Sources Of Sedimentary Organic Matter In And Around Cage Fish Farming Of Poyang Lake Using Stable Carbon And Nitrogen Isotopes

In recent years,the environmental quality of the water area in duchang of poyang lake has decreased,and some areas have obvious blooms and algal enrichment.Duchang is the largest cage culture base of poyang lake.In order to understand the effect of net-cage culture to the water body,this article selects Duchang aquaculture waters of Poyang Lake as the research object,respectively,in October 2017 and March 2018 and November 2018 samples in the water,the water environmental factors（DO,pH,Chla,TN,TP,etc.）on the analysis of the spatial and temporal variation characteristics of using comprehensive nutrition state index method for water body nutritional status was evaluated in the study area,and each month the correlation between environmental factors are analyzed,so as to reveal the effect of aquaculture on water;Meanwhile,the spatial and temporal variation characteristics of the physical and chemical factors（water content,pH,TOC and TON,etc.）and carbon and nitrogen isotopes in the cage culture sediments were analyzed.Through the above research,the water quality status of poyang lake duchang cage aquaculture waters and sediment sources have a preliminary understanding.The main conclusions are as follows:1.The characteristics of the water environment show that cage farming has almost no influence on the water temperature and pH in the breeding area;The effects on DO,Chla and nutrient content were obvious.In October and march,DO in the water was significantly lower than the control point,and Chla was significantly higher than the control point.In November,the DO concentration in the cage area was lower than that in the peripheral area and the control point,and Chla was significantly higher than that in the peripheral area.The aquaculture activity increased the nutrient salt content in the aquaculture area obviously.In October,march and November,the TN content in the cage area varied from 2.05 mg/L to 2.72 mg/L,from2.26 mg/L to 2.40 mg/L and from 2.20 mg/L to 2.57 mg/L,respectively.The TN content in the control points was 1.14 mg/L,1.49 mg/L and 2.05 mg/L,respectively.The variation range of TP was 0.11 mg/L⁰.23 mg/L,0.24 mg/L⁰.42 mg/L and 0.32mg/L⁰.73 mg/L,respectively.The TP content at the control points was 0.11 mg/L,0.18 mg/L and 0.13 mg/L,respectively.The maximum of TN and TP content appeared in the cage area.The range of NH₄⁺-N in the cage water was 0.18mg/L⁰.39 mg/L,0.66 mg/L¹.05 mg/L and 0.07 mg/L⁰.33mg/L,respectively.The variation ranges of NO₃^--N are 0.38 mg/L⁰.62 mg/L,0.72 mg/L¹.01 mg/L and 0.74mg/L¹.43 mg/L,respectively.In march,the content of NH₄⁺-N,NO₃^--N,TN and TP in the water in the study area was higher than that in October.In November,TP content in the cage area was significantly higher than that in October and march.The results of eutrophication evaluation showed that the aquaculture water in October and march was in a eutrophic state with good water quality.In November,the nutrient status of aquaculture water has reached eutrophication,and the water pollution is serious.2.The distribution characteristics of the stable carbon and nitrogen isotope values of the organic matter in the sediments show that the distribution range ofδ¹³C value in October,march and November is-27.81‰^-26.55‰,-27.67‰^-25.65‰and-27.61‰^-25.66‰,with the mean values of-27.18‰±0.3‰,-26.32‰±0.56‰and-26.52‰±0.49‰,respectively.The distribution range ofδ¹⁵N is 3.9‰¹3.21‰,5.19‰⁷.27‰,and 4.82‰⁶.92‰,with the mean of 6.15‰±2.06‰,6.38‰±0.49‰and 6.19‰±0.64‰,respectively.The C/N distribution ranges were 6.08⁷.95,6.85⁷.84 and 5.56⁶.55,respectively,and the mean values were 6.59±0.47,7.23±0.27 and 6.03±0.23,respectively.The maximum and minimum value ofδ¹³C will be in March and November respectively.The minimum and maximum value ofδ¹⁵N appeared in October,the minimum value of C/N appeared in November,the maximum value appeared in October,October and march C/N slightly higher,which may be related to the greater impact of external input at this stage,and the first degradation of organic matter is the release of nitrogen-containing protein.Theδ¹³C value of each month in the study area fluctuates significantly,while theδ¹⁵N value（except for the sampling point on October 4）and C/N value fluctuate less.3.The source of organic carbon in sediments showed that the organic carbon in sediments in October was relatively simple and mainly came from river plankton.The sources of organic carbon in sediments in March can be divided into two situations:（1）mainly from river plankton and feces;（2）the main sources of some stations are river plankton.The sources of organic carbon in sediments in November can be divided into two situations.（1）the main sources are river plankton and feces.（2）some stations are mainly derived from river plankton.4.The sources of organic nitrogen in sediments show that the main sources of nitrogen in sediments in October can be divided into two situations:（1）the main sources of nitrogen are soil organic matter and river plankton;（2）individual stations mainly come from the mixture of human and animal wastes,faeces of farmed fish and bait;There were also two sources of organic nitrogen in the sediments in March:（1）the main nitrogen sources were soil organic matter,feed and plankton;（2）the main source of organic nitrogen in some stations was feed.In November,the sediment organic nitrogen mainly comes from soil organic matter and plankton.