Chlorophyll-a concentrations in 82 large alpine lakes on the Tibetan Plateau during 2003–2017: temporal–spatial variations and influencing factors

As essential parts of the unique ecosystem of Tibetan Plateau(TP),the sizes and associated physical properties of alpine lakes have long been investigated.However,little is known about one of the most critical biogeochemical properties,i.e.the Chlorophyll-a(Chl-a)concentrations.Here,for the first time,we presented a comprehensive investigation of the temporal–spatial variations in Chl-a in 82 lakes(>50 km^(2))across the entire TP region,based on MODIS observations in the period of 2003–2017.The results showed that the 82 lakes exhibited an average longterm mean Chl-a of 3.3±4.3 mg m^(−3),with high Chl-a lakes concentrated in the eastern and southern inner TP basin and northeastern parts of the TP.An interannual trend analysis revealed that lakes exhibiting(significantly)decreasing Chl-a trends and(significantly)increasing Chl-a trends were comparable in numbers but differed in distribution patterns.A correlation analysis indicated that at least 70%of the interannual variability in Chl-a values of lakes was significantly correlated with one of the four environmental factors(wind speed,ice cover duration,lake water surface temperature and surface runoff)and lake size.In addition,glacier meltwater tended to reduce lake Chl-a while salinity levels showed minor influences.

Climate effects on an inland alpine lake in Xinjiang, China over the past 40 years

Inland lakes are important water resources in arid and semiarid regions. Understanding climate effects on these lakes is critical to accurately evaluate the dynamic changes of water resources. This study focused on the changes in Sayram Lake of Xinjiang, China, and addressed the effects of climate fluctuations on the inland lake based on long-term sequenced remote sensing images and meteorological data from the past 40 years. A geo- graphic information system （GIS） method was used to obtain the hypsometry of the basin area of Sayram Lake, and estimation methods for evaporation from rising temperature and water levels from increasing precipitation were proposed. Results showed that： （1）Areal values of Sayram Lake have increased over the past 40 years. （2） Both temperature and precipitation have increased with average increases of more than 1.8~C and 82 mm, respectively. Variation of the water levels in the lake was consistent with local climate changes, and the areal values show linear relationships with local temperature and precipitation data. （3） According to the hypsometry data of the basin area, the estimated lake water levels increased by 2.8 m, and the water volume increased by 12.9×108 m3 over the past 40 years. The increasing area of Sayram Lake correlated with local and regional climatic changes because it is hardly affected by human activities.

Can a dam type of an alpine lake be derived from lake geometry?A negative result

Glacial lake outburst floods(GLOFs)represent one of the most serious hazard and risk in deglaciating high mountain regions worldwide and the need for GLOF hazard and risk assessment is apparent.As a consequence,numerous region-and nation-wide GLOF assessment studies have been published recently.These studies cover large areas and consider hundreds to thousands of lakes,prioritizing the hazard posed by them.Clearly,certain simplification is required for executing such studies,often resulting in neglecting qualitative characteristics which would need manual assignment.Different lake dam types(e.g.,bedrock-dammed,moraine-dammed)are often not distinguished,despite they control GLOF mechanism(dam overtopping/dam breach)and thus GLOF magnitude.In this study,we explore the potential of easily measurable quantitative characteristics and four ratios to approximate the lake dam type.Our dataset of 851 lakes of the Cordillera Blanca suggests that while variances and means of these characteristics of individual lake types differ significantly(F-test,t-test),value distribution of different geometrical properties can’t be used for the originally proposed purpose along the spectra.The only promising results are obtained for extreme values(selected bins)of the ratios.For instance,the low width to length ratio indicates likely morainedammed lake while the high value of ratio indicating round-shape of the lake indicates increased likelihood of bedrock-dammed lake.Overall,we report a negative result of our experiment since there are negligible differences of relative frequencies in most of the bins along the spectra.

Monitoring open water area changes in a small tarn using historical orthophotomaps and a historical bathymetric map:a case study of the Litworowy Staw lake,the High Tatras

According to scientific evidence in general, the disappearance of the Tatras tarns as a result of external geomorphological processes has been accepted since the long term analyses of lake sediments and peat bogs, geomorphological mapping, geophysical measurements(– all indirect methods). It is paradoxical that the direct cartographic evidence of such changes has not existed until now. In this paper, we evaluate shore line changes of the small mountain glacial lake in the High Tatras-Litworowy Staw lake using a multitemporal analysis of a series of historical orthophotomaps and a historical map. Over the last 100 years, the tarn has lost 46.4% of open water level area. Significant visual changes were caused by vegetation growth of Carex species(sedges) on the water's surface. The accumulation of fine sediments and dead plant residues in the tarn basin create suitable conditions for this process, all together causing the tarn to become more shallow.

西藏羊卓雍错湖面水热通量变化及其与气象因子的关联

Lake surface water-heat exchange and its climatic attribution critically influence alpine lakes’evaporation mechanism and water storage balance with climate change.Here,this paper first explored the hourly,daily,and monthly water-heat flux variations of the lake surface and their correlations with meteorological factors based on the eddy covariance turbulent flux observation over the Yamzhog Yumco,an alpine lake in south Tibet in the non-freezing period(April-December)in 2016 and 2017.We found that the average latent heat flux was much higher than the sensible heat flux on the lake surface from April to December.Meanwhile,the water-heat flux exhibited remarkable seasonal variation,with a prominent role of higher air temperature and humidity in summer jointly controlling the lake-air energy exchange.Moreover,the main controlling meteorological factors for the water-heat flux variation of the lake surface differed with diversified timescales.First,the lake-air temperature difference was the most significant meteorological factor related to sensible heat flux on the half-hourly,daily,and monthly timescales.Second,the latent heat flux was strongly positively correlated with wind speed and the synergies of wind speed and water vapor pressure deficit on the daily and half-hourly timescales.Third,the lake surface heat flux was significantly negatively correlated with net radiation flux on the daily and monthly scales.The negative correlation can be attributed to the seasonal variation of the water surface net radiation,and the phase difference in heat flux intensity caused by the lake-air temperature difference and heat capacity contrast.Our findings will hopefully improve the understanding of energy exchange and evaporation mechanisms for alpine lakes in a warming climate.

The aliphatic hydrocarbon distributions of terrestrial plants around an alpine lake： a pilot study from Lake Ximencuo, Eastern Qinghai-Tibet Plateau

As part of an investigation of the sources of aliphatic hydrocarbons to the sediments of alpine Lake Ximencuo, leaves of the eight dominant vascular plants were collected and their hydrocarbon contents were analyzed. A series of unsaturated aliphatic hydrocarbons were identified in the plant leaves; in particular, Festuca sp. contain a series of n-alkadienes that have rarely been reported in previous studies. The comparison of n-alkane proxies （ACL27 33, ACLT, Paq, and CPI） and δ13Corg among plant leaves, surface soils, and lake sediments suggests that organic proxies have been altered to varying degrees during the transport and burial process of organic materials. It is believed that microbial reworking and source changes have great impacts on organic proxies in the alpine lake system. In addition, the cluster analysis for plant leaves depending on n-alkane compositions and the ACLT proxy generates similar results. Accordingly, we postulate that the average chain length of plant waxes might be a potential indicator of plant classification in regions such as the Qinghai-Tibet Plateau.

Indication of paleoecological evidence on the evolution of alpine vegetation productivity and soil erosion in central China since the mid-Holocene

Although alpine ecosystems have been commonly recognized as sensitive to recent climate change,few studies have examined its impact on the long-term productivity of vegetation and soil erosion.Using paleoecological records,these two aspects were examined in the alpine zone of the Taibai Mountains(elevation,3767 m)in monsoon-dominated East Asia since the middle Holocene.Proxies for the productivity of vegetation and severity of soil erosion from high-resolution alpine lacustrine records show that the productivity and soil erosion were closely related to mean annual temperature and summer precipitation from the East Asian Summer Monsoon(EASM),respectively.Specifically,when the mean annual temperature was low and precipitation was abundant,during 5800–4000 calendar years before the present(cal.yr BP),the alpine ecosystem was characterized by low vegetation productivity and severe soil erosion.However,the productivity increased and soil erosion decreased from 4000 cal.yr BP onwards.These results highlight the role of paleoecological evidence in studying ecosystem services on longer time scales,which is significant in making policies for sustainable development under climate change in regions for which such long-term monitoring data are not available.

Hydrochemical regime and its mechanism in Yamzhog Yumco Basin, South Tibet

The hydrochemistry of alpine lakes reflects water characteristic and its response to climatic change. Over 300 water samples had been collected from 52 sites of 5 lakes and 7 inflowing rivers in the Yamzhog Yumco Basin, South Tibet, during 2009-2014, basing which the hydrochemical regime and its mechanism were analyzed along with the adoption of hydrological investigations in 1979 and 1984 as well. Results revealed that the waters were hard with weak alkalinity for the Yamzhog Yumco Basin. Most of them were fresh, and the rest were slightly saline. The hydrochemical types of 5 lakes （i.e., Lake Yamzhog Yum Co, Puma Yum Co, Bajiu Co, Kongmu Co, and Chert Co） were SO4^2--HCO3^--Mg^2＋-Na^＋, HCO3^-- SO4^2--Mg^2＋-Ca^2＋, SO4^2--Mg^2＋-Na^＋, SO4^2--HCO3^--Ca^2＋, and SO4^2--Na^＋-Mg^2＋-Ca^2＋, respectively. As for rivers, HCO3^- and SO4^2- were the major anions, and Ca2. was the dominant cation. Lake Yamzhog Yum Co, the largest lake in the basin, exhibited remarkable spatial variations in hydrochemistry at its surface but irregular changes with depth. The weathering of evaporates and carbonates, together with evaporation and crystallization, were the major mechanisms controlling the hydrochemistry of waters in the Yamzhog Yumco Basin. Global warming also had significant impacts on hydrochemical variations.