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Study: China Reverses Widespread Freshwater Deoxygenation via Wastewater Management

Freshwater ecosystems worldwide have been suffering from declining oxygen levels—a trend known as deoxygenation—that threatens biodiversity, fisheries, and ecosystem stability. However, a new study published on June 26 in Nature Geoscience offers hope: targeted nutrient management via wastewater control can reverse this trajectory, even in the face of rapid climate warming.Led by Professor ZHOU Yongqiang from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, an international research team analyzed 18 years of monthly data (2005–2022) from 972 rivers and 354 lake sites across China. Their findings challenge the prevailing narrative that aquatic deoxygenation is an unstoppable consequence of human development and global warming.Although surface waters warmed by 1.2 °C per decade, a rate higher than the global average, the researchers found that dissolved oxygen (DO) concentrations increased across China's inland waters during the 18 years of the study—a phenomenon that is not typically associated with rising water temperatures.Specifically, DO levels rose by an average of 0.93 mg/L per decade in rivers and 0.38 mg/L per decade in lakes. This recovery led to a dramatic reduction in the incidence of hypoxia (low oxygen) and anoxia (no oxygen), with recorded hypoxic events in rivers falling from 170 occurrences in 2005–2010 to just 25 in 2017-2022.The primary driver of this remarkable recovery, the team found, was not related to increased photosynthesis from algae, but rather a reduction in organic pollution.Using variance partitioning and machine learning algorithms (XGBoost), the team found that decreases in biochemical oxygen demand (BOD), ammonium, and chemical oxygen demand (COD) were the best predictors of rising DO levels. In contrast, changes in phytoplankton abundance (measured as chlorophyll-a) showed no consistent relationship with DO trends, ruling out algal-driven oxygen supersaturation as a cause for the recovery.China's investments in environmental restoration, which surged from 1 trillion to 10 trillion RMB (approximately US$148 billion to US$1.48 trillion) annually between 2000 and 2022, expanded wastewater treatment coverage from 34.3% to 98.1% of the population. This resulted in nationwide declines in BOD, COD, and nutrients like nitrogen and phosphorus, the study suggested."While water temperature remains a strong predictor of oxygen solubility, our models show that reducing oxygen demand through pollution control has more than offset the oxygen loss expected from warming," ZHOU said."The correlations between provincial investment in sewer infrastructure, the volume of wastewater treated, and the magnitude of DO recovery are exceptionally strong," he added.The study also highlights where recovery is strongest—small headwater streams and the warm-temperate zones of central China. However, challenges remain in regions dominated by agricultural nonpoint-source pollution.According to the study, the rapid flushing rates of many Chinese freshwaters likely contributed to the swift response to management, as legacy pollutants stored in sediments were less of a factor than in deeper, stratified lakes."These results provide clear optimism for global restoration efforts," ZHOU said. "Effective water quality management can improve oxygen levels, protecting aquatic life and reducing the risk of deoxygenation while the climate continues to warm."

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Study Reveals Critical Temperature Thresholds for Lake Ice Cover Loss Across Northern Hemisphere
2026-07-28
Winter ice cover on lakes sets the timing and magnitude of most ecological processes in northern freshwaters, from energy balance and nutrient cycling to primary production and fisheries. Yet the exact response of lake ice cycles to rising air temperatures has remained poorly quantified.Now, an international research team led by scientists from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (NIGLAS), Nanjing Normal University, Bangor University, and the University of Regina has analyzed ice phenology records and air temperature data from 724 lakes across the Northern Hemisphere between 2000 and 2022.The study was published in Proceedings of the National Academy of Sciences USA on July 27.The researchers found that lakes exhibit a pronounced asymmetry in their response to warming. Spring ice-off dates are far more sensitive to rising air temperature than autumn freeze-up dates, meaning the melting process accelerates considerably faster than the freezing process as the climate warms. More critically, the analysis identified a set of winter temperature thresholds ranging from -13.7 to -6.8°C. Once mean winter air temperature climbs above these thresholds, the rate of ice loss increases sharply, with phenological sensitivity rising up to 22-fold."Below these thresholds, ice cover responds to warming in a gradual manner," said Dr. ZHOU Jian of Nanjing Normal University, first author of the study. "But once the threshold is crossed, the system enters a fundamentally different regime where each additional degree of warming triggers a disproportionately large loss of ice cover."The study also found that the thermal thresholds are regulated primarily by broad climatic factors, particularly winter air temperature and surface albedo, rather than by individual lake shape or depth. This indicates that the timing of threshold crossing depends more on large-scale climatic conditions than on local lake morphology.Looking ahead, the team projected that under a high-emission scenario, the duration of winter ice cover on Northern Hemisphere lakes will shrink by roughly 40 days by the end of this century. The proportion of lakes that have crossed critical thermal thresholds is expected to rise from 23 percent to 70 percent over the same period."Our findings suggest that many mid-latitude lakes are approaching a state of heightened vulnerability," said Prof. SHI Kun from the NIGLAS, co-corresponding author of the study. "Even modest additional warming could trigger abrupt and potentially irreversible shifts in lake ice regimes, with cascading consequences for water quality, aquatic ecosystems, and northern communities that depend on stable winter ice cover."The research provides a new mechanistic framework for predicting threshold-driven ice loss from northern lakes. The authors stress that incorporating such nonlinear thermal sensitivities into next-generation climate models is essential for reliable projections of freshwater ecosystem change.
Satellite Imagery Reveals Human Imprints on One-fifth of Global Riverfronts
2026-07-20
A new study led by researchers from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS) mapped 7.52 million kilometers of global riverfronts using satellite imagery and deep learning. They found that nearly 20% of these critical water-land transition zones are modified by agriculture and urban development. These human imprints, particularly concentrated in the "Low- and mid-latitude Afro-Eurasia Corridor" (LAEC), cause severe lateral fragmentation that degrades water quality and threatens freshwater biodiversity.Rivers support human societies and biodiversity. Global assessments of anthropogenic impacts on rivers have traditionally focused on longitudinal alterations, such as dams. However, the riverfront, as the dynamic lateral transition zone buffering pollutants and providing habitat, has remained poorly mapped due to spatial resolution constraints. To address this gap, a research team led by Prof. SONG Chunqiao at NIGLAS developed the Global Riverbank rOUting Dataset (GROUD). Published in Nature Communications, the study utilized deep learning and high-resolution satellite imagery to classify 7.52 million kilometers of global riverfronts.The study reveals that nearly 20% of global riverfronts have been transformed by human activity, predominantly driven by agricultural expansion (13.43%) and artificial built-up areas (6.29%). Notably, the researchers identified a continuous, highly developed belt termed the "Low- and mid-latitude Afro-Eurasia Corridor" (LAEC), which spans North Africa, Europe, West Asia, South Asia, and East Asia. This corridor contains approximately 60% of the world's human-imprinted riverfronts, exhibiting a modification density six times higher than regions outside the LAEC, which reflects a strong historical and modern societal dependency on these water resources.Human imprints, particularly from agriculture and infrastructure, have caused extensive lateral fragmentation of natural riverfronts. This lateral fragmentation disrupts the natural buffering capacity of riverfronts, leading to a decline in the river's self-purification capabilities. The study found strong spatial correlations between highly modified riverfronts and elevated levels of total dissolved solids (TDS) and biochemical oxygen demand (BOD), with rivers inside the LAEC exhibiting significantly higher concentrations of both.Furthermore, the loss of contiguous riverfront habitats poses a severe threat to biodiversity. The synergistic effects of lateral riverfront modification and longitudinal damming significantly escalate the risks for freshwater species, threatening 20.73% of amphibians and 16.60% of freshwater fishes where both stressors overlap globally. Within the LAEC specifically, this neglect contributes to the severe threat facing 34.02% of amphibians and 29.01% of freshwater fishes.The researchers advocate that future river conservation efforts must evolve beyond traditional channel management to explicitly include the ecological integrity of lateral riverfront zones, employing nature-based solutions to restore connectivity.The paper, "Human imprints on global riverfronts", was authored by Dr. ZENG Fanxuan, Prof. SONG Chunqiao, and international collaborators, and was published online in Nature Communications on July 18, 2026.
Study: China Reverses Widespread Freshwater Deoxygenation via Wastewater Management
2026-06-26
Freshwater ecosystems worldwide have been suffering from declining oxygen levels—a trend known as deoxygenation—that threatens biodiversity, fisheries, and ecosystem stability. However, a new study published on June 26 in Nature Geoscience offers hope: targeted nutrient management via wastewater control can reverse this trajectory, even in the face of rapid climate warming.Led by Professor ZHOU Yongqiang from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, an international research team analyzed 18 years of monthly data (2005–2022) from 972 rivers and 354 lake sites across China. Their findings challenge the prevailing narrative that aquatic deoxygenation is an unstoppable consequence of human development and global warming.Although surface waters warmed by 1.2 °C per decade, a rate higher than the global average, the researchers found that dissolved oxygen (DO) concentrations increased across China's inland waters during the 18 years of the study—a phenomenon that is not typically associated with rising water temperatures.Specifically, DO levels rose by an average of 0.93 mg/L per decade in rivers and 0.38 mg/L per decade in lakes. This recovery led to a dramatic reduction in the incidence of hypoxia (low oxygen) and anoxia (no oxygen), with recorded hypoxic events in rivers falling from 170 occurrences in 2005–2010 to just 25 in 2017-2022.The primary driver of this remarkable recovery, the team found, was not related to increased photosynthesis from algae, but rather a reduction in organic pollution.Using variance partitioning and machine learning algorithms (XGBoost), the team found that decreases in biochemical oxygen demand (BOD), ammonium, and chemical oxygen demand (COD) were the best predictors of rising DO levels. In contrast, changes in phytoplankton abundance (measured as chlorophyll-a) showed no consistent relationship with DO trends, ruling out algal-driven oxygen supersaturation as a cause for the recovery.China's investments in environmental restoration, which surged from 1 trillion to 10 trillion RMB (approximately US$148 billion to US$1.48 trillion) annually between 2000 and 2022, expanded wastewater treatment coverage from 34.3% to 98.1% of the population. This resulted in nationwide declines in BOD, COD, and nutrients like nitrogen and phosphorus, the study suggested."While water temperature remains a strong predictor of oxygen solubility, our models show that reducing oxygen demand through pollution control has more than offset the oxygen loss expected from warming," ZHOU said."The correlations between provincial investment in sewer infrastructure, the volume of wastewater treated, and the magnitude of DO recovery are exceptionally strong," he added.The study also highlights where recovery is strongest—small headwater streams and the warm-temperate zones of central China. However, challenges remain in regions dominated by agricultural nonpoint-source pollution.According to the study, the rapid flushing rates of many Chinese freshwaters likely contributed to the swift response to management, as legacy pollutants stored in sediments were less of a factor than in deeper, stratified lakes."These results provide clear optimism for global restoration efforts," ZHOU said. "Effective water quality management can improve oxygen levels, protecting aquatic life and reducing the risk of deoxygenation while the climate continues to warm."
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Int’l Cooperation News

2026-05-26

NIGLAS Delegation Attends 2026 ASLO-SIL Joint Conference in Canada
Sino-Tanzania collaboration boosts the improvement of water and air environment in the near-lake area of Lake Victoria
2026-01-30
Some bays of the Lake Victoria are now facing with the severe problem of Lake Eutrophication, which is mainly caused by the nitrogen (N) discharged from crop farming in the lake bay areas. While the farmers' behaviors affect directly on the N discharged from crop farming.The Sino-Tanzania cooperation project, a part of the International Partnership Program of the Chinese Academy of Sciences is led by Dr. XIONG Chuanhe from the Nanjing Institute of Geography & Limnology (NIGLAS). The project is dedicated to addressing the issues and improving the water and air environment in the near-lake area.Recently, the project conducted on-site investigations to three villages in the Mwanza Lake Bay area of Lake Victoria: Sweyavillage, Kisoko, and Kigoto. Through communication with village leaders, reviewing production records, and conducting on-site estimations, the project investigated the land use and input-output conditions.During the on-site investigations, the project team also conducted interviews and questionnaires with farmers, gaining a comprehensive understanding of the household characteristics, main agricultural production behaviors, awareness of agricultural non-point source pollution, and coping abilities of farmers in dealing with non-point source pollution.The project clarified the main farming behaviors of farmers in the Victoria Lake Basin that have an impact on nitrogen discharge in the area, as well as the extent of such impact. It also revealed the specific mechanisms by which farmers' capabilities and cognition play a role in decision-making regarding the main farming behaviors that have an impact on nitrogen discharge."These efforts enhance China's international reputation in environmental conservation, " said by Dr. XIONG. Economically, the research on nitrogen discharge in lake bay area will help improve water and air quality in surrounding regions, supporting agricultural productivity and local farmers' livelihoods. Socially, the project fosters capacity-building for environmental protection and sustainable development across Africa.
National Key Research and Development Program Plan (Sino-Mongolian international cooperation project) Successfully Convoked in Nanjing and Bayannur
2023-09-11

　　On September 4th to 7th, 2023, the symposiums on the establishment and implementation of the National Key Research and Development Program of China, i.e. Sino-Mongolian Intergovernmental Joint Research Program: The impacts of global change on nutrient cycles and ecosystem evolution in shallow lakes in Sino-Mongolian cold-arid area (No. 2023YFE0100500) were successfully held in Nanjing and Bayannur. 
　　Prof. ZHANG Ganlin, the director of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS), delivered a welcoming address on behalf of the host institution. Dr. Odsuren, the scientific secretary of the Institute of Geography and Geoecology, Mongolian Academy of Sciences (IGG, MAS) made a detailed introduction of IGG, MAS. 
　　After that, the project leader Dr. HAN Chao, introduced the current state of lake ecosystems in China and presented a comprehensive report on the overall research objectives, content, implementation plan, expected outcomes, innovative aspects, task decomposition, and progress schedule of the joint project. Followed, Dr. Narangerel from IGG, MAS presented a detailed introduction to the research team's prior achievements in the field of lake science in the cold-arid regions of Mongolia and the current ecological environment of Lake Ugii. 
　　During the local coordination symposium held in Bayannur, Inner Mongolia, Director Bao Wei from Wuliangsu Lake Ecological Protection Center extended a warm welcome to the attending experts. He emphasized that this project was a significant decision and deployment for the protection and comprehensive management of Wuliangsu Lake, which would effectively support the ecological environment protection on Wuliangsu Lake. Subsequently, the joint program teams carried out discussions and communications with the local administrators regarding the implementation plan, research platform status and infrastructure, data collection and sharing, preparation of field instruments and equipment, training of young researchers, and fieldwork plans. After the meeting, all members conducted the field surveys on Wuliangsu Lake. 
　　More than 20 experts and major leaders from NIGLAS, IGGMAS, and Wuliangsu Lake Ecological Protection Center participated in the symposiums. It further expanded the researcher exchanges and cooperation, promoted friendly contacts and mutual understanding between the two sides, which played a significant role in facilitating the smooth implementation of the joint project.
　　(photo by NIGLAS)
　　
Synergistic effects of warming and internal nutrient loading interfere with the long-term stability of lake restoration and induce sudden re-eutrophication
2023-02-27

　　Urban lakes are globally ubiquitous and are usually highly eutrophic, pointing to an increase in frequency, duration and magnitude of harmful algal blooms as wide-spread threats to ecological and human health. 
　　Over half a century, phosphate (P) precipitation is among the most effective treatments to mitigate eutrophication in these lakes. However, after a period of high effectiveness, re-eutrophication would possibly occur leading to the return of harmful algal blooms. While such abrupt ecological changes were presumably attributed to internal P loading, the role of lake warming and its potential synergistic effects with the internal loading, thus far, has been largely understudied. 
　　Researchers led by Dr. KONG Xiangzhen and Prof. Dr. XUE Bin from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, along with their international collaborators, have addressed the question by quantifying the contributions of lake warming and the potential synergistic effects with internal P loading in an urban lake located in central Germany, which suffered from the abrupt re-eutrophication and cyanobacterial blooms in 2016 (30 years after the first P precipitation). 
　　Their findings were published in Environmental Science & Technology on Feb. 20. 
　　In this study, a process-based lake ecosystem model (GOTM-WET) was established using a high-frequency monitoring dataset covering eutro-/oligo-trophic states over 30 years. 
　　Model analyses suggested that, for the abrupt occurrence of cyanobacterial blooms, internal P release accounts for 68% of the biomass proliferation, while lake warming contributed to 32%, including both direct effects via promoting growth (18%) and synergistic effects via intensifying internal P loading (14%). The model further revealed that the synergy was attributed to prolonged lake hypolimnion warming and oxygen depletion. 
　　“Our study exemplifies how process-based mechanistic modeling could help to tease apart important drivers of abrupt shifts and cyanobacterial blooms in lakes, particularly in an era of rapid global changes including climate change and human activities.” said Dr. Kong. 
　　This study unravels the substantial role of lake warming in promoting cyanobacterial blooms in re-eutrophicated lakes. The indirect effects of warming on cyanobacteria via promoting internal loading need more attention in future lake research and management. 
　　“Our findings will have far-reaching consequences for lake restoration and management as the nutrient targets we applied so far to reach or maintain a certain trophic state will not work in a far warmer future and need to be adjusted, i.e. stronger nutrient level reduction and higher efforts in restoration are demanded.” said Dr. Kong.
　　 
　　link: https://pubs.acs.org/doi/10.1021/acs.est.2c07181
　　 
　　 
　　Contact 
　　TAN Lei 
　　Nanjing Institute of Geography and Limnology 
　　E-mail: ltan@niglas.ac.cn
Sediment organic matter properties facilitate understanding nitrogen transformation potentials in East African lakes
2022-10-20

　　East African lakes include the most productive and alkaline lake group in the world. Yet, they generally receive fewer nutrient inputs than the densely populated subtropical and temperate lakes in the northern hemisphere. In these lakes with insufficient supplies of inorganic nitrogen, the mineralization of benthic organic matter can play an important role in driving the nutrient cycle and nitrogen loss. Using a suite of stable 15N isotope dilution and tracer techniques, we examined five main processes of the sediment nitrogen cycle in 16 lakes and reservoirs of Tanzania and Kenya, East Africa: gross nitrogen mineralization, ammonium immobilization, dissimilatory nitrate reduction to ammonium (DNRA), and the dinitrogen (N2) production via denitrification and anaerobic ammonium oxidation (anammox). Gross nitrogen mineralization and ammonium immobilization showed the maximum values of 9.84 and 12.39 μmol N kg-1 h-1 , respectively. Potential DNRA rates ranged from 0.22 to 8.15 μmol N kg-1 h-1 and accounted for 10 %–74 % (average 25 %) of the total dissimilatory nitrate reduction. Potential nitrate reduction rates in most lakes were dominated by denitrification with a contribution of 26 %–85 % and a mean of 65 %. We further found that the sediment nitrogen transformations were driven mainly by benthic organic matter properties and water column phosphate concentrations, reflecting microbial metabolic responses to the changing carbon and nutrients availability. For instance, autochthonous production of protein-like organic matter attributed to active sediment nitrogen mineralization, DNRA, and denitrification. In contrast, the high degree of humification caused by the inputs of terrestrial humic-like substances slowed down the sediment nitrogen transformations. The contribution of DNRA to total dissimilatory nitrate reduction was significantly positively correlated to sediment C: N ratios. These results indicate that predictions of sediment N supply and loss in East African lakes can be improved by incorporating sediment organic matter properties.
　　Xiaolong Yao, Zhonghua Zhao, Jianjun Wang, Qiqi Ding, Minglei Ren, Ismael Aaron Kimirei, Lu Zhang, Sediment organic matter properties facilitate understanding nitrogen transformation potentials in East African lakes, Science of The Total Environment, 841, 2022, 156607, https://doi.org/10.1016/j.scitotenv.2022.156607.
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