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The south-east margin of Tibet is highly sensitive to global environmental change pressures, in particular, high contemporary reactive nitrogen (Nr) deposition rates (ca. 40 kg ha−1 yr−1), but the extent and timescale of recent ecological change is not well prescribed. Multiproxy analyses (diatoms, pigments and geochemistry) of 210Pb-dated sediment cores from two alpine lakes in Sichuan were used to assess whether they have undergone ecological change comparable to those in Europe and North America over the last two centuries. The study lakes have contrasting catchment-to-lake ratios and vegetation cover: Shade Co has a relatively larger catchment and denser alpine shrub than Moon Lake. Both lakes exhibited unambiguous increasing production since the late 19th to early 20th. Principle component analysis was used to summarize the trends of diatom and pigment data after the little ice age ( LIA). There was strong linear change in biological proxies at both lakes, which were not consistent with regional temperature, suggesting that climate is not the primary driver of ecological change. The multiproxy analysis indicated an indirect ecological response to Nr deposition at Shade Co mediated through catchment processes since ca. 1930, while ecological change at Moon Lake started earlier (ca. 1880) and was more directly related to Nr deposition (depleted δ15N). The only pronounced climate effect was evidenced by changes during the LIA when photoautotrophic groups shifted dramatically at Shade Co (a 4-fold increase in lutein concentration) and planktonic diatom abundance declined at both sites because of longer ice cover. The substantial increases in aquatic production over the last ca. 100 years required a substantial nutrient subsidy and the geochemical data point to a major role for Nr deposition although dust cannot be excluded. The study also highlights the importance of lake and catchment morphology for determining the response of alpine lakes to recent global environmental forcing. [ABSTRACT FROM AUTHOR]

Understanding the strength and extent of the Asian summer monsoon (including the East Asian summer monsoon and the Indian summer monsoon) in the Tibetan Plateau (TP) region is crucial for predicting possible changes in the regional eco-environment and water resources under global warming. Due to the lack of well-dated and high-resolution paleoclimate records, long-term monsoon dynamics are still not well understood in the western TP, which is currently influenced by both the Indian summer monsoon (ISM) and the westerlies. Here we present a multi-proxy lacustrine record covering the past 10,500 years from Aweng Co, an alpine lake at the northern limit of the modern ASM in western Tibet. Our results show that the western TP was mainly controlled by the ISM during the Holocene and the regional ecosystem/environment was sensitive to climate change. The climate was the wettest between 10.5 and 7.3 cal kyr BP, when terrestrial plants in the catchment were productive and the biomass of benthic algae was low possibly due to limited sunlight at the lake bottom due to high lake level. From 7.3 to 5.0 cal kyr BP the climate shifted towards drier conditions, resulting in a decline in terrestrial plant cover. Between 5.0 and 3.1 cal kyr BP, the climate became even drier, resulting in a further decline in vegetation cover in the catchment. Between 4.6 and 3.1 cal kyr BP, 100% endogenic dolomite precipitated from the lake water, possibly induced by high Mg/Ca ratios. After 3.1 cal kyr BP, the climate was the driest and frequent centennial-scale droughts occurred. The lake level was low and would have resulted in more light reaching the lake bottom, favoring the growth of benthic algae. The reconstructed lake level change of Aweng Co agrees well with the paleo-shoreline records in the southern TP, demonstrating that the ISM evolution played a key role in lake hydrological processes in this region. A comparison of paleoclimate records shows the ISM reached 34.5° N in the western TP during the Holocene. • A high-resolution hydroclimate record from Aweng Co in the western TP since 10.5 cal ka BP. • δ18O carb -inferred lake level was high during 10.5–7.3 cal ka BP and declined afterwards. • Carbonate mineralogy indicates an extremely dry period in late-Holocene (4.6–3.1 cal ka BP). • Terrestrial-aquatic ecosystems in the western TP are sensitive to the climate change. [ABSTRACT FROM AUTHOR]

Modelling, monitoring, and experimental data have shown that global climate change can impact aquatic phytoplankton communities directly, through the effects of warming on primary producers, as well as indirectly through cascading effects from higher trophic levels. Although both concepts are common in modern limnological studies, it remains unclear whether the 'top-down' effects from higher trophic levels on phytoplankton exert strong effects in natural systems over long (centennial) timescales. Here, we use multiproxy data including pigments, zooplankton remains, nutrient concentrations, and paleoclimate indicators from a sediment core in Dagze Co, Central Tibet (a two-trophic level lake) to reconstruct algal production, zooplankton community, nutrient and salinity changes. Our results show that top-down effects of higher trophic levels offset effects from warming and nutrient addition on algal growth. Warming enhanced glacial meltwater inflow to the lake, and intensive human activities increased nutrient inputs. Changes in lake salinity and N:P ratios coincided with zooplankton community shifts during the past 600 years, and Daphnia tibetana replaced the brine shrimp, Artemia tibetiana , after the relocation of a town to upstream of the lake in the 1980s led to overharvesting of the brine shrimp. These shifts contributed strongly to changes in algal communities, with changes in zooplankton leading to strong top-down effects that decreased algal production through increasing grazing pressure despite increasing nutrient concentrations. Our results suggest that the typical external drivers (climate and nutrients) of lake ecosystems may be suppressed by internal shifts in plankton communities in lakes. • Salinity and N:P ratio dominated zooplankton community in past 600 years. • Warming enhanced grazing decreased algal production despite increasing nutrient. • External drivers may be suppressed by internal shifts in plankton communities. [ABSTRACT FROM AUTHOR]