Scientists Revealed the Molecular-level Impacts of Global Change on Natural Organic Matter for the First Time


  Dissolved organic matter (DOM) is the main form and active component of natural organic matter in lakes. DOM acts as a large reservoir of carbon. Furthermore, the processing of different organic compounds in DOM by aquatic organisms can be impacted by changes in temperature, which represents a potential climate feedback loop as the rate of carbon dioxide released into the atmosphere may be slowed or accelerated under higher temperatures. 
  Previous study simplified this complex issue in computational models by classifying different types of DOM into a few categories based on its response (or lack of response) to changing environmental conditions. 
  However, researchers led by Prof. WANG Jianjun from the Nanjing Institute of Geography and Limnology, Chinese Academy Sciences, recently developed an indicator that can be used to quantify the response of individual DOM constituents to changing environmental conditions. The indicator could allow for a more realistic and nuanced representation of DOM environmental responses, as opposed to a simple classification. 
  The study was published in Nature Communications on 17 January. 
  The researchers conducted field experiments on mountainsides of three distinct climate zones in Eurasian continent to assess how temperature affects the composition of DOM. The mountainsides ranged from a subtropical wet environment on the southeastern edge of the Tibetan Plateau, through a temperate arid environment in the northern Tibetan Plateau, to a subarctic environment in Northern Europe. 
  They discovered that individual DOM constituents showed a wide range of temperature responses. The thermal responses of DOM were further found to increase towards warmer conditions such as at low elevations. 
  Also, this warming effect could be strengthened by nutrient enrichment. The warming effect was strengthened by eutrophication, with increased sensitivity of up to 22% for each additional 1 mg L-1 of nitrogen loading. This suggests that the temperature responses of organic matter can be affected by other global change drivers, especially nutrient enrichment in complex ways. 
  Moreover, the research indicated that the thermal responses of individual organic molecules were associated with their chemical properties. Additionally, despite the differences in climate zones, the thermal responses of these molecules showed a remarkable level of consistency. Organic carbon molecules with lower thermodynamic favorability for microbial decomposition exhibited a higher positive response to temperature. Each organic carbon molecule exhibited similar thermal response across the three highly divergent mountain environments. The findings revealed that the responses of molecules to temperature were transferable and generalizable across regional and continental scales. 
  This is the very first application of molecular-level methods to assess the responses of organic matter to global change. The role of DOM in climate feedbacks and the earth system can be properly understood by this work, which will allow us to better prepare for an uncertain future.
  https://doi.org/10.1038/s41467-024-44813-2
  Contact TAN Lei Nanjing Institute of Geography and Limnology E-mail: ltan@niglas.ac.cn

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Scientists Revealed the Molecular-level Impacts of Global Change on Natural Organic Matter for the First Time
2024-01-18

  Dissolved organic matter (DOM) is the main form and active component of natural organic matter in lakes. DOM acts as a large reservoir of carbon. Furthermore, the processing of different organic compounds in DOM by aquatic organisms can be impacted by changes in temperature, which represents a potential climate feedback loop as the rate of carbon dioxide released into the atmosphere may be slowed or accelerated under higher temperatures. 
  Previous study simplified this complex issue in computational models by classifying different types of DOM into a few categories based on its response (or lack of response) to changing environmental conditions. 
  However, researchers led by Prof. WANG Jianjun from the Nanjing Institute of Geography and Limnology, Chinese Academy Sciences, recently developed an indicator that can be used to quantify the response of individual DOM constituents to changing environmental conditions. The indicator could allow for a more realistic and nuanced representation of DOM environmental responses, as opposed to a simple classification. 
  The study was published in Nature Communications on 17 January. 
  The researchers conducted field experiments on mountainsides of three distinct climate zones in Eurasian continent to assess how temperature affects the composition of DOM. The mountainsides ranged from a subtropical wet environment on the southeastern edge of the Tibetan Plateau, through a temperate arid environment in the northern Tibetan Plateau, to a subarctic environment in Northern Europe. 
  They discovered that individual DOM constituents showed a wide range of temperature responses. The thermal responses of DOM were further found to increase towards warmer conditions such as at low elevations. 
  Also, this warming effect could be strengthened by nutrient enrichment. The warming effect was strengthened by eutrophication, with increased sensitivity of up to 22% for each additional 1 mg L-1 of nitrogen loading. This suggests that the temperature responses of organic matter can be affected by other global change drivers, especially nutrient enrichment in complex ways. 
  Moreover, the research indicated that the thermal responses of individual organic molecules were associated with their chemical properties. Additionally, despite the differences in climate zones, the thermal responses of these molecules showed a remarkable level of consistency. Organic carbon molecules with lower thermodynamic favorability for microbial decomposition exhibited a higher positive response to temperature. Each organic carbon molecule exhibited similar thermal response across the three highly divergent mountain environments. The findings revealed that the responses of molecules to temperature were transferable and generalizable across regional and continental scales. 
  This is the very first application of molecular-level methods to assess the responses of organic matter to global change. The role of DOM in climate feedbacks and the earth system can be properly understood by this work, which will allow us to better prepare for an uncertain future.
  https://doi.org/10.1038/s41467-024-44813-2
  Contact TAN Lei Nanjing Institute of Geography and Limnology E-mail: ltan@niglas.ac.cn
Researchers Reveal the Impact of Extreme Heat on Lake Warming in China
2024-01-08

  In recent years, a pronounced global surge in the frequency and intensity of heat extremes has signaled the integration of climate change into a new normal. This shift poses a serious threat to both economic and societal sustainable development. 
  Moreover, lakes display remarkable sensitivity to climate change, serving as pivotal indicators and regulators of global environmental shifts and regional climates. Heat extremes can rapidly elevate the surface water temperature in lakes, swiftly compromising their physical, chemical, and biological attributes. This disruption, in turn, has profound and irreversible consequences for the entire lake ecosystem. 
  To quantify the contribution of heat extremes to the lake warming in China, Prof. SHI Kun and other researchers from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, collaborated with Dr. R. Iestyn Woolway from the University of Bangor, UK. They utilized the 1985-2022 ERA5-Land (European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5-Land) air temperature data as the forcing data, Landsat surface temperature products provided by NASA and USGS as the reference for lake surface water temperatures, and combined with the physical/statistical hybrid model Air2Water, to construct a long time-series lake surface water temperature dataset for 2,260 lakes. They conducted a comparative analysis of the patterns of change in summer air temperatures, lake surface air temperatures, and lake surface water temperatures. This analysis was performed both before and after the removal of heat extremes, allowing for the quantification of the contribution of heat extremes to the warming of China's lakes from 1985 to 2022. 
  The study was published in Nature Communications on January 2. 
  The results show that China experienced an average of about 454 days of extreme heat from 1985 to 2022. Overall, the number of days with heat extremes showed an increasing trend, about 2.08 days/decade, as well as a gradual increase in the intensity of heat extremes, about 0.03°C/day·decade. After removing the heat extremes, the average increase in air temperature in China decreased from 0.32 ± 0.10 °C/decade to 0.17 ± 0.08 °C/decade. 
  The research further quantified the effect of heat extremes on long-term changes in lake surface water temperatures. Removal of heat extremes resulted in a decrease in mean cumulative heat from 172.98°C to 66.65°C and a decrease in intensity of heat extremes from 0.43°C/day to 0.19°C/day. In addition, the average change in lake surface water temperature decreased from 0.16°C/decade to 0.13°C/decade, indicating that changes in heat extremes also contributed significantly to the increase in lake surface water temperature. 
  Although the number of days of heat extremes in the lake surface air temperature accounted for only 3% of the study period (~459 days), the contribution of heat extremes to the change of surface water temperature of China's lakes was as high as 36.5% from 1985 to 2022. It suggests that the occurrence of short-term heat extremes can have profound impacts on lake systems on seasonal, annual and even longer time scales. 
  This study promotes an insightful understanding of the impacts of climate change on lake ecosystems, and is of great scientific and practical significance for the construction of risk assessment and warning systems for heat extremes, and the establishment of new adaptation strategies and coping mechanisms for lakes under the new climate change norm of frequently occurrence of heat extremes.
  Contribution of heat extremes to lake surface water temperature (LSWT) (IMAGE by SHI Kun)
  DOI https://doi.org/10.1038/s41467-023-44404-7 
   Contact TAN Lei Nanjing Institute of Geography and Limnology E-mail: ltan@niglas.ac.cn
Greening land significantly reduced suspended sediment flux in China’s major rivers
2023-11-16

  Sediment in rivers plays an essential role in the river and watershed ecosystems. A recent paper (Science, 24 June 2022, p. 1447-1452) reported widespread decreases in river sediment fluxes for major rivers in the North Hemisphere and concluded that these decreases mainly resulted from dam construction. 
   Generally, the spatial and temporal variations in river sediment are regulated by climate change and human activities, including precipitation, land cover and land use, damming, and dredging activities. In addition to dams, what is the contribution of changes in land cover to the decrease in sediment flux in rivers? 
   Recently, a research team led by Dr. CAO Zhigang and Prof. DUAN Hongtao (Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, China) and Prof. YANG Hong (University of Reading, the UK), collected long-term in-situ sediment flux data from seven major rivers in China. The team found that large dams on the Yangtze River, Zhu River, and Yellow River significantly reduced the sediment fluxes in the first 5-10 years following dam construction. However, the sediment fluxes in these rivers did not continue to decrease as before but even increased. 
   Their findings were published in Science Bulletin on October 30, 2023. 
   “Besides dams, there must be other factors that continuously reduce sediment fluxes after dams have been operational for many years,” said Prof. DUAN. 
   The researchers further used satellites to obtain a long-term dataset of vegetation coverage and suspended sediment concentration in China’s major rivers. The team found that vegetation cover in river catchments significantly increased in the 1980s and correlated well with the variations in sediment concentration. 
   “The variations in sediment fluxes in estuaries resulted from the combined effects of dams and land cover change in the watershed,” said Dr. CAO. 
   “Undoubtedly, anthropogenic activities have altered the landscape in the watershed in various ways. Our analysis demonstrated a significant relationship between vegetation cover in the watershed and river sediment, suggesting that the greening of Chinese land mainly induced by afforestation in the watersheds significantly reduces sediments in rivers,” said Prof. YANG. 
   This study illustrates that greener land exerted essential controls on reducing soil erosion and sediment loads into China's rivers. To a certain extent, while the influence of dam construction on river sediment was dramatic, the impact of land use change was even more profound. 
  In the past two decades, China has implemented several massive ecological restoration programs, including afforestation and the “Grain for Green” project, which significantly increased terrestrial vegetation. Greening land is another crucial strategy to reduce sediments in rivers, and it can also increase carbon sequestration and enhance other ecological services. 
   See the article: 
  Remarkable effects of greening watershed on reducing suspended sediment flux in China major rivers 
  https://doi.org/10.1016/j.scib.2023.08.036
  Contact 
   TAN Lei 
   Nanjing Institute of Geography and Limnology 
   E-mail: ltan@niglas.ac.cn
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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.
A comprehensive evaluation of organic micropollutants (OMPs) pollution and prioritization in equatorial lakes from mainland Tanzania, East Africa
2022-05-17

  A lack of understanding the fate of highly toxic organic micropollutants (OMPs) in the equatorial lakes of Tanzania hinders public awareness for protecting these unique aquatic ecosystems, which are precious water resources and stunning wildlife habitats. To address this knowledge gap, the occurrence of 70 anthropogenically-sourced OMPs, including phthalates (PAEs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), was investigated in the water and sediment of 18 lakes in Tanzania. Similar residue concentrations were found in both compartments, showing higher pollution of PAEs ranging from 835.0 to 13,153.1 ng/L in water and 244.6–8691.8 ng/g dw in sediment, followed by PAHs, while OCPs and PCBs were comparatively lower. According to the multi-criteria scoring method for prioritization, the final OMP priority list for the lake environment in Tanzania comprised 25 chemicals, specifically 5 PAEs (DEHP, DIBP, DBP, DCHP and DMPP), 6 PCBs (PCB153, PCB105, PCB28, PCB156, PCB157 and PCB167), 6 PAHs (BaP, BaA, BbF, Pyr, DahA and InP) and 8 OCPs (cis-chlordane, trans-chlordane, p,p’-DDD, p,p’-DDE, p,p’-DDT, endrin, methoxychlor and heptachlor epoxide), suggesting the key substances for conventional monitoring and pollution control in these equatorial lakes, with an emphasis on PAEs, especially DEHP, due to the top priority and endocrine disruptor properties.
  Zhonghua Zhao, Xiaolong Yao, Qiqi Ding, Xionghu Gong, Jianjun Wang, Saadu Tahir, Ishmael Aaron Kimirei, Lu Zhang, A comprehensive evaluation of organic micropollutants (OMPs) pollution and prioritization in equatorial lakes from mainland Tanzania, East Africa, Water Research, Volume 217, 2022, 118400, ISSN 0043-1354, https://doi.org/10.1016/j.watres.2022.118400.
Re-evaluation of Wetland Carbon Sink Mitigation
2022-03-22

  A new review of coastal and inland wetland carbon sink services reveals current mitigation concepts for greenhouse gas emissions and measurements are not what they seem. Accumulation of buried organic carbon is not a measure of carbon sequestration; stable organic carbon inputs require subtraction and are undervalued; and carbon mitigation from wetland restoration is less than their preservation. 
  The study was published in the journal Wetlands as a flagship Mark Brison Review, from Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS) in collaboration with Borneo Marine Research Institute (BMRI) Universiti Malaysia Sabah (UMS), and lead by the Institute for Marine and Antarctic Studies (IMAS) University of Tasmania (UTAS).  
  Associate Reseacher Dr John Barry Gallagher (IMAS) said that the sediment organic carbon accumulation down inland and coastal wetlands has always been regarded as a convenient means of measuring trends and average rates of sequestration over climatic scales. Wetlands, however, are open to organic inputs from catchments and adjacent water bodies. These can be labile and easily consumed or decomposed, and recalcitrant outside the carbon loop that is not consumed or decomposed. 
  Consequently, what is required from the sediment record is not the total organic burial, but the burial rate of what remains of the wetlands plant production from the amount of the labile organics inputs consumed, and the remains of those recalcitrants inputs, largely black or pyrogenic carbon. To estimate this we modified a general decomposition model to hindcast the original input rate and to project what remains for all organic sources after 100 years of burial. 
  For a mangrove and a seagrass ecosystem, we found that carbon accumulation was on average 33.5 and 7.2 times greater than their respective sequestration rates. We also noted that sequestration relative to its non-canopy replacement or alternative stable state is not included for voluntary or compliance carbon markets, instead, only the rate of loss and gain of organic stocks for wetlands likely be disturbed or restored. This limitation would otherwise undervalue the wetlands systems mitigation potential with one caveat: the rate of gain in sediment stocks for a restored system is similarly constrained as a mitigation service by consumption and decomposition of those external organic inputs. 
  Dr Gallagher says that the review is important from two standpoints. Firstly, natural carbon sequestration solutions require re-evaluation. This is required to avoid GHG emissions above their capacity or indeed reduce the ability to fulfil Nations’emission targets, as set by COP26. Secondly, the model provides a new Paleoecological tool. It has the potential to measure and predict how wetlands' ability to function as a carbon sink can change with both climate and catchment agricultural and industrial development from changes to government policy.
  Paper link: https://link.springer.com/article/10.1007/s13157-022-01539-5 
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