Cumulative Effects Of Ecological Environment And Ecosystem Service Responses By Continuous-Damming In The Huangshui River

At present,the construction of small cascade dams such as rubber dams and hydraulic dams is the current situation generally faced by domestic rivers,the small cascade dams are also different from natural rivers in the hydrology,water quality and water ecology of the river,then the specific impact on the river ecosystem,the mechanism of cumulative effects,and the feedback mechanism on ecosystem services are not clear,and the research scope is relatively narrow.The research takes the Huangshui River city section as the research area,and obtains basic data on hydrology,water quality,aquatic organisms,and residents’satisfaction through field surveys,indoor experiments,and questionnaires,and analyzed the impact of dam construction on the river ecosystem.The deviation coefficient and accumulation coefficient models were constructed,it quantitatively judged whether the monitored ecological environmental indicators had significant deviation and intensity accumulation in time and space.Aiming at the ecological environment factors with cumulative effects,a material flow process model is constructed,and ecological network analysis is used to analyze the key accumulation process of the ecosystem from a system perspective.Finally,established an ecosystem service evaluation model,andevaluated the response of river ecosystem services to dam construction from a qualitative and quantitative perspective.（1）Small-scale cascade dams drastically changed the hydrological and water quality characteristics of the dammed river section.After the dam was built,the river flow rate and DO decreased by an average of 75%and 12%,respectively,and the water depth and water temperature increased by 67%and 50%on average.The COD_Mnand BOD indicators in the water quality of the continuous dam area decreased by 28%and 21%,respectively,and the TN,TP,and TOC in the water body sediments increased to 40%,23%,and 64%on average.Changes in hydrology and water quality have also influenced the growth environment of plankton in the dammed river section.The population structure of plankton has changed significantly,and the types,density and biomass of plankton have increased by an average of 1.7,71,and12 times.In terms of time and space,the impact of cascade dams on rivers in normal water periods is 78%higher than in normal and high water periods.（2）The deviation degree of hydrology,water quality and water ecology in the first index is obviously different,the deviation coefficients of aquatic ecology are the largest（4.69）,followed by hydrology（1.35）and water quality（1.32）.In the secondary indicators,the flow rate,total phosphorus,nitrogen content and total organic carbon in the sediment,the biomass,density and diversity index of plankton,and the biochemical oxygen demand in the water body have a high degree of deviation.The deviation coefficients of other indicators are all around 1,and the degree of deviation is not obvious.（3）Cascade dams increase the emission flux of greenhouse gases.From a time scale,the emission flux during the wet season is higher than that during the dry season.From a spatial perspective,the greenhouse gas emission flux in the lower dam area is higher than that in the upper dam area,and the deviation coefficients of greenhouse gas emission fluxes are all much higher than 1,and the deviation coefficient of N₂O is the highest,followed by CH₄and CO₂,and the average deviation coefficients are 4.78,1.34 and 2.12 respectively.（4）In the river ecosystem indicators,total nitrogen,phytoplankton density and biomass,zooplankton diversity index,and greenhouse gas emission flux accumulation coefficient are more significant in different periods or different spaces.The main ecological processes in response to greenhouse gas accumulation are as follows:flow rate decrease→sediment deposition→increase of methanogenic microorganisms in anaerobic environment→CH₄emission increase;flow rate decrease-nutrient accumulation→aquatic plants increase→CO₂emission;flow rate decrease→phytoplankton increase→the increase in CO₂emissions,more and more carbon chains and increased internal flow strength,both promote greenhouse gas emissions.（5）The cascade dams bring positive and negative feedback values to the river ecosystem services,the positive feedback is mainly reflected in the convenience brought to human production and life,while the negative feedback is reflected in the transformation of the river’s self-regulation function.Among the indicators for the existence of positive feedback value include flood regulation and storage,water supply,provision of biological habitat,increased landscape appreciation,and scientific research value.Indicators for the existence of negative feedback value include adjustment of atmospheric composition function,river self-purification capacity,increased chances of river alien species invasion,changes in river sediment transport and nutrient transport.