Temporal and Spatial Distribution and Fluorescence Spectra of Dissolved Organic Matter in Plateau Lakes -- A Case Study of Qinghai Lake


 Dissolved organic matter (DOM) has a great influence on the main pollution indexes of lakes (such as COD). Therefore, DOM research is the basis for understanding the water environmental quality and the law of pollutant migration and transformation in the basin. In this study, the water quality monitoring data of Qinghai Lake water body and 8 rivers around the lake from 2010 to 2020 were collected, and the dissolved organic matter (DOM) was synchronously sampled in May, September and October 2020. The optical characteristics of DOM, the temporal and spatial distribution of CDOM and the fluorescence spectrum and fluorescence component characteristics of FDOM were analyzed and studied. The results show that: (1) From 2010 to 2020, the annual mean value of CODCr of Qinghai Lake water body fluctuates in the range of Class III to Class V according to the environmental quality standard of the surface water, and shows a downward trend first and then an upward one. In general, the mean value of CODCr concentration in Qinghai Lake water body is at a high level and varies slightly among different months. (2) The mean value of CODCr concentration of the eight main rivers entering Qinghai Lake from 2010 to 2020 can be sorted from lowest to highest as follows: Ganzi River, Buha River, Jilmeng River, Hargai River, Shaliu River, Quanji River, HeMa River, and DaoTang River. (3) The concentration of CDOM in Qinghai Lake shows not only obvious seasonal variation (October, September and May, in the descending order) but also spatial variation. (4) The three-dimensional fluorescence spectrum matrix data of DOM in Qinghai Lake were analyzed by PARAFAC model, and four DOM fluorescence components with single maximum emission wavelength were analyzed.

(DOC is dissolved organic carbon; CDOM is colored dissolved organic matter; FDOM is uorescent dissolved organic matter; DBC is dissolved black carbon.) CDOM is the main light absorbing substance in dissolved organic matter (DOM) in natural water, which can affect the migration and transformation of heavy metals and organic pollutants (Patidar et al., 2015).
The part with uorescence characteristics in CDOM is called uorescent dissolved organic matter (FDOM), which is of great signi cance to aquatic ecosystems (Mostofa et al., 2013). As an important part of DOM, its important role in natural water environment has long been concerned. At present, ultraviolet, visible and uorescence spectra are mainly used to study CDOM of water bodies from different sources at home and abroad. Foreign studies on CDOM have been carried out earlier, and the research elds include CDOM origin, destination, distribution, migration and transformation, and spatio-temporal changes. The Tibet Plateau is the highest plateau on earth and has always enjoyed the reputation of "Asian water tower" and "third pole". Its high terrain and unique geographical location have a signi cant impact on the global climate (Yang., 2009; Wang et al., 2003). Qinghai Lake is an important water body to maintain the ecological security of the Tibet Plateau. It plays a great role in improving and regulating climate and promoting a virtuous cycle of Regional Ecology (Xinhua news agency., 2020). However, there are few investigations or regional environmental studies on the water environment of Qinghai Lake Basin.
In recent years, with the acceleration of urbanization and the rapid development of tourism, Qinghai Lake Basin is facing great ecological pressure. Therefore, the investigation on the water quality of Qinghai Lake and the study on the temporal and spatial distribution and optical characteristics of dissolved organic matter in the water body are helpful to deeply understand the current situation of the water environment of Qinghai Lake, the carbon cycle process within the basin and its potential impact on water quality, and can provide background data support for lake water environment analysis and water body assessment.

Study area
Qinghai Lake is located in the northeast of Qinghai Tibet plateau at the junction of Gangcha County, Gonghe County and Haiyan County. It is the largest inland salt water lake in China. In 2019, the lake area reached 4519 km 2 . The lake water is weakly alkaline, low oxygen content and salt content is 14.1 G L −1 , transparency below 3M. Qinghai Lake Basin is between 97°50 ′~101°20′ E and 36°15 ′~38°20′ N, with a drainage area of about 29661 km 2 (Yu et al., 2021). The overall outline is oval, and the terrain is high in the northwest and low in the northeast. It is located at the intersection of the East Asian monsoon, the northwest arid region and the alpine region of the Qinghai Tibet Plateau. It belongs to the semi-arid and alpine climate of the plateau, and the precipitation is unevenly distributed throughout the year(Yu et al., 2021). Qinghai Lake is located in the southeast of the basin and is mainly supplied by rivers and precipitation. Most of the rivers entering the lake are seasonal rivers and concentrated in the West and north of the lake. The main supply rivers are Buha River, Shaliu River, Quanji River, hargai River, Ganzi River and HeMa river.
In this study, Qinghai Lake Basin is taken as the research object, 14 sampling points are set, and the layout of sampling points is shown in Figure 2-1. In May, September and October 2020, Qinghai Lake Basin was comprehensively sampled three times, and 2.5L surface water samples (0 ~ 0.25m from the water surface) were collected, stored in brown glass bottles (constant 4 ° C) and sent back to the laboratory for testing.

Sample Method
Seawater samples are collected by Niskin water collector, and the parameters such as water depth, temperature and salinity are measured synchronously by CTD sensor during seawater collection. The river water samples are taken manually, and the parameters such as temperature, salinity and pH value of the on-site river water are synchronously measured in the river by the portable water quality analyzer. Dom and CDOM use ltering devices to lter the water samples to be collected in the shortest possible time, and the ltered samples can be frozen and stored. Thaw in the laboratory before measurement, and transfer su cient samples to doc measurement sample bottle (24 ml or 40 ml) with burned Pasteur glass pipette (Corning). If the sample cannot be measured immediately, add acid to the sample (pH<2) and store it in cold storage for measurement.

Sample Analysis
In order to facilitate the examination of the instruments used in the experiment and the experimental process, the instruments used for parameter determination correspond to Table 1, and the process corresponds to the back. Hitachi, Tokyo, Japan DOM and CDOM before analysis, the water samples were ltered using pre-combusted (450°C for 5 h) glass ber lters (0.7µm, GF/F, Whatman).
The absorption coe cient of CDOM at 254 nm (α254, m −1 ) was used as a proxy for CDOM concentration. According to the historical monitoring data ( Fig. 3-3), the permanganate index concentration of Qinghai lake water from 2010 to 2020 is between 2.14 and 5.53 mg/L, with an average of 3.89 mg/L, which is at the surface class II level. Except for the large uctuation of permanganate index concentration in water body in 2012, the uctuation of permanganate index concentration in Qinghai lake water body in other years is relatively small. From the mean value of permanganate index at each historical monitoring point, the mean value of permanganate index at Shadao point is relatively the highest, which is 5.39 mg/L.
From 2010 to 2015, the annual mean value of permanganate index of Qinghai lake water body uctuated as a whole, but all met the class III level of surface water. From 2016 to 2020, the annual average value of permanganate index of Qinghai lake water body showed a downward trend and was basically at the class II level of surface water. Among them, the permanganate index remained at the level of 2.14 ~ 2.35 mg/L from 2018 to 2020, slightly higher than the class I water quality limit. In recent years, the average value of permanganate index in 2011 is relatively the highest, which is 5.53 mg/L, close to the upper limit of class III water quality standard for surface water, and the average annual value of permanganate index in 2019 is relatively the lowest.
Time variation of CODcr and permanganate index of rivers entering Qinghai Lake from 2010 to 2020 From 2010 to 2020, CODcr of main rivers entering Qinghai Lake Basin was 7.50~13.94 mg/L, with an average of 10.99 mg/L; The CODcr of rivers entering the lake is lower than that of lakes as a whole (  Fig. 4-1).
Except for the DaoTang River, the mean value of CODcr of major rivers is relatively similar from 2010 to 2020, and the interannual variation trend is relatively consistent. The average CODcr of the eight main rivers entering Qinghai Lake from 2010 to 2020 shows the law of Ganzi River < Buha River < jilmeng River < hargai River < Shaliu River < Quanji River < HeMa River < DaoTang river.
From 2010 to 2020, the permanganate index of main rivers entering the lake in Qinghai Lake Basin was 1.44~2.98 mg/L, with an average of 2.18 mg/L( Fig. 4-2); The permanganate index of rivers entering the lake is lower than that of lakes as a whole.
Except for Daotang River and Heima River, the mean value of permanganate index of main rivers is relatively similar from 2010 to 2020, and the interannual variation trend is relatively consistent. The average value of permanganate index of the eight main rivers entering Qinghai Lake from 2010 to 2020 shows the law of Buha River < Shaliu River < hargai River < jilmeng River < Ganzi River < Quanji River < Heima River < DaoTang river. Except Shaliu River and Ganzi River, the law of other rivers is consistent with CODcr.

Spatial distribution of COD cr and permanganate index in Qinghai Lake
There are obvious differences in the spatial distribution of COD Cr concentration in Qinghai Lake ( Fig. 4-3).
In May, COD Cr in the East is higher than that in the west, and that in the center of the lake is higher than that in the coast; In September and October, CODcr of water body showed two high areas, namely, near Jiangxi ditch wharf and Sand Island, Jiangxi ditch wharf and Qinghai Lake shing ground wharf. From the perspective of spatial distribution characteristics, the higher values in the three months are mainly concentrated in Jiangxi ditch wharf and Qinghai Lake shery wharf. Jiangxi ditch wharf and Qinghai Lake shery wharf are located in the south of Qinghai Province, and the planting industry along the south coast is mainly. Therefore, the input of external sources leads to the higher concentration of CODCr in the water body in the south of Qinghai Lake than in other regions.
There are obvious differences in the spatial distribution of permanganate index concentration in Qinghai Lake ( Fig. 4-4). In May, the permanganate index concentration range is 1.80 ~ 4.50 mg/L, with an average of 2.21 mg/L, in September, the permanganate index concentration range is 1.90~14.50 mg/L, with an average of 2.83 mg/L, and in October, the permanganate index concentration range is 1.80~4.50 mg/L, with an average of 2.29 mg/L. In May and September, the permanganate index in the East was higher than that in the West; In October, the permanganate index of water body showed three high areas, namely the center of the lake, the East and the west, and the shore area close to the center of the lake. From the perspective of spatial distribution characteristics, except for the center of the lake in October, the highvalue areas appear in the areas close to the shore. Therefore, the external input may be the reason for the higher concentration of permanganate index in the water body of Qinghai Lake than in other areas.
Optical Properties Of Dom In Qinghai Lake Qinghai Lake not only has obvious seasonal differences, but also shows different spatial differences. Among them, the highest value of CDOM at each sampling point in the water body in May, September and October appeared near erlangjian in the south of the lake. In addition, the value of CDOM in the water body on the West Bank of Qinghai Lake in May was also high.
(2) Fluorescence spectrum and uorescence component characteristics of FDOM The three-dimensional uorescence spectrum matrix data of DOM in Qinghai Lake were analyzed by PARAFAC model, and four DOM uorescence components with single maximum emission wavelength were analyzed. The maximum excitation / emission wavelength distribution of the four uorescent components and the three-dimensional uorescence spectra of the principal components are shown in the gure below ( Fig. 5-2).
The excitation wavelength of component C1 is at 242 nm and the maximum emission wavelength is near 422 nm, which re ects the land-based high molecular weight humic acid, mainly from the degradation of higher plants or soil leaching; Component C2 has two obvious excitation wavelengths at 220 nm and 272 nm, and the maximum emission wavelength is 422 nm, which re ects the low molecular weight fulvic acid like uorescence peak, which mainly comes from the uorescence peak formed by biodegradable organic compounds; Component C3 has two obvious excitation wavelengths at 232 and 286 nm, and the maximum emission wavelength is 338 nm. It belongs to the binding peak of protein like and fulvic acid like, which has a red shift compared with the conventional tryptophan like peak; Component C4 has an obvious excitation wavelength at 270 nm and the maximum emission wavelength is 478 nm, re ecting the uorescence peak formed by polymer humic acid.
Overall, the proportion of each component in the total uorescence intensity of FDOM in May and September is C2>C1>C4>C3, and the proportion of each component in the total uorescence intensity of FDOM in October is C3>C2>C1>C4, which has certain seasonal differences ( Fig. 5-3).
In the three sampling surveys of Qinghai Lake in May, September and October (Fig. 5-4), the average total uorescence intensity of FDOM in the water body of Qinghai Lake was 22.20, 24.17 and 42.80 R.U., showing a signi cant seasonal difference law of October>September>May. The spatial distribution of the total uorescence intensity of FDOM in water is also different in different seasons. The spatial difference in October is more obvious than that in May and September. However, in the three surveys, the highest value of the total uorescence intensity of FDOM in water appears near erlangjian in the south of Qinghai Province.
The mean values of C1 components in the overlying water of Qinghai Lake in May, September and October are 5.85, 6.86 and 7.23 R.U. (Fig. 5-5), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest value appears near erlangjian in the south of Qinghai and the lowest value appears in the center of the lake.
The mean values of C2 components in the water body of Qinghai Lake in May, September and October were 6.93, 6.96 and 9.93 R.U. (Fig. 5-6), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values in May and October appear in the south of Qinghai, and the highest values in September appear in the southwest of the lake.
The mean values of C3 components in the water body of Qinghai Lake in May, September and October were 4.59, 4.96 and 20.28 R.U. (Fig. 5-7), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values appear near erlangjian in the south of Qinghai and Hunan.
The mean values of C4 components in the water body of Qinghai Lake in May, September and October were 4.82, 5.39 and 5.36 R.U. (Fig.5-8) respectively, showing a different law from other components, that is, September>October>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values appear near erlangjian in the south of Qinghai and Hunan. Both COD and BOD5 are used to quantitatively re ect the degree of organic pollution in water. Previous studies have shown that when BOD5 / CODcr≥30%, it is biodegradable sewage; If BOD 5 / CODcr<30%, it is di cult to biodegrade sewage. In this study (Fig. 6), the BOD 5 / CODcr value of Qinghai Lake is between 1.90%~5.29%, with an average of 3.36%, which is obviously low, indicating that the bioavailability of organic matter in Qinghai Lake is poor and di cult to decompose. This non decomposable organic matter will continue to accumulate into the sediment over time and migrate between water and sediment under the disturbance of wind, waves and sh, which also re ects the reason for the high COD of Qinghai Lake from another aspect.

Conclusions
The monitoring data from 2010 to 2020 showed the high COD Cr values and the high COD Mn and BOD values in the Qinghai Lake. From 2010 to 2020, the annual mean value of COD Cr of Qinghai lake water body rst decreased and then increased, and uctuated greatly, uctuating between class III ~ V levels of surface water. Spatially, the COD Cr mean value of sand island is relatively the highest, which is 43.84 mg/L. In terms of time nodes, the average value of COD Cr in 2011 is relatively the highest, close to the upper limit of class V water quality standard for surface water, and the annual average value of COD Cr in 2012 is relatively the lowest; In 2020, COD Cr will rise to class IV water level, with an average of 36.02 mg/L. From 2010 to 2015, the annual mean value of permanganate index of water body in Qinghai Lake uctuated as a whole, but they all met the class III level of surface water. It is noteworthy that the mean value of permanganate index at Sand Island is relatively the highest, which is 5.39 mg/L. From 2016 to 2020, the average annual value of permanganate index of Qinghai lake water showed a downward trend and was basically at the level of class II surface water. Among them, the permanganate index remained at the level of 2.14~ 2.35 mg/L from 2018 to 2020, slightly higher than the class I water quality limit. In recent years, the average value of permanganate index in 2011 is relatively the highest, which is 5.53 mg/L, close to the upper limit of class III water quality standard for surface water, and the average annual value of permanganate index in 2019 is relatively the lowest. The average CODcr of the eight main rivers entering Qinghai Lake from 2010 to 2020 shows the law of Ganzi River <Buha River <jilmeng River <hargai River <Shaliu River <Quanji River <HeMa River <DaoTang river (3) The concentration of CDOM in Qinghai Lake not only has obvious seasonal differences (showing the law of October>September>May), but also shows different spatial differences. The three-dimensional uorescence spectrum matrix data of DOM in Qinghai Lake were analyzed by PARAFAC model, and four DOM uorescence components with single maximum emission wavelength were analyzed. Component C1 re ects terrestrial high molecular weight humic acid, which mainly comes from the degradation of higher plants or the leaching of soil; Component C2 re ects the low molecular weight fulvic acid like uorescence peak, which mainly comes from the uorescence peak formed by biodegradable organic compounds; Component C3 belongs to the protein like and fulvic acid like binding peak, which has a red shift compared with the conventional tryptophan like peak; Component C4 re ects the uorescence peak formed by polymer humic acid.     spatial distribution characteristics of CDOM concentration (a254, m-1) in water body of Qinghai Lake in different seasons characteristics of DOM uorescence components in Qinghai Lake uorescence intensity characteristics of DOM components in Qinghai Lake spatial distribution of total uorescence intensity of FDOM in Qinghai Lake in different seasons spatial distribution characteristics of C1 uorescence intensity in Qinghai lake water in different seasons spatial distribution characteristics of C2 uorescence intensity in Qinghai lake water in different seasons spatial distribution characteristics of C3 uorescence intensity in Qinghai lake water in different seasons spatial distribution characteristics of C4 uorescence intensity in Qinghai lake water in different seasons Figure 6 Bioavailability of DOM in Qinghai Lake