Integrating indices based on different chemical extractions and bioaccumulation in Bellamya aeruginosa to assess metal pollution and ecological risk in sediment.

Various indices based on metal chemical data are used to evaluate pollution and ecological risk, but the consistency of the assessment results is usually unsatisfactory, and it is unclear if the ecological risk from sediment metals is accurately represented in in situ zoobenthos. Herein, the pollution and ecological risk associated with As, Cd, Cr, Cu, Ni, Pb and Zn in the sediments of two adjacent lakes (Datun (DT) and Changqiao (CQ)) were comprehensively evaluated by integrating metal concentrations, chemical forms and bioaccumulation in Bellamya aeruginosa (B. aeruginosa). The metal concentrations and chemical compositions varied widely in the sediments. Over 50% of the Cd, Pb and Zn in the sediments was present in bioavailable forms, followed by 28% of Cu and less than 25% of As, Cr and Ni. According to the enrichment factor (EF) and concentration enrichment ratio (CER) assessments, Cr and Ni were natural in origin, while the other metals were at minor to extremely high pollution levels, with average EFs of 1.5-77.6 and CERs of 1.1-113.4. The pollution levels for Cd, Cu and Pb from the EF and CER assessments were similar, while those for As and Zn were higher according to CER than EF (p = 0.05), likely due to the baseline underestimation associated with the potential diagenetic remobilization of bioavailable metals. The ecological risk index (Er), sediment quality guidelines (SQGs) and risk assessment code (RAC) showed a high eco-risk for Cd, while no similar risk was found for the other metals. By integrating risk indices with the chemical forms and pollution levels of metals, we deduced high eco-risks for As and Pb and moderate eco-risks for Cu and Zn in DT Lake and moderate eco-risks for As, Pb and Zn in CQ Lake. The other metals in the sediments of the two lakes presented low eco-risks. No significant positive correlations (p = 0.05) between metal accumulation in B. aeruginosa and the indices of pollution and eco-risk were observed except for the case of As, implying that measuring the metal concentrations in B. aeruginosa would not accurately characterize the metal pollution and ecological risk of sediments.

Network parameters quantify loss of assemblage structure in human-impacted lake ecosystems.

Lake biodiversity is an incomplete indicator of exogenous forcing insofar as it ignores underlying deformations of community structure. Here, we seek a proxy for deformation in a network of diatom assemblages comprising 452 species in 273 lakes across China. We test predictions from network theory that nodes of similar type will tend to self-organize in an unstressed system to a positively skewed frequency distribution of nodal degree. The empirical data reveal shifts in the frequency distributions of species associations across regions, from positive skew in lakes in west China with a history of low human impacts, to predominantly negative skew amongst lakes in highly disturbed regions in east China. Skew values relate strongly to nutrient loading from agricultural activity and urbanization, as measured by total phosphorus in lake water. Reconstructions through time show that positive skew reduces with temporal intensification of human impacts in the lake and surrounding catchments, and rises as lakes recover from disturbance. Our study illustrates how network parameters can track the loss of aquatic assemblage structure in lakes associated with human pressures.

Flickering gives early warning signals of a critical transition to a eutrophic lake state.

There is a recognized need to anticipate tipping points, or critical transitions, in social-ecological systems. Studies of mathematical and experimental systems have shown that systems may 'wobble' before a critical transition. Such early warning signals may be due to the phenomenon of critical slowing down, which causes a system to recover slowly from small impacts, or to a flickering phenomenon, which causes a system to switch back and forth between alternative states in response to relatively large impacts. Such signals for transitions in social-ecological systems have rarely been observed, not the least because high-resolution time series are normally required. Here we combine empirical data from a lake-catchment system with a mathematical model and show that flickering can be detected from sparse data. We show how rising variance coupled to decreasing autocorrelation and skewness started 10-30 years before the transition to eutrophic lake conditions in both the empirical records and the model output, a finding that is consistent with flickering rather than critical slowing down. Our results suggest that if environmental regimes are sufficiently affected by large external impacts that flickering is induced, then early warning signals of transitions in modern social-ecological systems may be stronger, and hence easier to identify, than previously thought.

Consistent C3 plant habitat of hominins during 400-300 ka at the Longyadong Cave site (Luonan Basin, central China) revealed by stable carbon isotope analyses of loess deposits.

The proportions of woody and grassland taxa in terrestrial ecosystems played an important role in the origin and evolution of early Palaeolithic hominins. However the influence of ecosystem changes on hominin behavior and adaptations in Asia has not been studied in detail. Hominins have exploited the Luonan Basin in the Eastern Qinling Mountains, central China, since the early Paleolithic. Dated sites, consisting of alternating loess and soil deposits with in situ artefacts, are common in the region, and provide a detailed record of Early to Middle Pleistocene hominin environments. Here, we present the results of measurements of the stable carbon isotopic composition of soil organic matter (δ13CSOM) in the loess-paleosol sequences from the Longyadong Cave site. Our analyses of δ13CSOM show that for at least 400 ka the Longyadong Cave site and its surroundings were dominated by C3 woody plants, whereas the nearby Liuwan site was dominated by C4 and C3 mixed grassland or woody grassland vegetation. These findings demonstrate that between 400 and 300 ka in the Luonan Basin, hominins occupied a habitat consisting of a mosaic of grassland and woodland/forest. Although the vegetation of the region changed in response to the glacial-interglacial climatic cycles, patches of woody vegetation in landscapes such as at Longyadong Cave site persisted continuously. Such environments seem to be have been favored by hominins living in the Luonan Basin, possibly because they provided a diverse range of food resources during both glacial and interglacial intervals of the Middle Pleistocene, when most of northern China was experiencing an increasing trend of drying and cooling and steppe environments were expanding. Thus, the Luonan Basin would have served as a refugium for hominin occupation in China during the Middle Pleistocene.

Early warning of critical transitions in biodiversity from compositional disorder.

Global environmental change presents a clear need for improved leading indicators of critical transitions, especially those that can be generated from compositional data and that work in empirical cases. Ecological theory of community dynamics under environmental forcing predicts an early replacement of slowly replicating and weakly competitive "canary" species by slowly replicating but strongly competitive "keystone" species. Further forcing leads to the eventual collapse of the keystone species as they are replaced by weakly competitive but fast-replicating "weedy" species in a critical transition to a significantly different state. We identify a diagnostic signal of these changes in the coefficients of a correlation between compositional disorder and biodiversity. Compositional disorder measures unpredictability in the composition of a community, while biodiversity measures the amount of species in the community. In a stochastic simulation, sequential correlations over time switch from positive to negative as keystones prevail over canaries, and back to positive with domination of weedy species. The model finds support in empirical tests on multi-decadal time series of fossil diatom and chironomid communities from lakes in China. The characteristic switch from positive to negative correlation coefficients occurs for both communities up to three decades preceding a critical transition to a sustained alternate state. This signal is robust to unequal time increments that beset the identification of early-warning signals from other metrics.

Extending the timescale and range of ecosystem services through paleoenvironmental analyses, exemplified in the lower Yangtze basin.

In China, and elsewhere, long-term economic development and poverty alleviation need to be balanced against the likelihood of ecological failure. Here, we show how paleoenvironmental records can provide important multidecadal perspectives on ecosystem services (ES). More than 50 different paleoenvironmental proxy records can be mapped to a wide range of ES categories and subcategories. Lake sediments are particularly suitable for reconstructing records of regulating services, such as soil stability, sediment regulation, and water purification, which are often less well monitored. We demonstrate the approach using proxy records from two sets of lake sediment sequences in the lower Yangtze basin covering the period 1800-2006, combined with recent socioeconomic and climate records. We aggregate the proxy records into a regional regulating services index to show that rapid economic growth and population increases since the 1950s are strongly coupled to environmental degradation. Agricultural intensification from the 1980s onward has been the main driver for reducing rural poverty but has led to an accelerated loss of regulating services. In the case of water purification, there is strong evidence that a threshold has been transgressed within the last two decades. The current steep trajectory of the regulating services index implies that regional land management practices across a large agricultural tract of eastern China are critically unsustainable.

Human impacts overwhelmed climate as the dominant factor controlling lacustrine organic matter accumulation in Erhai Lake 2000 years ago, Southwest China.

Climate and human activity are two important factors in regulating organic matter (OM) accumulation in the lake environment. However, when and how anthropogenic impacts have affected lacustrine OM accumulation in southwest China during the late Holocene have not yet been well defined. Here, a 16.3-kyr n-alkane record derived from Erhai Lake was used to trace OM sources and explore their connections to climate and human activity. The n-alkane distributions indicated that the dominant sediment sources shifted from terrestrial and aquatic plants to algae in the late Holocene. OM accumulation was closely related to catchment soil erosion, sediment transport, and deposition processes regulated by climate conditions before 5.0 cal. kyr B.P., following the patterns that stronger monsoon precipitation favoured more terrestrial and less aquatic OM input, and vice versa. From 5.0 to 2.0 cal. kyr B.P., the synchronous downwards trends in terrestrial OM input and precipitation intensity indicated that climate remained a major driving force for OM accumulation. However, sediment sources experienced large-magnitude and centennial-scale oscillations between allochthonous and autochthonous inputs, reflecting early human impacts appeared and lake ecosystems retained the self-regulated ability to recover from the basin-wide early moderate human disturbances. Afterwards, the increased (decreased) OM contributions from terrestrial (aquatic) plants contradicted the weakening monsoon precipitation since 2.0 cal. kyr B.P., indicating a dominant effect of human activities on OM accumulation. This change was accompanied by highly improved algae productivity and gradually elevated lacustrine trophic status, and the lake ecosystem eventually shifted into another state largely deviating from its climate-driven background due to intensified deforestation and agricultural cultivation. Regional comparison indicated that anthropogenic disturbances have temporal differences in southwest China. This study will further improve our understanding of past climate-human-environment interactions in southwest China.

The first DNA barcode library of Chironomidae from the Tibetan Plateau with an evaluation of the status of the public databases.

The main aim of this study was to curate a COI barcode library of Chironomidae from the Tibetan Plateau (TP) as an essential supplement to the public database. Another aim is to evaluate the current status of the public database of Chironomidae in aspects of taxonomic coverage, geographic representation, barcode quality, and efficiency for molecular identification, the Tibetan Plateau, China. In this study, 512 individuals of Chironomidae from the TP were identified based on morphological taxonomy and barcode analysis. The metadata of public records of Chironomidae were downloaded from the BOLD, and the quality of the public barcodes was ranked using the BAGS program. The reliability of the public library for molecular identification was evaluated with the newly curated library using the BLAST method. The newly curated library comprised 159 barcode species of 54 genera, of which 58.4% of species were likely new to science. There were great gaps in the taxonomic coverage and geographic representation in the public database, and only 29.18% of barcodes were identified at the species level. The quality of the public database was of concern, with only 20% of species being determined as concordant between BINs and morphological species. The accuracy of molecular identification using the public database was poor, and about 50% of matched barcodes could be correctly identified at the species level at the identity threshold of 97%. Based on these data, some recommendations are included here for improving barcoding studies on Chironomidae. The species richness of Chironomidae from the TP is much higher than ever recorded. Barcodes from more taxonomic groups and geographic regions are urgently needed to fill the great gap in the current public database of Chironomidae. Users should take caution when public databases are adopted as reference libraries for the taxonomic assignment.

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.

Spatiotemporal patterns of organic carbon burial over the last century in Lake Qinghai, the largest lake on the Tibetan Plateau.

Lakes are important carbon sinks in terrestrial environments. However, the estimation of the global lake carbon sink has large uncertainty. Data from plateau and remote lakes are rare, and most studies of carbon sequestration in large lakes have been based on single or a few sediment cores. Here, twenty-five sediment cores were collected by grid sampling covering Lake Qinghai, the largest lake on the Tibetan Plateau. Age models were established by combining radionuclide 210Pb137Cs dating with magnetic susceptibility chronostratigraphy of sediment cores. Furthermore, the spatiotemporal variations of the organic carbon burial rate (OCBR) over the past century were investigated. The OCBR of Lake Qinghai has increased significantly since the 1990s in association with warm-humid climates, increased nutrient supply and, enhanced land-use changes. The spatial distributions of OCBR were insignificant during the 1900s-1960s and 1960s-1990s and then shifted to a pattern of high values occurring in the southwestern lake areas during the post-1990s period. The spatial distribution of OCBR was mainly determined by sediment mass accumulation rate, primary production, and potential mineralization. The average OCBR in the all sediment cores showed no correlation with water depth (12-30 m) and was within one standard deviation of the whole-lake average value for most cores. These results suggest that the average OCBR of a sediment core in a relatively flat lakebed can generally represent the whole-basin level of Lake Qinghai. The average OCBR was 22.5 ± 5.5 g m-2 yr-1, which is close to those values reported previously for lakes of boreal forest and taiga but significantly higher than those reported for tundra lakes. Our findings highlight that the remote lakes on the Tibetan Plateau have great carbon sequestration potential in sediments and may act as a significant natural carbon sink.

[Spatial-temporal Variations and Pollution Assessment of Heavy Metals in Sediments of Qionghai Lake in Xichang City].

Based on the analysis of the total concentrations of 10 metals in the sediment core and total concentrations and chemical fractions of seven metals in the surface sediments of Qionghai Lake in Xichang City, Sichuan Province, the spatial-temporal characteristics of metal accumulation and pollution over the past century and the potential ecological risk of metals in surface sediments were studied. Before the 1970s, metal concentrations in the sediment core were stable. The total concentrations of Al, Fe, K, and Cr in the sediment core exhibited visible peaks in the 1970s, which were related to the enhanced input of fine-grained topsoil caused by increasing precipitation, lake reclamation, and deforestation. Since the 1990s, the total concentrations of Al, Fe, K, and Cr decreased with the reduced topsoil erosion, whereas the total concentrations of As, Cd, Cu, Pb, and Zn gradually increased or remained stable. The enrichment factor results showed that Cd, Pb, and Zn were the main contaminants, with Cd as the typical contaminant in the sediment core. The Cd contamination started in the 1960s and has remained at a moderate level since the 1990s. In the surface sediments, the total concentrations of Cd were higher in the northwest lake area, and no visible spatial concentration trends of the other metals were displayed. The bioavailable fractions of Cd, Pb, and Zn accounted for 95%, 63%, and 48% of the total metal concentrations on average. Among the bioavailable fractions, Cd was mainly in the acid-soluble fraction, and Pb and Zn were mainly in the reducible and oxidized fractions. The bioavailable fractions of the other metals were less than 27%. The results of total concentrations and bioavailable fractions of metals revealed that Pb and Zn in the surface sediments were slightly or moderately contaminated, and Cd was moderately contaminated on average. Cd contamination was at a severe level in the northwest lake area. The concentrations of anthropogenic Cd, Pb, and Zn in the surface sediments estimated from the total and bioavailable concentrations were comparable (P>0.05), indicating that anthropogenic metals primarily existed in bioavailable fractions in the sediment. Integrating the assessment results from sediment quality guidelines, potential ecological risk index, and chemical forms of metals, Cd in surface sediments may pose a high ecological risk, whereas the other metals has a low ecological risk.

Spatial distribution and human contamination quantification of trace metals and phosphorus in the sediments of Chaohu Lake, a eutrophic shallow lake, China.

Distinguishing and quantifying anthropogenic trace metals and phosphorus accumulated in sediment is important for the protection of our aquatic ecosystems. Here, anthropogenic proportion and potential sources of trace metals and phosphorus in surface sediments of Chaohu Lake were evaluated based on the exhaustive geochemical data. The analysis shows that concentrations of major and trace metals, and phosphorus, displayed significant spatial diversity and almost all elements were over the pre-industrial background value, which should be related to the variations of sediment composition partially. Therefore, conservative element normalization was introduced and calculated enrichment factors (EFs) of the elements were referenced highlighting the human contamination. EFs of the major and trace metals, except Zn, Pb, and Cu, were all nearly 1.0, indicating the detrital origin. The EFs of Zn, Pb, Cu and phosphorus were 1.0-10.4, 1.0-3.8, 1.0-4.9, and 1.0-7.6, respectively, showing moderate to significant contamination. Higher EFs of Zn, Pb and Cu occurred in the mouth areas of Nanfei River and Zhegao River, and they decreased to the lake center in the northwest and northeast lake areas, respectively. We deduced that anthropogenic Zn, Pb, and Cu were mainly from urban and industrial point sources and the non-point sources of atmospheric deposition contributed little to their contamination. The EFs of phosphorus showed similar spatial degradation with that of Zn, Pb, and Cu. Moreover, higher EFs (>1) of phosphorus also occurred in other areas adjacent to the river mouths besides Nanfei River and Zhegao River. This indicated that the non-point agricultural source may also be responsible for the contamination of phosphorus in Chaohu Lake in addition to the urban sewage sources. Anthropogenic phosphorus was mainly concentrated in the speciation of NaOH-P, which had higher potential biological effects than the detrital proportion. Concentrations of Zn, Pb and Cu surpassed the threshold effect concentrations (TEC) of consensus-based sediment quality guidelines of freshwater ecosystems, especially in the contaminated northwest area of Chaohu Lake. This highlighted the contributions of anthropogenic contamination to the elevated potential biological effects of trace metals. Though there had been no obvious human contamination of Cr and Ni in Chaohu Lake, concentrations were all over the TECs, which may be due to higher background levels in the parent materials of soils and bedrocks in Chaohu Lake catchment.

One century sedimentary record of lead and zinc pollution in Yangzong Lake, a highland lake in southwestern China.

Reconstruction of trace metal pollution histories and sources may help us to regulate current pollutant discharge. This is especially important for the highland lakes in southwestern China, which are facing trace metals pollution. We present sedimentary records of 11 metals accumulated in Yangzong Lake since the 1870's, a highland lake in southwestern China. Pollution of lead and zinc (Pb and Zn) was differentiated based on principal component analysis, geochemical normalization, and lead isotope ratios. Nearly all the metals as well as grain size composition show generally constant values before the mid-1980's, denoting stable detrital input in the catchment. Fluctuations in the concentrations of the metals as well as grain size composition since the mid-1980's indicate an increase in soil erosion with strengthened human disturbance in the catchment. After geochemical normalization, Pb and Zn showed constant values before 1990 AD and then a gradual increase in parallel with the variations in 208Pb/206Pb and 207Pb/206Pb ratios, indicating that Pb and Zn pollution occurred. Combining the data of 208pb/206Pb and 207Pb/6Pb ratios in the sediments of Yangzong Lake, leaded gasoline, Pb-Zn ore and coal, and consumption or production historical trends, we deduced that the enhanced Pb and Zn pollution in Yangzong Lake is caused primarily by ore mining and refining.

Aquatic ecosystem responds differently to press and pulse nutrient disturbances as revealed by a microcosm experiment.

Due to climate change and increasing anthropogenic activities, lakes are disturbed frequently, usually by press (e.g., diffused pollution, rising temperatures) or pulse (e.g., storms, rainfall, pollution events) disturbances. Both press and pulse disturbances can affect abiotic and biotic environments, changing the structure of ecosystems and affecting ecosystem services. To confront with the effects of climate change and increasing anthropogenic activities, understanding the different effects of press and pulse disturbances on lake ecosystems is essential. This study assessed the effect of press and pulse disturbances of phosphorus on a microcosmic aquatic ecosystem by measuring the total phosphorus (TP), algae density, and physiological indicators of submerged macrophytes. We found that the microcosmic aquatic ecosystem responded differently to press and pulse disturbances. Our results suggested that it had a lower resistance to pulse phosphorus disturbances than to press phosphorus disturbances. There were significantly higher nutrient concentrations and algal densities in the pulse treatment than in the press treatment. Positive feedback was found between the biomass of submerged macrophytes and the water quality. There was a higher submerged macrophytes biomass at low TP concentration and algal density. In the context of climate change, press and pulse disturbances could have severe impacts on lake ecosystems. Our findings will provide some insight for further research and lake management.

Historical trends in atmospheric metal(loid) contamination in North China over the past half-millennium reconstructed from subalpine lake sediment.

Trace metal (loid) contamination in the atmosphere is widely monitored, but there is a gap in understanding its long-term patterns, especially in North China, which is currently a global contamination hotspot mainly caused by heavy industry emissions and coal combustion. Herein, historical trends of atmospheric As, Cd, Cr, Cu, Hg, Ni, Pb and Zn contamination in North China over the past ∼500 years are comparatively studied with sediment cores from two subalpine lakes (Gonghai and Muhai). Arsenic, Pb, Cd and Hg were main pollutants according to Pb isotopes and enrichment factors. Mercury contamination has increased continuously since the late 1800s and increasing As, Pb and Cd contamination started in the 1950s in Gonghai. In contrast, the contamination in Muhai lagged two decades for As, Cd and Pb and a half-century for Hg behind that in Gonghai, although the trends were similar. This contamination lag was attributed to the low sensitivity of Muhai sediment to early weak atmospheric metal contamination under 2.1-fold higher detrital sedimentation. As, Pb and Cd contamination has intensified since the 1980s, and the metals showed similar sedimentary fluxes in the cores. However, sedimentary fluxes of Hg contamination were 3.4-fold higher in Gonghai than in Muhai due to combination with organic matter. No obvious Cr, Cu and Ni contamination in the cores was mainly because of the low atmospheric deposition from anthropogenic sources relative to detrital input, although some of their atmospheric emissions were higher than those of As, Cd and Hg. Atmospheric As, Pb and Cd contamination was mainly from domestic sources of coal combustion and nonferrous smelting. Mercury contamination was mainly from global and Asian sources in the first half of the 20th century, and domestic emissions gradually dominated Hg contamination after the mid-1900s.

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.

Spectral attenuation of ultraviolet and visible radiation in lakes in the Yunnan Plateau, and the middle and lower reaches of the Yangtze River, China.

Solar radiation spectra were measured in China in 2006 and 2007 in situ in 16 lakes with 42 stations in the Yunnan Plateau (YP) and in 4 lakes with 41 stations in the middle and lower reaches of the Yangtze River (YR) to determine the principal factors influencing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). The diffuse attenuation coefficient for PAR (K(d)(PAR)) varied from 0.19 to 2.88 m(-1) in the YP, which was significantly lower than the values that were measured in the YR, which ranged from 0.55 to 4.52 m(-1) (t-test, p < 0.001). In the UV-B region (320 nm), K(d)(320) ranged from 0.59 to 18.56 m(-1) in the YP, significantly lower than that in the YR, which ranged from 4.80 to 20.64 m(-1) (t-test, p < 0.001). The corresponding 1% attenuation depths ranged from 0.25 to 7.81 m in the YP lakes and 0.22 to 0.96 m in the YR lakes (t-test, p < 0.001). Partial correlation analyses were used to partition the importance of every optically-active substance on UVR and PAR attenuation in the two different regions. In the YP lakes, chromophoric dissolved organic matter (CDOM) absorption explained most of the variability in the UV-B (320 nm) and UV-A (360 nm) attenuations (partial correlation: r = 0.911, p<0.001; r = 0.820, p<0.001, respectively). Tripton and phytoplankton made no significant contributions to UVR attenuation. Tripton, phytoplankton and CDOM all made significant contributions to PAR attenuation with the highest partial correlation corresponding to tripton. In the YR lakes, tripton, phytoplankton, and CDOM all made significant contributions to UVR attenuation. The contribution of CDOM to UVR decreased with increasing wavelength, but the contribution of tripton and phytoplankton to UVR increased with increasing wavelength. Tripton contributed the most to UV-A attenuation. Tripton and phytoplankton made significant contributions to PAR attenuation with a higher partial correlation for tripton but CDOM had no significant contribution to PAR attenuation. The absorption and attenuation data of UVR and PAR in this study showed a shift from absorption-dominated K(d) in the YP lakes to scattering-dominated K(d) in YR lakes.

Comprehensive assessment of heavy metal pollution and ecological risk in lake sediment by combining total concentration and chemical partitioning.

Total concentration and chemical partitioning of heavy metals are commonly used in environmental quality assessment; however, their comparability and comprehensive application are far less discussed. Herein, bioavailability, pollution and eco-risk of As, Cd, Cr, Cu, Ni, Pb and Zn in surface sediments of Erhai Lake were evaluated referring to multiple indices following the experimental methods of complete digestion, optimized Community Bureau of Reference (BCR) and 1.0 M HCl extractions. Results of bioavailability for most metals were similar and comparable from BCR and HCl extractions. While bioavailable concentrations of Cd and Pb from HCl extraction were significantly (p < 0.01) lower than those from BCR extraction, indicating BCR extraction is more efficient. Results of enrichment factor (EF) and concentration enrichment ratio (CER) suggested that Cd was the highest polluted element followed by As, Pb and Zn, whereas Cr, Cu and Ni were mainly natural in origin. Similar concentrations of anthropogenic As from EF and CER assessments indicated anthropogenic As mainly existed in bioavailable form. However, anthropogenic Cd, Pb and Zn existed in both bioavailable and residue forms, resulting in the underestimation of anthropogenic metals by the CER assessment. The sediment quality guidelines (SQGs), potential ecological risk index (Er) and risk assessment code (RAC) showed inconsistent eco-risks for each of the metals except Cd. Combining pollution level and chemical partitioning with SQGs, Er and RAC assessments, high eco-risk of Cd, moderate eco-risk of As and Pb, and low eco-risk of Cr, Cu, Ni and Zn were graded. Our study highlights the limitation of single index and the necessity of integrating multiple indices following total concentration and chemical partitioning in metal pollution and eco-risk assessments.

Possible obliquity-forced warmth in southern Asia during the last glacial stage.

Orbital-scale global climatic changes during the late Quaternary are dominated by high-latitude influenced ~100,000-year global ice-age cycles and monsoon influenced ~23,000-year low-latitude hydroclimate variations. However, the shortage of highly-resolved land temperature records remains a limiting factor for achieving a comprehensive understanding of long-term low-latitude terrestrial climatic changes. Here, we report paired mean annual air temperature (MAAT) and monsoon intensity proxy records over the past 88,000 years from Lake Tengchongqinghai in southwestern China. While summer monsoon intensity follows the ~23,000-year precession beat found also in previous studies, we identify previously unrecognized warm periods at 88,000-71,000 and 45,000-22,000 years ago, with 2-3 °C amplitudes that are close to our recorded full glacial-interglacial range. Using advanced transient climate simulations and comparing with forcing factors, we find that these warm periods in our MAAT record probably depends on local annual mean insolation, which is controlled by Earth's ~41,000-year obliquity cycles and is anti-phased to annual mean insolation at high latitudes. The coincidence of our identified warm periods and intervals of high-frequent dated archaeological evidence highlights the importance of temperature on anatomically modern humans in Asia during the last glacial stage.