Jia Peng

Atmospheric deposition and climate change are altering alpine lake ecosystems at an accelerating rate. Biotic responses to the interactions of multiple stressors are underexplored in alpine lakes. This study explored diatom responses to climate change and nutrient supply based on multi-proxy sedimentary records in Erye and Sanye Lakes of the Taibai Mountain at the boundary between the subtropical and temperate climate zones of eastern China. Diatom communities of the two lakes shifted from large-sized benthic taxa to small fragilarioid species at the beginning of the 21st century, synchronous with an increase in diatom accumulation rate. Redundancy analyses showed that diatom communities were significantly correlated with catchment processes and nutrient inputs in both lakes, as well as climate warming in the upstream lake. Partial least squares path modelling revealed that catchment processes were more important than direct effects of climate warming and nutrient supply for explaining the observed shifts in diatom flora in the two alpine lakes. The comparison with diatom records in similar alpine lakes in the East Asian monsoon region reveals that responses of alpine lake ecosystems to warming and atmospheric deposition vary with different catchment settings and lake morphometry. Given that these alpine lakes serve as important water sources, recent biotic reorganization suggests that the protection of headwater systems should be given high priority, especially in rapidly developing and densely-populated regions.

Over the last century, climate warming has increased vegetation coverage in alpine regions, having major implications for associated terrestrial and aquatic ecosystems. Due to the remoteness of mountainous regions, there is a paucity of monitoring data regarding the response of catchment processes to climate change. Particle size distribution, magnetic properties, and elemental contents were investigated in 210Pb-dated sediment cores (34 cm in length) collected from two hydrologically-connected cirque lakes in the Taibai Mountain (the highest mountain in east mainland China). Both lakes have experienced directional environment changes since the 1980s, suggested by concurrent increases in magnetic susceptibility and organic matter. Prolonged growth season under a scenario of warmer climate has promoted vegetation development and soil conservation, subsequently increasing the contribution of terrestrial detritus derived from top soils that are rich in magnetic minerals and organic matter. Contents of clay and silt remained high throughout the two sediment cores, suggesting that depositional environment was relatively stable in both lakes. However, few peaks in coarse particles probably resulted from the cold and wet climate between the 1940s and the 1970s. During the relatively cold and wet episodes, prolonged snow-packs and enhanced melt-water discharge would strengthen bedrock weathering and erosion intensity, subsequently increasing the influxes of coarse particles. This is the first study to reveal decadal-scale changes in catchment processes of the Taibai Mountain, and the results highlight positive effects of climate warming on environmental protection of our study site and other similar alpine mountains through promoting water and soil conservation.

Subalpine lakes are important reserves of biodiversity and freshwater resource, and their unique ecosystems are responsive to environmental changes. Yunzhong Lake, a subalpine reservoir located in the Jiugong Mountains（southeast Hubei province）, provides important ecosystem services of tourism, water supply, irrigation and electricity generation. However, the lake has suffered environmental degradation（e.g., algal bloom） since 2000, resulting from wastewater input from local tourist hotels. In order to reveal the response of aquatic biota to anthropogenic and climatic disturbances, multi-proxy analysis（element content, particle size and diatom） and210 Pb dating were applied on a 27-cm length core collected from Yunzhong Lake. The results revealed that sedimentary diatom assemblages were co-dominated by planktonic forms（Aulacoseira and Cyclotella） and benthic forms（Achnanthes and Fragilaria）.The results of redundancy analysis（RDA） revealed that sedimentary diatom assemblages of Yunzhong Lake were significantly related to sedimentary elements（i.e., Al, Fe and Pb） and temperature. The cluster analysis of diatom assemblages showed that environmental changes in Yunzhong Lake had experienced two major phases. During the early period after the damming between 1958 and 1993, high abundances of the species preferring alkaline water（i.e.,Achnanthes minutissima and Fragilaria nanana） and relatively high values in chemical index of alteration（CIA） mainly responded to strong weathering in the catchment. After 1993,climate warming and nutrient enrichment induced by the development of tourism were two main driving forces of the proliferation of mesotrophic and eutrophic species, such as Cyclotella pseudostelligera and Aulacoseira alpigena. This study reconstructed the evolution of the subalpine lake ecosystem under the combined effects of climate change and human activities. The results can provide baseline information for eco-environmental protection of subalpine lakes and the restoration of water quality in the watershed.