Response Of Nitrogen And Phosphorus Within Danjiangkou Reservoir Bays To Background Characteristics Of Landscape

A reservoir bay is an area sensitive to water quality in reservoirs.The watershed of a reservoir bay is a direct source of water pollutants.Watershed landscapes,including topography,and land use,profoundly influence the water quality of reservoir bays.Understanding the specific responding mechanisms of spatial and seasonal variability in water quality within reservoir bay background characteristics of landscape,is important to explore water quality evolution.In this study,we selected 62 typical reservoir bays with different landscape of the Danjiangkou Reservoir as the research objects.Based on continuous monitoring of water quality and dataset of landscape features including topography,pattern and composition of land use,we took information technologies such as remote sensing and geographic information system,reconnaissance field surveys,landscape feature analysis and partial least square regression method to analyze the specific responding mechanism of spatial and seasonal variability in water quality within reservoir bay to background characteristics of landscape.The research results will be beneficial to water quality protection for drinking water reservoirs.The main results and conclusions of the present study are as follows:(1)The total nitrogen concentration ranged from 0.46 to 7.83 mg L-1in the wet season,and 0.53 to 7.84 mg L-1 in the dry season,respectively.The average total nitrogen concentrations were generally lower in the wet season(1.29 mg L-1)than in the dry season(1.49 mg L-1).The total nitrogen concentration of each bay was seasonally different,the variation coefficient was 72.2% in the wet season,and 77.4% in the dry season.The bays with higher total nitrogen concentration were Shendinghe bay,Sihe bay,and the north of the Danjiangkou Reservoir.(2)The total phosphorus concentration varied from 0.01 to 0.56 mg L-1 in the wet season,and 0.02 to 0.62 mg L-1 in the dry season,respectively.The average total phosphorus concentrations were generally higher in the dry season(0.08 mg L-1)than those in the wet season(0.06 mg L-1).The variation coefficient was 105% in the wet season,and 155.2% in the dry season,respectively.The bays with higher total phosphorus concentration were Shendinghe bay,Laoguanhe bay,Xianghuazhen bay.(3)The main land use type in the study area were forest and agricultural,followed by shrubs,grass,residential.Land use structure in each watershed was different from each other,in particular,the proportion of grass area,shrubs area,and residential area shows greater variances than other variables,the variation coefficient were 121%,93%,and 92.8%,respectively.In addition,land use pattern variables and topography variables had small variation,only the coefficient variation of mean patch size,basin relief,profile curvature,landscape shape index,patch density,slope were more than 50%.A lot of background characteristics of landscape variables were co-linear.(4)The total nitrogen and total phosphorus were mainly determined by the proportion of residential and agricultural areas,hypsometric integral,slope,patch cohesion index,contagion,edge density,and shannon's diversity index.The main contribution of variables to the total nitrogen concentration is seasonal variability.In the wet season,the total nitrogen was mainly determined by the hypsometric integral(VIP=2.016),patch cohesion index(VIP=1.370),the proportion of residential area(VIP=1.225),slope(VIP=1.080),the proportion of agricultural area(VIP=1.055);the total phosphorus was mainly determined by the proportion of agricultural area(VIP=1.433),patch cohesion index(VIP=1.397),shannon's diversity index(VIP=1.351),the proportion of residential area(VIP=1.210),hypsometric integral(VIP=1.148),slope(VIP=1.131),and edge density(VIP=1.017).In dry season,the total nitrogen was mainly determined by the proportion of agricultural area(VIP=1.760),the proportion of residential area(VIP=1.283),edge density(VIP=1.216),contagion(VIP=1.088),and shannon's diversity index(VIP=1.014)in the dry season;the total phosphorus was determined by the proportion of agricultural area(VIP=1.575),the proportion of residential area(VIP=1.133),shannon's diversity index(VIP=1.461),contagion(VIP=1.051),and edge density(VIP=1.026).