Spatiotemporal distribution of nitrogen and phosphorus in alpine lakes in the Sanjiangyuan Region of the Tibetan Plateau

To provide a theoretical basis for alpine source lake protection, ten samples were taken from each lake annually from 2012 to 2015. Each year, the various species of nitrogen and phosphorus nutrients were measured. The average contents of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen in the four lakes are 0.195–0.0 mg/L, 0.038–0.143 mg/L, 0.004–0.168 mg/L, 0.006–0.740 mg/L, and 0.050–0.547 mg/L, respectively. The total phosphorus contents in Eling Lake, Longbao Lake and Sea Star were higher than Class I water quality standards, and the total nitrogen contents in Eling Lake, Sea Star and Zhaling Lake were higher than Class I water quality standards as well. The concentration contour maps of the nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen showed that the indicators of the four lakes in the east, the west, and the center of the lake did not have the same trend. From 2012 to 2015, each of the measured nutrients showed a rising trend year by year. The four lakes are polluted by both endogenous and exogenous pollution, and it is necessary to limit the exogenous pollution and protect the alpine lakes immediately.

INTRODUCTION

Lake eutrophication occurs when a lake receives a large amount of nutrients within a certain period of time. For example, nitrogen and phosphorus inputs can cause nutrient levels in the lake to exceed the maximal load and thereby cause algal overgrowths, which can cause water quality to deteriorate and upset the ecosystem's balance, and accelerate the decay processes in the lake as well (Liu 2004). The issue of lake eutrophication has become one of the most active research topics in limnology.

During the past 20 years, lake eutrophication has become a serious problem in China. The proportion of lakes with eutrophication in China increased from 41% in the 1970s, to 61% in the 1980s and to 77% in the 1990s. According to a survey of 39 large and medium reservoirs in China, the proportions of reservoir amount and reservoir capacity that have become eutrophic are 30.8% and 11.2%, respectively, while those that have been mesotrophic are 43.6% and 83.1%. In addition, some rivers in China also show the phenomenon of eutrophication, such as the Han River and Pearl River (Zhang 2004). Therefore, water eutrophication is one of the major current environmental problems to be solved in China.

The Sanjiangyuan Region contains the headwaters of the Yangtze River, the Yellow River and the Lantsang River. Its geographic location is between E 89 °24′ and E 102 °23′, N 31 °39′ and N 36 °16′. The area of Sanjiangyuan Region is 363,000 km², which accounts for 50.3% of area in Qinghai Province (Qu et al. 2011). There are more than 180 rivers, 16,543 lakes, 66,600 km² swamp and 73,300 km² wetland in the Sanjiangyuan Region. It is the largest water resource terminus and the most abundant ecological adjustment region in Asia, even in the whole world (Li & Xue 2014).

Since the 1980s, ecosystems and watersheds have undergone significant change in the Sanjiangyuan Region due to the impacts of climate change and human activities. Phenomena such as snow line rising, acceleration of glacial ablation, thinning of tundra, groundwater recession, surface water capacity reduction and wetland degradation have caused the ecological environment to become fragile (Liu et al. 2008; Song et al. 2009). The areas of rivers and lakes have tended to decrease in typical alpine wetlands in the Sanjingyuan Region in the last 20 years (Wu & Chang 2008).

So far, studies of wetlands in the Sanjiangyuan Region have mainly focused on hydrologic circulation processes, soil hydrothermal processes, carbon cycle, alpine wetland degradation, the effect of wetland change on regional climate, and so on (Degani et al. 1998; Yuan et al. 2002; Luo et al. 2007), while studies on the status of nutrients, such as nitrogen and phosphorus, in alpine lakes are scarce.

Changes in the regional climate, water resources and ecological environment in the Sanjiangyuan Region will not only influence regional ecological safety, water safety and the sustainable economic development of society but will also influence the balance of water resources, ecological safety and even production and living in the middle and lower reaches.

In this research, alpine lakes (Eling Lake, Longbao Lake, Sea Star, Zhaling Lake) were used as the research subject. The contents of nutritive salts, such as nitrogen and phosphorus salts, were compared from 2012 to 2015 in order to evaluate the current status of nutritive salts, to provide control measures for alpine lakes' nutritive salts and to provide the theoretical foundation of the current status of nutritive salts in alpine lakes for creating and implementing methods to protect the ecological environment of alpine lakes.

OVERVIEW OF STUDY AREA

Eling Lake District

Eling Lake is located in Maduo County, which belongs to the Guoluo Tibetan autonomous prefecture (E 97 °40′31″–97 °42′27″ and N 34 °53′4″–34 °54′30″). Eling is the biggest lake in the watershed of the Yellow River (Zhang et al. 2010). It is 190 km away from Kariqu, one of the headwaters of the Yellow River, and its south side is 60 km from the Maduo County line, while its west side is 15 km from Zhaling Lake. Eling Lake and Zhaling are known as ‘sister lakes’ in the headwaters of the Yellow River (Chen & Ma 1991a, 1991b). The Lake area is 610 square kilometers, approximately 31.6 km east to west and 32.3 km north to south. Its average water depth is 17.6 meters, and the deepest depth, in the north middle of the lake, is 30.7 meters, with a volume of 10.7 billion cubic meters. Eling Lake is shaped like a bell, the long axis extends from north to south, and it is located in the late graben basin between Bayan Har Mountain and Buqing Mountain, which belong to the Kunlun Mountain range (Shen et al. 2011); the climate of this area is influenced by southwest monsoon, southeast monsoon and west wind. Annual average temperature is −4.6 °C and annual precipitation is 200–400 mm (Chen & Ma 1991a, 1991b) (Figure 1).

Zhaling Lake District

Zhaling Lake and Eling Lake are the largest freshwater lakes in the upper reaches of the Yellow River. Zhaling Lake is also known as Chaling Sea, meaning ‘white long lake’ in Tibetan. It is located in the Maduo depression in the western Qinghai plateau, on the west side of Eling lake (E 97 °02′–97 °27′ and 34 °49′–35 °01′) (Lin 2003). Zhaling Lake is long from east to west and narrow from north to south. Its area is up to 52 km², its average depth is approximately 9 m, its storage capacity is 4.6 billion m³, and its altitude is 4,291 m. Zhaling Lake has a typical plateau continental climate with a long cold season, dramatic temperature variations, strong winds and significant frost crack weathering. The annual average temperature is 4.6 °C, annual rainfall is 340.1 mm, annual evaporation capacity is 1,327.9 mm, annual average relative humidity is 59%, annual sunshine is 2,685 h, and average wind speed is 3.4 m/s (Chen & Ma 1991a, 1991b) (Figure 2).

Sea Star Region

Sea Star is also known as Xingsu Sea, referring to Cuocha in the Tibetan language, meaning ‘flower sea’. It is located in the source region of the Yellow River. Its east side is adjacent to Zhaling Lake, and its west side is linked to another source of the Yellow River, Maqu. Its altitude is higher than 4,000 m. It is a long, narrow sea, 30 km long from east to west and 10 km wide from north to south. The climate is cold, dry, and occasionally wet. The annual average temperature is 0.8 °C, annual minimum temperature is −42 °C and annual maximum temperature is 28 °C. The annual average rainfall is 463.7 mm, and the air oxygen levels are 1/3 to 1/2 that at sea level (Duan et al. 2015) (Figure 3).

Longbao Lake Region

The Longbao lake wetland (N 33 °09′ – 33 °17′, E 96 °24′ – 96 °37′) is a typical alpine wetland in the Sanjiangyuan Region, located in northwestern Yushu County in Qinghai Province (Long & Xu 2007). Its length is 18.7 km from east to west and its width is 3 km from north to south, and the total area is up to 50 km². Cangzongchayi Mountain (4,760 m) and Ninggairenqibengba Mountain (5,182 m) are to the south and north, respectively, and the lake area is also surrounded by Yaqinyaqiong Mountain and Xiaohaolajia Mountain (Long & Xu 2007). Longbao Lake has a high plateau continental cold climate. The annual average temperature is 2.9 °C, and the annual minimum temperature is – 26 °C, while the annual maximum temperature is 28.7 °C, and the annual rainfall is 730 mm (Long & Xu 2007). It is windy, dry and cold in winter, and rain predominantly occurs from June to August; it is wet during this time. The total sunshine throughout the year is approximately 2,300 h. Longbao Lake freezes in November and melts in April or May (Lin 2003) (Figure 4).

The specific location of Eling Lake, Zhaling Lake, Sea Star and Longbao Lake in the Sanjiangyuan Region is shown in Figure 5.

RESEARCH METHODS

Sampling times

Lake water samples were collected during the normal level period (May), wet season (August) and dry season (November) from 2012 to 2014 and during the normal level period (May) in 2015 (Table 1).

Design of sampling point

Sampling methods

A bucket or bottle was used to collect surface water, and a heavy hammer was used to collect deep water. The surface and deep water samples from the same sampling location were mixed.

Measurements and methods

Nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen were measured. UV spectrophotometry was used to determine nitrate nitrogen. Nessler's reagent spectrophotometry was used to determine ammonia nitrogen. A spectrophotometric determination method utilizing hydrochloric acid and N-(1-naphthyl) ethylenediamine dihydrochloride was used for nitrite nitrogen. An ammonium molybdate spectrophotometric method was used to determine total phosphorus. Alkaline potassium persulfate digestion and ultraviolet spectrophotometry were used to determine the total nitrogen content.

Evaluation criteria

The surface water environment quality standards (GB3838-2002) were used as the evaluation criteria (Table 2).

Evaluation methods

Retention times in ultra-high performance liquid chromatography (HPLC) in comparison with standards were used as the evaluation method (Cui 1995).

Data processing

Excel 2010 was used to generate descriptive statistics for the experimental data (including average, maximum value and minimum value), variable statistical characteristics, and correlations between variables. Surfer 13 was used to make contour maps of the lakes.

RESULTS AND ANALYSIS

Nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen in the different alpine lakes

Nitrate nitrogen in the different alpine lakes

Nitrate nitrogen concentration was 0.253–0.320 mg/L with a mean of 0.280 mg/L in Eling Lake, 0.192–0.199 mg/L with a mean of 0.195 mg/L in Longbao Lake, 0.236–0.310 mg/L with a mean of 0.270 mg/L in Sea Star, and 0.213–0.300 mg/L with a mean of 0.250 mg/L in Zhaling Lake (Figure 6). Nitrate nitrogen concentration in Eling Lake was higher than in any of the other three lakes. Nitrate nitrogen contents in Sea Star and Zhaling Lake increased from 2012 to 2015, and the rates of increase for Sea Star for the three years were 8.47%, 3.9% and 16.5%, respectively, while the rates of increase for Zhaling Lake were 10.8%, 4.2% and 21.9%, respectively. The maximum change of nitrate (29%) occurred in Zhaling Lake, while the minimum change (3.5%) occurred in Longbao Lake.

Ammonia nitrogen in the different alpine lakes

Ammonia nitrogen concentration was 0.221–0.113 mg/L with a mean of 0.106 mg/L in Eling Lake, 0.136–0.176 mg/L with a mean of 0.143 mg/L in Longbao Lake, 0.026–0.06 mg/L with a mean of 0.038 mg/L in Sea Star, and 0.044–0.052 mg/L with a mean of 0.047 mg/L in Zhaling Lake (Figure 7). Ammonia nitrogen concentrations in Eling Lake, Sea Star, and Zhaling Lakes showed an increasing trend during the measurement period, and the increase in Sea Star was higher than those in Eling Lake and Zhaling Lake (rates of increase for Eling Lake for the three years were 1.9%, 3.8% and 4.6%, respectively, and those for Zhaling Lake were 2.3%, 4.4% and 10.6%, respectively, while those for Sea Star were 7.4%, 42.8% and 7.4%, respectively). On the other hand, ammonia nitrogen content in Longbao Lake decreased from 2013 to 2014, and the rate of ‘increase’ was −13.1%.

Nitrite nitrogen in the different alpine lakes

Nitrite nitrogen concentration was 0.002–0.008 mg/L with a mean of 0.0042 mg/L in Eling Lake, 0.164–0.173 mg/L with a mean of 0.168 mg/L in Longbao Lake, 0.003–0.007 mg/L with a mean of 0.004 mg/L in Sea Star, and 0.002–0.008 mg/L with a mean of 0.004 mg/L in Zhaling Lake (Figure 8). Nitrite nitrogen concentration in Longbao Lake was the highest among the four lakes, while those in Eling Lake, Sea Star, and Zhaling Lake were each very low and almost did not change from 2012 to 2015.

Total phosphorus in the different alpine lakes

Total phosphorus concentration was 0.009–0.025 mg/L with a mean of 0.020 mg/L in Eling Lake, 0.710–0.780 mg/L with a mean of 0.740 mg/L in Longbao Lake, 0.009–0.050 mg/L with a mean of 0.030 mg/L in Sea Star, and 0.002–0.008 mg/L with a mean of 0.006 mg/L in Zhaling Lake (Figure 9). Total phosphorus concentration in Longbao Lake was the highest among the four lakes. Total phosphorus concentration in Zhaling Lake was almost below the detection limit in all four years. The total phosphorus concentrations in Eling Lake and Sea Star were similar, with values of 0.009 mg/L and 0.008 mg/L in 2012, 0.010 mg/L and 0.010 mg/L in 2013, 0.025 mg/L and 0.035 mg/L in 2014, and 0.04 mg/L and 0.05 mg/L in 2015, respectively.

Total nitrogen in the different alpine lakes

Total nitrogen concentration was 0.530–0.580 mg/L with a mean of 0.547 mg/L in Eling Lake, 0.020–0.090 mg/L with a mean of 0.050 mg/L in Longbao Lake, 0.443–0.520 mg/L with a mean of 0.475 mg/L in Sea Star, and 0.356–0.440 mg/L with a mean of 0.393 mg/L in Zhaling Lake (Figure 10). Total nitrogen concentrations in Eling Lake, Longbao Lake, Sea Star and Zhaling Lake all showed increasing trends from 2012 to 2015, while the lowest total nitrogen concentration was in Longbao Lake.

Distributions of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen within each of the different alpine lakes

Distributions of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen in Eling Lake

In general, the patterns of nitrate, ammonia and nitrite were all similar in Eling Lake; they presented an initial increase and then decrease from west to east and reached the maximum near the center, and the values for the west, center, and east were 0.27 mg/L, 0.113 mg/L, and 0.004 mg/L, respectively (Figure 11). The contour map of total phosphorus showed that the total phosphorus in Eling Lake decreased from northwest to southeast and reached the maximum of 0.01 mg/L near the center. The contour map of total nitrogen showed that total nitrogen in Eling Lake decreased from east to west.

Distributions of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen in Longbao Lake

The contour maps of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen are shown in Figure 12. In general, the patterns of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen were all similar to one another in Longbao Lake, decreasing at first and then increasing from east to west, and reaching minima of 0.192 mg/L, 0.146 mg/L, 0.164 mg/L, 0.728 mg/L and 0.037 mg/L, respectively, near the center and reaching maxima of 0.196 mg/L, 0.164 mg/L, 0.168 mg/L, 0.743 mg/L and 0.06 mg/L, respectively, near the west of the lake.

Distributions of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen in Sea Star

The contour maps of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen are shown in Figure 13. In general, the patterns of nitrate nitrogen and ammonia nitrogen were almost the same; they presented a decrease at first and then increased from east to west and reached minima of 0.236 mg/L and 0.026 mg/L, respectively, near the lake center and reached maxima of 0.266 mg/L and 0.04 mg/L near the east of the lake. Nitrite nitrogen and total nitrogen decreased gradually from east to west, which illustrated that the distributions of nitrite nitrogen and total nitrogen were similar. Total phosphorus decreased gradually from northwest to southeast.

Distributions of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen in Zhaling Lake

The contour maps of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen are shown in Figure 14. In general, the patterns of nitrate nitrogen, ammonia nitrogen, total phosphorus and total nitrogen were all similar; they presented a decrease at first then an increase from east to west, and reached minima of 0.213 mg/L, 0.042 mg/L, 0.004 mg/L and 0.356 mg/L, respectively, near the lake center. Nitrate nitrogen, ammonia nitrogen, and total nitrogen reached maxima of 0.246 mg/L, 0.067 mg/L and 0.396 mg/L, near the west of the lake. Total phosphorus reached a maximum of 0.008 mg/L near the east of the lake. Nitrite nitrogen concentration decreased gradually from east to west and reached the minimum of 0.002 mg/L near the west of the lake. In summary, the analysis of contour maps illustrated that the distributions of nitrate nitrogen, ammonia nitrogen, and nitrite nitrogen in Eling Lake were all similar; all increased at first then decreased from east to west. Total phosphorus decreased gradually from northwest to southeast, while total nitrogen decreased gradually from east to west. Patterns of all the indices (except nitrite nitrogen) in Longbao Lake and Zhaling Lake were similar, decreasing at first then increasing from east to west. The nitrite nitrogen concentration in Zhaling Lake decreased gradually from east to west. Nitrate nitrogen and ammonia nitrogen in Sea Star decreased at first then increased from east to west. Nitrite nitrogen and total nitrogen decreased gradually from east to west, and total phosphorus decreased gradually from northwest to southeast.

Water quality evaluation of alpine lakes

Compared with the surface water environment quality standards (GB3838-2002), nitrate nitrogen, ammonia nitrogen and nitrite nitrogen contents in Eling Lake, Longbao Lake, Sea Star and Zhaling Lake were lower than the water quality Class I standards. The total phosphorus and total nitrogen contents in Eling Lake were higher than the water quality Class I standards, and the fold-change of total phosphorus and total nitrogen contents compared with ULTRA standards was 1 and 1.735, for rates of 58.3% and 100%, respectively. They were, however, lower than the water quality Class II standards. Total phosphorus content in Longbao Lake was higher than the water quality Class I standard; the fold-change compared with ULTRA standards was 73, and the ratio was 66.7%. Total phosphorus and total nitrogen contents in Sea Star were higher than the water quality Class I standards, and the fold-change compared with ULTRA standards of total phosphorus and total nitrogen content was 2 and 1.375, for rates of 41.7% and 100%, respectively. They were, however, lower than the water quality Class II standards. Total nitrogen content in Zhaling Lake was higher than the water quality Class I standard, and the fold-change compared with ULTRA standards was 0.965, and the rate was 100%. However, it was lower than the water quality Class II standard (Table 3).

Discussion

Nutritive salts, as important materials for ecosystem development in lakes, also provide important indications of a lake's future state. Excessive nutrients in water are the fundamental cause of eutrophication. Thus, in order to research the eutrophication status and ecosystem development direction of a lake, we have to research nitrogen and phosphorus content distributions in the water body first, and then evaluate it using effective evaluation methods (Sundareshwar et al. 2003; Qin et al. 2006). Yong-fen He has studied the changes of eutrophication status over time (2012–2013) in Long Lake in Hubei and applied canonical correlation analysis in order to reveal the correlations between eutrophication and the physical and chemical characteristics of the lake (He et al. 2015). Wang Jun and others studied the basic characteristics and nutrient drive mechanism of Baiyangdian eutrophication (Wang et al. 2010). S. S. S. Lau and S. N. Lane proposed that phytoplankton and zooplankton biomass was related to nitrogen and phosphorus (Lau & Lane 2002). Marjo Palviainen et al. noted that the combination of carbon, nitrogen, phosphorus and other nutrients can cause the emissions of greenhouse gases and the lake's eutrophication status by both biological and photochemical mechanisms in their study of 12 lakes in eastern Finland (Palviainen et al. 2016). R. Shinohara researched the effects of different organic phosphorus and inorganic phosphorus in Xiapu Lake sediments on the survival of phytoplankton and bacterial cells in 2016 (Shinohara et al. 2016); these studies revealed that the phytoplankton and zooplankton biomass, along with the eutrophication status of lakes, reservoirs, and rivers, correlated well with the nitrogen and phosphorus contents of the water. The present study reported four alpine lakes' concentrations of various nutrient species and described yearly trends; the following section will discuss how sources of pollution and prevention measures can be proposed for alpine lakes, by considering the temporal and spatial distributions of nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, total nitrogen, total phosphorus and other indicators for these alpine lakes.

In our study, the concentrations of ammonia nitrogen, nitrite nitrogen and total phosphorus were high in Longbao Lake, which might be the result of discharge of domestic sewage from the residential areas upstream in Longbao Town. Longbao Town is 17 km away from Longbao Lake, located on the west side of Longbao Lake, and has 8,623 residents. The north side of Longbao Lake is close to Yuzhi Road, which is only 500 m away from the lake center. The traffic on this road also produces pollution from automobile exhaust and dust runoff to the lake. There are wetlands near Longbao Lake that are suitable for grazing. Thus, animal waste and debris might dissolve in water and flow in to the lake, which could cause the ammonia nitrogen, nitrite nitrogen and total phosphorus concentrations to increase.

Nitrite nitrogen and total nitrogen content in Zhaling Lake and Eling Lake were high, which might be due to the development of tourism in these areas. Garbage, human and animal feces and other pollutants could pollute the lake and lead to an increase of nitrate nitrogen and total nitrogen content. In addition, the density of vegetation coverage in this area is low, and soil nutrients might be leached by rainwater, which would then flow into the lake, which would also lead to nitrate nitrogen and total nitrogen content increase. The nitrite nitrogen and total nitrogen contents in Eling Lake are higher than those in Zhaling Lake, which may be because Eling Lake is located downstream of Zhaling Lake and Eling Lake is closer to Maduo County, so Eling Lake is more likely to be affected by human beings. The nitrite nitrogen and total nitrogen contents in Sea Star are high. The first reason for this is that this lake is close to national road 214, and thus automobile exhaust and dust could impact the water quality. Another reason is that animal waste and debris and garbage from human activities could lead to nitrite nitrogen and total nitrogen increase polluting the lake.

The analysis of nutrient contour maps showed that nitrate nitrogen, ammonia nitrogen and nitrite nitrogen in Eling Lake; nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and total nitrogen in Longbao Lake; and nitrate nitrogen, ammonia nitrogen, total phosphorus and total nitrogen in Zhaling Lake had lower values in the east and west of the lakes than in the center of the lakes. This indicated that the effect of exogenous pollution in Eling Lake, Longbao Lake and Zhaling Lake was weaker than the effects of endogenous nutrient in these lakes. Bottom sediment contains a certain amount of nitrogen and phosphorus inside the water body, which can be dissolved into the water. Total phosphorus and total nitrogen in Eling Lake, nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, and total phosphorus and total nitrogen in Sea Star have lower values in the center of these lakes than in the east and west of these lakes. This indicated that the exogenous pollution is the main reason for the high content of nitrogen and phosphorus nutrients in these lakes.

In this study, the nitrate nitrogen and total nitrogen contents in Eling Lake were the highest. In Longbao Lake, ammonia nitrogen, nitrite nitrogen, and total phosphorus content were the highest. The nutrient indices of Longbao Lake and Eling Lake were higher than the others. This is probably because Longbao town is only 40 m upstream of Longbao Lake; there are 800 residents in Longbao town, and Longbao discharges its sewage into the lake. Eling Lake is surrounded by grazing pastures, from which manure can run off into the lake with the rain. In addition, in recent years, the government has allowed visitors to the vicinity of the Eling Lake, and the litter left by the tourists can be blown by the wind or washed by the rain into the lake, causing additional pollution. The contents of nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and total nitrogen in all four lakes showed an increasing trend year by year from 2011 to 2015. This is associated with the surrounding overgrazing, increased tourism, increased amounts of garbage, and increased amounts of sewage, resulting in increased overall pollution of the lake.

Much research has shown that human activities are behind the fragmentation of the natural wetland and species extinctions. Livestock feces produced by overgrazing and nutrients in farm land runoff are discharged into the lakes, so that the ecological balance of water is disrupted and biological populations and species numbers are reduced, greatly accelerating the process of water eutrophication (Han et al. 2012). In this research, the nitrogen and phosphorus contents of the lake increased year by year because of the rapid development of tourism, overgrazing, discharge of animal wastes, discharge of sewage and garbage, vehicular exhaust emissions, etc.

Research has shown that global warming and continuous drought could lead to alpine lake area reduction, causing the mineral enrichment and salinization of wetlands. Sanjiangyuan Region is located in a high altitude area, and the effect of human activity is relatively smaller, so climate change is one of the important reasons for the degradation of alpine wetlands (An et al. 2008). In this study, the effects of climate change on nitrogen and phosphorus in alpine lakes have not been investigated, though they will be examined in future research.

CONCLUSION

The average nitrate nitrogen concentrations in Eling Lake, Longbao Lake, Sea Star, and Zhaling Lake are 0.280 mg/L, 0.195 mg/L, 0.270 mg/L, and 0.250 mg/L, respectively; the average ammonia nitrogen concentrations are 0.106 mg/L, 0.143 mg/L, 0.038 mg/L, and 0.047 mg/L, respectively; the average nitrite nitrogen concentrations are 0.0042 mg/L, 0.168 mg/L, 0.004 mg/L, and 0.004 mg/L, respectively; the average total phosphorus concentrations are 0.020 mg/L, 0.740 mg/L, 0.030 mg/L, and 0.006 mg/L, respectively; and the average total nitrogen concentrations are 0.547 mg/L, 0.050 mg/L, 0.475 mg/L, and 0.393 mg/L, respectively. Each index tends to increase yearly from 2012 to 2015. The four lakes are polluted by both endogenous and exogenous pollution, but it is necessary to prevent the exogenous pollution and protect the alpine lakes immediately.

The ecosystem in the Sanjiangyuan Region is fragile. Thus, in order to protect alpine lakes, we should cut off the sources of exogenous pollution. We should prevent excessive grazing and random discharge of domestic sewage, and reduce the pollution of the lake caused by automobile exhaust and dust. We should restore the grassland, prevent soil erosion and improve wetland restoration technology. We should also limit the travel and tourism activities in order to guarantee that humans cannot pollute the lake area so easily. We should actively carry out scientific research on plateau wetlands, increase reserve funds, strengthen the construction of protection facilities, improve restoration techniques for degraded wetlands, and establish a unified leadership organization for national wetland conservation to regulate within the legal system and strengthen law enforcement.

ACKNOWLEDGEMENTS

This research was financially supported by The National Natural Science Funds Fund (No. 31260128), International Cooperation Projects of Qinghai Province Department (No. 2016-HZ-811), Program for Changjiang Scholars and Innovative Research Team in University, MOE Ecological Evolution and Environmental Protection of Sanjiangyuan (No. IRT13074), China New Zealand Plateau Grassland Nutrient Flow and Sustainable Production Research (2015DFG31870) International Cooperation Project of the Ministry of Science and Technology, State Key Laboratory of Plateau Ecology and Agriculture of Qinghai University. This study was conducted in the Laboratory of Eco-environmental Engineering College of Qinghai University, whom we thank for providing the testing facility and equipment for the laboratory measurements.

REFERENCES