Health Assessment Of Urban Stream Ecosystem In The North Canal River Basin,Beijing,China

The candidate indicators were estimated on the basis of sampling data including water quality,habitat and riparian zone,as well as aquatic organisms in the North Canal River basin in Beijing.Principal component analysis was used to select the core health assessment indices of stream ecosystem from the candidate indicators,and the weight of each core index was estimated by using entropy method.In this study,the spatial distribution characteristics of the core indices were analyzed.The ecological health composite index and the health assessment criterion were calculated and determined to carry out the health assessment of stream ecosystem in the North Canal River basin.In comparison with the results of community structure characteristics for aquatic organisms and water quality assessment,the reliability of health assessment results was analyzed.The main contents and conclusions are as follows:(1)A total of 32 aquatic ecology monitoring metrics was selected to be the candidate indicators,including hydrology,water quality,aquatic organisms,habitat and riparian zone,on the basis of the field sampling results in the North Canal River basin in July 2015.The candidate indicators were divided into 1 target layer,5 criterion layers and 13 element layers.The results of discrimination ability analysis,characteristic contribution degree analysis and correlation analysis showed that a total of 10 metrics was selected as the core indices to support the health assessment of stream ecosystem in the North Canal River basin in Beijing.The results of entropy method showed that the maximum weight of the 5 criterion layers was aquatic organisms,while the maximum weight of the 13 element layers was Shannon-Wiener diversity index of benthic macroinvertebrates.They provided the most effective information for the health assessment of stream ecosystem.(2)The spatial distribution characteristics of the core indices showed that except the mountainous areas in the north of Changping district,the further it was away from the central district,the higher water temperature was.Qualitative habitat evaluation index presented a progressive increase from the midstream to the upstream and dowmstream.Biochemical oxygen demand and total nitrogen were at a lower level which meaned that the overall condition was good,while Shannon-Wiener diversity index of benthic macroinvertebrates was at a lower level which meaned that the overall condition was bad.In addition,the spatial heterogeneity of other 5 core indices was strong.After calculating the ecological health composite index,five-grade method was used to determine the health assessment criterion on the basis that the ecological health composite index showed a distribution characteristic of “double pyramid style”.According to the spatial distribution characteristics of health assessment classification,the stream ecosystem in the North Canal River basin was generally unhealthy.The upstream was better than the dowmstream,and the midstream was the worst.The southwest of the midstream showed a great difference from the other areas and the spatial heterogeneity of the health condition was strong in this area.The health condition in the downstream of the Ba River and the upstream of the Tonghui River was the worst,while the mountainous areas in the north of Changping district as well as the junction of the southwest of the central district and Fengtai district was the best.(3)The community structure of aquatic organisms in the upstream of the Qing River,the upstream of the Wenyu River,the downstream of the Ba River and other areas in the upstream and downstream of the basin was simple.Furthermore,the species distributed inhomogeneously in the areas as mentioned above.The water quality in the whole basin was relatively bad especially in the Dongsha River,the Beisha River and the Nansha River in the upstream of the Wenyu River,the upstream of the Qing River,the Ba River,the Tonghui River and the dowmstream of the Liangshui River.On the contrary,the water quality was better in the mountainous areas in the north of Changping district,the Qing River,the central district and other surrounding areas.Health assessment results and the analysis results mentioned above showed the same tendency generally in spite of some differences.Consequently,health assessment results based on the ecological health composite index were reliable to some extent.(4)Both results of community structure characteristics for aquatic organisms and water quality assessment were compared each other.The Berger-Parker dominance index result of plankton was a little better than that of benthic macroinvertebrates,which indicated that most of the reaches were being regulated and restored,while a few were still in the stage of degradation.The results of comprehensive water quality identification index was a little better than that of biotic index,which indicated that most of the reaches were in the early stage of restoration and the major achievement was the improvement of water quality.The health condition of stream ecosystem was yet to be further enhanced in the North Canal River basin.