Lakes constitute more than 90% of all the liquid fresh water on the Earth’s surface. Water quality of a lake plays a key role in providing ecological functions. However, the rapid development of industries and wide use of agrochemicals have subjected lakes to an increased stress, deteriorating water quality.

Increasing wastewater discharge and water regulation of upstream dams affect the water quality of lakes, particularly for those with high hydrological alterations.

Based on the Mann-Kendall (M-K) test and self-organizing map (SOM), a team of researchers from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences characterized and evaluated water quality trends in Dongting Lake, China, from 1991 to 2018, before and after the inauguration of the Three Gorges Dam (TGD).

They selected six water quality parameters, namely pH, permanganate index (COD_Mn), ammonia nitrogen (NH₃-N), total nitrogen (TN), total phosphorus (TP), and the five-day biochemical oxygen demand (BOD₅).

The concentrations of TN and BOD₅ increased significantly throughout the study period. The number of abrupt change points for the six water quality parameters in the post-TGD period was greater than that in the pre-TGD period, indicating an increased risk of water deterioration in the post-TGD period.

In addition, the SOM results suggested that the water quality in the pre-TGD period was better. The classification of TN and TP ranged from Level IV-V among the clusters, which did not satisfy the level III standard for potable water, thereby posing a higher ecological risk to the Dongting Lake.

This research indicated the deterioration of the water quality in Dongting Lake during the post-TGD period under the influences of pollution load and hydrological regulation. It is meaningful to control the external nutrient loading and hydrological regulations in Dongting Lake.

The study was published in Environmental Pollution. It was supported by the Joint Fund for Regional Innovation and Development of NSFC, the Youth Innovation Promotion Association of CAS, the Water Conservancy Science Project of Hunan Province, the Changsha Science and Technology Project, the Youth Innovation Development Program of Changsha, and the Hunan innovative province construction projection.