Polluted lake restoration to promote sustainability in the Yangtze River Basin, China

China has made a concerted effort to successfully improve water quality of rivers, but lake water quality has not improved. Lakes require controls on both catchment external nutrient loads and in-lake internal loads, where nature-based solutions are coupled with engineered systems to achieve the United Nations Sustainable Development Goals (SDGs).

China has sought to address water pollution in the last decade by introducing a wide range of laws and regulations (Table S1), which led to nationwide water quality improvement [1]. However, recent quantitative assessment of progress toward Sustainable Development Goals (SDGs) in China suggests that some SDGs underpinning goals for water pollution and biodiversity have not been met [2]. Specifically, tests on water quality improvement in lakes have had contradictory results [3,4], leading to confusion about water quality improvement in China.
Here, we synthesize water pollution governance and evaluate the effectiveness of pollution mitigation actions for rivers and lakes in China. We focus on the main stem of the Yangtze River and 25 associated lakes from the mid to lower reaches of the Yangtze River (MLRYR). Water quality in 2008-2018 is assessed using concentrations of chemical oxygen demand (COD), ammonium-nitrogen (NH 4 + -N) and total phosphorus (TP) for rivers, and COD, total nitrogen (TN), TP and chlorophyll a (Chla) concentration for lakes. Lake trophic state is evaluated using a comprehensive index, and lake ecological condition is assessed with the Shannon-Weaver biodiversity index for benthic macroinvertebrates (Supplementary Materials).
We found that the water quality of rivers has improved across the country. There has been a significant increase in the percentage of grade I (highest water quality) to III cases and a decrease in grade V + (poorest water quality) cases for river cross sections (Fig. S1) and river lengths (Fig. S2). Monthly monitoring of 14 cross sections along the Yangtze River main stem (Table S2) in 2007-2018 demonstrates that COD, NH 4 + -N and TP concentrations have generally declined (Fig. 1b). It is evident that China has made substantial progress towards combating water pollution of rivers.
In contrast, the water quality and ecological condition of lakes have not improved due to low hydraulic flushing rates. Evaluation of lake trophic conditions across the country in 2008-2018 showed an increase in the percentage of eutrophic lakes and a decrease in the percentage of oligotrophic lakes (Fig. S3), indicating that deterioration of lake water quality resulting from eutrophication has neither been halted nor reversed.
We further examined the water quality of 24 lakes from MLRYR (Table  S3, Fig. 1c) between 2008 and 2018, and found no change in COD and TN (p > 0.05) in nearly half of the lakes (45.8% for COD and 54.2% for TN), and a significant increase (p < 0.05) in TP and Chla in most lakes (50% for TP and 66.7% for Chla) (Fig. 1c). Moreover, biodiversity of benthic fauna (Table S4) showed no change in an overwhelming majority of lakes (81.0%) (  Table  S5) indicated that more than half had increased in COD (6 out of 11 lakes), TN (7 out of 11 lakes), TP (9 out of 11 lakes) and Chla (8 out of 10 lakes) (Fig. S4, Table S5). The water quality and ecological state of most lakes seem unresponsive to pollution governance in 2008-2018.
Lake Taihu exemplifies such unresponsiveness to pollution governance. Monitoring of the water quality of rivers feeding the lake in 2008-2018 showed an increase in the number of grade I to III cases and a decrease in the number of grade V and V + cases (Fig. S5). Water quality monitoring of the lake revealed decreased concentrations of TN, no trend of TP and increased Chla (Fig. S6). The Shannon-Weaver biodiversity index of benthic macroinvertebrates showed a slight decline from 2013 to 2018 (Fig. S7). Responses of water quality and benthic biodiversity in Lake Taihu suggest that the lake restoration has been unsuccessful, although this lake has received the largest investment towards pollution governance in China.
Yangtze River to increase the flushing rate and water supply [6]. The effluent diversion mainly focused on control of point source pollution in urban areas. However, these efforts have not resulted in a significant decrease in external loading (Fig. S6), which is attributed to three factors. The first is low wastewater treatment standards and increasing water consumption linked to economic growth (Table S6), resulting in the contribution of point source pollution ranging from one-third to one-half of the external load. The second is the non-point source pollution in rural areas, which accounts for more than 50% of external loading. To date, only ca. 10% of nonpoint source pollution has been reduced through restoring wetlands. The third is the inter-basin water diversion from the Yangtze River, which increases the external loading as nitrogen and phosphorus concentrations of the Yangtze River are higher than Lake Taihu.
Internal loading from sediment is another key issue for shallow lakes. In Lake Taihu, ∼60%-70% of external phosphorus is retained at the lake bottom. This 'legacy phosphorus' is increasingly mobilized with the proliferation of cyanobacterial blooms [7]. The shallow depth (maximum <3 m) and frequent sediment resuspension mean that in-situ measures such as sediment capping or flocculation are largely ineffective. Furthermore, ex-situ treatments, such as sediment dredging, which had been conducted in a limited area (<100 km 2 ), are hindered by inadequate storage capacity for dredged sediments and the secondary pollution risk.
In addition, climate warming has increased cyanobacterial blooms and extended the 'window' to almost yearround for blooms to form and persist. In turn, cyanobacterial bloom decay has led to anoxia and mobilization of nutrients from the sediments, promoting additional cyanobacterial blooms in a positive feedback loop [7].
A similar consequence of lake restoration is evident in Lake Chaohu [8], another large, shallow and eutrophic lake from MLRYR, suggesting that the dilemma of lake restoration in Lake Taihu is representative of a broader challenge in China. The reason for the lake restoration dilemma is the inadequate pollution control of both external and internal loading, which has its roots in the conflict of economic growth and pollution governance in developing areas.
To sustainably improve the water quality of eutrophic lakes in developing areas, pollution governance should be aligned with social and economic development. Considering Lake Taihu as an example, a water-food-energy-climateeconomy nexus could systematically address the SDGs by including pollution governance within the watershed, in association with water quantity management (droughts, floods and inter-basin diversions), green farming and manufacturing (less point and non-point source pollution), sediment management, economic transformation, and adaption to climate change. However, tools addressing diverse sectors at larger, integrated scales are lacking, and need to be developed urgently. Long-term persistent improvement of the water quality and ecosystems of lakes requires nature-based solutions coupled with engineered systems to pursue sustainable development [9]. One positive sign is