According to the report of ‘The Fifth Global Environment Outlook’ by the United Nations Environment Program, more than 40% of water bodies all over the world are suffering eutrophication. Lake eutrophication is a great international concern because of its economic and ecological consequences.
　　Anthropogenic eutrophication of lake ecosystems is a widely acknowledged, but largely unresolved, especially for some large shallow lakes (Okeechobee, Winnipeg, Erie, Great Salt, and Champlain in North America, Lough Neagh, Peipsi, and Malaren in Europe, and Taihu, Chaohu, and Dianchi in China). It is worth thinking about why shallow lakes appear to be prone to eutrophication and resistant to restoration.
　　It is well recognized that the prevalence of lake eutrophication varies with respect to watershed geology, climate, land use, landscape position, connectivity, and lake morphology, each of which varies with lake district or ecoregion. However, most researches studied their effects on lake eutrophication separately, and relatively little is known of how these factors interact. A holistic understanding of lake eutrophication and improved lake management strategies requires freshwaters to be considered as part of an integrated socio-ecological system. There is a profound need to better understand the susceptibility of lakes to eutrophication to safeguard these systems for a sustainable future.
　　To protect lake ecosystems and achieve the Sustainable Development Goals identified by the United Nations, a group of Chinese scientists led by Prof. Boqiang Qin from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences analyzed 1151 lakes with area ≥ 0.5 km2 located within the Europe and the United States of America to identify how lake morphology and regional social-ecological systems interact to affect the susceptibility of lakes to anthropogenic eutrophication.
　　Their findings were published in Water Research.
　　The findings demonstrate that lake depth is linked to the ecoregion and land use of lake ecosystems, which largely determines the intensity of human activities and, consequently, lake productivity. Specifically, lakes distributed in agricultural plain and densely populated lowland areas were generally shallow and subjected to intense human activities with high external nutrient inputs. In contrast, deep lakes frequently occurred in upland regions, dominated by natural landscapes with little anthropogenic nutrient input. Moreover, lake depth appeared to not only reflect external nutrient load to the lake, but also acted as an amplifier that increased shallow lake sensitivity to anthropogenic disturbance.
　　The susceptibility of lake to anthropogenic eutrophication, and consequently the risk of water quality issues, is not the same for all lakes. The findings suggest that shallow lakes are more susceptible to human forcing and are predisposed to receiving large quantities of nutrients, their eutrophication may be not an occasional occurrence. Special attention should be given to shallow lakes that are at high risk of waters quality degradation and eutrophication, yet may be more resistant to restoration compared to deep lakes.
　　In the future, increased agricultural production to meet demands for food and energy will intensify both point and diffuse sources nutrients. Many shallow lakes already exhibit eutrophic conditions, yet lie in watersheds where further eutrophication may be an inevitable rather than an occasional occurrence.
　　This information may help clarify why shallow lakes are prone to eutrophication and why some efforts to control eutrophication have resulted in frustratingly slow or modest effects in shallow productive lakes. This study helps set realistic goals and adjusts community expectations to advance the protection and restoration of lakes globally. It may be a challenge that convincing stakeholders continue to invest in nutrient reductions without evidence of rapid improvement, but it is necessary for long-term water quality improvement. The findings are expected to increase understanding for limnologists, stakeholders, and managers.
　　Lake depth relates the effects of external nutrient input and in-lake biogeochemical processes to regulate the productivity of lake ecosystems.
　　Generally, shallow lakes lie in naturally fertile plain and lowland regions, where they are exposed to strong anthropogenic disturbances (agriculture and urban development) and are predisposed to receiving large quantities of nutrients due to extensive drainage networks. In contrast, deep lakes are frequently concentrated in poor upland regions (mountains and highlands) with mainly natural land cover (e.g., forest and shrubland), low degrees of human disturbance, and limited nutrient input. Compared to deep lakes, shallow basins often have a small volume and weak capacity to dilute input nutrients resulting in high sensitivity to anthropogenic forcing. In addition, strong water-sediment interactions are more common and sediment is more prone to resuspension in shallow lakes, leading to elevated internal nutrient loading and higher productivity. Collectively, shallow lakes in agricultural or populated regions may be particularly susceptible to eutrophication and their eutrophication may be not an occasional occurrence.
　　Paper link: https://www.sciencedirect.com/science/article/pii/S0043135422006819
　　 
　　 
　　 
　　 
　　 
　　 
　　 
　　 
　　 
　　 
　　Contact
　　TAN Lei
　　Nanjing Institute of Geography and Limnology
　　E-mail: ltan@niglas.ac.cn

　　Researchers from Institute of Soil Science Chinese Academy of Sciences (ISSCAS), Nanjing Institute of Geography and Limnology Chinese Academy of Sciences (NIGLAS), and other 25 universities and research institutes have revealed that soil inorganic carbon (SIC) is not temporally stable due to soil acidification, which is different from the traditional viewpoint. The study was published recently in National Science Review.
　　Unlike soil organic carbon (SOC), SIC was frequently disregarded, as its cycling is much slow and the mean residence time is about 78,000 years.
　　“Environmental changes, including the use of chemical fertilizers, global warming and atmospheric acid deposition, can cause significant global soil acidification, especially in cropland, which may accelerate SIC turnover,” said Dr. ZHANG Gan-Lin from NIGLAS.
　　“We didn’t know how much SIC has been lost during last decades at a continental scale like China, due to data limitations.” To quantify these changes, researchers conducted a national resampling campaign in the last decade and collected legacy soil data as much as possible. The national soil data in the 1980s were taken as control plots. By comparing the SIC data in the 1980s and 2010s, they found that approximately half of the pairs exhibited a clear declining trend.
　　“It is a clear evidence of SIC loss,” said Dr. Zhang. “This result is consistent with the findings of similar studies conducted at the regional scale and can provide an important observational benchmark.”
　　They also performed the modeling to map the spatial distribution of SIC and estimated the total changes in SIC stock over the last 30 years. The machine learning technique was used to combine soil data at different sites with the environmental covariates that control SIC turnover. It is evidenced that soil acidification caused by enhanced nitrogen precipitation, together with nitrogen fertilizer application is accounted for SIC reduction. A mass of SIC was found in the subsoils regardless of the ecosystems. The total SIC has decreased by approximately 1.37±0.37 Pg C. About 19% of current SIC stocks are projected to disappear by 2100.
　　Most of the lost SIC is believed to be converted to CO2 and released to the atmosphere. In general, the SIC losses across China and in cropland can offset approximately 18%-24% of the terrestrial biomass organic carbon sink and 57% of the SOC sink in cropland, respectively.
　　The study reveals that the consumption of SIC may offset a large portion of the global efforts aimed at ecosystem carbon sequestration, which emphasizes the importance of better understanding the indirect coupling mechanisms of nitrogen and carbon cycling and of effective countermeasures to minimize SIC loss.
　　Paper link:https://academic.oup.com/nsr/article/9/2/nwab120/6313293
　　Contact
　　TAN Lei
　　Nanjing Institute of Geography and Limnology
　　E-mail: ltan@niglas.ac.cn

　　Quantitative reconstruction of Holocene paleotemperature is crucial for understanding the evolution of the climate system in an ice-free world and assessing the position of recent global warming within the context of natural climate variability. During the Holocene (between 11.5 and 0 ka BP), the initial compilation of reconstructed global temperature records showed a steady cooling trend, conversely, the simulated Holocene temperature exhibited a general warming trend. The discrepancy between model simulations and proxy reconstructions is colloquially referred to as the “Holocene temperature conundrum”.
　　The Tibetan Plateau (TP), located in southwest China, is the highest plateau of the Earth and is one of the most sensitive areas to climate changes with twice as much warming as the global average for the past decades. Many efforts have been taken to reconstruct temperature changes using multiple proxies from various types of geological archives on the TP during the Holocene. However, these paleotemperature reconstructions show complex and even contrasting results and the mechanism underlying such contrasting patterns of temperature changes still remains unclear.
　　A scientific research group, composed of Nanjing Institute of Geography and Limnology, Nanjing University, Institute of Tibetan Plateau Research and other institutions, presents a well-dated, high resolution, quantitative ice-free-season temperature record over the past 19 ka from a small alpine lake on the southeastern TP based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) proxy. The reconstructed temperature displays a long-term ~4°C warming trend during the past 19 ka with a deglacial increase of ~3°C and Holocene increase of ~1°C.
　　This study reviews 16 published quantitative paleotemperature records since the Last Deglaciation, to better understand the pattern and mechanism of postglacial temperature changes on the TP.
　　“Results suggest that a general warming pattern without seasonal biases during the Last Deglaciation but divergent trends of seasonal temperatures during the Holocene with a gradual cooling pattern in summer temperature, an overall warming pattern in winter temperature, and a complicated annual temperature,” said ZHANG Can, first author of the study.
　　The inconsistence among annual temperature records is mainly attributed to the length of ice-free season by comparing the geographic background of proxy-reconstructed data. For the ice-free lakes at low altitude regions, lake water temperature covaries with local air temperature and proxy-reconstructed temperature can really reflect the annul mean temperature changes. In contrast, for the high-altitude lakes with the frozen nature, proxy-reconstructed temperature mainly records air temperature changes during the ice-free season due to the isolation between lake water temperature and atmospheric temperature during the cold season.
　　According the season differences of proxy-reconstructed temperature, the results reveal an overall cooling pattern for summer temperature, a warming trend for winter temperature, annual temperature, and spring-autumn temperature, and a warming-cooling-warming pattern in warming-season temperature. The reconstructed temperature gradually shifts from the “annual temperature-like” pattern to the “summer temperature-like” pattern with decreasing length of ice-free season.
　　“In addition, the TraCE-21ka model results show a remarkable similarity with our composite temperature records of proxy reconstructions for all seasons, and further indicate that the Holocene temperature changes are mainly controlled by the coupling of local seasonal insolation and GHGs on the TP,” ZHANG said.
　　Their findings were published in Earth-Science Reviews.
　　Paper link: https://www.sciencedirect.com/science/article/pii/S0012825222000113
　　 
　　Location and topographical setting of the Tibetan Plateau. (a) Map showing the location of the Cuoqia Lake (CQ Lake, red circle, No. 1) and other paleotemperature records (No. 2–16) reviewed in this study. Filled circle, triangle, and square denote lake, peatland, and ice core, respectively.
　　Comparison of the composite and simulated temperature records for different seasons on the TP. (a) summer temperature, (b) winter temperature, (c) annual temperature, (d) ice-free-season (March–October) temperature (TM-O), and (e) warm-season (May–September) temperature (TMJJAS). (f) Global temperature anomaly from proxy reconstruction and model-simulation. (g) Dome C ice-core record of the atmospheric CO2 concentration.
　　Schematic diagram showing the proxy-reconstructed temperature changes and the driving mechanisms. The length of ice-free season decreases with increasing altitude so that proxy-reconstructed temperature is increasingly skewed toward warm-season. Holocene temperature changes for different seasons are mainly controlled by the coupling of local seasonal insolation and GHGs.TS = summer temperature; TMJJAS = temperature from May to September; TM-O = temperature from March to October; TA = annual temperature; ﹢ = positive correlation; ﹣ = negative correlation.
　　 
　　 
　　 
　　Contact information
　　TAN Lei
　　Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences
　　E-mail: ltan@niglas.ac.cn
　　Web: http://english.niglas.cas.cn/