Diatom evidence for mid-Holocene peatland water-table variations and their possible link to solar forcing.

Peatlands located at the northern edge of the East Asian monsoon (EAM) are well placed to provide a terrestrial record of past climate and hydrological changes for this globally sensitive region. Here we present a middle to late Holocene, diatom-derived water-table records from a peatland in the Greater Hinggan Mountains, northeastern China. An age-depth model was achieved through AMS14C dating and Bayesian piece-wise linear accumulation modelling. The diatom-based water-table reconstructions show that the peatland water-table rose from 5100 to 3500 cal. yr BP, but fell approximately 3500 cal. yr BP. From about 2800 to 1500 cal. yr BP, the peatland water-table stabilized. After about 1500 cal. yr BP, several rapid hydrological shifts, which correspond with global climate anomalies such as ice-rafted debris (IRD) events, were registered in the reconstructed water-tables. Compared with other paleoclimate records in East Asia, the general trend of peatland water-table fluctuations follows the variations in the East Asian summer monsoon (EASM) intensity. Spectrum analysis of the water-table profile yielded a statistically significant periodicity of 470-year that may be related to the "~500-year" inherent solar irradiation cycles. In addition, positive correlation between the peatland water-table levels and cosmic-isotope-reconstructed sunspot numbers underscores the role of the sun in regulating hydrological processes in the EASM margin area. The data suggest that the regional climate and hydrological variations at the EASM margin were first triggered by changes in solar output, but may have been amplified by interactions with oceanic and atmospheric circulations.

Assessing the impact of long-term changes in climate and atmospheric deposition on a shallow alpine lake from southeast Tibet.

Regional warming and atmospheric nitrogen deposition have been widely recorded to impact remote catchments and alpine lakes; however, their independent roles and interactions have rarely been identified. Here, we combined down-core analyses of sedimentary mercury (Hg) and aluminum (Al) with multiple proxies (i.e. nitrogen stable isotope, chlorophyll a pigments, diatoms) for a radiometrically-dated sediment core of an alpine lake in southeast Tibet to track the atmospheric deposition of pollutants, and to examine possible effects of climate and catchment forcing over the past three centuries. The sediment data revealed that airborne deposition of Hg was recorded from the ~1860s, with an accelerating increase in anthropogenic Hg flux since the ~1960s. A synchronous decrease in reconstructed lake-water TOC indicated that acid deposition may have affected lake-water carbon concentrations and impaired catchment export of decomposed organic matter (OM). A moderate depletion of bulk sediment δ15N started from the ~1820s, but was followed by an enriching trend after the ~1970s. This positive shift of δ15N was associated with elevated sediment OM and decreased catchment runoff of clastic materials (as inferred by Al). Sediment OM content displayed an accelerating increase from the ~1960s, with an increased input of autochthonous sources (i.e. lower bulk sediment C:N ratios), such as algae (as inferred by sedimentary chlorophyll a pigments). Meanwhile, climate warming and decreased lake-water TOC enhanced the production of algae, which was characterized by a more enriched δ15N signal than that of allochthonous OM. Furthermore, atmospheric acid deposition was significantly related to diatom assemblage changes, with an increase in acidophilous taxa. Our sediment evidence revealed the dominating impact of climate and catchment processes on lake-water chemistry and algal shifts in the context of atmospheric nitrogen deposition, and highlighted an increasing link of external forcing with in-lake processes in enriching sediment δ15N signal over the last few decades.

Impoundment intensity determines temporal patterns of hydrological fluctuation, carbon cycling and algal succession in a dammed lake of Southwest China.

Hydrological control of lakes has been increasingly practiced in many parts of the world, however, the long-term ecological impact of hydrological regulation and their dependence on lake impoundment intensity has been rarely examined. We combined a spatial survey of surface sediments with sediment core analyses to quantify the limnological changes over the last two centuries for an oligo-mesotrophic lake, which was dammed in 1957 and reinforced during 1987-1990, respectively. A water depth inference model constructed from surface sediment clay components was applied to a well-dated sediment core for water level reconstruction. The inferred water depth increased from 6.2 ±â€¯0.9 m to 8.7 ±â€¯1.7 m after dam construction and further to 13.6 ±â€¯2.6 m after dam enforcement, resulting in an increase in the magnitude of water level fluctuation (WLF). Accordingly, bulk sediment C/N ratio and median grain size spiked in ∼1957 and ∼1990, respectively, reflecting a large input of terrestrial sources due to impoundment. With a consistent loss of littoral zone and benthic diatoms over time, a significant decrease in C/N ratio and an abrupt depletion of carbon isotopic signal suggested a shift of carbon transfer towards a pelagic pathway after ∼1990. While there was a significant increase in algal production since ∼1990, the accumulation rate of carbon and nitrogen burial displayed an accelerating drop since ∼1957, reflecting a diluting effect derived from expanding water storage. Furthermore, there was a significant increase in both the ratio between inorganic and organic carbon fluxes and sediment burial of inorganic carbon, reflecting enhanced degradation and low storage of aquatic organic carbon in stratified deep waters since ∼1990. Hydro-morphological variables were found to exert strong impact on diatom communities, with an increasing interplay with nutrient and climate variables over time. While there existed a significant shift of diatom composition in ∼1960, species richness and community dissimilarity showed a significant decrease when water depth was raised to above ∼10 m or the magnitude of WFL was above ∼2 m. Thus, our sediment surveys provide evidence on the significant impact of lake regulation on hydro-morphology, carbon burial and ecological shift over time, as well as its stronger interaction with other forcing with increased impoundment intensity.

[Spatio-temporal Variations of Diatom Community and Their Relationship with Water Environment in Fuxian Lake].

Fuxian Lake is an important deep lake in China with trophic status generally categorized as class I. However, there exists a trend of degradation in its ecological and environmental health under the increasing impacts derived from catchment development and recent global warming. Therefore, it is important to evaluate the community composition, distribution patterns, and driving factors of lake biota. In order to uncover the spatio-temporal variations in water quality and diatom communities, a monthly survey of diatom community composition and limnological characteristics was carried out in the surface waters of three sampling sites from the south, middle, and north of the lake basin in Fuxian in 2015. A total of 166 diatom species belonging to 31 genera were identified, which was dominated by planktonic diatoms with the extirpation of Cyclotella rhomboideo-elliptica, an endemic species for Yunnan. Diatom community structure showed significant seasonal succession across the three sites, which were dominated by Fragilaria crotonensis in January and February, by Aulacoseira granulata in March, by Cyclotella ocellata and F. crotonensis in April, and by C. ocellata for other months (i.e., relative abundance of ~80%). Over the spatial scale, the distribution of the dominant diatoms displayed a high degree of similarity. Principal component analysis further showed that there existed significant difference in the diatom community structure and lake environment over the temporal scale, but no significant deviance was found across the three sites. The results of redundancy analysis and variation partitioning revealed that the key driver included meteorological and physical factors (i.e., water temperature, wind velocity, and Secchi depth), which accounted for 27.6% of the total variance in diatom community changes. In combination with the thermocline analysis, the change in lake hydrodynamics may have influenced the thickness and duration of the thermal stratification of lake water, resulting in the reorganization of the diatom community. At the same time, the lake-water nutrients and ions also exerted an important influence on community structuring, which explained the 21.2% and 9.4% of the diatom variation, respectively. Therefore, regional warming and catchment development have significantly structured the biological community and ecosystem health of Fuxian Lake. In all, measures should be taken not only to control the watershed input of pollutants but also to actively mitigate the long-term impact of climate change for the protection and ecological remediation of Fuxian Lake.

Sediment evidence of industrial leakage-induced asynchronous changes in polycyclic aromatic hydrocarbons and trace metals from a sub-trophic lake, southwest China.

It has been well established that regional patterns of atmosphere-borne polycyclic aromatic hydrocarbons (PAHs) and trace metals were predominantly associated with the trajectory of socio-economic development; however, they could be potentially modulated by anthropogenic fingerprint of local sources such as industrial spill. Here, we established historical pollution data of both PAHs and trace metals from a well-dated sediment core from Yangzong Lake of Southwest China, which experienced a severe tailing leakage accident derived from a zinc concentrate smelting plant in 2007, aiming to evaluate the heterogeneity in their temporal trajectories and their sources of contamination in the context of regional deposition patterns. Sedimentary records show that the concentrations and fluxes of both PAHs and trace metals remained a consistently low level before the 1950s. An increasing trend and the synchronous changes of both PAHs and trace metals during ~ 1950-2002 were well consistent with the temporal pattern of socio-economic development in western China, with coal combustion and smelting industries as the main sources of contamination in this region. However, arsenic (As) and PAHs exhibited a concurrent spike for the period of ~ 2007-2013, contrasting strongly to the regional pattern of these contaminants. The modern concentrations of As revealed a 5- to 14-fold increase over the pre-1950 level, with the contemporary concentrations of PAHs rising by ~ 10-14 times. The sediment records reveal that local fingerprints of smelting activities in the catchment of Yangzong Lake have overridden the temporary pattern of regional atmosphere-borne As and PAHs over the last decade. This highlights the important role of local pollution sources in modulating or even overriding the regional pattern of anthropogenic contamination in highly impacted systems.