Anthropogenic activities altering the ecosystem in Lake Yamzhog Yumco, southern Qinghai-Tibetan Plateau.

Lakes on the Qinghai-Tibet Plateau (QTP) have been subject to multiple environmental pressures from rapid climate change and intensified human activity in recent decades. However, their ecological effects on the lake ecosystem remain largely unclear due to the lack of long-term monitoring data. This study presented the environmental and ecological changes of the lake Yamzhog Yumco (Southern QTP) over the past three decades based on multi-proxy analysis (geochemistry and sedaDNA) on a high-time resolution sediment core. The result showed that the lake exhibited a continuous eutrophication process from 2004 CE, which has accelerated since 2014 CE. The nutrient enrichment was mainly attributed to anthropogenic emissions from the catchment. The sedimentary ancient DNA (sedaDNA) metabarcoding data registered a sensitive response of aquatic communities to the additional nutrient supply. Eukaryotic algae and aquatic invertebrate communities exhibited similar temporal dynamics, characterized by the increase in eutrophic taxa and the decrease in oligotrophic taxa. Change points analysis suggested that lake ecosystems underwent a slight ecological shift in 2003 CE and an abrupt shift in 2012 CE driven by nutrient enrichment. Quantitative analysis revealed that nutrients and human activity accounted for 27.9 % and 21.7 % of the temporal variation in aquatic communities, whereas climate change only explained 6.9 % of the total variation. From a paleolimnological view, our study supported that regional human activity could distinctly alter the nutrient level and aquatic community structure of lake ecosystems in the QTP. Considering that anthropogenic disturbance will continuously increase, it is crucial to strengthen the field monitoring of the lakes on the plateau and make effective management measures to avoid irreversible ecological consequences.

Uncertainties in Pollution and Risk Assessments of Heavy Metals in Lake Sediments Using Regional Background Soils in China.

Background soils are frequently utilized as a surrogate to assess pollution levels and environmental risks of heavy metals in Chinese lakes. However, there remains a lack of understanding regarding the reliability and uncertainty of such assessments. Here, we determined heavy metals (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn) in sediment cores from five rural lakes in North China to evaluate the reliability and uncertainty of the assessments using background soils by comparing them with assessments based on background sediments. Comparative studies reveal large uncertainties in the assessments using background soils. Among these metals, uncertainties for Hg and Cd are relatively large, whereas those for the other metals are minor. This discrepancy is due to the considerably higher natural variability of Hg and Cd in soils and sediments in comparison to the other metals. Generally, assessments utilizing background soils underestimate pollution levels and risks of Hg but overestimate those of Cd in these lakes. Despite limited human activities around the lakes, they still received a considerable influx of heavy metals via regional atmospheric transport. Assessments of the nine metals indicate moderate to considerable ecological risks in these lakes. The risks are contributed primarily (78-89%) by Hg and Cd. This study underscores the substantial uncertainties in assessing heavy metal pollution and risks using regional background soils and emphasizes the importance of controlling atmospheric emissions of Hg and Cd to mitigate pollution in rural and remote water bodies in China.

Two-century sediment records of atmospheric mercury variations in North China and their relations with regional and global emissions.

Sedimentary mercury (Hg) records from remote areas are significant for revealing historical variations of regional Hg and understanding the influence of regional and global Hg emissions. In this study, sediment cores were retrieved from two subalpine lakes in Shanxi Province in North China and employed to reconstruct atmospheric Hg variations over the last two centuries. The two records show similar anthropogenic Hg fluxes and evolution trends, corresponding with that they were affected mainly by regional atmospheric Hg deposition. Before ~1950, the records show negligible Hg pollution signals. Atmospheric Hg in the region had increased rapidly since the 1950s, lagged more than a half-century compared to the global Hg. This indicates that they were seldom affected by Hg emissions dominated by Europe and North America after the industrial revolution. The Hg increases since the 1950s in the two records corresponded well with rapid industrial developments in and around Shanxi Province after the founding of the China, implying the dominant contribution of domestic Hg emissions. By comparing other Hg records, we find that widespread increases in atmospheric Hg in China likely occurred post ~1950. This study rouses to re-examine historical variations in atmospheric Hg at various settings, which is significant to understanding global Hg cycling in the industrial era.

Sediment records of black carbon variations over the last two centuries in North China.

Reconstructing historical black carbon (BC) variations based on sedimentary records are significant for understanding long-term BC emissions, tracing sources, and establishing effective strategies for pollution control. By comparing BC profiles between four lake sediment cores, historical BC variations were reconstructed on the southeastern Mongolian Plateau in North China. Except one, the other three records show close fluxes and similar temporal trends of soot, indicating their repetitiveness on revealing historical variations at a regional scale. Unlike soot, char and BC in these records, derived mainly from local sources, reflected the occurrence of natural fires and human activities near the lakes. Before the ∼1940s, these records showed no well-established anthropogenic BC signals except some occasional natural-related increases. This was different from the global BC increased since the Industrial Revolution, indicating a negligible influence of transboundary BC on the region. Anthropogenic BC in the region had increased since the 1940s-1950s ascribed to emissions from Inner Mongolia and nearby provinces. The increases were moderate in the 1950s-1970s, corresponded with the initial development of industry after the founding of the P.R. China. The most pronounced BC increases occurred in 1980s-2016, commensurate with rapid socio-economic development after the Reform and Opening-up in 1978. Different from model estimations on Chinese BC emissions, our records show unexpected BC increases in recent two decades caused by pollutant emission rises in this undeveloped region. This suggests that BC emissions in relatively small cities and rural areas in China were likely underestimated and their role on national BC cycling needs to be reassessed.

A Chironomid Record of Early-Middle Holocene Environmental Evolution in the Darhad Basin, Northern Mongolia.

Under the influence of various circulation systems, the Holocene humidity conditions on the Mongolian Plateau are spatially heterogeneous and the underlying mechanism is still ambiguous. The complexity of climate change may affect the accuracy of assessing lake ecosystem evolution. In this study, based on the precise chronology, a chironomid assemblage sequence from the Darhad Basin in northern Mongolia is analyzed to elucidate the hydroclimate variation during the early-middle Holocene. The results show that the chironomid communities changed suddenly from littoral taxa to sublittoral/profundal taxa at about 9 cal kyr BP, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment. Thereafter, most parts of the paleolake remained at a relatively high level until 4.5 cal kyr BP. This hydrological pattern resembles the typical humidity variations in the Westerlies affected regions, except that the onset of wetter conditions occurred one thousand years earlier as reflected in our results. The melting of glaciers and permafrost in the basin resulting from the early increased summer solar insolation could be a feasible explanation for these time advances.

Behavior of iron and other heavy metals in passivated sediments and the coupling effect on phosphorus.

In situ passivation, which is easy to operate and affordable, is one of the most commonly used methods for sediment phosphorus (P) remediation. Understanding the behavior of iron and other heavy metals in passivated sediments is important for alleviating lake eutrophication and for ensuring drinking water safety. In this study, we investigated the behavior of P, Fe, Mn, Cd, Co, and Pb in lanthanum modified bentonite (LMB, Phoslock®) and polyaluminum chloride (PAC)-passivated sediments using intact sediment cores. Rhizon sampler and diffusive gradients in thin films technology (DGT) were respectively used to collect soluble and labile substances in sediment; a modified sequential selective extraction method was used to characterize metal forms. Results showed that LMB reduced soluble reactive phosphorus (SRP) at sediment depths of 0 ~ -15 mm and DGT-labile P flux at 0 ~ -50 mm. Correlation between DGT-labile P and Fe (R2 = 0.71) indicated that P mobility in the LMB group was affected by the behavior of Fe. PAC decreased SRP at sediment depths of 0, -5, -10, -15, -20, -25, and -50 mm with removal rates of 100%, 90%, 45%, 35%, 81%, 89%, and 100%, respectively. DGT-labile P flux was decreased by PAC at 0 ~ -10 mm and -50 ~ -110 mm, but increased at -10 ~ -50 mm; this is a result of synthetical effect by Al flocs adsorption and Fe(III) reductive dissolution. LMB decreased Cd, Co, and Pb in LMB layer in carbonate, reducible, and oxidizable forms. PAC decreased Cd mobility but caused the transformation of Co and Pb from reducible to other forms because of Fe(III) reductive dissolution. Those results indicate that sedimentary Fe plays an important role in in situ passivation. We suggest modifying passivators to Fe(II) adsorbents and increasing DO permeability of sediment to promote the formation of an Fe(III) passivation layer and hence the effectiveness of P control.

A historical record of trace metal deposition in northeastern Qinghai-Tibetan Plateau for the last two centuries.

Owing to rapid socio-economic development in China, trace metal emissions have increased and lakes even in remote areas have experienced marked changes in the last century. However, there are limited studies revealing long-term trends, anthropogenic fluxes, and spatial characteristics of trace metals in lakes. In this study, we present a geochemical record from Lake Qinghai in the northeastern Qinghai-Tibetan Plateau and reconstruct trace metal pollution history during the last two centuries. The lacustrine sediment core was dated by 137Cs and 210Pb, and sediments deposited prior to the 1850s were selected as the pre-industrial background. Factor analysis and enrichment factor indicated Cr, Cu, and Ni generally originated from natural sources, while Cd, Pb, and Zn have been influenced by human contamination since the mid-1980s. The anthropogenic Cd mainly derived from non-ferrous metal smelting in Gansu Province, and fluxes to Lake Qinghai sharply increased after the mid-1980s. The metal reconstruction is similar to other lake sediment records from China and corresponds well with rapid economic development in China. The spatial pattern of anthropogenic Cd fluxes to lakes is primarily attributed to regional industrial emission, phosphate fertilizers, and manure applied in agriculture.

Sedimentary Evidence of Environmental Degradation in Sanliqi Lake, Daye City (A Typical Mining City, Central China).

To reconstruct the history of environmental degradation in Sanliqi Lake (Daye City, central China), multiple proxies were analyzed in a sedimentary core which was dated using (137)Cs and spheroidal carbonaceous particles (SCPs). The results show that Sanliqi Lake has experienced serious degradation during the past 60 years, resulting from a large influx of metals and nutrients. Expansion of agricultural and industrial activities between 1945 and 1993 enhanced nutrient and metal enrichment, indicated by increases in metals, SCPs, magnetic susceptibility, total phosphorus, total nitrogen and total organic carbon. Further enrichment of Zn, Cd, Ni and Cr after 1993 was linked to a recent intensification of mining activities. Decreases in Cu and Pb after 2006 probably resulted from recent environmental remediation. This study verified the coupling between lake sediment pollution and human activities in Daye City during the past 60 years. The reconstructed history of lake pollution can provide reference information for continued restoration of Sanliqi Lake and other similar heavily polluted lakes in the developing regions.