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

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.

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).

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.

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.

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

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

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.

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).

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.

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.

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.