Optimization Of River Health Assessment Method Based On Key Function Group Selection

At present,most river ecosystems are negatively affected by human activities,and it is urgent to comprehensively promote river ecological restoration.Quantitative evaluation of river ecosystem health is of great significance for scientific guidance of river ecological restoration.Biodiversity is an important manifestation of river health.Protecting key functional groups is an effective measure to improve biodiversity and is very important for improving river ecosystem health.In this paper,the Ecopath model,which is commonly used to study the structure and function of ecological system,is used as the main research tool.The optimization technical route of river ecological health evaluation method based on the screening of key ecosystem function groups is explored,and the shortcomings of relatively less ecological health evaluation of small and medium rivers are improved.This study selects the typical small and medium rivers-Qingyi River(Xuchang section)as the application verification.Combined with the final evaluation results,the main research contents and conclusions of river ecological restoration in the study area are as follows:(1)Based on the survey results of aquatic organisms,the food web model of Qingyi River in summer and autumn was constructed.The Qingyi River water ecosystem survey was carried out in July and October 2021,and a total of 10 survey sites were set up to obtain hydrological,water quality,habitat and aquatic biological data.Based on the biological investigation,the model parameters were determined by further calculation and literature review,and the Ecopath food web model of Qingyi River in different seasons was constructed.The simulation results show that the food network of Qingyi River in summer and autumn is composed of pastoral food chain and clastic food chain,and most of them begin with phytoplankton and debris.The ecosystem characteristic parameters showed that the sum of ecosystem output,total respiration,total consumption and total flow into debris in summer and autumn of Qingyi River was 2571.06 t/km~2and 1472.58 t/km~2,respectively,and the ecosystem scale in summer was larger than that in autumn.Further analysis of ecosystem maturity index showed that the values of FCL and FML in summer were2.15 times and 1.33 times of those in autumn.The maturity of ecosystem in summer was higher than that in autumn,but it was still lower than that of near-natural rivers.(2)The structure and function of ecological system of Qingyi River in summer and autumn were analyzed,and the key functional groups were screened and determined.From the perspective of ecosystem structure,the nutrient level distribution and niche characteristics of fish in summer and autumn of the Qingyi River were consistent,and the distribution range of nutrient level was 1.00–3.29.The niche overlap mainly occurred between small fish,and the niche overlap degree of small fish in the upper and middle and lower layers was the highest,while there was no niche overlap in predatory fish,and the overlap index of predators and baits was close to 1,indicating that fish resources were miniaturized.From the perspective of ecosystem function,the total conversion efficiency of ecosystem in summer and autumn was very low,1.08%and 1.82%respectively,which was significantly lower than the average transmission efficiency of ecosystem 9.2%.According to the species criticality index,the influence of species on the energy flow of food web was quantified.Based on the ranking results of species criticality index of different functional groups of primary producers,benthic animals and fish,the key functional groups of Qingyi River in summer and autumn were selected as phytoplankton,mollusks and culter.Through the correlation analysis between the key functional groups and environmental factors,the main environmental factors affecting the key functional groups in summer and autumn were identified as nitrogen(P<0.05).The distribution of Culter alburnus was mainly affected by ammonia nitrogen,the distribution of mollusks was mainly affected by nitrate nitrogen,and the distribution of phytoplankton was affected by ammonia nitrogen and nitrate nitrogen.(3)Optimize the existing river health evaluation methods,and put forward ecological restoration suggestions according to the loss of points.Based on the water health evaluation in the Technical Guidelines for Watershed Ecological Health Assessment,combined with the screening results of key functional groups in the Qingyi River,the evaluation indexes of biological criterion layer were optimized,and the weight of each index was optimized by combination weighting method.A comprehensive health evaluation standard system of’habitat-key functional groups-pressure’suitable for the Qingyi River was established.Methods After optimization,the health evaluation results changed obviously.The health status of the whole Qingyi River in summer and autumn were good and poor,and the health status of the whole Qingyi River after optimization was general and poor.From the health evaluation results of different river sections,the health evaluation results of the main stream,tributary Baling River and Xiaoni River before optimization were good,general and good in summer,and the evaluation results of autumn were poor.The health evaluation results of summer after optimization were general,poor and general,and the evaluation results of autumn were poor.Compared with the score of the criterion layer of the health evaluation method system,it was found that the score of aquatic organisms in autumn was significantly lower than that in summer after the optimization of the evaluation method,but the score was not changed before the optimization,indicating that the optimized evaluation results can better reflect the response of aquatic organisms to external conditions,and reflect the change of ecosystem health status.Finally,taking into account the protection of key functional groups and the restoration of main lost items in health evaluation of different river sections,ecological restoration suggestions such as source control,pollution interception and habitat restoration,food chain restoration based on key functional groups,and limitation of human disturbance are proposed.