Adaptability Of Fish In Different Reaches Of Lancang River Based On Carbon And Nitrogen Isotopes

In recent years,the Lancang River cascade hydropower station has exerted enormous comprehensive benefits,but it has also caused the cumulative impact on the ecological environment,which has caused varying degrees of impact on the hydrological situation,water temperature,water quality,aquatic organisms,habitats,etc.Water ecological problems have gradually become prominent.Based on the in-situ monitoring of the ecological environment indicators of Lancang River,the temporal and spatial variation characteristics of environmental factors in different waters of Lancang River were analyzed,and the environmental characteristics of different waters were determined based on hierarchical clustering method.Meanwhile,the contribution ratio of different organic matter sources to surface sediments in Lancang River Basin,and the contribution ratio of primary producers and consumers to fish in different waters in Lancang River Basin were calculated quantitatively by using stable isotope technology and a multi-source linear mass mixed model.The effects of reservoir construction on the source of organic matter and the change of fish feeding habits in different waters were discussed.The measures and countermeasures of fish protection in Lancang River were put forward,which can provide basic reference for fish protection in the basin.The main work and conclusions are as follows:（1）According to the temporal and spatial variation of nutrients in Lancang River Basin,there were significant spatial differences in TN concentration in surface water,TN concentration in the reservoir section was significantly higher than that in the natural river section,and the overall level was in the Class III surface water standard.There was no temporal and spatial difference in TP concentration,and the overall level was in Class II surface water standard.D-Si concentration had obvious temporal and spatial differences,D-Si concentration in the reservoir section was significantly higher than that in the natural river section,and it was higher in the wet season than in the dry season.（2）According to the temporal and spatial variations of habitat factors in Lancang River,there were significant differences in water environment characteristics between the natural river section and the reservoir section.Conductivity,flow rate,turbidity and dissolved oxygen concentration in the natural river section were higher than those in the reservoir section,whereas transparency,water temperature and chlorophyll a concentration in the natural river section were lower than those in the reservoir section.（3）Theδ¹³C andδ¹⁵N values in suspended organic matter in Lancang River Basin had obvious temporal differences,which were greater in the dry season than in the wet season.There was a linear correlation between theδ¹³C andδ¹⁵N values in the suspended organic matter,which was linear negative correlation in the wet season and weak linear negative correlation in the dry season.There were obvious spatial differences inδ¹³C andδ¹⁵N values in the sediment organic matter.Theδ¹³C value of the natural river section was greater than that of the reservoir section,while theδ¹⁵N value of the natural river section was smaller than that of the reservoir section,and theδ¹³C andδ¹⁵N values were negatively correlated linearly,the correlation was more significant in the wet season than in the dry season.（4）There were obvious differences in the sources of sediment organic matter in different waters of Lancang River Basin.The source of sediment organic matter of the natural river section was mainly the upstream organic matter,while the source of sediment organic matter of the reservoir section was mainly the externally input vegetation soil,cultivated land soil and litter.（5）In July 2017,there were obvious temporal and spatial differences inδ¹³C andδ¹⁵N values of catfish,crucian carp and silver carp in the basin.Theδ¹³C value was significantly greater in the natural river section than that in the reservoir section,while theδ¹⁵N value was significantly smaller in the natural river section than that in the reservoir section.There was no significant difference in theδ¹³C andδ¹⁵N values of schizothorax between the natural river section and the reservoir section.There were obvious temporal and spatial differences in suspended organic matter,attached algae and zoobenthos.Theδ¹³C value was greater in the natural river section than in the reservoir section,and theδ¹⁵N value was smaller in the natural river section than in the reservoir section.