Tibetan Plateau Lakes Research Articles

Assimilation of ancient organic carbon by zooplankton in Tibetan Plateau lakes is depending on watershed characteristics

AbstractAncient (i.e., radiocarbon depleted) organic carbon (OC) is exported from ice sheet, glacier, and permafrost systems and may be buried, respired, or assimilated in downstream aquatic systems. Few studies have explored the potential use of this ancient OC in lake food webs. We combined natural abundance radiocarbon and stable carbon isotope data (Δ14C andδ13C) to study ancient OC utilization by zooplankton in six lakes covering a large climate gradient on the central and peripheral Tibetan Plateau. A depleted Δ14C signature of dissolved and particulate OC was found in the inflowing streams and lakes, ranging from – 49‰ to – 569‰, corresponding to radiocarbon ages between 403 yr and 6757 yr. The Δ14C values for zooplankton in the lakes ranged from – 45‰ to – 264‰, reflecting that zooplankton obtain14C‐depleted signatures through assimilation of ancient OC and/or indirectly through consumption of phytoplankton or aquatic plant utilizing14C‐depleted inorganic carbon. Moreover, ancient OC from inflowing streams contributed more to zooplankton diets in the temperate glacier area than in the cold glacier area. Assimilation of ancient OC by zooplankton in lakes is not only affected by drainage basin characteristics, such as the recharge coefficient of the lake, but also by the biogeochemical properties of OC. Use of ancient OC by zooplankton in high‐altitude lakes may constitute an important link between the contemporary aquatic food webs and the glaciated watersheds. Our findings have important implications for the contribution of ancient carbon to the modern lake food webs of high‐altitude and polar lakes.

Investigation of the ice surface albedo in the Tibetan Plateau lakes based on the field observation and MODIS products

ABSTRACTThe Tibetan Plateau (TP) lakes are sensitive to climate change due to ice-albedo feedback, but almost no study has paid attention to the ice albedo of TP lakes and its potential impacts. Here we present a recent field experiment for observing the lake ice albedo in the TP, and evaluate the applicability of the Moderate Resolution Imaging Spectroradiometer (MODIS) products as well as ice-albedo parameterizations. Most of the observed lake ice albedos on TP are <0.12, and the clear blue ice albedo is only 0.075, much lower than reported in the previous studies. Even that of ice covered with snow patches is only 0.212. MOD10A1 albedo product has the best agreement with observations, followed by those of MYD10A1. MCD43A3 product is consistently higher than the observations. Due to an error of snow flag and inconsistent time windows in MCD43A2 and MCD43A3, at certain times, the albedo of the ice without snow is even higher than that covered with snow. When the solar zenith angle is not considered, there is no significant correlation between the albedo and the ice surface temperature. None of the existing ice-albedo parameterizations can reproduce well the observed relationship of the albedo and surface temperature.

Characterization of CDOM absorption of reservoirs with its linkage of regions and ages across China.

The absorption of chromophoric dissolved organic matter (CDOM) is an important part of light absorptions in aquatic systems. The increasing eutrophication of reservoirs and regional characteristics would affect the CDOM properties sensitively which would be important for the application of remote sensing monitoring. The highest (4.07 ± 2.31m-1) and lowest (0.79 ± 0.67m-1) CDOM concentrations of reservoirs were observed in the northeastern lake region (NER) and Tibetan Plateau lake region (TPR), respectively. The differences between S275-295 among the five lake regions were significant (p < 0.05) in which the steepest S275-295 (0.0173 ± 0.0026nm-1) was observed in TPR and the shallowest (0.0326 ± 0.0152nm-1) in Yungui Plateau lake region (YGR). The strong relationships between aCDOM(355) and DOC appeared in the NER (R2 = 0.43), eastern lake region (EAR) (R2 = 0.69), Mengxin lake region (MXR) (R2 = 0.61), and YGR (R2 = 0.79) which would be a good proxy for DOC in regional reservoirs. Most of all, the correlation between reservoir's establishing time and CDOM absorption under oligotrophic states was relatively strong in the EAR and MXR regions. It indicated that the establishing time of reservoirs affected the CDOM absorption to some extent under the oligotrophic states without much human disturbance. Our results indicate CDOM absorption varies with regions, and the relationships between CDOM and DOC are variable for different regions. Therefore, DOC estimation in reservoirs through CDOM absorption needs to be considered according to lake regions and trophic states.

An Investigation of Ice Surface Albedo and Its Influence on the High-Altitude Lakes of the Tibetan Plateau

Most high-altitude lakes are more sensitive to global warming than the regional atmosphere. However, most existing climate models produce unrealistic surface temperatures on the Tibetan Plateau (TP) lakes, and few studies have focused on the influence of ice surface albedo on high-altitude lakes. Based on field albedo measurements, moderate resolution imaging spectrometer (MODIS) albedo products and numerical simulation, this study evaluates the ice albedo parameterization schemes in existing lake models and investigates the characteristics of the ice surface albedo in six typical TP lakes, as well as the influence of ice albedo error in the FLake model. Compared with observations, several ice albedo schemes all clearly overestimate the lake ice albedo by 0.26 to 0.66, while the average bias of MODIS albedo products is only 0.07. The MODIS-observed albedo of a snow-covered lake varies with the snow proportion, and the lake surface albedo in a snow-free state is approximately 0.15 during the frozen period. The MODIS-observed ice surface (snow-free) albedos are concentrated within the ranges of 0.14–0.16, 0.08–0.10 and 0.10–0.12 in Aksai Chin Lake, Nam Co Lake and Ngoring Lake, respectively. The simulated lake surface temperature is sensitive to variations in lake ice albedo especially in the spring and winter.

Quantifying recent precipitation change and predicting lake expansion in the Inner Tibetan Plateau

Lake expansion since the middle of the 1990s is one of the most outstanding environmental change events in the Tibetan Plateau (TP). This expansion has mainly occurred in the Inner TP, a vast endorheic basin with an area of about 708,000 km2 and containing about 780 lakes larger than 1 km2. The total lake area of the Inner TP has increased from 24,930 km2 in 1995 to 33,741 km2 in 2015. The variability of the lake area in the coming decades is crucial for infrastructure planning and ecology policy for this remote region. In this study, a lake mass balance model was developed to describe the lake area response to climate change. First, the model was used to inversely estimate the change in precipitation from the change in lake volume. The result shows that precipitation has increased by about 21 ± 7% since the middle of the 1990s, as seen in GPCC global data set. Then, the lake size in the coming two decades was predicted by the model driven with either current climate or a projected future climate, showing the lake area would expand continuously, but at a lower rate than before. Both predictions yield a total lake area of 36150 ± 500 km2 in 2025 and a rise of average lake level by about 6.6 ± 0.3 m from 1995 to 2025. However, the two predictions become disparate in the second decade (2026–2035), as the future climate is more warming and wetting than the current climate. It is noted that the prediction of lake expansion is robust for the entire inner TP lake system but not always applicable to individual subregions or specific lakes due to their spatiotemporal heterogeneity.

Simulations of the Impact of Lakes on Local and Regional Climate Over the Tibetan Plateau

ABSTRACTBecause of the high elevation and complex topography of the Tibetan Plateau (TP), the role of lakes in the climate system over the Tibetan Plateau is not well understood. For this study, we investigated the impact of lake processes on local and regional climate using the Weather Research and Forecasting (WRF) model, which includes a one-dimensional physically based lake model. The first simulation with the WRF model was performed for the TP over the 2000–2010 period, and the second was carried out during the same period but with the lakes filled with nearby land-use types. Results with the lake simulation show that the model captures the spatial and temporal patterns of annual mean precipitation and temperature well over the TP. Through comparison of the two simulations, we found that the TP lakes mainly cool the near-surface air, inducing a decreasing sensible heat flux for the entire year. Meanwhile, stronger evaporation produced by the lakes is found in the fall. During the summer, the cooling effect of the lakes decreases precipitation in the surrounding area and generates anomalous circulation patterns. In conclusion, the TP lakes cool the near-surface atmosphere most of the time, weaken the sensible heat flux, and strengthen the latent heat flux, resulting in changes in mesoscale precipitation and regional-scale circulation.

Spatial structure and β-diversity of phytoplankton in Tibetan Plateau lakes: nestedness or replacement?

Spatial patterns and β-diversity of phytoplankton assemblages depend on the relative importance of species dispersal capacity and species-sorting. Variability in species composition, composed by differences in species richness (nestedness) and/or species replacement, may be caused by niche availability and environmental selection. A field survey was carried out in Tibetan plateau on 38 lakes. Tibetan plateau lakes, located at high elevation, are harsh ecosystems characterized by low temperatures, low available nutrients, high UV amount, and strong salinity gradients. Only well-adapted species can survive in these environments. We therefore hypothesized that environmental filtering was the main factor determining phytoplankton assemblage structure in these lakes, and that species nestedness largely contributed to β-diversity. Spatial analysis showed that both environmental variables and broad-scale spatial variables accounted for the variation of phytoplankton assemblages. In particular, phytoplankton was mainly shaped by spatially structured environmental heterogeneity, which resulted in low α-diversity and high β-diversity. In contrast to our assumption, species turnover contributed to the majority of β-diversity of the studied assemblages. Local environments selected species with the corresponding stress-tolerant traits. Within the narrow range of available niches, environmental filtering of phytoplankton traits induced species’ replacement along environmental gradients.

INFORMATION EXTRACTION AND CHANGE ANALYSIS OF MAJOR LAKES IN TIBETAN PLATEAU BASED ON LANDSAT REMOTE SENSING IMAGES

Abstract. The water resources of Tibetan plateau, particularly the lakes, has been influenced by global climate change and also reacted to global change. It is important to study the lake changes in the Tibetan plateau. This paper aimed to analyse the changes detected from remote sensing images for the typical lakes in Tibetan Plateau, including Qinghai Lake, Nam Co and Selin Co, using different information extraction methods, such as the single-band threshold method, NDWI, MNDWI, MSCT, MLC, SVM and object-oriented classification method. Results show that object-oriented classification method is better at removing ice, snow, cloud and mountain shadow, while the NDWI and MNDWI generated the worst results for lake extraction. After that, the lake's long temporal changes for Qinghai Lake, Nam Co and Selin Co derived from 12 images in different periods have been analysed using GIS technology. Results show that between 1985 and 2015, Qinghai Lake has expanded after a steady expansion; Nam Co was in the state about stable to expansion to stable. The area increased by 70.03 km2, a growth rate of 2.33 km2 a-1 and had a small change in area; Selin Co expanded after a stable state, and the area is always growing, and Selin Co's growth rate is larger than Qinghai Lake and Nam Co at the same time.

Effect of roughness lengths on surface energy and the planetary boundary layer height over high-altitude Ngoring Lake

The special climate environment creates a distinctive air-lake interaction characteristic in the Tibetan Plateau (TP) lakes, where the variations of surface roughness lengths also differ somewhat from those of other regions. However, how different categories of roughness lengths affect the lake surface energy exchange and the planetary boundary layer height (PBLH) remains unclear in the TP lakes. In this study, we used a tuned Weather Research and Forecasting (WRF) model version 3.6.1 to investigate the responses of the freeze-up date, turbulent fluxes, meteorological variables, and PBLH to surface roughness length variations in Ngoring Lake. Of all meteorological variables, the lake surface temperature responded to roughness length variations most sensitively; increasing roughness lengths can put the lake freeze-up date forward. The effect of momentum roughness length on wind speed was significantly affected by the fetch length. The increase in the roughness length for heat can induce the increment of the nightly PBLH in most months, especially for the central lake area in autumn. The primary factors that contribute to sensible heat flux (H) and latent heat flux (LE) were the roughness lengths for heat and momentum during the ice-free period, respectively. Increasing roughness length for heat can increase the nightly PBLH, and decreasing roughness length for moisture can also promote growth of the PBLH, but there was no obvious correlation between the momentum roughness length and the PBLH.

Hydrogeochemistry and δ13CDIC and δ18OH2O composition of three Chinese Tibetan Plateau lakes

ABSTRACTThis study established the hydrochemistry and stable isotope variations in three lakes, namely brackish Zigetang Co, freshwater Cona and Ranwu lakes located in the central and southeastern Tibetan Plateau of China. Vertical profile fluctuations in the water column, such as temperature and dissolved oxygen (DO) concentration, displayed significant differences which were linked to the thermocline. The hydrochemistry of the three lakes showed that as the dominant anion, whereas Na+ is the main cation in Zigetang Co and Cona lake and Ca2+ is the prevailing cation in Ranwu lake. In Zigetang Co, K+ and Na+ concentrations decreased by 42 % from 1999 to 2012, caused by the enlargement of the lake area, documented by field investigations carried out in 1998, 2002, 2006 and 2012. The δ13CDIC and δ18OH2O values analysed from the three lakes varied from −6.0 to 2.0 ‰, and from −14.8 to −6.4 ‰, respectively. The closed Zigetang lake showed higher δ13CDIC and δ18OH2O values compared to those of the rivers, the semi-closed Cona and open Ranwu lakes. The δ13CDIC values of lake water in Zigetang Co were mainly controlled by CO2 exchange between lake water and atmosphere; the δ18OH2O values were dominated by the evaporation/freshwater input ratios.

Investigating changes in lake systems in the south-central Tibetan Plateau with multi-source remote sensing

Lakes in the Tibetan Plateau are considered sensitive responders to global warming. Variations in physical features of lake systems such as surface area and water level are very helpful in understanding regional responses to global warming in recent decades. In this study, multi-source remote sensing data were used to retrieve the surface area and water level time series of five inland lakes in the south-central part of the Tibetan Plateau over the past decades. Changes in water level and surface area of the lakes were investigated. The results showed that the water level of three lakes (Puma Yumco, Taro Co, Zhari Namco) increased, with expanding surface area, while the water levels of the other two lakes (Paiku Co, Mapam Yumco) fell, with shrinking area. The water levels of the lakes experienced remarkable changes in 2000–2012 as compared with 1976–1999. Spatially, lakes located at the southern fringe of the Tibetan Plateau showed consistency in water level changes, which was different from lakes in the central Tibetan Plateau.

Air–lake boundary layer and performance of a simple lake parameterization scheme over the Tibetan highlands

The Tibetan Plateau (TP) is covered by thousands of lakes which affect the regional and global heat and mass budget with important implications for the current and future climate change. However, the heat and mass budget of TP lakes and the performance of contemporary lake models over TP have not been quantified to date. We utilise 3-yr observations from Ngoring Lake, the largest lake in the Yellow River source region of TP, to investigate the typical properties of the lake–air boundary layer and to evaluate the performance of a simplified lake scheme from the Community Land Model version 4.5 (SLCLM) as one of the most popular lake parameterization schemes in atmospheric models. The strong boundary layer instability during the entire open-water period is a distinguishing feature of the air–lake exchange, similar to the situation over tropical and subtropical lakes, while contrasting to the generally stable atmospheric conditions commonly observed over ice-free temperate and boreal lakes from spring to summer. The rather simple algorithm of SLCLM demonstrated good performance in these conditions. A series of sensitivity simulations with SLCLM revealed strong shortwave solar radiation and cold air temperatures because of high altitude as the primary factors causing the boundary layer instability. The outcomes of the study (1) demonstrate the role of TP lakes as accumulators of shortwave solar radiation releasing the heat into the atmosphere during the entire open-water period; (2) justify the use of simple lake models for the Tibetan highlands, while revealing remarkable uncertainties in the estimations of the latent heat flux; (3) qualify the strong cool-skin effect on the lake surface which results from permanent negative air–lake temperature difference, and should be taken into account when interpreting remote sensing data from highland areas.

Historical Records of Mercury Stable Isotopes in Sediments of Tibetan Lakes.

The Tibetan Plateau (TP), known as the “Third Pole”, is a critical zone for atmospheric mercury (Hg) deposition. Increasing anthropogenic activities in the globe leads to environmental changes, which may affect the loading, transport and deposition of Hg in the environment. However, the deposition history and geochemical cycling of Hg in the TP is still uncertain. Our records of Hg and Hg isotopes in sediment profiles of the two largest lakes in the TP, Lake Qinghai and Nam Co, show increased Hg influx since last century, with the maximum Hg influx enrichment ratios of 5.4 and 3.5 in Lake Qinghai and Nam Co, respectively. Shifts in negative δ 202Hg in Lake Qinghai (−4.55 to −3.15‰) and Nam Co (−5.04 to −2.16‰) indicate increased atmospheric Hg deposition through rainfall, vegetation and runoff of soils. Mass independent fractionation of both even-Hg (∆ 200Hg: +0.05 to +0.10‰) and odd-Hg (∆ 199Hg: +0.12 to +0.31‰) isotopes were observed. Positive Δ 200Hg suggest high proportion of precipitation-derived Hg in the TP, whereas the positive Δ 199Hg results from Hg(II) photo-reduction. Both lakes show increasing Δ 199Hg since the 1900 s, and we conclude that with the decrease of ice duration, Hg(II) photo-reduction may have been accelerated in these TP lakes.

Salinity drives archaeal distribution patterns in high altitude lake sediments on the Tibetan Plateau.

Archaeal communities and the factors regulating their diversity in high altitude lakes are poorly understood. Here, we provide the first high-throughput sequencing study of Archaea from Tibetan Plateau lake sediments. We analyzed twenty lake sediments from the world's highest and largest plateau and found diverse archaeal assemblages that clustered into groups dominated by methanogenic Euryarchaeota, Crenarchaeota and Halobacteria/mixed euryarchaeal phylotypes. Statistical analysis inferred that salinity was the major driver of community composition, and that archaeal diversity increased with salinity. Sediments with the highest salinities were mostly dominated by Halobacteria. Crenarchaeota dominated at intermediate salinities, and methanogens were present in all lake sediments, albeit most abundant at low salinities. The distribution patterns of the three functional types of methanogens (hydrogenotrophic, acetotrophic and methylotrophic) were also related to changes in salinity. Our results show that salinity is a key factor controlling archaeal community diversity and composition in lake sediments on a spatial scale that spans nearly 2000 km on the Tibetan Plateau.

Tracing contributions to hydro‐isotopic differences between two adjacent lakes in the southern Tibetan Plateau

AbstractHydrological processes of lakes in the Tibetan Plateau are an important indicator of climate change. Due to the high elevation, inaccessibility and limited availability of historical observations, water budget evaluation of typical lake basins has been inadequate. In this study, stable isotopes are used to trace the multiple water sources contributing to two adjacent lakes on the north slope of the Himalayas, Gongmo‐tso and Drem‐tso. The two lakes have nearly the same elevation, lake area and climatic condition. However, the isotopic composition of the two lakes presents significant differences. Qualitative observations attribute the differences to hydrological discrepancies: Gongmo‐tso is a through‐flow lake, whereas Drem‐tso is a terminal lake. Quantitative analyses, including water and isotope mass balance modelling, clarify the fluxes and isotopic compositions among the various hydrological elements. The isotopic composition of input water, calculated as the summation of rainfall and upstream runoff, is estimated using the local meteoric water line (LMWL) combined with the time series of lake water isotope values. The isotopic composition of evaporation is calculated with a linear resistance model using local meteorological data. The results show that Drem‐tso is a closed lake in a hydrological steady state with relatively more enriched lake water isotope values resulting mainly from evaporation. In contrast, through‐flow accounts for more than 88% of the water input into Gongmo‐tso. The large amount of upstream runoff with lower isotopic composition and enrichment due to evaporation are the major contributions to the observed lake water isotope values. Isotopic modelling of the two neighbouring lakes is effective for isotopic and hydrological research in this region with limited in situ observations. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Dynamic Variation Analysis on Tibetan Lake Wetlands Based on RS

The wetlands of Tibet plateau have typical characteristics of highest elevation, largest area and most concentrated distribution. Tibetan Plateau Lake Group is one of the main five lake groups in China. TM images of 2000 and 2010 are analyzed by ERDAS and ARCGIS. And Landscape Ecology theory is applied to analyze the spatial and dynamic characteristics of Tibetan lake wetlands. The results show: there are 66643 lakes in 2000 and 54655 lakes in 2010, which means there have disappeared 11988 lakes. Xigaze and Lhokas lakes patches presented decrease trend. The lake wetland of Nagri has increased 3.32%.The lake wetland of Lhoka and Xigaze has decreased 7.37% and 3.52%. MPS, MPE, PSCov and PSSD indexes can reflect the lake wetland dynamic changes than other indexes. Tibetan Plateau is very sensitive to global cliamte change, espectially in Nagqu.

Spatial variability of 14C reservoir effects in Tibetan Plateau lakes

Radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect (LRE, also ‘dead carbon’ or ‘old carbon’ effect) especially in dry and cold regions with a sparse plant cover in the catchment. The Tibetan Plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present-day LREs is a crucial prerequisite for the establishment of reliable radiocarbon age–depth relationships for lake sediment cores. This paper examines the spatial variability of LREs within individual lakes, through a discussion of new data for Lake Donggi Cona and a compilation of previously published data for five additional lakes where LRE data are available for different sites. Lake reservoir effects for Lake Donggi Cona on the northeastern Tibetan Plateau were determined for shells of aquatic snails collected alive close to the lake's shore. The largest determined LRE of 20,000 14C years is significantly larger than previously reported LREs from the central part of the lake, and larger than any previously published LRE for the Tibetan Plateau. Relatively low LREs in the central regions of lakes, higher LREs towards the margins, and high LREs in tributaries and spring waters are apparently a common pattern of Tibetan Plateau lakes. The differences in LREs within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water's dissolved inorganic carbon with the atmospheric CO2 in central lake regions on the one hand and the increasing influence of 14C free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins. Generally higher LREs were recorded in the three tectonically active lake regions of the six examined catchments, and it is speculated that rising crustal CO2 further contributes to the LREs in these catchments. In addition to these observations and inferences, elevated 14C levels of the atmosphere as a result of nuclear bomb testing are often ignored if LREs for modern materials are reported by convention relative to the atmospheric 14C activity of the year 1950. LRE data reported in this way represent unrealistic minimum estimates.

Hydrodynamic process of Tibetan Plateau lake revealed by grain size: Case study of Pumayum Co

Under the background of global warming, some lakes on the Tibetan Plateau (TP) are potentially sensitive to temperature change. With a case study of Pumayum Co, where glacier meltwater is important to supply (we call this a glacier-fed lake hereafter), we analyze the sensitivity of lake sediment grain size to temperature change. This is done by resolving the modern hydrodynamic process, coupled with comparison of paleoclimatic proxies. According to the spatial distributions of parameters, percentage of grain size and the grain size frequency distribution curve, hydrodynamic processes are analyzed. Five clastic sedimentation types are thereby discriminated. In the open lake area, suspended load transport is the main transport agent. Grain Size Trend Analysis (GSTA), a sediment dynamics model, reveals a trend toward eastward transport. This indicates that the largest and glacier-fed river, the Jiaqu River, influences the entire lake (not just the subsurface alluvial fan), and that lake sediment grain size may serve as a temperature indicator. Time series comparison between grain size of a short core from the central lake and meteorological data confirms this temperature indication, which in turn shows reliability of the method discriminating the hydrodynamic process. This case study will improve the ability of paleoclimatic reconstruction using lake sediment in glacier-fed lakes on the Tibetan Plateau.

Sedimentary evidence for recent increases in production in Tibetan plateau lakes

The Tibetan Plateau is a vast, elevated plateau in Central Asia with an average elevation of over 4,500 m and contains the world’s third largest store of ice. It occupies a climatic transition zone between the Asian monsoons and westerly airflow. As a result of this location, the region is sensitive to changes in climate on timescales of decades to millennia and longer. Long-term data are needed to evaluate climatic changes and their impact on ecosystems, but in areas as remote as the Tibetan Plateau, long-term instrumental records of environmental change are geographically sparse and monitoring has only been undertaken in recent times. Paleolimnological approach might be then one of the few means by which environmental variability can be ascertained at scales that allow comparison with contemporary monitoring data and future model projections. Therefore, a paleolimnological study was undertaken in eight different lakes sampled along a North–South transect across the Tibetan Plateau analysing geochemistry and algal pigment in order to assess longer term variability in the trophic condition of these systems and their potential to reconstruct changes in relation to recent climate evolution and possible human impacts. Chronologies for the last century were based on radiometric techniques (210Pb, 241Am and 137Cs). Results show that inorganic sediment dominates the composition of the cores used in this study. Organic carbon constitutes less than 5% d.w. in all the lake cores, except for Kemen Co core where concentrations up to 14% d.w., are observed. Corg:N ratios are generally in the order of 5–10, indicating that autochthonous algal production is the principal biological source of organic matter. Pigment preservation is generally good throughout the cores from all lakes as shown by the 430:410 nm ratio that is generally around 1.0 or higher. Six out of eight lakes show an increase in primary production in recent times. High pre-1800 AD pigment concentrations were detected only in Qinghai Lake. Since most of the lakes show a similar behaviour in the most recent section of the core, we interpret this as a response to climate and land-use changes that have increased autochthonous production throughout the Tibetan Plateau.

Holocene land-cover changes on the Tibetan Plateau

Information on changes in land-surface features on the Tibetan Plateau (TP) during the Holocene may help our understanding of the forcing of monsoonal circulation. We analyse vegetation changes during the last 9000 years from pollen records of four lakes (Hurleg Lake in the Qaidam Basin; Qinghai Lake on the northeastern TP; Zigetang Lake on the central TP and Koucha Lake on the eastern TP) which represent different regions and vegetation types on the Plateau today. A set of modern pollen assemblages from 111 lake sediment samples originating from different vegetation types is used for the interpretation of the Holocene pollen records. Four types of numerical analyses are used to infer different aspects of pollen-stratigraphical and inferred vegetation changes: biome reconstruction; ordination (to infer palynological compositional turnover); rate-of-assemblage-change analysis, and a pollen-LAI (leaf area index) transfer function. Our results show strong regional differences in inferred vegetation change in terms of timing, strength and the nature of change. The greatest changes in compositional turnover and LAI are found in the Qinghai Lake record, indicating that forests were replaced by steppe vegetation in a step-wise fashion since the mid Holocene. Alpine steppe vegetation on the central and eastern TP was relatively stable throughout the Holocene, only showing a gradual replacement of temperate steppe by high-alpine meadows. The Qaidam Basin was dominated by temperate desert throughout the Holocene. The documented palynological and inferred vegetation changes can be most parsimoniously explained by climate change, although human impacts cannot be excluded as a contributing factor. Comparison of our findings with sensitivity analyses of numerical climate simulations suggests that long-term vegetation change may have had only a weak influence on regional climate change on the Tibetan Plateau.