Study On Ecosystem Assessment Method Of Large Rivers

The estuary area of large rivers is one of the most densely populated areas,which is characterized by complex hydrodynamic force and aquatic ecology.The estuary area have witnessed rapid economic development,and frequent human activities,landsource pollutant discharge,offshore aquaculture and engineering construction,which have exerted an important impact on the area.At present,there are some prominent problems in the environmental management of large rivers estuary,such as the lack of ecological evaluation and the inapplicability of estuarine water quality evaluation standards and methods.Two sets of standards of surface water and seawater are often used for evaluation in management.The two sets of standards have a big difference in assessment indicators and threshold,which leads to the assessment results are not objective and difficult to reflect the real water quality of estuarine,and also leads to the difficulty of estuary administration.It may even lead local governments to incorrect protection measures.Therefore,it is an urgent task to delineate the estuarine range,formulate estuarine water quality standard and assessment indicators system.This paper focuses on the Yangtze River estuary region.Firstly,biological and physical factors are used to divide the scope of the Yangtze River estuary,and on the basis of the estuary division,point layout optimization and ecological environment assessment are carried out.The specific contents are as follows:A method for delimit the estuary area of large rivers based on physical-biological factors is proposed.Based on the water quality and biological data from 2009 to 2021,By analyzing the benthic dominant species in the Yangtze River estuary and combined with the spatial-temporal distribution characteristics of annual mean salinity,the thesis divides the Yangtze River Estuary on physical-biological factors.The results show that the gradient line of annual mean salinity of 5‰ is the distribution boundary of freshwater organisms,and the connection line between 25‰ salinity and topographic node is the distribution boundary of Marine organisms.The area between annual mean salinity 5-25‰ is taken as the Yangtze River estuary area.Based on the spatial balanced sampling method,the optimal layout method of water quality monitoring sites is established,the sites in the estuary area are optimized to 38,the suitability of sampling sites was analyzed by significance test,variance and spatial autocorrelation.Results show that,the variance and Morans’ I of the optimized sites are smaller,which indicated that the spatial balanced sampling method is suitable for the layout and optimization of the sites in estuaries and bays.The estuarine water quality evaluation indicators system is established,including inorganic nitrogen,active phosphate,active silicate,total nitrogen,total phosphorus,dissolved oxygen,organic carbon,oil,permanganate index and heavy metal indicators are determined,and the standard value of estuarine water quality indicators are standardized through frequency distribution method.Thiessen polygons method is used to determine the representative area of monitoring points,and the water quality of the Yangtze River estuary area was evaluated by the data from 2020 to 2021;Based on the estuarine water quality standard and combined with biological indicators,a health assessment indicators system and health assessment for the Yangtze estuary ecosystem are finished,The assessment results showed that the ecosystem health of the Yangtze River Estuary is average and showed a trend of improvement on the whole.