Comparison Of Environmental Difference Between Pengxi And Modao River In The Three Gorges Reservoir Based On Severity Of Algal Blooms

After the impoundment of the Three Gorges Reservoir(TGR)region in 2003,the velocity of the tributaries water reduced,the water became deeper,and the self-purification capability of the water decreased,eutrophication was becoming more and more serious,and many tributary blooms have become the norm,which has aroused widespread concern in society.Pengxi River is the largest tributary in TGR in its northern bank,water blooms are particularly frequent,and the degree is increasing year by year.Modao River is the primary tributary in TGR in its southern bank,whose geographical location,nature,and human conditions are similar to the Pengxi River,but there are few reports of blooms.This study takes Pengxi River and Modao River as examples,comparative analysis of the water environment in the two basins,explores the main factors affecting the blooms,and the direct and indirect effects of water blooms on the water environment,in order to provide a scientific basis for the prevention and control of water blooms in the Three Gorges reservoir area.This study monitored the water bodies of Pengxi River and Modao River in TGR in April,July,October 2017 and January 2018 respectively.The results show:(1)According to the revised Carlson index,the nutritional status of Pengxi River and Modao River was evaluated.In general,the water nutrition level of Pengxi River was higher than that of Modao River,and the water body of Pengxi River was mainly eutrophication,while the whole body of Modao River was presented as a medium eutrophication level.In the spring,the water quality of Pengxi River was the worst,reaching the level of super-eutrophication,the Modao River reached the eutrophication level in summer.(2)The levels of N and P in Pengxi and Modao River were higher than the international eutrophication thresholds(N?0.2 mg/L;P?0.02 mg/L).During the study period,the water body of Pengxi River was in a phosphorus-restricted state,and phosphorus was still an important limiting factor for the blooms of Pengxi River.The Modao River was not in a nitrogen or phosphorus restricted state.(3)The concentration of chlorophyll a in the water body of Pengxi River is significantly higher than that of Modao River.In spring,the concentration of chlorophyll a reached the highest in the whole year,and the phenomenon of bloom was obvious,then it continued to decline.The chlorophyll a concentration in Modao River reaches the highest in summer.In the spatial horizontal dimension,the chlorophyll a concentration in the upstream backwater area of the two rivers was higher than that at the estuary.(4)Affected by changes in temperature and water level,the Pengxi River and Modao River began to stratify in the spring.In summer,the water body stratified obviously.In the autumn,the water body stratification began to disappear,and the winter water body was completely mixed.On the whole,the stratification of Pengxi River(April)was earlier than that of Modao River(May),and the layered structure was more complete.This is one of the reasons why the spring of the Pengxi River was proned to bloom.(5)Water stratification is the main factor affecting the water environment in backwater area of TGR.The stable thermocline effectively limited the exchange between the upper and the lower water layers.When the water was well mixed,the DO,contents of Chl-a,conductivity and contents of soluble nutritive salts were homogeneous in the surface,middle and bottom layers.When the water stratification occurred,the content of chlorophyll a at the surface increased to 148.90 ?g/L in the spring in Pengxi River,and the conductivity was decreased,the DO was greatly increased.The content of chlorophyll a at the surface of Modao River also reached the highest of 74.63 ug/L in the whole year in summer(At this time,the water body is layered).(6)The conductivity value represents the amount of soluble nutrients that can be utilized by the phytoplankton.Therefore,the conductivity of the surface water can be used to predict the outbreak scale of the algal blooms under stratified conditions.These results also show that the control of soluble nutrients(such as soluble nitrogen and phosphorus)is more important than that of the total nutrients(such as total nitrogen and total phosphorus)in the prevention and control of non-point source pollution.In-situ experiment on soil nutrient dynamics in Gaoyang and Longjiao water-level-fluctuation zones under flooding and drying conditions in September 2017 and February,March,April,May 2018.The results show:(1)The soil organic matter in the two water-level-fluctuation zones showed an upward trend during the whole flooding-drying cycle.The rise of the Gaoyang water-level-fluctuation zone was 78.34%,and the rise of the Longhorn water-level-fluctuation zone was 88.52%.It can be seen that the rot decomposition of the flooded vegetation during the flooding period can accelerate the accumulation of organic matter in the soil in the water-level-fluctuation zone.(2)The total nitrogen content in the two water-level-fluctuation zones showed an upward trend throughout the flooding and drying cycles.The rise of the Gaoyang water-level-fluctuation zone was 45.45%,and the rise of the Longhorn water-level-fluctuation zone was 89.06%.The total phosphorus content was stable and there was no significant difference with time.The soil available nutrient in the water-level-fluctuation zone showed an upward trend throughout the flooding-drying cycle.It can be seen that in the flooding-drying cycle,the soil in the water-level-fluctuation zone was characterized by the accumulation of nutrients.(3)The change of soil nutrient increment is different under different flooding time.In this study,the soil nutrient in the 4-month flooding fluctuation zone showed a downward trend,showing as the "source" of water nutrient.With the increase of flooding time,the soil nutrient in the fluctuation zone showed an overall upward trend.