Macroinvertebrate Community Characteristics And Its Relationship With Environmental Factors In River-lake Ecotone

The river-lake ecotone is an important pollution buffer and water purification zone in the freshwater ecosystem,which can not only maintain the regional ecological balance,but also provide good habitats for aquatic organisms,and is of great significance in protecting biodiversity and other aspects.However,in recent years,natural climate change and human activities have caused problems such as reduced biodiversity and impaired ecological services in the watershed.Macrobenthic invertebrates,as an important aquatic ecosystem taxon,are affected by various environmental factors and have a strong sensitivity to changes in the water environment,which can reflect the quality of the water environment over time,and have been widely used to evaluate the health of aquatic ecosystems.In this study,the water quality,sediment quality and benthic community were investigated in September 2020,April 2021 and October 2021,and the chemical composition of water and sediment,the community structure characteristics of benthic fauna,changes in biodiversity and the main environmental factors affecting benthic fauna were investigated.The health of the water environment in the study area was also evaluated using the biodiversity index,the biological pollution tolerance sensitivity index（BMWP）and the physicochemical index of water bodies to provide a theoretical basis for the ecological restoration of the river-lake confluence area.The main results are as follows:（1）The water quality and sediment indexes in the confluence area of Nansi Lake are better in autumn than in spring.The main pollutants of water quality are chemical oxygen demand(COD_Cr),ammonia nitrogen（NH₃-N）and total phosphorus（TP）,and the water quality is generally in Class III and Class IV;based on the results of sediment pollution index,the sediment pollution mainly comes from nitrogen,and the overall pollution level is moderate;five types of fluorescence peaks appear in the water body and sediment DOM,including tyrosine-like,tryptophan-like,soluble microbial The degree and concentration of DOM humification in the water column were higher in autumn than in spring,and the concentration of DOM in the sediment was higher in spring than in autumn,and the main components were humic substances.（2）The benthic community structure in the study area was spatially and temporally heterogeneous.A total of 55 species of macrobenthos were identified during the survey,belonging to 6 phyla,15 orders and 39 families,mainly including Arthropoda,Mollusca and Annelida.A total of 51 species of benthic animals were identified in autumn 2020,19species in spring 2021,and 15 species in autumn 2021,with Neocaridina denticulata sinensis appearing as the co-dominant species in the three sampling events.Neocaridina denticulata sinensis and Indoplanorbis exustus had the highest frequency（100%）,and the results of non-metric multidimensional scale analysis（NMDS）showed that there were significant differences in the composition of the benthic community between the sites in autumn 2020 and spring and autumn 2021.（3）There were seasonal differences in the main environmental factors affecting the benthic community structure in the river-lake confluence area.The main environmental factors affecting benthic community structure were total sediment carbon（STC）and sediment nitrate nitrogen（SNO₃-N）in autumn 2020,COD_Cr and STC in spring 2021,and CODCr,total sediment phosphorus（STP）and STC in autumn 2021.and COD_Crcontributed 11.6%,7.3%,5.9%,6.7%,and 5.0%,while there was no significant correlation between other environmental factors and benthic community（P>0.05）.（4）There were significant positive correlations between Shannon-Wiener diversity index（H’）and Simpson diversity index（d）,Margalef richness index（D）,as well as Pielou evenness index（J）,BMWP index,and benthic abundance;there were significant positive correlations between BMWP index and benthic abundance.The BMWP index and H’index had a high linear fit relationship,and the J index did not vary much in time and space,indicating that the species composition of the river-lake confluence area in the South Lake is relatively uniform at this stage.The results of water quality health evaluation based on biological indices for the river-lake confluence area of Nansi Lake the health of the water environment is mainly between"subhealthy"and"poor",and seasonally,the health of the water environment is autumn 2020>spring 2021>autumn2021.