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Lake eutrophication in northeast China induced by the recession of the East Asian summer monsoon
Abstract
Lakes are one of the most important freshwater resources on Earth and they provide a wide range of ecosystem services. However, due to rapid economic development and the intensification of human activities, many lakes have become eutrophic, which may threaten their status as water resources. Human activities have played a significant role in lake eutrophication, but whether this role is independent of, or coupled with, natural climate change requires further study. We selected Dali Lake, a large lake affected by human activity within the ancient warfare borders, to clarify the ecological response of a lake to climate change and human activity. We used analyses of sedimentary n-alkanes and AMS ¹⁴C dating to reconstruct the paleolimnological evolution of Dali Lake since 15 cal kyr BP, and specifically to assess the timing and causes of eutrophication. The results show that the short-chain n-alkanes (C17–19-alkanes) in Dali Lake are mainly produced by bacteria and algae within the lake, and that the sedimentary absolute abundance of short-chain n-alkanes (A17–19-alkanes) can be used as a proxy for assessing the ecological status of the lake. The ecological status of Dali Lake was the most stable during the early to middle Holocene, when the East Asian summer monsoon was strong, but bacterial and algal outbreaks occurred during three episodes of a weakened summer monsoon—the Older Dryas, Younger Dryas, and the Common Era—when the lake experienced different degrees of eutrophication. During the recession of the East Asian summer monsoon, the weakening of precipitation recharge of the lake led to a reduction in lake area and an increase in nutrient concentrations in the lake water, while aeolian dust input was an additional nutrient source, leading to bacterial and algal outbreaks. During the Common Era, lake eutrophication occurred in the context of both summer monsoon recession and enhanced human activities, but their combined effects did not lead to more intense lake eutrophication than was caused by monsoon recession during the Younger Dryas. We conclude that, although human activities have enhanced the eutrophication of Dali Lake, the reduction in lake size due to monsoon recession and the resulting increase in the salinity and nutrient concentration of the lake water, combined with increased aeolian inputs, were a more important trigger of lake eutrophication.
... Soil Thaumarchaeota are dominated by Group 1.1b, which can produce abundant crenarchaeol (Cren) and crenarchaeol regioisomer (Cren') 29,30 ; whereas aquatic Group 1.1a Thaumarchaeota, in addition to the production of Cren and Cren', can also produce OH-GDGTs. These observations indicate that OH-GDGTs are produced in situ in lakes 31 . The distribution patterns of isoGDGTs in soils and surface sediments are similar, but OH-GDGTs were only detected in the surface sediments ( Supplementary Fig. 1), which is consistent with results from the Lugu Lake basin 32 . ...
... Additionally, Hunshandake Sandy Land was an extremely wet environment during the early Holocene 14 . A nutrient-poor freshwater environment at this time also indicated a maximum in lake volume 31,35 . An early Holocene lake level maximum was also determined from the lakeshore embankments of Chagan Nuur 21 and Angulinao Lake 15 . ...
Understanding the causes of abrupt hydrologic declines in the Asian summer monsoon region may help limit the adverse effects of such events on the region’s economy and ecology. Here, we provide a high-resolution record of hydrological evolution in the East Asian monsoon margin region since the last deglaciation. Our synthesis of hydrological records in the East Asian monsoon region reveals that northern and southern China were generally wet during the early Holocene, while two significant drought events occurred in northern China under the suppressive effect of Northern Hemisphere ice sheets. The hydrological pattern of “southern drought–northern floods” developed during the middle Holocene, and the southward withdrawal of the Intertropical Convergence Zone, which was controlled by the inter-hemispheric temperature gradient, along with the enhanced suppression of the East Asian monsoon by the Hadley Circulation, combined to cause synchronous extreme droughts during the mid- to late-Holocene.
... The impacts of climate change propagate throughout hydrological systems (Rozemeijer et al., 2021), and significantly impact groundwater and surface water circulation (Jiang et al., 2015;Kim et al., 2023;Zeydalinejad & Dehghani, 2023), water quality (Robertson et al., 2016), and water ecology (L. Chen et al., 2022;Qin et al., 2021). Climate factors affect algal in both direct and indirect ways. ...
Assessing future chlorophyll‐a (Chl‐a) changes is crucial for developing effective lake management programs. Future assessments should integrate socio‐economic and climatic factors, including extreme climate impacts. However, such assessments are lacking. This study developed a novel framework and integrated model to analyze Chl‐a response in Chaohu Lake to climate (e.g., precipitation, temperature, and wind) and socio‐economic (e.g., population, fertilizer application, and livestock farming). The results indicated that by around 2050, the Chl‐a concentration would change by −6.5% to −0.1% compared to around 2020. While socio‐economic and climatic factors significantly altered nitrogen and phosphorus loading, exogenous loading had a minimal effect on Chl‐a due to high endogenous releases. The decrease in Chl‐a is mainly due to the increase in precipitation. Chl‐a reduction was primarily driven by increased precipitation (0.6%–12.9%), leading to −9.4% to −4.4% Chl‐a changes. The runoff increase also provided an opportunity to treat the heavily polluted lake area. Air temperature increases of 5.1%–9.2% resulted in Chl‐a increases of 0.9%–3.1%. On annual scales the effects of precipitation and temperature were mainly due to changes in mean values. The effects of variability were significant at seasonal scales, for example, lower spring temperatures favored a decrease in summer Chl‐a. There was uncertainty about the impact of future wind speeds. This study emphasized the importance of comprehensively quantifying the impacts of external pressures in lake water quality assessment and provided a reference and pathway support for the assessment and management of large, shallow lakes.
... Lakes situated in semi-arid regions are particularly vulnerable to environmental degradation due to low precipitation and high evaporation rates, posing a significant threat to water security for both human populations and ecosystems (Maceda-Veiga et al. 2018;Jeppesen et al. 2020;Cao et al. 2021). In recent decades, the number of lakes on the Mongolian Plateau has not only significantly decreased, but the degradation trend of lake ecosystems has also become increasingly prominent due to external environmental disturbances, including increased precipitation variability, climate warming, and intensified human activity (Tao et al. 2014;Chen et al. 2022;Liu et al. 2022;Ostrowski et al. 2022;Xu et al. 2022;Yan et al. 2022;Wang et al. 2023). For example, the over-extraction of freshwater resources and changes in surface vegetation cover in the lake basin causing an increase in external nutrients and organic matter entering the lake are driven by factors such as overgrazing, accelerated urbanisation and increasing population density Liu et al. 2023). ...
Aim
Freshwater lake ecosystems in semi‐arid regions are highly fragile and complex. However, the driving forces behind changes in temperate lake ecosystems at a regional scale in China remain largely unclear, primarily due to the lack of multidecadal to centennial regional studies. This study focuses on Hulun Lake, the largest freshwater lake in northern China's semi‐arid region, with the aim of elucidating the characteristics and driving mechanisms of freshwater lake ecosystems in such areas.
Location
Hulun Lake, the largest freshwater lake in northern China's semi‐arid region (48.55° ~ 49.33° N, 116.97° ~ 117.81° E).
Time Period
From the 1840s to 2010s AD.
Major Taxa Studies
Zooplankton Cladocera.
Methods
High‐resolution multiproxy palaeoecological data (including biological and geochemical indices), human activities parameters and observational datasets from the lake basin are used to explore ecosystem shifts and Cladocera responses in Hulun Lake, located in a semi‐arid region, over the past 170 years. The hypothesis that changes in the zooplankton Cladocera community composition within the ecosystem are driven by synergistic interactions among multiple factors was tested using effective approaches based on generalised additive models (GAM) and redundancy analysis (RDA).
Results
The results indicate that both internal factors (such as lake sedimentary environment, fish catches, lake water level, organic matter and nutrients) within the lake and external factors (including population, urbanisation rate and livestock husbandry) in the surrounding basin have been key drivers influencing the structure of Cladocera in Hulun Lake over the past 170 years. The originally predominant littoral Cladocera species (Chydorus sphaericus. sl.) were replaced by planktonic (Bosminidae) since the 1980s accompanied by a decline in diversity, which corresponds with the process of eutrophication and pollution in the lake.
Main Conclusions
The dynamics of the Cladocera assemblages indicate that the lake ecosystem is vulnerable to further imbalance since the 2000s due to the ongoing synergistic effects of declining fishery catches, increasing livestock numbers, rising populations, urbanisation rates and declining water levels in semi‐arid regions.
... Eutrophication of water bodies is a significant environmental issue worldwide. The level of eutrophication is influenced by human activities [6], and certain water quality parameters, such as phosphorus and nitrogen concentrations, need to be focused on [7,8]. The concentration of dissolved oxygen significantly influences the aquatic environments such as rivers [9] and ponds [10], having a direct correlation with the quality of water. ...
Accurately predicting the state of surface water quality is crucial for ensuring the sustainable use of water resources and environmental protection. This often requires a focus on the range of factors affecting water quality, such as physical and chemical parameters. Tree models, with their flexible tree-like structure and strong capability for partitioning and selecting influential water quality features, offer clear decision-making rules, making them suitable for this task. However, an individual decision tree model has limitations and cannot fully capture the complex relationships between all influencing parameters and water quality. Therefore, this study proposes a method combining ensemble tree models with voting algorithms to predict water quality classification. This study was conducted using five surface water monitoring sites in Qingdao, representing a portion of many municipal water environment monitoring stations in China, employing a single-factor determination method with stringent surface water standards. The soft voting algorithm achieved the highest accuracy of 99.91%, and the model addressed the imbalance in original water quality categories, reaching a Matthews Correlation Coefficient (MCC) of 99.88%. In contrast, conventional machine learning algorithms, such as logistic regression and K-nearest neighbors, achieved lower accuracies of 75.90% and 91.33%, respectively. Additionally, the model’s supervision of misclassified data demonstrated its good learning of water quality determination rules. The trained model was also transferred directly to predict water quality at 13 monitoring stations in Beijing, where it performed robustly, achieving an ensemble hard voting accuracy of 97.73% and an MCC of 96.81%. In many countries’ water environment systems, different water qualities correspond to different uses, and the magnitude of influencing parameters is directly related to water quality categories; critical parameters can even directly determine the quality category. Tree models are highly capable of handling nonlinear relationships and selecting important water quality features, allowing them to identify and exploit interactions between water quality parameters, which is especially important when multiple parameters together determine the water quality category. Therefore, there is significant motivation to develop tree model-based water quality prediction models.
... According to the 2023 China Ecological Environment Status Bulletin, among the 205 important lakes (reservoirs) undergoing nutritional status monitoring, 8.3% are categorized as oligotrophic, 64.4% as mesotrophic, 23.4% as experiencing light eutrophication, and 3.9% as undergoing moderate eutrophication. Relevant studies indicated that eutrophication water bodies in China are mainly concentrated in the middle and lower reaches of the Yangtze River Plain, the Northeast Plain and the Yunnan-Guizhou Plateau (Chen et al., 2022a;Liu et al., 2023;Zhou et al., 2021). ...
The main influencing factors of water environment and the spatiotemporal differences of eutrophication were identified in the Dianchi Lake, the results indicated a comparatively poor and fluctuating the water environmental condition in the Caohai compared to the Waihai, and differences in the correlation between water environment indicators were observed in Caohai and Waihai. The absolute contribution rates of the inner sources to water temperature, pH, electrical conductance, total nitrogen, chlorophyll a and algal density were the largest in Caohai, while offshore sources are pH, electrical conductance, permanganate index, total phosphorus and chlorophyll a in Waihai. The eutrophication level is relatively high near the Xiyuan Suidao section, and the comprehensive trophic level indexes are 61.14, 64.45 and 64.45 in spring, summer and autumn, respectively, which all reach the state of moderate eutrophication; the comprehensive trophic level index is 55.72 in winter, which reaches the state of light eutrophication. The algal density near the Xiyuan Suidao and Luojiaying sections exhibited high levels, reaching a state of moderate algal bloom in summer. Spatial autocorrelation analyses highlighted significant positive and negative spatial autocorrelation for comprehensive Trophic Level Index and algal density, respectively, in Dianchi Lake. The High-High aggregation of the comprehensive Trophic Level Index and algal density was mainly concentrated in the Caohai, while the Low-Low aggregation of the comprehensive Trophic Level Index was primarily observed in the Waihai. Consequently, the risk of eutrophication and algal bloom outbreak in Caohai surpassed that in Waihai. Therefore, it is imperative to propose appropriate treatment measures based on the varying eutrophication level and algal bloom outbreak during different time periods and in distinct regions of the aquatic ecological environment in Dianchi Lake.
... A study along the west coast of India revealed that barnacles can shift their diet during the monsoon, a survival strategy that ensures their persistence (Gaonkar and Anil 2010). Elevated nutrient levels during monsoon can promote the growth of algae (Chen et al. 2022;Jargal and An 2023) which can compete with corals for space and resources. This can affect the availability of suitable substrates for epifauna and impact their abundance and distribution (Becerro et al. 2006). ...
Coral reefs are highly biodiverse ecosystems, enriched by a range of biofouling species. Temporal variations in biofouling can affect ecosystem stability, but these diverse coral-associated communities remain underexplored in some regions. In the present study, biofouling assemblages of coral reefs in the Chabahar Bay were investigated during a summer monsoon at three deployment periods. In total, 26 taxa were identified with barnacles and polychaetes being the dominant taxa during the whole study. The coverage percentage was driven mostly by the encrusting taxa such as bryozoans and algae while biomass was determined by the dominance of shell-forming taxa. The results of PERMANOVA showed that the effects of the submersion period were significant on the assemblage structure. Biofouling assessment plays a pivotal
role in safeguarding the intricate balance and long-term health of coral reef ecosystems. For a comprehensive understanding of biofouling dynamics and interactions with coral-associated
species, conducting long-term studies is vital.
... These microorganisms, which are known for their sensitivity to environmental fluctuations, serve as valuable paleoenvironmental indicators and provide profound insights into ancient aquatic ecosystems by uniquely responding to specific ecological conditions through changes in their abundance, presence or absence, and morphology (BATTARBEE et al. 2002, SMOL 2022. Consequently, they have made significant contributions to the elucidation of critical past environmental changes including climate oscillations (FONTANA et al. 2019, RODRÍGUEZ-ZORRO et al. 2022, ALMARAZ-RUIZ et al. 2023, BALASUBRAMANIAM et al. 2023, SAROS et al. 2023, limnological/ecological changes (WENGRAT et al. 2019, hydrological regimes (RIBEIRO et al. 2010, FAYÓ et al. 2018, HAN et al. 2023, alterations in land-use practices (TRÁBERT et al. 2020, GARDOKI et al. 2023, and the historical deposition of pollutants and eutrophication (BENNION et al. 2004, COSTA-BÖDDEKER et al. 2012, FONTANA et al. 2014, SIMMATIS et al. 2022, CHEN et al. 2022, GREGERSEN et al. 2023. ...
The use of diatoms in paleoenvironmental research is of the utmost importance for reconstructing past ecosystems because it provides crucial insights into the adaptability and resilience of these systems under ongoing climate change and increased human impacts. Maintaining a comprehensive dataset distribution is crucial to ensure the validity and relevance of these reconstructions on a global scale. This scientometric study aimed to analyze trends in diatom-based paleoenvironmental research on a global scale with an emphasis on South America. This study aimed to investigate disparities in the geographical distribution of diatom-based paleoenvironmental studies, collaboration networks in the field, and regions in which this research is lacking. The Scopus search criteria excluded certain types of publications, such as conference articles and notes. The search focused on articles, reviews, books and chapters of books using specific keywords aligned with the objectives of this study. Using the Scopus database, we identified 7,091 publications (1979-2023) that employed diatoms as a primary proxy for paleoenvironmental reconstruction. The results show significant changes in this field recent decades. The analysis highlights a marked upsurge in publication output during the 1980s and a substantial increase in publications after 2000. This upward trend in scientific documentation reflects the growing interest in and recognition of the importance of this field in the context of palaeoecology and environmental sciences. After 2011, the results revealed a shift in thematic focus from historical and geological reconstruction to ecological, anthropogenic, and climatic interactions. Key researchers, including Smol JP, Leng MJ, and Birks HJB, have been at the forefront of international collaborations. Since the 2000s, South American studies have focused primarily on lakes, climate change effects, and volcanic activity. In this region, researchers such as Maidana N.I., Urrutia, R., and García-Rodriguez, F. emphasized the significance of regional ecosystems and their paleoenvironmental history in the global context of the field. Despite its high biodiversity, South America contributed only 6.7% of the global research output, placing it 19th globally. Socioeconomic factors and other obstacles faced by researchers in the region could explain the observed disparity in scientific output in South America compared to other regions. To unlock South America’s vast potential for diatom-based paleoenvironmental research, which boasts the highest global biodiversity yet remains underrepresented in the field, stakeholders must implement equitable partnerships, increase funding and improve their academic infrastructure and opportunities. These strategies are crucial for advancing the global paleoenvironmental understanding, informing future conservation and environmental policies in South America, and promoting scientific progress on a global scale.
... Lakes and reservoirs serve as vital water sources for human irrigation and drinking purposes (Du et al., 2011;Wu and Liu, 2015). However, the persistent increase in nitrogen and phosphorus loading within water bodies, driven by both human activities and natural climate changes (Zhang et al., 2020;Lai et al., 2021;Chen et al., 2022), has resulted in the rising occurrence of eutrophication and algal blooms in lake and reservoir waters. This severely threatens human livelihoods, flora, and fauna habitats (Cook et al., 2023). ...
Remote sensing is a promising technology for global water eutrophication monitoring, and the quantity and representativeness of the observed samples influence its accuracy. Due to limitations in high costs in situ monitoring, current remote sensing of lakes often relies on models trained with water quality samples from a single season. However, the generalization ability of these single-season trained models to other seasons has not been extensively studied. In this study, focusing on 38 major lakes in the Yangtze River Basin, we utilized monthly chlorophyll-a (Chla) monitoring data and Sentinel-2 image datasets from 2016 to 2021 to assess the impact of different seasonal data-trained models on the performance of Chla concentration retrieval in lakes. The results indicate that: (a) The sampling time for lake Chla significantly affects the performance of the retrieval model. Based on the seasonal data, the trained models show an R 2 range of 0.523 to 0.699, with a Bias ranging from − 7.06 % to 7.74 %, while the models trained with full-season samples perform between the various seasonal models. (b) When models built with seasonal data are applied to full-season data retrieval, there is a noticeable decrease in performance. The R 2 of the spring-winter data trained model drops by 18.2 %, with RMSL and MAPE increasing by 11.4 % and 4.2 %, respectively. Similarly, the model trained from summer-autumn data shows a 4.02 % decrease in R 2 , with RMSLE and MAPE rising by 2.76 % and 13.56 %, respectively. This suggests that using models established from seasonal samples to infer full-season Chla concentrations can amplify errors. (c) Compared to seasonal models, full-season models based on different seasonal sampling exhibit better performance (R 2 = 0.585, RMSLE = 0.337, Bias =-3.12 %, MAPE = 34.71 %). R 2 , RMSLE, and MAPE are all superior to seasonal models. Therefore, when performing full-time and long-term retrieval of Chla concentrations in water bodies, it is necessary to use data sampled in different seasons to reduce the errors introduced by seasonal sampling. (d) Chla retrieval models were established for 38 typical lakes and reservoirs in the Yangtze River Basin using the full-time dataset, revealing that 25 % (7/28) of the lakes and 20 % (2/10) of the reservoirs showed significant alteration trends. The number of lakes with increasing Chla concentrations exceeded those with decreasing concentrations, indicating that there is still a substantial risk of eutrophication in the lakes and reservoirs of the Yangtze River Basin.
... Cultural eutrophication featured by excess algal blooms is the leading environmental problem faced by global lakes (Ayele and Atlabachew 2021), and the main drivers are climate change (Chen et al. 2022a) and human activities (Liu et al. 2021). Eutrophication is due to the enrichment of nutrients such as nitrogen and phosphorus in water bodies, resulting in increased primary productivity and a frequent outbreak of phytoplankton. ...
Cyanobacterial blooms in lakes fueled by increasing eutrophication have garnered global attention, and high-precision remote sensing retrieval of chlorophyll-a (Chla) is essential for monitoring eutrophication. Previous studies have focused on the spectral features extracted from remote sensing images and their relationship with chlorophyll-a concentrations in water bodies, ignoring the texture features in remote sensing images which is beneficial to improve interpreting accuracy. This study explores the texture features in remote-sensing images. It proposes a retrieval method for estimating lake Chla concentration by combining spectral and texture features of remote sensing images. Remote sensing images from Landsat 5 TM and 8 OLI were used to extract spectral bands combination. The gray-level co-occurrence matrix (GLCM) of remote sensing images was used to obtain a total of 8 texture features; then, three texture indices were calculated using texture features. Finally, a random forest regression was used to establish a retrieval model of in situ Chla concentration from texture and spectral index. Results showed that texture features are significantly correlated with lake Chla concentration, and they can reflect the temporal and spatial distribution change of Chla. The retrieval model combining spectral and texture indices performs better (MAE = 15.22 μg·L⁻¹, bias = 9.69%, MAPE = 47.09%) than the model without texture features (MAE = 15.76 μg·L⁻¹, bias = 13.58%, MAPE = 49.44%). The proposed model performance varies in different Chla concentration ranges and is excellent in predicting higher concentrations. This study evaluates the potential of incorporating texture features of remote sensing images in lake water quality estimation and provides a novel remote sensing method to better estimate lake Chla concentration.
... Numerous studies have shown that hydroclimatic changes (i.e., temperature and precipitation) are important factors that influence lake trophic status (Sinha et al., 2017;Chen et al., 2022). Increased precipitation leads to more terrestrial nutrients entering the lake, thereby improving lake trophic status (Sinha et al., 2017). ...
Lake eutrophication seriously threatens water quality and human health. Under continuous global warming and intensified human activity, increasing attention is being paid to how lake trophic status responds to climate change and anthropogenic impacts. Based on the sedimentary organic matter (SOM) molecular composition determined by the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) technology, and combined with the SOM stable nitrogen isotopes (δ15Norg), we studied how lake trophic status and ecology respond to both climatic changes and anthropogenic impacts of the past 500 yrs at Lake Daihai, Inner Mongolia. The results show that the relative abundance of lipids, proteins, and carbohydrates in lake sediments kept relatively low before AD ∼1850, and increased gradually thereafter, especially after AD ∼1950, suggesting that the lake trophic status was low before AD ∼1850, but obviously increased during the past one more century. On the other hand, the relative abundance of allochthonous condensed aromatics and vascular plant-derived polyphenols compounds gradually decreased after AD ∼1850, which is most likely due to the intensified land-use changes in the catchment. Our results show that the SOM molecular composition is more sensitive to trace the land-use changes than the δ15Norg ratios, suggesting a potential use of this technique to trace even earlier human land uses (e.g., during the prehistorical times) in a catchment. The results of this study suggest that intensified land-use change, increased discharges of human sewage and industrial wastewater, cropland runoff, and concentrated effects caused by lake level drops may have combinedly increased nutrient concentration and accelerated lake eutrophication at Lake Daihai. Therefore, proper policy is necessary to slow down anthropogenic impacts and limit further eutrophication for lakes like Lake Daihai.
... Lake eutrophication is a serious problem that is largely caused by human modifications to the natural biogeochemical cycles of nitrogen and phosphorus (P). P is a primary cause of eutrophication because it amplifies positive feedback in eutrophication [1]. One of the most important factors influencing the P concentration of overlying water is the release of P from sediment, which may have a significant impact on water quality and result in continuing eutrophication [2]. ...
Three submerged macrophytes, Hydrilla verticillata, Vallisneria natans, and Potamogeton crispus, were planted in winter and spring to investigate the environmental mechanism of phosphorus transformation during submerged macrophyte growth periods. The experimental buckets each contained 4,832.80 g of dry sediment (depth: 10 cm) and 100 L water. The phosphorus (P) fractions in the water and sediment were investigated along with the environmental factors, including pH, dissolved oxygen (DO), and redox potential (ORP). The concentrations of the different forms of P in the water and sediment were lower in the submerged macrophyte groups. Submerged macrophyte uptake was the primary reason for the decrease in P in the water and sediment. At the end of the experiment, the total amounts of phosphorus accumulated by H. verticillata, V. natans, and P. crispus were 156.05 g, 282.24 g, and 115.84 g, respectively, and the three submerged macrophytes enriched P by 4.18%, 7.56%, and 3.10%. The levels of the environmental factors pH, DO, and ORP were higher in the submerged macrophyte groups than the control groups. The environmental factors had different effects on the different forms of P during growth of the submerged macrophytes. H. verticillata primarily acted on the soluble reactive phosphorus (SRP) through environmental factors, while P. crispus acted on the dissolved total phosphorous, SRP, dissolved organic phosphorus, and particulate phosphorous through these factors. V. natans had a weak impact on P content in the water through the environmental factors. Hence, the transformation of P between the sediment and water was affected by environmental factors. Overall, the results of this study reveal the effects of environmental factors under submerged macrophyte growth on water purification as well as the migration and transformation of P in the sediment and on the surface and provide a theoretical basis for the ecological restoration of eutrophicated shallow lakes.
... In recent years, the eutrophication of water bodies has become a global environmental problem (Chen et al. 2022;Karlson et al. 2022;Wei et al. 2022). The dominant species in eutrophic water bodies are generally cyanobacteria (Vilar et al. 2022). ...
The effects of different concentrations (100, 150, 200, 250 mg/L) and different particle sizes (0–75 μm, 75–120 μm, 120–150 μm, 150–500 μm) on the soluble protein content, superoxide dismutase (SOD) and catalase (CAT) activity, malondialdehyde (MDA) content, chlorophyll a (Chla) content, and photosynthetic parameters of Microcystis flos-aquae were studied, and the mechanism of the effect of suspended particulate matter on the physiology and biochemistry of Microcystis flos-aquae was discussed. The results showed that the soluble protein content of Microcystis flos-aquae did not change noticeably after being stressed by suspended particles of different concentrations/diameters. The SOD activity of Microcystis flos-aquae first increased and then decreased with increasing suspended particulate matter concentrations. The SOD activity of Microcystis flos-aquae reached 28.03 U/mL when the concentration of suspended particulate matter was 100 mg/L. The CAT activity of Microcystis flos-aquae increased with increasing concentrations of suspended particles and reached a maximum value of 12.45 U/mg prot in the 250 mg/L concentration group, showing a certain dose effect. Small particles had a more significant effect on SOD, CAT, and MDA in Microcystis flos-aquae than large particles. The larger the concentration was and the smaller the particle size was, the stronger the attenuation of light and the lower the content of Chla. Both the maximum quantum yield of PSII (Fv/Fm) and the potential photosynthetic activity of PSII (Fv/F0) of Microcystis flos-aquae increased at first and then decreased under different concentrations/sizes of suspended particles. The relative electron transfer rate gradually returned to a normal level over time. There was no significant difference in the initial slope (α) value between the treatment group and the control group, and the maximum photo synthetic rate (ETRmax) and the semilight saturation (Ik) decreased.
... Seventeen AMS 14 C dates (fifteen bulk samples and two terrestrial plant materials) were used to generate an independent chronology Zhang et al., 2022). After subtracting the reservoir ages and converting corrected radiocarbon ages to calendar ages by using the Intcal 20 curve (Reimer et al., 2020), the Bayesian age-depth model for the DLH19B core was established by using the Bacon program with "R" software ( Fig. S1; Chen et al., 2022;Zhang et al., 2022). In this paper, we focus on the top 7.92 m of the core (the Holocene period) and analyze long chain alkenones (LCAs) for reconstructing past temperature and salinity changes. ...
The influence of Holocene East Asian Summer Monsoon (EASM) rainfall on ecosystems over northern China remains controversial: delay or synchronization. Here we present Holocene quantitatively reconstructed temperature and salinity records based on alkenone proxies in the Dali Lake from northern China. Our data indicate that the Dali Lake was significantly diluted to a freshwater lake (0.8 ± 0.4 g/L) during~11.4e3.8 cal ka BP, relative to the modern brackish state. From the early Holocene (before 8.0 cal ka BP) to the middle Holocene (8.0e5.0 cal ka BP), the average salinity value of Dali Lake decreased bỹ 0.3 g/L (ca. 30%), but our reconstructed temperature record shows an increase of~2.1 C. In the context of climate warming, the evaporation was enhanced, and thus the decline in salinity is attributed to the increased precipitation, indicating a mid-Holocene EASM rainfall maximum. After comparing our reconstruction with vegetation data, we proposed that the stable and abundant EASM rainfall promoted the forest expansion and vegetation recovery over northern China during the mid-Holocene, suggesting the coupling of ecosystems and EASM rainfall in North China.
Climate change exerts both direct and indirect influences on the eutrophication of surface water ecosystems in various ways. This study aimed to evaluate the impact of temperature fluctuations on trophic levels through various interdisciplinary coupling analysis methods after land use change, which including water and sediment sample analysis, hydraulic model, remote sensing, and historic data analysis. For the historical analysis, six satellite images of Lake Dianshan were examined to assess algal bloom occurrences and the coverage of Eichhornia crassipes from 2013 to 2023. The correlation between trophic indicators and temperature was analyzed using statistical methods. For the monthly analysis, a total of 27 sediment samples and 54 water samples collected from Lake Dianshan were assessed to determine how seasonal temperature variations influence eutrophication status. The trophic indicators have higher concentration at inlet sampling sites compared to outlet sites, which validated the potential external pollution source. The trophic level of Lake Dianshan is influenced not only by climate change but also significantly by urban human activities. The management of eutrophication has substantially improved the water quality of Lake Dianshan over the past few decades. Furthermore, increasing temperatures demonstrate a positive correlation with the proliferation of cyanobacteria, particularly in urban areas.
The dual pressures of global warming and increased anthropogenic activities pose significant threats to terrestrial vegetation ecosystems. To better understand the impacts of climate change and human activities on terrestrial vegetation ecosystems, we reconstructed the changes in vegetation and plant biomass over the past 4500 years using n-alkane records from sediments of two alpine lakes in northwestern China: Xiannv Lake and Tianchi Lake. Our results indicate that changes in the spatial variability of plant biomass are not related to temperature and precipitation. Furthermore, CO2 fertilization and nutrient inputs from dust contributed to the observed changes in plant biomass. We also compared the history of human activities in the Tianshan Mountains, the eastern Silk Road, and the Hunsandak Sandy Land, and find that the demand for plant resources—whether for human settlement, military construction, or warfare—may have caused a sudden decline in terrestrial vegetation, thereby disrupting the positive effects of dust on plant biomass growth.
The northern Chinese steppe is situated in the periphery of the East Asian Summer Monsoon and exhibits high sensitivity to variations in monsoon intensity and global climate change. Therefore, comprehending the climatic evolution history of this region is pivotal for elucidating early human civilization development. The research focuses on a fully enclosed crater lake located in the northern Chinese steppe by utilizing n-alkanes, total organic carbon (TOC), total nitrogen (TN) in lake sediments, based on AMS- ¹⁴ C dating analysis, the climatic and environmental evolution history of this region from the last deglaciation to the middle Holocene has been reconstructed, and the impact of climate change on the evolution of early human civilization is elucidated. The findings indicate that the EASM driven by summer solar radiation, exerts control over the climate and hydrology in the steppe region of northern China. Since the last deglaciation, the gradual warming and humidification of the climate environment have significantly enhanced the animal and plant resources, thereby establishing a solid foundation for the transition of early human populations in northern Chinese steppe from high-mobility hunter-gatherers to settled communities. Additionally, this climatic shift has provided an optimal backdrop for the emergence of primitive agriculture. The changes of stone tool assemblages unearthed at related sites also confirm the above views. However, the climate model underwent rapid changes during the period of 7.2–6.4 cal kyr BP, leading to the occurrence of extreme arid climate events. Consequently, regional precipitation decreased, lake ecology became imbalanced, woody plants retreated in the basin, and vegetation coverage was greatly reduced. The aforementioned factor may have played a pivotal role in the disappearance of early populations within the region. Consequently, climate and environmental change not only serve as the primary driving force behind human civilization’s development but also act as an inexorable catalyst for its decline.
The Qilian Mountains (QLM) act as an “ecological security barrier” in western China, impacting the downstream ecosystems and water resource utilization. However, the hydrological evolution of the QLM during the last millennium remains controversial, and their ecological response to climate change is poorly understood. We present a pH record based on the brGDGTs (branched glycerol dialkyl glycerol tetraethers) of a 14 C -dated sediment core from Lake Maozangtianchi in the QLM. We combined this record with element contents determined by scanning XRF and grain size to reconstruct the summer monsoon precipitation variability over the last 900 years. We also reconstructed the history of eco-environmental changes from the total n-alkane contents. On centennial scales, local precipitation exhibited peaks during the intervals of 1100‒1300 CE and 1750‒2000 CE, as well as between 1400‒1750 CE. Additionally, abrupt decreases in precipitation occurred during the transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA) (1300‒1400 CE). This pattern coherent with other hydroclimatic records from the monsoonal margin of northern China, likely resulted from the combined impact of the El Niño‒Southern Oscillation on tropical Pacific sea-surface temperatures and the meridional shift of the Intertropical Convergence Zone. In addition, a coupled relationship between plant biomass in the Lake Maozangtianchi watershed and fluctuations in monsoon precipitation was observed, with higher plant biomass during 1100‒1200 CE, 1750‒1900 CE, and 1950‒2000 CE, and lower biomass during 1200‒1400 CE and 1900‒1950 CE. However, during 1400‒1750 CE, plant biomass exhibited a minor increasing trend, deviating from its usual correlation with monsoon precipitation. Despite precipitation usually being the primary climatic factor influencing plant biomass in the QLM, during the LIA, nutrients transported by dust and decreased evapotranspiration became pivotal in bolstering plant growth. Our research emphasizes the significant moderating effects of exogenic dust on vegetation changes in alpine ecosystems.
The alpine grasslands and meadows of the Tibetan Plateau are critical for maintaining biodiversity, carbon storage, and regional ecological balance. However, the ecological response of the Tibetan Plateau to climate change and anthropogenic activity is poorly understood. We selected two alpine lakes at different altitudes on the northeastern Tibetan Plateau and used n-alkanes from lacustrine sediments to probe the process of ecological response to climate change (temperature and precipitation) during typical climatic periods of the past millennium, as well as impacts on the ecosystem caused by anthropogenic activities. Our results showed that mid- and long-chain n-alkanes in Lake Dalzong and Lake Bihu sediments, which could be used to evaluate plant biomass variations, were mainly derived from terrestrial plants around the watershed. For both the low-altitude Lake Dalzong (3120 m above sea level (a.s.l.)) and the high-altitude Lake Bihu (4360 m a.s.l.), plant biomass of the watershed increased with enhanced temperature and precipitation during the Medieval Warm Period (MWP), whereas they decreased during the Little Ice Age (LIA), under conditions of lower temperature and precipitation. A further comparison with regional climatic records indicates that temperature and precipitation may have played a more important role in plant biomass changes in the low- and high-altitude regions in the northeastern Tibetan Plateau, respectively. In addition, during the Current Warm Period (CWP), when temperature and precipitation were higher than that in the MWP, plant biomass was lower than that in the MWP in both the Lake Dalzong and Lake Bihu watersheds. The intensification of regional grazing activities and the increase in religious rituals were the primary factors contributing to CWP vegetation degradation. This study highlights the fact that a progressively warmer and wetter climate will not offset the negative impacts of anthropogenic activities on vegetation.
Cyanobacterial blooms in lakes fueled by increasing eutrophication have garnered global attention, and high-precision remote sensing retrieval of chlorophyll-a (Chla) is essential for monitoring the blooms. Previous studies have focused on the spectral features extracted from remote sensing images and their relationship with chlorophyll-a concentrations in water bodies, ignoring the texture features in remote sensing images which is beneficial to improve interpreting accuracy. This study explores the texture features in remote-sensing images. It proposes a retrieval method for estimating lake Chla concentration by combining spectral and texture features of remote sensing images. Remote sensing images from Landsat 5, and 8 were used to extract NIR-Red, GREEN-BLUE, MNDWI, and KIVU bands ratio. The gray-level co-occurrence matrix (GLCM) of remote sensing images was used to obtain a total of 8 texture features; then three texture indices were calculated using texture features. Finally, a random forest regression was used to establish a retrieval model of in-site Chla concentration from texture and spectral index. Results showed that texture features are significantly correlated with lake Chla concentration, and they can reflect the temporal and spatial distribution change of Chla. The retrieval model combining spectral and texture indices has better performance (R ² = 0.801, RMSE = 16.0 µg·L − 1 ) than the model without texture features(R ² = 0.746, RMSE = 16.2 µg·L − 1 ). The proposed model performance varies in different Chla concentration ranges and is excellent in predicting higher concentrations. This study evaluates the potential of incorporating texture features of remote sensing images in lake water quality estimation and provides a novel remote sensing method to better estimate lake Chla concentration.
Coastal waters are often influenced by seawater intrusion and terrestrial emissions because of its special location. In this study, the dynamics of microbial community with the role of nitrogen cycle in sediment in a coastal eutrophic lake were studied under a warm season. The water salinity gradually increased from 0.9‰ in June to 4.2‰ in July and 10.5‰ in August because of seawater invasion. Bacterial diversity of surface water was positively related with salinity and nutrients of total nitrogen (TN) as well as total phosphorus (TP), but eukaryotic diversity had no relationship with salinity. In surface water, algae belonging to Cyanobacteria and Chlorophyta were dominant phyla in June with the relative abundances of >60%, but Proteobacteria became the largest bacterial phylum in August. The variation of these predominant microbes had strong relationship with salinity and TN. In sediment, the bacterial and eukaryotic diversity was greater than that of water, and a significantly different microbial community was observed with dominant bacterial phyla Proteobacteria and Chloroflexi, and dominant eukaryotic phyla Bacillariophyta, Arthropoda, and Chlorophyta. Proteobacteria was the only enhanced phylum in the sediment with the highest relative abundance of 54.62% ± 8.34% due to seawater invasion. Denitrifying genera (29.60%-41.81%) were dominant in surface sediment, then followed by microbes related to nitrogen fixation (24.09%-28.87%), assimilatory nitrogen reduction (13.54%-19.17%), dissimilatory nitrite reduction to ammonium (DNRA, 6.49%-10.51%) and ammonification (3.07%-3.71%). Higher salinity caused by seawater invasion enhanced the accumulation of genes involved in dentrificaiton, DNRA and ammonification, but decreased genes related to nitrogen fixation and assimilatory nitrogen reduction. Significant variation of dominant genes of narG, nirS, nrfA, ureC, nifA and nirB mainly caused by the changes in Proteobacteria and Chloroflexi. The discovery of this study would be helpful to understand the variation of microbial community and nitrogen cycle in coastal lake under seawater intrusion.
Plain Language Summary
The Younger Dryas, circa 12,900 to 11,700 years ago, is a hemispheric abrupt climate change event that occurred at rates similar to those projected by the 21st century. Its cause has been linked to a reduction in northward oceanic heat transport in the Atlantic that led to Northern Hemispheric cooling and Southern Hemispheric warming. Here we present detailed reconstructions of Younger Dryas temperature variations in eastern North America that suggest a more complex spatial fingerprint than predicted by the standard global model. New England, Maritime Canada, and the Great Lakes Region all cooled, like Greenland and elsewhere in the Northern Hemisphere. However, regions south of Virginia experienced little temperature change and Florida warmed slightly. Possible mechanisms include atmospheric processes that enhanced advection from the subtropics and oceanic processes that transported heat northward from the equatorial Atlantic. These reconstructions also help explain the hotspot of biodiversity and endemic species in the southeastern United States, by showing that this region was buffered from past abrupt millennial‐scale climate reversals.
Radiocarbon (C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
The Asian monsoon (AM) played an important role in the dynastic history of China, yet it remains unknown whether AM-mediated shifts in Chinese societies affect earth surface processes to the point of exceeding natural variability. Here, we present a dust storm intensity record dating back to the first unified dynasty of China (the Qin Dynasty, 221–207 B.C.E.). Marked increases in dust storm activity coincided with unified dynasties with large populations during strong AM periods. By contrast, reduced dust storm activity corresponded to decreased population sizes and periods of civil unrest, which was co-eval with a weakened AM. The strengthened AM may have facilitated the development of Chinese civilizations, destabilizing the topsoil and thereby increasing the dust storm frequency. Beginning at least 2000 years ago, human activities might have started to overtake natural climatic variability as the dominant controls of dust storm activity in eastern China.
The late Holocene is generally regarded as an interval of monsoon recession and decreased precipitation in North China. However, the extent to which this affected the status of lakes in North China is unclear. In the study, we analyzed multiple proxies (δ¹³C, magnetic susceptibility [χlf], and exogenous element concentrations) from Lake Mayinghai, an undisturbed alpine lake in North China. We also compared the results with previously published paleoenvironmental records from the Chinese Loess Plateau. We aimed to determine the extent of the influence of variations in the East Asian summer monsoon (EASM) on the status of Lake Mayinghai during the late Holocene. The results demonstrate that negative δ¹³C values at Lake Mayinghai correspond to high sedimentary tree pollen content and high reconstructed precipitation, indicating that variations in δ¹³C likely reflect variations in vegetation and the EASM in North China. A comparison of the δ¹³C record with other proxies from the same core reveals an abrupt change in the EASM at ~3 kyr before present (BP), which is also evident in other regional climate records from the Chinese Loess Plateau. The rapid decline in EASM intensity at ~3 kyr BP is also consistent with changes in the status of Lake Mayinghai, Lake Gonghai, Lake Ganhai, and Lake Daihai, in that these lakes underwent an abrupt retreat after 3 kyr BP, demonstrating that the rapid decline in EASM intensity at ~3 kyr BP had a large impact on lake evolution in North China.
Plain Language Summary
We present a 15,000‐year record of southern China precipitation based on well‐dated (n=130 radiometric ages) lake level variations of a closed basin lake. The reconstructed long‐term southern China precipitation trend is out of phase with boreal summer insolation, which has been shown to drive ASM precipitation, but broadly follows trends in tropical Pacific sea surface temperatures (SSTs), the related zonal SST gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high and are juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia, including an unexpected southern China mid‐Holocene drought. The results of this study are unique in that (1) precipitation was robustly reconstructed from beach evidence, which is a first‐order measure of the balance between rainfall amount and evaporation. (2) A new mechanism driving long‐term precipitation changes over subtropical East Asia is proposed. Providing the mechanism stands, increasing SST gradients under the expected global warming scenario could enhance the role of western Pacific subtropical high and lead to less precipitation over subtropical East Asia.
Freshwater phytoplankton blooms affect public health and ecosystem services globally1,2, with harmful impacts resulting either from a bloom’s high intensity or the presence of toxin-producing phytoplankton species. Freshwater blooms result in economic losses of over US$4 billion annually in the United States alone, primarily from harm to aquatic food production, recreation and tourism, and drinking-water supplies3. Studies documenting bloom conditions in lakes have either focused only on individual or regional subsets of lakes4–6, or have been limited by lack of long-term observations7–9. Here, we use three decades of high-resolution Landsat 5 satellite imagery to investigate long-term trends in intense summertime near-surface phytoplankton blooms for dozens of large lakes globally. We find that peak summertime bloom intensity has increased in a majority (68 per cent) of the lakes studied, revealing a global exacerbation of bloom conditions. Lakes that have experienced a significant (P < 0.1) decrease in bloom intensity are rare (8 per cent). The reason behind the increase in phytoplankton bloom intensity remains unclear, however, as temporal trends do not track consistently with temperature, precipitation, fertilizer-use trends, or other previously hypothesized drivers. We do find that lakes with a decrease in bloom intensity warmed less compared to other lakes, suggesting that lake warming may already be counteracting management efforts to ameliorate eutrophication10,11. Our findings support calls for water-quality management efforts to account better for the interactions between climate change and local hydrologic conditions12,13.
Human impacts on biodiversity are well recognized, but uncertainties remain regarding patterns of diversity change at different spatial and temporal scales. Changes in microbial assemblages are, in particular, not well understood, partly due to the lack of community composition data over relevant scales of space and time. Here, we investigate biodiversity patterns in cyanobacterial assemblages over one century of eutrophication and climate change by sequencing DNA preserved in the sediments of ten European peri-Alpine lakes. We found species losses and gains at the lake scale, while species richness increased at the regional scale over approximately the past 100 years. Our data show a clear signal for beta diversity loss, with the composition and phylogenetic structure of assemblages becoming more similar across sites in the most recent decades, as have the general environmental conditions in and around the lakes. We attribute patterns of change in community composition to raised temperatures affecting the strength of the thermal stratification and, as a consequence, nutrient fluctuations, which favoured cyanobacterial taxa able to regulate buoyancy. Our results reinforce previous reports of human-induced homogenization of natural communities and reveal how potentially toxic and bloom-forming cyanobacteria have widened their geographic distribution in the European temperate region.
There is a controversy regarding whether the high precipitation delivered by an intensified East Asian summer monsoon occurred during the early Holocene, or during the middle Holocene, especially in the context of the monsoonal margin region. The conflicting views on the subject may be caused by chronological uncertainties and ambiguities in the interpretation of different climate proxies measured in different sedimentary sequences. Here, we present a detailed record of the Holocene evolution of vegetation in northern China based on a high-resolution pollen record from Dali Lake, located near the modern summer monsoon limit. From 12,000e8300 cal BP, the sandy land landscape changed from desert to open elm forest and shrubland, while dry steppe dominated the hilly lands and patches of birch forest developed in the mountains. Between 8300 and 6000 cal BP, elm forest was extensively distributed in the sandy lands, while typical steppe covered the hilly lands and mixed coniferousebroadleaved forests expanded in the mountains. Our pollen evidence contradicts the view that the monsoonal rainfall increased during the early Holocene; rather, it indicates that the East Asian summer monsoon did not become intensified until ~8000 cal BP in northern China. The low precipitation during the early Holocene can be attributed to the boundary conditions, i.e., to the remnant high-latitude Northern Hemisphere ice sheets and the relatively low global sea level.
Anthropogenic aerosol increases over the past few decades have weakened the Asian summer monsoon 1–3 with potentially far-reaching socioeconomic and ecological repercussions. However, it is unknown how these changes will aaect freshwater ecosystems that are important to densely populated regions of Asia. High-resolution diatom records and other proxy data archived in lake sediment cores from the Chinese Loess Plateau allow the comparison of summer monsoon intensity, lake trophic status and aquatic ecosystem responses during warming periods over the past two millennia. Here we show that an abrupt shift towards eutrophic limnological conditions coincided with historical warming episodes 4,5 , marked by increased wind intensity and summer monsoon rainfall leading to phosphorus-laden soil erosion and natural lake fertilization. In contrast, aerosol-aaected Anthropocene warming catalysed a marked weakening in summer monsoon intensity leading to decreases in soil erosion and lake mixing. The recent warm period triggered a strikingly diierent aquatic ecosystem response with a limnological regime shift marked by turnover in diatom species composition now dominated by oligotrophic taxa, consistent with reductions in nutrient fertilization, reduced ice cover and increased thermal stratification 6. Anthropogenic aerosols have altered climate–monsoon dynamics that are unparalleled in the past ∼2,000 years, ushering in a new ecological state.
Significance
The magnitude, rate, and extent of past and future East Asian monsoon (EAM) rainfall fluctuations remain unresolved. Here, we present a rainfall reconstruction based on the surface area of a closed-basin lake located at the modern northwestern boundary of the EAM. Our record shows that fluctuations of EAM intensity and spatial extent covaried over the past 125 ka. This record contributes to the resolution of a current controversy concerning the response of the EAM to external climatic forcings. We propose that a substantial decrease in rainfall at 5.5 ka was a major factor leading to a large cultural collapse of the Early Neolithic culture in north China.
The Great Khingan Mountain range, Northeast China, is located on the northern limit of modern East Asian Summer Monsoon (EASM) and thus highly sensitive to the extension of the EASM from glacial to interglacial modes. Here, we present a high-resolution pollen record covering the last glacial maximum and the early Holocene from a closed crater Lake Moon to reconstruct vegetation history during the glacial-interglacial transition and thus register the evolution of the EASM during the last deglaciation. The vegetation history has gone through distinct changes from subalpine meadow in the last glacial maximum to dry steppe dominated by Artemisia from 20.3 to 17.4 ka BP, subalpine meadow dominated by Cyperaceae and Artemisia between 17.4 and 14.4 ka BP, and forest steppe dominated by Betula and Artemisia after 14.4 ka BP. The pollen-based temperature index demonstrates a gradual warming trend started at around 20.3 ka BP with interruptions of several brief events. Two cold conditions occurred around at 17.2-16.6 ka BP and 12.8-11.8 ka BP, temporally correlating to the Henrich 1 and the Younger Dryas events respectively, 1and abrupt warming events occurred around at 14.4 ka BP and 11.8 ka BP, probably relevant to the beginning of the Bølling-Allerød stages and the Holocene. The pollen-based moisture proxy shows distinct drought condition during the last glacial maximum (20.3-18.0 ka BP) and the Younger Dryas. The climate history based on pollen record of Lake Moon suggests that the regional temperature variability was coherent with the classical climate in the North Atlantic, implying the dominance of the high latitude processes on the EASM evolution from the Last Glacial Maximum (LGM) to early Holocene. The local humidity variability was influenced by the EASM limitedly before the Bølling-Allerød warming, which is mainly controlled by the summer rainfall due to the EASM front covering the Northeast China after that.
Integration of DNA derived from ancient phototrophs with their characteristic lipid biomarkers has been successfully employed to reconstruct paleoenvironmental conditions. However, it is poorly known that whether the DNA and lipids of microbial functional aerobes (such as ammonia-oxidizing archaea: AOA) can be used for reconstructing past environmental conditions. Here we identify and quantify the AOA amoA genes (encoding the alpha subunit of ammonia monooxygenases) preserved in a 5.8-m sediment core (spanning the last 18,500 years) from Qinghai Lake. Parallel analyses revealed that low amoA gene abundance corresponded to high total organic carbon (TOC) and salinity, while high amoA gene abundance corresponded to low TOC and salinity. In the Qinghai Lake region, TOC can serve as an indicator of paleo-productivity and paleo-precipitation, which is related to historic nutrient input and salinity. So our data suggest that temporal variation of AOA amoA gene abundance preserved in Qinghai Lake sediment may reflect the variations of nutrient level and salinity throughout the late Pleistocene and Holocene in the Qinghai Lake region.
Dust storms in northern China strongly affect the living and health of people there and the dusts could travel a full circle of the globe in a short time. Historically, more frequent dust storms occurred during cool periods, particularly the Little Ice Age (LIA), generally attributed to the strengthened Siberian High. However, limited by chronological uncertainties in proxy records, this mechanism may not fully reveal the causes of dust storm frequency changes. Here we present a late Holocene dust record from the Qaidam Basin, where hydrological changes were previously reconstructed, and examine dust records from northern China, including the ones from historical documents. The records, being broadly consistent, indicate the onset of frequent dust storms at ~AD 1100. Further, peaked dust storm events occurred at episodes of high total solar irradiance or warm-dry conditions in source regions, superimposed on the high background of frequent dust storms within the cool LIA period. We thus suggest that besides strong wind activities, the centennial-scale dust storm events over the last 1000 years appear to be linked to the increased availability of dust source. With the anticipated global warming and deteriorating vegetation coverage, frequent occurrence of dust storms in northern China would be expected to persist.
The organic matter contents of sediments deposited in Lakes Erie and Ontario contribute to the record of changes in the lake watersheds and aquatic ecosystems which have resulted from European settlement and cultural eutrophication of these systems over the past two centuries. Compositions of n-alkanes and n-alkanoic acids extracted from lake sediments track the clearing of the original natural forests and the appearance of agriculture in the watershed areas beginning about 1820. Aquatic productivity increased as runoff of soil nutrients increased. Cultural eutrophication in the 1950s is recorded in increases in organic C and in n-C17 alkane concentrations. Diagenetic effects overprint source changes as shorter chain length and unsaturated lipid components are preferentially removed from the sedimentary record.
One contribution of 12 to a Theo Murphy Meeting Issue 'New models and observations of the Southern Ocean, its role in global climate and the carbon cycle' . Fluxes of lithogenic material and fluxes of three palaeo-productivity proxies (organic carbon, biogenic opal and alkenones) over the past 100 000 years were determined using the 230 Th-normalization method in three sediment cores from the Subantarctic South Atlantic Ocean. Features in the lithogenic flux record of each core correspond to similar features in the record of dust deposition in the EPICA Dome C ice 2014 The Author(s) Published by the Royal Society. All rights reserved. fluxes correlate with lithogenic fluxes in each sediment core. Our preferred interpretation is that South American dust, most probably from Patagonia, constitutes a major source of lithogenic material in Subantarctic South Atlantic sediments, and that past biological productivity in this region responded to variability in the supply of dust, probably due to biologically available iron carried by the dust. Greater nutrient supply as well as greater nutrient utilization (stimulated by dust) contributed to Subantarctic productivity during cold periods, in contrast to the region south of the Antarctic Polar Front (APF), where reduced nutrient supply during cold periods was the principal factor limiting productivity. The anti-phased patterns of productivity on opposite sides of the APF point to shifts in the physical supply of nutrients and to dust as cofactors regulating productivity in the Southern Ocean.
The Yunnan Plateau of southwest China is strongly influenced by the Asian monsoon and is a critical area in terms of determining its development and variability since the Last Glacial Maximum. High-resolution diatom analysis of a 14C-dated sediment core retrieved from Lugu Lake, Yunnan provides a detailed history of palaeoenvironment changes from 30,000 to 10,000 cal. yr BP. A model of climate–diatom interactions (linking thermal stratification, ice cover and catchment-derived nutrients) is proposed to explain the observed changes. Prior to 24,500 cal. yr BP the dominance of the planktonic Cyclotella rhomboideo-elliptica indicates a period of stable stratification and low nutrient levels, with a moderate change of temperature and precipitation in the southwest monsoon region. During the LGM (24,500–14,500 cal. yr BP), the loss of the planktonic diatoms and a switch to small Fragilaria spp. suggest an altered thermal regime in the lake (i.e. weaker stratification and possibly ice cover). From ∼18,000 cal. yr BP, the expansion of planktonic taxa (Stephanodiscus and the Asterionella formosa) and increased %TOC are indicative of increased nutrient availability reflecting altered vegetation and soil processes due to rising regional temperatures. After 14,500 cal. yr BP, a rapid reversal to dominance by planktonic diatoms indicates a further increase in temperature and continued development of catchment vegetation and soils. The abrupt changes in diatom species composition around 24,500 and 14,500 cal. yr BP suggest that the ecological status of the lake exceeded limnological thresholds. Interestingly, however, the Younger Dryas event is not clearly recognizable in either the diatom or pollen records in the Lugu Lake sediment record. In a partial redundancy analysis, 44.2% of the variability in the total diatom assemblages in the period 30,000–10,000 cal. yr BP was accounted for by changes in summer solar radiation. The relationships between the diatom assemblages and the C/N and pollen data are further evidence of climate–catchment–vegetation interactions and their impact on lake functioning. The composition of diatom assemblages in the Lugu Lake sediment core reflects direct and indirect (i.e. via catchment processes) climate forcing and in particular, the strength of the southwest monsoon, which in turn corresponds to orbitally-induced variability in Northern Hemisphere summer solar insolation from 30,000 cal. yr BP to 10,000 cal. yr BP.
A pollen diagram from a 6.62 m sediment sequence in Lake Erhai in northwest Yunnan, China was obtained to examine the roles of climate change and human impact on the development of the Erhai lake-catchment system since 12950 cal. yr BP. The record extends back into the Younger Dryas, where the dominance of Betula and deciduous Quercus points to a relatively cold and wet winter climate. After 11 750 cal. yr BP, a warming climate coupled with enhanced summer monsoon precipitation results in the expansion of Tsuga and evergreen broadleaved trees (Cyclobalanopsis, Lithocarpus and Castanopsis). An increase in evergreen oaks and dry-tolerant species after 10 320 cal. yr BP suggests a greater seasonality in rainfall, reflecting a southward shift in the winter front across the region. This trend of increasing temperatures and seasonality is seen to continue through into the mid-Holocene and the onset of the Holocene optimum. A marked decline in arboreal taxa coupled with increased levels of grass (Poaceae) and other disturbance taxa provides the first evidence for human impact in the catchment at c. 6370 cal. yr BP. This early phase of forest clearance leads to the collapse of the natural altitudinal vegetation gradient that existed in the catchment from the Lateglacial. The subsequent expansion of secondary pine forest suggests that these early clearances were part of a sustained period of shifting agriculture. Archaeological and historical records for the region point to a gradual increase in immigration into the region throughout the late Holocene. The increased pressure on the catchment is reflected in the pollen record by a series of clearance phases, which increase in intensity after 2140 cal. yr BP, linked presumably to intensification of agriculture and early urbanization. This trend continues through the last millennium, before a sharp increase in arboreal pollen at the top of the core reflects a phase of reforestation that took place in the catchment over the last 25 years.
DH99a sediment core recovered at the center of Daihai Lake in north-central China was analyzed at 4-cm intervals for pollen assemblage and concentration. The pollen record spanning the last ca 10,000yr revealed a detailed history of vegetation and climate changes over the Daihai Lake region during the Holocene. From ca 10,250 to 7900calyr BP, arid herbs and shrubs dominated the lake basin in company with patches of mixed pine and broadleaved forests, indicating a mild and dry climatic condition. Over this period, the woody plants displayed an increasing trend, which may suggest a gradual increase in warmth and humidity. The period between ca 7900 and 4450calyr BP exhibits large-scale covers of mixed coniferous and broadleaved forests, marking a warm and humid climate. Changes in the composition of the forests indicate that both temperature and precipitation displayed obvious fluctuations during this period, i.e., cool and humid ca 7900–7250calyr BP, warm and slightly humid ca 7250–6050calyr BP, warm and humid between ca 6050 and 5100calyr BP, mild and slightly humid ca 5100–4800calyr BP, and mild and humid ca 4800–4450calyr BP. The period can be viewed as the Holocene optimum (characterized by a warm and moist climate) of north-central China, with the maximum (dominated both by warmest temperatures and by richest precipitations) occurring from ca 6050 to 5100calyr BP. During the period of ca 4450–2900calyr BP, the woody plants declined, and the climate generally became cooler and drier than the preceding period. This period is characterized by a cold, dry episode from ca 4450 to 3950calyr BP, a warm, slightly humid interval between ca 3950 and 3500calyr BP and a mild, slightly dry episode from ca 3500 to 2900calyr BP, and appears to be a transition from warm and humid to cold and dry climatic conditions. Since ca 2900calyr ago, the forests disappeared and the vegetation density decreased, reflecting a cool and dry climate. However, a relative recovery of the woody plants occurring between ca 1700 and 1350calyr BP may denote an increase both in temperature and in precipitation. Fluctuations in the climatic condition of the Daihai Lake region were not only related to changes in the seasonal distribution of solar insolation and in the axis and intensity of the ocean current in the western North Pacific but were also closely linked to variations in the position and strength of polar high-pressure systems and in the pattern and intensity of the Westerly winds.
Testate amoebae are a diverse and abundant group of protozoa that constitute a large proportion of biomass in many ecosystems and probably fill important roles in ecosystem function. These microorganisms have attracted the interest of paleoecologists because the preserved shells of testate amoebae and their known hydrological preferences enable reconstruction of past hydrological change. In ombrotrophic peatlands, surface wetness reflects hydroclimate, so testate amoebae can play an important role in reconstruction of Holocene climate change. Previous studies, however, have been geographically restricted, mostly to North America and Europe. We studied the ecology of testate amoebae in peatlands from central China in relation to hydrology, pH and metal concentrations. We found that testate amoeba community structure was correlated with depth to water table (DWT) and that the hydrological preferences of species generally matched those of previous studies. We developed a weighted average DWT transfer function that enables prediction of water table depth with a cross-validated mean error of <5 cm. Our results demonstrate the potential for using testate amoebae to reconstruct paleohydrology in China. Such studies could contribute to our understanding of Holocene climate changes in China, particularly regarding past Asian monsoon activity.
Asian dust storms have long been a major environmental concern in China, affecting the lives of about one billion people. However, it is unclear whether the mechanisms responsible for Asian dust storms during the Holocene varied on different timescales, and thus it is unclear whether there was a shift from a natural forcing to an anthropogenic forcing of dust storms. We reconstructed a high-resolution Holocene record of dust storms from the sediments of an undisturbed alpine lake on the Chinese Loess Plateau. We found that Asian dust storm activity generally increased during the Holocene, with the largest fluctuations occurring during the past 2000 years. The increase in dust storm activity was in contrast to the decrease in East Asian winter monsoon (EAWM) intensity during the Holocene, indicating that the EAWM played a limited role in modulating dust storms. By contrast, the increase in dust storms corresponded to a decrease in East Asian summer monsoon (EASM) precipitation. This demonstrates that EASM precipitation was the dominant control of dust storm activity on a millennial timescale, because decreased EASM precipitation expanded the desert area and thus increased the dust storm activity. The increasing intensity of human activity in the region since the Bronze Age resulted in an acceleration of dust storm activity against the background of decreased EASM precipitation. As human disturbance continued to intensify, beginning at least at ∼2 cal ka BP, increased dust storms were closely linked to increasing human populations in the dust source regions, and there is a strong temporal coherence between increased dust storms and higher EASM precipitation. This was completely different from when natural processes are dominant. During that period, fewer dust storms occurred during periods of a strengthened EASM. Therefore, there was a shift from a natural forcing to an anthropogenic forcing of dust storms on a multi-decadal to centennial timescale, and was a mode in which “human activity overtook the EASM as the dominant control of the Earth surface system”.
It is unclear whether the Holocene climatic optimum (HCO) occurred synchronously throughout the East Asian summer monsoon (EASM) region, and if it did, whether it occurred in the early Holocene (EH, 11.7–8.0 ka, ka = 1000 years before present, where the “present” is defined as the year CE 1950), or in the middle Holocene (MH, 8.0–4.0 ka). This is because the definition of the HCO in previous paleoclimatic studies was mainly based on the criterion of maximum temperature or some “optimum” combination of temperature and precipitation. Since the development of prehistoric societies and human welfare are closely linked to climatic variability, it may be more meaningful to define the HCO from the perspective of climatic stability, which may provide fresh insight into the debate on the timing of the HCO. Here we use calculations of squared chord distance (SCD), an effective method for quantifying temporal dissimilarity of the variance of samples in a time series, to quantitatively compare the climatic stability of the EH and the MH, based on a diverse range of the EASM-associated paleoclimatic records from the EASM region. The results show that the SCD values decreased significantly from the EH to the MH, indicating a more stable climate which occurred synchronously during the MH in the EASM region. We suggest that the stable climate of the MH provided an optimum environment for the development of agriculture and animal husbandry, which contributed to rapid population growth and the flourishing of the Yangshao culture which was the most prosperous Neolithic culture in the EASM region of China during the Holocene. Thus we propose that the stable MH climate can be used to define a synchronous HCO of the EASM which was more supportive of societal development and population growth than that during the EH. The results of TraCE-21 ka climatic modeling reveal that, with the retreat of the Northern Hemisphere ice sheets, the variability of the westerly jet (WJ), which was previously perturbed by large ice sheets, was significantly reduced from the EH to the MH. This reduced WJ variability during the MH enhanced the stability of the climate of the EASM region, via the interactions of the EASM and the WJ.
Variability in East Asian summer monsoon precipitation during the Holocene remains of debate. In this study, we use a closed lake with well‐dated lake beach ridges located on the margin of the East Asian summer monsoon, a region highly sensitive to monsoon precipitation changes, to obtain a temporal sequence of water volume in North China. The elevation of each beach ridge calibrated to the modern lake level was surveyed. Optically stimulated luminescence dating of undisturbed sediments of beach ridges was performed. The lake area and water volume corresponding to each beach ridge were calculated using a digital elevation model. This study reveals relatively reduced monsoon precipitation from ~12 to 7 ka interrupted by strengthening of the monsoon circulation to a maximum from ~7 to ~5 ka and followed by greatly reduced monsoon intensity until the present day. These results demonstrate that changes in the East Asian summer monsoon precipitation may not be directly driven by global temperature or atmospheric CO2 content. Rather, we suggest that variation in the the monsoon margin precipitation is probably mainly driven by ice volume and subordinately by the summer solar insolation difference between mid‐latitude land and low‐latitude ocean.
A multi-proxy record of Holocene and late-Pleistocene aeolian mineral dust is reconstructed using a combination of geochemical (trace elements), mineralogical and grain-size analyses on cores from the Hani peatland in north-eastern (NE) China. The dust record displays a sharp increase in dust deposition during the late Holocene in comparison to the rest of the Holocene. This trend is in line with climatic records from the Chinese dust source regions and their downwind areas, which generally show an increase in aridity and aeolian activity during the late Holocene. The larger part of the Chinese dust source regions experienced a gradual increase in effective moisture and vegetation cover reaching maxima during the middle Holocene (6.0–8.0 kyr cal. BP) co-occurring with the minima in dust deposition in Hani. These changes in the dust source regions are likely to have been modulated by the variations in the East Asian summer monsoon (EASM), which is the principal mechanism controlling climate in the region. The intensified EASM during the middle Holocene is also likely to have resulted in a sediment recharge at the margin of the Chinese drylands providing additional material and enhancing the atmospheric dust load after the late-Holocene aridification of the region. Combined together, these changes promoted a remobilization of dust sources increasing the amount of material available for transport by the East Asian winter monsoon (EAWM) and the Westerlies. Human activities might also have played a role in the increased dust emissions during the late Holocene, but further research is needed to assess the extent of those impacts at a regional level.
Numerous studies have focused on environmental changes and lacustrine ecological transitions in the modern industrial era. However, little is known about the processes of ecological transitions during early human disturbances. The current lake ecosystem state is the result of long-term integrated human influences on lakes. In this study, we present multi-proxy evidence of changes in the lacustrine environment, ecosystem, and human activities inferred from a sediment core collected from Beilianchi Lake in the southwestern Chinese Loess Plateau, to document the lacustrine ecological transitions with increasing human disturbances during the past 5000 years. Our data reveal the lacustrine ecosystem was quite stable with unchanging terrestrial versus aquatic sourced organic matter before major human disturbances (4400-1400 BP), although there was a decline in precipitation and a transition from tree-to herb-dominated vegetation. From 1400 to 300 BP, when early humans began to occupy and disturb the catchment, the lake experienced high-magnitude, centennial-to decadal-scale oscillations between aquatic and terrestrial sourced organic contributions. Thereafter, as human activities further increased, the lacustrine ecosystem finally stabilized at a state with terrestrial dominated organic matter. Our results suggest that the lake experienced large-magnitude ecological oscillations during the period of 1400–300 BP, probably indicating the lake ecosystem retained the capability of recovery from the modest early human disturbances on the catchment. The recovery of the ecosystem to these external disturbances lasted hundreds of years with centennial to decadal scales oscillations. With further increased human influences since 300 BP, the lacustrine ecosystem probably finally shifted to another state largely deviating from its natural (climate-driven) background.
The composition, distribution, indexes and budget of saturated lipid biomarker aliphatic hydrocarbons (AHs) in sediments of the East China Sea (ESC) were analyzed to identify their indications and sources. The resolved n-alkane (R) contents were 0.70-2.89 μg/g, with long-chain n-alkanes as the dominant composition in the ECS. The high R values mainly appeared at south inner shelf and north outer upwelling area, corresponding to the high mud, total organic carbon (TOC) and Chl a contents there. The composition, distribution pattern, combined with indexes of AHs, suggested no petroleum contamination and predominant biogenic sources in the ECS. The biogenic sources mainly were the mixed terrestrial higher plant, marine plankton and bacteria and aquatic macrophyte origins. Biotic source apportionment suggested that terrestrial higher plants were the dominant source of AHs, followed by marine planktons, with the lowest of submerged/floating macrophytes. Quantitative evaluation of R sources suggested that the Changjiang River input was the primary terrestrial contributor, accounting for 67.9% of total terrestrial input. The burial flux of R was 1.11 × 103 t/yr, with inner shelf and estuary as main accumulation areas. Although there was a huge amount of R influx from terrestrial and marine sources, only 9.8% could be preserved in sediments.
Aliphatic hydrocarbon biomarkers from the surface sediments of lakes along the lower reaches of the Yangtze River (Eastern China) were analyzed in order to determine the origin and composition of organic matter (OM) in the sediment, which is necessary to understand how anthropogenic environmental change in the region is impacting on OM dynamics and carbon cycles. The results indicate that OM in lake sediments is derived from biogenic sources including phytoplankton, aquatic macrophytes, bacteria and terrestrial plants, and abiotic sources of petroleum hydrocarbon contamination. The significance of each source varies greatly between lakes. Inputs of autochthonous OM to sediments were closely dependent on nutrient levels and the organisms present in the lake water column. Cyanobacterial OM input, signified by the distribution of short-chain n-alkanes, was most abundant in sediments from the most severely polluted lake, West Lake Chaohu. OM derived from diatoms, indicated by C25 highly branched isoprenoids (HBIs), was relatively abundant in sediments from lakes Gucheng and Shijiu, the two studied lakes with the lowest nutrient levels. Growth of macrophytes in lakes Gucheng, Shijiu, Gehu and Yangcheng resulted in a significant accumulation of OM from these organisms in the sediments. In contrast, the input of allochthonous OM was controlled mainly by natural and anthropogenic activities around the lakes. For example, terrestrial plant OM input, deduced from the abundance and proportion of long-chain n-alkanes, was exceptionally high in the sediment of Lake Nanyi, perhaps stemmed from frequent severe flooding in the catchment. Input of fossil fuel OM reflected direct petroleum contamination of the water or the anthropogenic petroleum burning in the catchment. In addition, the allochthonous refractory OM tends to be enriched in sediments of lakes with strong hydrodynamic process. The results of this study provided a lot of details in explaining the variation of terrestrial OM dynamics and carbon cycle in response to anthropogenic activities.
Organic geochemical studies of surface sediments from Laizhou Bay, the Yellow River mouth, northeast China, were conducted to reveal the sources, distributions, and burial efficiency of terrigenous OC exported by the Yellow River. Bulk OC contents and δ¹³Corg values from 204 samples show that terrigenous OC mainly accumulates in two muddy patches, basically controlled by hydrodynamic conditions. Distributions and stable carbon isotopic compositions of individual n-alkanes from 45 samples suggest that short-chain n-alkanes, with even carbon-number predominance, are most likely microbial products from degradation of riverine fossil OC, whereas long-chain odd-numbered n-alkanes (n-C27, n-C29 and n-C31) and long-chain even-numbered n-alkanes (n-C26,n-C28 and n-C30) are representative of riverine soil OC and fossil OC, respectively. Integrated with previous reports of ∼100% burial efficiency of soil OC in the Yellow River mouth, concentration ratios of n-C27+29+31 to OC and n-C26+28+30 to n-C27+29+31 from the riverine particles to surface sediments in Laizhou Bay were constructed to indicate relative terrigenous OC contributions and fossil OC burial efficiency, respectively. The results suggest that the terrigenous OC contribution ranges from 14% to 57% in Laizhou Bay, with the highest values in the two muddy patches as well as in areas near the river mouth. The fossil OC burial efficiency ranges from 36% to 76%, with an TOC-weighted mean value of 42%. The 58% fossil OC lost during the transport and burial processes in the Yellow River mouth may provide a significant CO2 source.
We present a high-resolution oxygen isotope record from authigenic carbonate (δ¹⁸Ocarb) from Lake Xiarinur (Inner Mongolia) since the last deglaciation. The lake is located at the modern northern limit of the monsoon, and is therefore sensitive to the extension of the East Asian summer monsoon. Based on calibration against the instrumental record, the δ¹⁸Ocar variation has been interpreted as changes in atmospheric circulation pattern on decadal time scales. On longer time scales, the δ¹⁸Ocarb in lake sediments could be mainly regulated by the relative contribution of nearby (remote) water-vapor sources associated with subtropical (tropical) monsoon through changes in the distance from sources to the site of precipitation. Increased remote water vapors from tropical monsoon would lead to lighter isotope value in our study site. Through time the δ¹⁸Ocarb record in Lake Xiarinur indicate a notable weak tropical monsoon during the Younger Dryas, a gradual increasing monsoon from the early Holocene and weakening monsoon after the middle Holocene. Oxygen isotope records from lakes and stalagmite in the Asian monsoon region across different localities show a general similar temporal pattern since the last deglaciation, and highlight a fundamental role of the tropical monsoon.
Anthropogenic climate change has the potential to alter many facets of Earth's freshwater resources, especially lacustrine ecosystems. The effects of anthropogenic changes in Lake Superior, which is Earth's largest freshwater lake by area, are not well documented (spatially or temporally) and predicted future states in response to climate change vary. Here we show that Lake Superior experienced a slow, steady increase in production throughout the Holocene using (paleo)productivity proxies in lacustrine sediments to reconstruct past changes in primary production. Furthermore, data from the last century indicate a rapid increase in primary production, which we attribute to increasing surface water temperatures and longer seasonal stratification related to longer ice-free periods in Lake Superior due to anthropogenic climate warming. These observations demonstrate that anthropogenic effects have become a prominent influence on one of Earth's largest, most pristine lacustrine ecosystems.
Lake Sifangshan (49°22'32.97″N, 123°27'49.90″E, altitude: 933 m asl) is a nearly dried up volcanic lake, located on the central-northern part of the Great Khingan Mountain, northeast China. This study obtained n-alkane samples from Holocene sediment cores in the lake, and analysed their distribution and compound specific carbon isotope compositions. The distribution of n-alkanes showed the four following characteristics: (1) a homologous series of n-alkanes was detected with carbon numbers ranging from nC17 to nC33; 2) Most samples show a single peak, while others have double peeks; 3) For single peak samples, the peak is nC27, and for double peak samples, the first peak is nC21 and the second peak is nC27; 4) For short chain (<nC21) n-alkanes do not have obvious carbon number characteristics. For middle (nC23~nC25) and long (>nC27) chain n-alkanes have an odd-over-even carbon number predominance. These characteristics show that the organic matter in the lake came from both terrestrial and aquatic plants, the former of which is the main input. The isotope ratios (δ¹³C27~31) of n-alkanes in these samples are negative, and gradually decreases along the time, in good accordance with changes of summer solar radiation in the Northern Hemisphere during the Holocene. This implies that effective precipitation changes on the orbital scale were directly controlled by changes in summer solar radiation. Based on above proxy indicators, the environmental evolution of Lake Sifangshan during Holocene is divided into five stages: (1) In the 11.2-8.0 ka BP, effective precipitation was low and proportion of woody plants in terrestrial C3 plants increased slightly.The lake water level fluctuated frequently, but the primary productivity and nutritional status of the lake deteriorated. (2) In the 8.0-6.4 ka BP, effective precipitation increased and proportion of woody plants in terrestrial C3 plants increased slightly. The lake surface area shrunk and water level dropped.The primary productivity and nutritional status of the lake were stable. (3) In the 6.4-3.4 ka BP, effective precipitation was higher than that in the previous stage, and the proportion of woody plants increased but that of herbs decreased.The lake surface area expanded and water level rised.The primary productivity increased, and the tropical level of the lake started to be lower; (4) In the 3.4-2.4 ka BP, effective precipitation continued to rise, and the proportion of herbs increased. The lake surface area shrunk and water level dropped. The primary productivity decreased, and depletion of nutrition occurred again. (5) In the 2.4-0.9 ka BP, effective precipitation was in a high level and the proportion of woody plants increased gradually. The lake surface area expanded and water level rised.The primary productivity increased and the tropical level of the lake started to get lower. The climatic evolution produced by this study is in good agreement with other high-resolution climate records of Northeast China, and the differences show unique regional climate characteristics of the Lake Sifangshan.
Both the palaeoenvironmental and diagenetic significance of these lipid distributions have been assessed and found to be consistent with their suspected origins, ie turbidites from the upper slope of the western Bay of Bengal and the Ganges-Brahmaputra delta. -from Authors
High-resolution palynological analysis on annually laminated sediments of Sihailongwan Maar Lake
(SHL) provides new insights into the Holocene vegetation and climate dynamics of NE China. The robust
chronology of the presented record is based on varve counting and AMS radiocarbon dates from
terrestrial plant macro-remains. In addition to the qualitative interpretation of the pollen data, we
provide quantitative reconstructions of vegetation and climate based on the method of biomization and
weighted averaging partial least squares regression (WA-PLS) technique, respectively. Power spectra
were computed to investigate the frequency domain distribution of proxy signals and potential natural
periodicities. Pollen assemblages, pollen-derived biome scores and climate variables as well as the
cyclicity pattern indicate that NE China experienced significant changes in temperature and moisture
conditions during the Holocene. Within the earliest phase of the Holocene, a large-scale reorganization
of vegetation occurred, reflecting the reconstructed shift towards higher temperatures and precipitation
values and the initial Holocene strengthening and northward expansion of the East Asian summer
monsoon (EASM). Afterwards, summer temperatures remain at a high level, whereas the reconstructed
precipitation shows an increasing trend until approximately 4000 cal. yr BP. Since 3500 cal. yr BP,
temperature and precipitation values decline, indicating moderate cooling and weakening of the EASM. A
distinct periodicity of 550e600 years and evidence of a MideHolocene transition from a temperaturetriggered
to a predominantly moisture-triggered climate regime are derived from the power spectra
analysis. The results obtained from SHL are largely consistent with other palaeoenvironmental records
from NE China, substantiating the regional nature of the reconstructed vegetation and climate patterns.
However, the reconstructed climate changes contrast with the moisture evolution recorded in S China
and the mid-latitude (semi-)arid regions of N China. Whereas a clear insolation-related trend of
monsoon intensity over the Holocene is lacking from the SHL record, variations in the coupled
atmosphere-Pacific Ocean system can largely explain the reconstructed changes in NE China.
Both in-lake and catchment measures designed to reduce phosphorus (P) loading were implemented as part of a 12.3 million USD restoration project for the Minneapolis Chain of lakes in Minnesota (USA). Treatment wetlands, 'in-pipe' measures, and in-lake aluminum sulfate (alum) treatment were applied to restore water quality in the four urban lakes. Different alum dosing methods led to between 4 and 20+ (modeled) years of water quality improvements in these lakes after treatment and only one of the four lakes continues to meet water quality goals approximately 25 years after the project started. Due to limited space and poor performance, reduction of total external loads was low (1-13%) for three lakes. Changes to internal P sediment release rates after application of alum correlated well with epilimnetic total P (TP) concentrations in these lakes, indicating that improvements in water quality were mainly driven by reduced internal loading via in-lake measures. Substantial reductions to external P loading were only achieved at Cedar Lake (49%) via conversion of an existing natural area to a treatment wetland, but even Cedar Lake is no longer meeting management goals. When expressed in terms of dollars spent per unit P removed, in lake alum treatment was on average 50 times more effective than in-catchment measures. The results of this study indicate that substantial external nutrient reductions may not be adequate to sustainably maintain water quality in urban lakes and that continued in-lake management of P accumulated in lake sediment will not only be necessary, but will also be more cost efficient relative to in-catchment measures.
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The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennialscale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.
Lake Jingpo is located in Ning'an County, Southeast Heilongjiang Province, northeast China and is strongly influenced by the East Asian monsoon system. Diatom analysis of the last 5000 cal. yr BP according to the results of AMS C-14 dating provides a detailed history of paleoenvironment changes. The relative abundance of diatom species, Asterionella formosa, Aulacoseira, Cydostephanos, Stephanodiscus and Discostella species can be used as a proxy of spring wind speed, which is supported by the results of seasonal diatom and reviews of literature on the autoecologies of these species. In the sediment, high relative abundance of A formosa, Aulacoseira species indicates high wind-driven turbulence of the water column. The diatom record of the past 5000 years shows that the spring wind speed shifted from weak to strong to weak. In our result, the relative abundance of Aulacoseira species decreased sharply, while that of small-cell Cyclotella sensu lato (including Stephanodiscus, Cyclostephanos and Discostella) increased since 2000 cal. yr BP. The broad alternations between these species are probably keyed to mean temperature and wind variations that control lake circulation and stratification. Our results support the hypotheses that abiotic drivers affect the size structure of planktonic communities and that a warmer climate favors small-sized diatom cells. The change in the ratio of A. formosa and Aulacoseira species to Cyclostephanos. Stephanodiscus and Discostella species (AA/SC) basically correspond to change of the spring insolation (May) at 45 degrees N. Abundance of diatoms in Lake Jingpo roughly corresponds to changes in the Mid- to Low-latitude Circulation Index of the northern hemisphere as indicated by WIZ concentration from the GISP2 ice-core, the strength of the Asian summer monsoon as recorded from Hani Peat in Northeastern China, and from a stalagmite found in the Dongge cave in southern China. The results are important in demonstrating the sensitivity of diatoms to climate change, and providing proxy evidence for spring wind speed marked by shifts of diatom type.
Phytoplankton species are the primary producers in lakes and play important roles in food-web structures. Any shift in their diversity and productivity has an impact on other aquatic life forms. We use a range of environmental variables to explore the possible drivers influencing phytoplankton composition over the last 14,500 years in a temperate lake Lielais Svētiņu, eastern Latvia. Using pollen, non-pollen palynomorphs, temperature reconstructions and lithological information as proxies of environmental factors, we statistically test their associations with the fossil phytoplankton community composition. Our results reveal that during the Late Glacial, the climate warming, the decrease in landscape openness, and increase in organic matter were significant environmental variables affecting dynamics of phytoplankton communities
, especially in the prevalence of Botryococcus, Tetraedron, Scenedesmus and Pediastrum. According to the Redundancy Analysis and Generalized Least Squares models, Pediastrum, Scenedesmus and Tetraedron were positively associated with waterlogging tolerance that indicates moist soils in surroundings of the lake, during the Early Holocene. The 8.2 ka cold event with a 2–3 °C cooling led to a strong environmental disturbance for nearly 700 years, indicated by an increased chlorophyta accumulation rates and a decrease in the organic matter. Our results indicate that Coelastrum
reticulatum and C. polychordum are characteristic for the 8.2 ka cold event. Positive association between cyanobacteria and mean air summer temperature suggests that a warming favoured cyanobacteria over other phytoplankton taxa between 8000 and 4000 cal yr BP. High nutrient loads and water turbidity were more important for the dynamics of cyanobacteria from 4000 to 2000 cal yr BP. Human-driven trophic level change was recorded in the last 2000 years by abundances of fungi Sporormiella and Sordaria, and by the peaks of Gloeotrichia pisum, C. reticulatum and C. polychordum indicating eutrophication.
Algal production in Maumee Bay in western Lake Erie is highly affected by inputs of nitrogen (N) and phosphorus (P) from the Maumee River, which drains predominantly agricultural lands, leading to the formation of cyanobacterial blooms. In a 3-year study, precipitation and discharge ranged from relatively low (2012) to relatively high (2011) with corresponding changes in the size of the cyanobacterial bloom. This study aimed to quantify the relation between river discharge and algal nutrient limitation in Maumee Bay. During the summer growing seasons, 20 nutrient enrichment bioassays were performed to determine which nutrient (P or N) might limit phytoplankton growth; and ambient N and P concentrations were monitored. The bioassays suggested that phytoplankton growth shifted from P-limited to N-limited during summer of the low and intermediate discharge years (2012 and 2010, respectively), whereas during the high discharge year (2011) phytoplankton were nutrient-replete before becoming N-limited. Phosphorus-replete growth during the high discharge year likely was due to high P loads from the river and dissolved P concentrations greater than 1 μmol/L. Symptoms of N-limited growth occurred during August and September in all three years and during July of 2012 when NO3− plus NH4+ concentration was less than 7.29 μmol/L suggesting low or no correspondence between N-limitation and size of the cyanobacterial bloom. Occurrence of a relatively small cyanobacterial bloom in 2012 following the record-breaking bloom in 2011 suggests the possibility of fast-reversal of eutrophication in Maumee Bay if P loading from the watershed could be decreased.
Chaohu Lake, the fifth largest freshwater lake in the Yangtze floodplain, is faced with multiple stresses from anthropogenic disturbances and climate change. To explore the ecological changes in Chaohu Lake since the mid 20th century, we examined diatoms, geochemical indicators and particle size in 210Pb-dated sediment core from the lake. Diatom succession revealed that the lake had switched to a eutrophic state since the late 1970s. Redundancy analysis using limnological data, hydrological and meteorological variables showed that sedimentary total phosphorus (TP) and total organic carbon (TOC), annual mean temperature, annual mean wind velocity, and water-level amplitude (WLA) were five significant factors influencing diatom succession. Diatom assemblages from 1950 till 1978 were driven by WLA and wind. The establishment of Chaohu Dam baffled hydrological connectivity between the lake and the Yangtze River in 1962, and reducing water exchange-induced flow. Meanwhile, weak wind velocity reduced the wind-induced flow in the 1960s. Due to the weak hydrodynamic intensity, the dominant species (Aulacoseira granulata, a species with high sinking rate) became less important during this period. From 1979 till 2006, diatom assemblages were mainly driven by TP, TOC and temperature, which were highly correlative. Increasing nutrient loading promoted the blooms of eutrophic species (e.g., Cyclostephanos dubius). In addition, rising temperature would indirectly influence diatom assemblages by mediating nutrient release process. As a consequence, multiple stresses in concert have caused the lake switch to a further eutrophic state indicated by prominent increases in more eutrophic species (e.g., Stephanodiscus parvus) since 2000. This study provided information on complex trajectories of aquatic ecosystem shifts driven by increasing nutrient loading, hydrological alteration and climate warming in the Yangtze floodplain lake.
A sediment core from Lake Arapisto, Finland, was examined for fossil diatom assemblages to reconstruct changes in Holocene nutrient availability. Our aim was to investigate the long-term relationship between lake trophic status and climate by comparing the diatom-based phosphorus reconstruction with paleoclimatic proxies. Our results showed that the cold early Holocene was characterized by elevated nutrient conditions concurrent with newly exposed fertile ground. As the climate rapidly warmed and ice sheet further retreated, the catchment vegetation developed, which resulted in decreased nutrient flux into the lake. The Holocene Thermal Maximum (HTM), between ~ 8000 and 4000 cal yr BP, was characterized by oligotrophic conditions, which may have been caused by low effective precipitation and stable watershed vegetation. After the HTM, the lake became more productive. There was no particular increase in the trophic state that could be connected to more recent human influence. Although lake productivity has been shown to be affected by temperature, our record indicated that the nutrient dynamics were driven by complex interactions between changes in temperature, precipitation, catchment, and in-lake processes. Understanding of long-term nutrient dynamics and the associated processes can help in resolving relationships between lake productivity and climate during past and present climate changes.
Sediment has been recognized as a gigantic sink of organic materials and therefore can record temporal input trends. To examine the impact of anthropogenic activities on the marginal seas off China, sediment cores were collected from the Yellow Sea, the inner shelf of the East China Sea (ECS), and the South China Sea (SCS) to investigate the sources and spatial and temporal variations of organic materials, i.e., total organic carbon (TOC) and aliphatic hydrocarbons. The concentration ranges of TOC were 0.5-1.29, 0.63-0.83, and 0.33-0.85%, while those of Σn-C14-35 (sum of n-alkanes with carbon numbers of 14-35) were 0.08-1.5, 0.13-1.97, and 0.35-0.96 μg/g dry weight in sediment cores from the Yellow Sea, ECS inner shelf, and the SCS, respectively. Terrestrial higher plants were an important source of aliphatic hydrocarbons in marine sediments off China. Spatial distribution analysis of Σn-C14-35 concentrations and source diagnostic ratios suggested greater load of terrestrial organic materials in the Yellow Sea than in the ECS and SCS. Temporally, TOC and Σn-C14-35 concentrations increased with time and peaked at either the surface or immediate subsurface layers. This increase was probably reflective of elevated inputs of organic materials to marginal seas off China in recent years, and attributed partly to the impacts of intensified anthropogenic activities in mainland China. Source diagnostics also suggested that aliphatic hydrocarbons were mainly derived from biogenic sources, with a minority in surface sediment layers from petroleum sources, consistent with the above-mentioned postulation.
Lake size is sensitive to both climate change and human activities, and therefore serves as an excellent indicator to assess environmental changes. Using a large volume of various datasets, we provide a first complete picture of changes in China's lakes between 1960s–1980s and 2005–2006. Dramatic changes are found in both lake number and lake size; of these, 243 lakes vanished mainly in the northern provinces (and autonomous regions) and also in some southern provinces while 60 new lakes appeared mainly on the Tibetan Plateau and neighboring provinces. Limited evidence suggested that these geographically unbalanced changes might be associated primarily with climate change in North China and human activities in South China, yet targeted regional studies are required to confirm this preliminary observation.
This chapter reviews that the Late Wisconsinan North American ice sheet complex consisted of three major ice sheets: (1) the Laurentide Ice Sheet, which was centred on the Canadian Shield, but also expanded across the Interior Plains to the west and south, (2) the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia, and (3) the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 7°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres rather than from the Canadian Shield. All of the peripheral ice sheets were confluent at the Last Glacial Maximum (LGM) with the Laurentide Ice Sheet, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector, named for areas of inception mid probable areas of outflow at LGM and located respectively east, west and north of Hudson Bay. The chapter presents revised maps of North American deglaciation at 500-year and finer resolution. These maps represent an updating of a series prepared nearly two decades ago for the INQUA 1987 Congress.
The objective of this study is to summarise limnological effects of cultural eutrophication in some Chinese lakes. Detailed data are shown for Lake Tai Hu as an example for a very large shallow lake affected by anthropogenic impacts. Urban and industrial waste water discharged to the lake result in high nutrient concentrations, large algal blooms dominated by cyanobacteria, mainly species of Microcystis and strong horizontal gradients. Algal growth in this lake is mainly ligh and/or phosphorus limited. This situation is typical for most of the lakes included in this review although some of the large deep lakes seem to be nitrogen deficient. In some cases toxicological effects may be the reason for observed discrepancies. Lakes are of vital importance for populations in China as drinking water and nutritional resources. To prevent water shortage because of conflicting usage, anticipation, control and management of water pollution is urgently needed. Moreover, efforts must be taken to rehabilitate deteriorated freshwater ecosystems.
In future scenarios for a doubled CO2 climate, air temperature increases in summer and winter and precipitation decreases (summer) in western Ontario but increases (winter) in wester Ontario, northern Minnesota, Wisconsin and Michigan. Warmer climates ie 2× CO2 climates, would lower net basin water supplies, stream flows and water level owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0.2 to 2.5 lower. Warmer climates would decrease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Temperature simulation for stratified lakes are 1-7oC warmer for surface water, and 6oC cooler to 8oC warmer for deep waters. These physical changes would in turn affect the phytoplankton, zooplankton, benthos and fishes. Annual phytoplankton production may increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nutrient inputs would be expected. Zooplankton biomass would increase, but again, many complex interactions are expected. Genrally, the thermal habitat for warm-, cool-, and even cold-water fishes would increase in size in deep stratified lakes, but would decrease in shallow unstratified lakes and in streams. Growth and production would increase for fishes that are now in thermal invironments cooler than their optimum but decrease for those that are at or above their optimum, provided they cannot move to a deeper or headwater thermal refuge. The zoogeographical boundary for fish species could move north by 500-600 km; invasions of warmer water fishes and extirpations of colder water fishes should increase.