Plateau Lake Water Quality and Eutrophication: Status and Challenges

The continuous and widespread deterioration of lake water quality and eutrophication is not only a local problem, but also a global phenomenon [...]


Introduction
The continuous and widespread deterioration of lake water quality and eutrophication is not only a local problem, but also a global phenomenon.It is not only destroying, or at least limiting the valuable water resources for daily life, but threatening the water security for sustainable social development.More importantly, along with the rapid accumulation of the alga, including novel hypertoxic viruses and new toxic chemicals as well as other organic compounds, these stressors threaten aquatic life, biodiversity and endanger our health [1].The scientific community, especially environmental scientists and ecologists, needs to pay special attention to this issue and alert the public of the potential ecological and human health effects.The unknown consequences of utilizing trans-watershed or longdistance water diversion to dilute highly polluted lake water as a solution to eutrophication and contamination, an approach often preferred by engineers and hydrologists, must be avoided to provide sustainable water resources.
The costs necessary to control lake pollution and eutrophication are high, not to mention the invisible influences on ecosystem productivity and potential persistent threat to public health.There is a need for watershed management approaches where remedial build on each other to restore and protect water quality in an efficient manner.
The so-called Nine Large Lakes (>30 km 2 ) in Yunnan Province of southwestern China have experienced dramatic changes, seven of the nine lakes have become heavily polluted during last few decades [1].These closed and/or semi-closed inland lakes are strongly influenced by the monsoonal climate, which results in a rainy season from mid-May to October and a dry season from November to early May, with a south-southwesterndominated wind.At the same time, the elevated lake surfaces are exposed to strong ultraviolet radiation.The distinct seasonal contrast leads to large seasonal lake-level fluctuations, influencing the function and structure of the ecological system.These climatoenvironmental scenarios are what make the lakes in the Yunnan Plateau different to and more sensitive than lakes in other locations.This situation implies that we need to adopt different approaches to manage water pollution and eutrophication, instead of replicating management paradigms from other systems.Understanding the status, evolutionary processes, and mechanisms of lake water systems, as well as predicting future trends, are critical for us to address current and future problems and challenges.

Articles
In total, fifteen papers were published in this Special Issue.The article titles, authors and keywords are summarized in Table 1.In these papers, the seasonal variation in the water quality and eutrophication of Lake Lugu [2], Lake Chenghai [3], Lake Erhai [4], Lake Yangzong [5], Lake Xingyun [6], Lake Qilu [7], Lake Yilong [8], and a small water body (Lake Jian) [9] in Yunnan Plateau of Southwestern China was examined using current monitoring and research data.As all these lakes are heavily polluted, the situation is such that even if there might exist a nutrient "threshold" to control the eutrophication in one lake [10], it is difficult to identify a common management approach for all lakes.Fundamentally, many lake processes are almost "dead".Restoring the ecological functioning of the lake should be the priority before, or at least in parallel with, other engineering management approaches.
The quantifying the dynamics of phosphates in lake systems is a crucial first step in controlling eutrophication.La-modified materials and La/Al-co-modified attapulgite along with macrophytes, Hydrilla verticillata royle and Ceratophyllum demersum L., were investigated and tested as an approach to modify P dynamics.The results indicate that mineralization of organophosphates is an important factor for regulating high internal P loadings and P concentrations.The combination of LMM and macrophytes led to synergistic effects in the efficiency of aquatic ecological restoration compared with individual treatments.It was also concluded that LMM enhanced the conversion rates of redoxsensitive P forms in surface sediments [11].
Organic matter is is a keey component of lakes, and the origins or organic matter vary from one lake to another.The temporal and spatial distributions of particulate organic matter (POM) prior to and after ecosystem degradation in the karst lake (Caohai Lake) were analyzed using a combination of carbon and nitrogen stable isotopes (δ 13 C-δ 15 N).This study revealed that environmental factors, including DO, turbidity, water depth, and water temperature, that regulate both photosynthesis and sediment resuspension, are key factors determining the spatiotemporal distribution of POM.Meanwhile, the POM in water is closely related to the dissolved oxygen concentrations and pH, such that decreased dissolved oxygen (DO) concentrations and pH values resulted in an increase of POM [12].
The impact of cascade hydro-power development on water quality was investigated at six hydropower stations that have been in joint operation for seven years along the main course of the middle reach of the Jinsha River in Yunnan and Sichuan Provinces.This study reveals that cascade hydropower development resulted in a decrease in TP concentrations but an increase in the concentration of CODMn and NH 3 -N along that section of the river.The concentrations of CODMn and TP are higher during the rainy season and lower in the dry season, which is directly related to the input of non-point-source pollutants in the basin during the period of high surface runoff [13].
To understand the pollution status of the surface water of the Tibet Plateau, the spatial and temporal variation of nutrients, heavy metals with respect to, water quality conditions and pollutant sources were studied in surface water from 41 cross-sectional monitoring sites in 2021.The results revealed that 12 polluting elements, except lead (Pb), had significant seasonal variation.In general, the water quality in most parts of Tibet was observed to be good.The water quality of the 41 monitoring sections met the Class I water standard as per the entropy -fuzzy evaluation method [14].
Models have also been applied to better understand the effects of external nutrient loading impacts on the water quality of the lakes.It was found that the annual inputs of total nitrogen (TN) had higher variability than total phosphorus (TP) in Lake Xingyun, and the highest loadings were during the wet season and the lowest during the dry season.The poor correlation between in-lake nutrient concentrations and tributary nutrient inputs at monthly and annual time scales suggests that both external and internal loadings were regulating lake eutrophication [15].

Conclusions
This Special Issue highlights and discusses major threats to Plateau Lakes water quality, and provides an update on both lake current status as well as future challenges.Lake problems, such as pollution and eutrophication, require that we quantify how serious the situation is, identify the probable causes, and recommend how to control the pollution in order to restore and protect water quality.More importantly, this issues stresses the need to manage the lakes and the watershed under a unified approach.
It is important to point out that there are still many other aspects of plateau lake water quality and eutrophication that require further research and monitoring.Although water pollution and lake eutrophication conditions have recently been protected from further deterioration, this is not due to actions we have taken; instead, it is the result of reduced human disturbance of the plateau lake water and the discharge of nutrients into the lake, which is attributable to the COVID-19 pandemic as well as the abnormal climatic conditions during the last three years.According to our monitoring data, the nutrient concentrations in the plateau lake water are still higher than acceptable values in other lakes.Nevertheless, the fundamental problems still need to be addressed, and therefore, it is not the time to take pride and become complacent.The war continues, even though the battle fields are almost empty.We still have a long way to go to regain the beauty of harmony in mountain-river-lake-water-plants scenery.

Table 1 .
Summary of the papers published in the Special Issue entitled "Plateau Lake Water Quality and Eutrophication: Status and Challenges" for the journal Water.