₂^--N and NO₃^--N play an important role in the distribution of bacterial community structure at genus level.TN,NH₃-N and NO₃^--N were positively correlated with Photobacterium,while TP was negatively correlated with Photobacterium.The effects of NH₃-N,TP,NO₃^--N and TN on bacterial functional groups were significant,and the effects of NH₃-N and TP were extremely significant.(5)The results of single factor standard index method showed that TP pollution was more serious at most points in the estuary and TN pollution was more serious in the offshore sediments.The results of comprehensive pollution index show that the estuarine sediments are in the range of clean to moderate pollution,and the offshore sediments are in the range of light to moderate pollution.Comprehensive analysis shows that the comprehensive pollution of the offshore sediment is relatively serious.