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The ways in which natural ecosystems respond to global change have become an urgent concern to society. Lakes play a pivotal role in ecosystem changes because of their central position within watersheds and their involvement in various endogenous and cross-system ecological and biogeochemical processes, leaving natural archives accumulated in sediments over time. However, how lake ecosystems have responded to atmospheric changes during the past few centuries is still not fully understood. To analyse the process in more detail, we conducted a case study at Lake Genco, a remote alpine on the southeastern Qinghai-Tibetan Plateau (QTP) and recovered a 170-year history of lake's ecosystem from a 210Pb-dated sediment core. The air temperature increased between 1900 and 1950 CE and 1980 and 2020 CE. Geochemical and physical proxies (magnetic susceptibility, Sr/Ba, K/Al, Na/Al, Ti, Pb, P flux, TN and δ15N org) indicated greater degrees of weathering and erosion, and higher nutrient loadings as terrestrial systems responded to atmospheric changes after 1900 CE. Higher primary aquatic productivity, richer biodiversity and weaker structural stability were found in Lake Genco between 1852 and 2017 CE, although the ecological response was quiet in terms of the compositional change in the diatom community, which was generally dominated by small planktonic Cyclotella ocellata. During warmer periods, diatom PC1 changed directionally due to the overall increase in the abundance of tychoplanktonic Fragilaria construens and periphytic Achnanthes minutissima and the decreasing abundance (∼10%) of C. ocellata. This study provides empirical evidence that ecological changes in remote lakes rely on interactions between the atmosphere and the landscape and helps us to understand the complexity of the various impacts of global climate change. More local and regional studies are needed to generalise the mechanism of biological responses to atmospheric changes in remote lakes, especially in the QTP. • Remote lake ecosystems' responses to climate change have been a key concern in the global warming context. • Modest ecological changes were revealed over the past 170 years in a small high-altitude lake of the southeast Tibetan Plateau. • The relative abundance of planktonic Cyclotella ocellata decreased while primary productivity and biodiversity increased. • Lake surface area may have determined the local ecological response of lakes to atmospheric changes in this region. [ABSTRACT FROM AUTHOR]

For the lack of historical water ecology monitoring data and the limitations of traditional morphology-based water ecology reconstruction methods, the response mechanism of lake phytoplankton to climate change in the Tibetan Plateau remains unclear. In this study, we reconstructed the succession of phytoplankton community in Lake Gyaring, located in the northeastern Tibetan Plateau, since the 6th century by using sediment DNA metabarcoding. We found that the diversity and structure succession stages of phytoplankton community were consistent with the global temperature oscillation events, namely, the Medieval Warm Period (800–1300 CE). Little Ice Age (1300–1850 CE) and Current Warm Period (1850–2000 CE). The community structure dominated by Chlorophyta in the Medieval Warm Period gradually changed to that dominated by Bacillariophyta in the Little Ice Age. However, the Current Warm Period did not observe the dominant community structure of Chlorophyta similar to that of the Medieval Warm Period. This finding may be due to the climate change of the Current Warm Period was influenced by multiple human activities, such as increased anthropogenic aerosols, which promoted obvious weakening of the intensity of the summer monsoon, leading to decreased soil erosion and lake mixing, and thus may trigger a strikingly different aquatic ecosystem compared to that during the Medieval Warm Period. In this paper, the combination of genomics and palaeollimnology is helpful to explore the response mechanism of phytoplankton to climate change. [Display omitted] [ABSTRACT FROM AUTHOR]

Profundal lake sediment cores are often interpreted in line with diverse and detailed sedimentological processes to infer paleoenvironmental conditions. The effects of frozen lake surfaces on terrigenous sediment deposition and how climate changes on the Tibetan Plateau are reflected in these lakes, however, is seldom discussed. A lake sediment core from Hala Lake (590 km 2 ), northeastern Tibetan Plateau spanning the time interval from the Last Glacial Maximum to the present was investigated using high-resolution grain-size composition of lacustrine deposits. Seismic analysis along a north–south profile across the lake was used to infer the sedimentary setting within the lake basin. Periods of freezing and melting processes on the lake surface were identified by MODIS (MOD10A1) satellite data. End-member modeling of the grain size distribution allowed the discrimination between lacustrine, eolian and fluvial sediments. The dominant clay sedimentation (slack water type) during the global Last Glacial Maximum (LGM) reflects ice interceptions in long cold periods, in contrast to abundant eolian input during abrupt cold events. Therefore, fluvial and slack water sedimentation processes can indicate changes in the local paleoclimate during periods of the lake being frozen, when eolian input was minor. Inferred warm (i.e., ∼22.7 and 19.5 cal. ka BP) and cold (i.e., ∼11–9 and 3–1.5 cal. ka BP) spells have significant environmental impacts, not only in the regional realm, but they are also coherent with global-scale climate events. The eolian input generally follows the trend of the mid-latitude westerly wind dynamics in winter, contributing medium-sized sand to the lake center, deposited within the ice cover during icing and melting phases. Enhanced input was dominant during the Younger Dryas, Heinrich Event 1 and at around 8.2 ka, equivalent to the well-known events of the North Atlantic realm. [ABSTRACT FROM AUTHOR]

The reconstruction of human-driven ecosystem dynamics under the background of climate change is crucial for understanding past human-environment interactions and gaining insights into the evolution of aquatic ecosystems that are currently facing eutrophication. However, disentangling the effects of human activities from climate warming on lake ecosystem presents challenge due to limited knowledge of spatial and temporal response mechanisms of ecosystem evolution from climate-driven processes to those influenced by both climate and mild human intervention, and ultimately to those primarily driven by intense human activities. Here, we use diatom analysis of a sediment core from Dianchi Lake on the southeastern margin of the Tibetan Plateau to reconstruct environmental changes, with particular emphasis on the influences of climate, changing vegetation, and land-use on lake ecosystems. By comparing fossil diatom assemblages with previous reconstructions of vegetation and catchment processes, we demonstrate three successive stages of nutrient enrichment in Dianchi Lake over the past 20,000 years. The first major transition, occurring at the late glacial/Holocene boundary, witnessed an increase in nutrient inputs primarily driven by Holocene climate warming. The most pronounced nutrient enrichment, characterized by significant changes in diatom assemblages around 2.3 cal kyr BP, resulted from intensified agricultural cultivation. The arrival of the Han people to the Dianchi lake basin has been followed by extensive burning, land clearance, and increased soil erosion associated with agricultural production, marking the early detectable human impact on aquatic ecosystems since 2.3 cal kyr BP. Despite the eutrophication observed in Dianchi Lake in the last century, the long-term diatom record indicates that the lake ecosystem experienced abrupt state transformations as early as 2.3 cal kyr BP. The decline in water level due to dredging for flood control, coupled with intensified soil erosion and exposure of alluvial deposits during the period of 750–250 cal yr BP, triggered exceptionally low lake productivity and vegetation changes, indicating a collapse of the lake ecosystem due to intensified human impacts. This underscores the limnological effects of transitioning from predominantly agricultural practices to large-scale hydrological modifications for irrigation and agricultural development. The accelerated soil erosion and rapid water level decline since 2.3 cal kyr BP represent a tipping point for both the lake ecosystem and catchment vegetation, with significant implications for present-day land and lake degradation. To prevent further degradation of the lake and mitigate the impacts of climate warming, we propose: (1) enhancing lake ecosystem management through the promotion of diverse forests and crops within the landscape, and (2) fostering diverse cultural relationships between people and their water and land resources. Our study provides evidence that the impact of local anthropogenic activities on aquatic ecosystem functions and services can outweigh that of climate change, particularly since the onset of significant human influence. • Diatom analysis reconstructs 20,000-year environmental history of Dianchi Lake, disentangling human and climate impacts. • Intensified agriculture around 2.3 cal kyr BP caused significant nutrient enrichment and detectable human impact on Dianchi Lake's aquatic ecosystems. • Accelerated soil erosion and water level decline since 2.3 cal. kyr BP mark a tipping point for the lake ecosystem. [ABSTRACT FROM AUTHOR]

• Shrub - grassland ecotone acts as a net CO 2 sink, with the maximum NEE occurring in July. • Shrub - grassland ecotone degradation diminishes the carbon sequestration capacity, and grazing exclusion enhanced carbon sequestration capacity, particularly in heavily degraded ecotone. • Soil moisture and soil temperature are pivotal drivers influencing NEE. • NEE was negatively correlated with soil temperature during April to June and September to October, but was positively correlated with soil temperature during July to August. • NEE displays a consistent negative correlation with soil moisture during April to October. Grassland degradation is a prevalent issue at the interface of shrubland and grassland ecosystems, particularly susceptible to environmental changes. To counteract the degradation, grazing exclusion has been proposed as a dominant restoration strategy on the Tibetan plateau, the world's largest and highest plateau. However, the impact of degradation and subsequent restoration on the carbon dynamics consist of carbon sink / source capacity of shrub-grassland ecotones remain unclear. Therefore, in this study, using a transparent chamber (0.8 m in length × 0.8 m in width × 0.6 m in height) attached to an infrared gas analyzer, the in situ field investigation was performed to measure the gross ecosystem primary productivity (GEP), ecosystem respiration (Re), net ecosystem CO 2 exchange (NEE), and environmental factors over a four-year continuous period in lightly and heavily degraded shrub-grassland ecotones during recover process on the Tibetan Plateau. Our result showed that: 1). Shrub-grassland ecotone acts as a net CO 2 sink, with the maximum NEE occurring in July during the growing season. 2). Ecotone degradation diminishes the carbon sequestration capacity. Specifically, the annual GEP, Re, and NEE rates decrease by 14.1 %, 7.3 %, and 27.2 %, respectively, in the heavily degraded ecotone compared to the lightly degraded counterpart. 3). Grazing exclusion positively influenced the carbon sequestration capacity, particularly in heavily degraded ecotone, which leads to a substantial increase in the annual GEP, Re, and NEE rates of 15.4 % and 33.4 %, 12.3 % and 20.9 %, 22.25 % and 64.9 % for lightly and heavily degraded ecotones, respectively. 4). A piecewise structural equation model reveals that soil moisture and soil temperature are pivotal drivers influencing NEE. 5) Correlation analysis shows that NEE was negatively correlated with soil temperature during April to June and September to October but was positively correlated with soil temperature during July to August. In contrast, NEE displays a consistent negative correlation with soil moisture throughout the growing season. This research provides new insights on grassland degradation and restoration for carbon sequestration in alpine shrub-grassland ecotones and highlights their implications for sustainable management of alpine ecosystems. [ABSTRACT FROM AUTHOR]

The Tibetan Empire, the only united regime in Tibetan history, existed between 618 and 877 CE, when Tibet was a unified world power, contesting control of Central Asia with the Tang Empire and the Abbasid Caliphate. Before its collapse in the 9th century, this was the world's highest elevation empire, ruling a vast territory that extended well beyond the Tibetan Plateau. However, factors leading to the rapid decline of this powerful highland empire remain elusive. Here we use a highly resolved lake sediment record from the western Tibetan Plateau to demonstrate that pronounced fluctuations in the climate regime between the 7th and 9th centuries played a key role in the fragmentation of the Tibetan Empire. Specifically, we demonstrate that a nearly complete shift from planktonic to benthic diatom life strategy signified markedly drier conditions and lower lake-levels. The height of the empire's power (ca. 600–800 CE) coincided with two centuries of uncharacteristically warm and humid conditions, whereas the empire's collapse (ca. 800–877 CE) co-occurred with an abrupt onset of severe drought conditions that were clearly registered in the paleolimnological records. Severe drought conditions combined with decreased social resilience might have led to rapid declines in crop production, which fostered a surge in domestic conflicts and expediated the final collapse of the Tibetan Empire. Although religious and political factors are commonly cited to explain the fall of this formidable great power, our paleoenvironmental analyses underscore that harsh environmental factors might have hastened the cultural decline of this highland region. • Paleolimnological data from a rare freshwater lake in the western Tibetan Plateau. • Abrupt collapse of the Tibetan Empire around 9th century linked to severe drought. • Rise and fall of empire linked to marked climate shifts between 7th and 9th centuries. • These changes were detrimental to the political unity of the Tibetan Plateau. • Diatoms closely tracked changes in lake level. [ABSTRACT FROM AUTHOR]