No abstract

In order to assess how best to manage impacted lake systems, one needs to understand the trophic functioning of the lake system and the recent states through which the lake may have transitioned. Lakes in the middle and lower reaches of the Yangtze have been heavily impacted over recent decades. In order to understand recent changes in functional status, we examined sediment cores covering the last 120 years from two lakes in the same catchment with differing status: one algal-dominated (Taibai Lake) and the other macrophyte-dominated (Longgan Lake). Chironomid head capsules were identified from both sites and an expanded chironomid-total phosphorus (TP) transfer function (21 sites were added to the 30-lake model previously developed by Zhang et al. 2006) was used to assess the lakes' response to recent anthropogenic change. Quantitative chironomidinferred TP (CI-TP) reconstructions showed that Taibai Lake experienced clear changes in trophic status since the 1860s. Before the 1950s, the CI-TP concentration was relatively stable around 50-80 lg L -1 , while it reached to 80-130 lg L -1 in the latter period. CI-TP for Longgan Lake, however, showed a relative decline from the range of 50-75 lg L -1 since the 1880s to 30-40 lg L -1 in recent years, accompanied by strong evidence from the chironomids for increased macrophyte biomass as TP levels declined. Both reconstructions agreed with diatom inferences of TP from the same lakes. The stark difference between these two sites is thought to reflect a function of macrophyte development, with Taibai Lake losing its plants through increased nutrient levels and internal recycling, whereas Longgan Lake, which is much bigger in area and hence potentially more resilient to change, was able to develop macrophyte communities over the same time period. The positive feedbacks associated with abundant macrophytes retained the clear water state of Longgan Lake, but a further increase in nutrients might lead to decrease in resilience of the relatively stable macrophyte state and loss of benthic pathways of primary production, which would push the lake towards eutrophication. Unless nutrient inputs to Longgan are controlled, Longgan Lake might lose macrophyte communities and follow a developmental pathway similar to that observed in Taibai Lake.

No abstract

We present a sub-centennial resolution summer temperature record from the SE Tibetan Plateau (Tiancai Lake, SW China, 3900 m a.s.l.) derived from a chironomid stratigraphy covering the last c.19.5 ka. The record highlights the interaction between tropical and highlatitude climate forcing through the changes in atmospheric circulation during the last deglaciation. The scale of the last glacial maximum (LGM) cooling is consistent with other tropical mountain regions at c.5 °C and a rapid recovery of temperatures at 19.0 ka is related to changes in adiabatic lapse rates at the end of the glaciation. The overall pattern of change shows that North Atlantic deglaciation climate events (the Heinrich 1 and Younger Dryas cooling events and the Bølling-Allerød warm period) are all recorded, but the influence of the events decline as the deglaciation progresses. We relate these patterns to North Atlantic Deep water ventilating the Southern Ocean and to the consequent movements of the Intertropical Convergence Zone (ITCZ) in the Indian Ocean Basin as transmitted through changes in the southern mid-latitude circulation. 2004). This reduction in AMOC would have caused a southward shift in the Intertropical Convergence Zone (ITCZ), which in turn changed latitudinal precipitation and wind patterns, as well as upwelling regimes (Lea et al. 2003; Yancheva et al. 2007; Zhang and Delworth 2005). δ 18 O records of Chinese cave speleothems show a weakened monsoon during this period (e.g. Wang et al. 2001; Dykoski et al. 2015). These changes, during a weakened AMOC, would result in reduced northward heat transport and it has been suggested that heat may be preferentially stored in the subtropics and tropics (Carlson et al. 2008; Rühlemann et al. 1999), causing at least regional warming in the tropics during North Atlantic cooling events. These ideas are little tested in terrestrial areas of the tropics, and subtropics and terrestrial temperature reconstructions in these areas rarely extend back to the LGM. Here we develop the first terrestrial summer temperature record from lake sediments in the Yunnan region of the Qinghai-Tibetan Plateau (QTP) that extends back to the end of the last glacial maximum (LGM). in order to examine climate responses in this region during the deglaciation, while the Asian summer monsoon was inferred to be weakened (Shakun et al. 2007; Wang et al. 2001). In addition, we compare our summer temperature reconstruction with a model simulation of regional mean annual temperatures based on the output data from the TraCE-21000 model experiment which is forced by meltwater flux, orbital changes, CO 2 , and ice sheet volume (Liu et al. 2009). We aim to understand the rate, magnitude and timing of changes in terrestrial summer temperatures to better understand the long-term nexus of the regional summer temperature and the Asian summer monsoons. The Yunnan region of the QTP forms an important eastern extension of the Tibetan high country (Fig. 1). It differs from the main QTP in that it is primarily influenced by warm ...

Large river-floodplain systems which provide a variety of societal, economic and biological benefits are undergoing extensive and intensive human disturbance. However, floodplain lakes responses to multiple stressors are poorly understood. The Yangtze River and its floodplain which provide water and food resources for more than 300 million people are an important region in China. Hydrological regulation as well as socio-economic development have brought profound negative influence on this ecologically important area. To improve understanding of decadal-scale responses of floodplain lakes to multiple stressors, lake sediment proxies including particle size, geochemical elements, diatoms and chironomids were analysed in a lead-210 dated core from Futou Lake. The analyses show that dams constructed in 1935 and the early 1970s stabilized hydrological conditions in Futou Lake and impeded the interaction with the Yangtze River, resulting in a decrease in major elements (e.g., Mg, Al, Fe) transported into the lake and an increase of macrophyte-related chironomids (C. sylvestris-type, P. penicillatus-type and Paratanytarsus sp.). After the late 1990s, further decreases in major elements and increases in median grain size are attributed to the erosion of the Yangtze riverbed and declining supply of major elements-enriched sediments from the upper Yangtze caused by the impoundment of the Three Gorges Dam. Chironomid and diatom assemblages indicate that hydrological stabilization caused by dam constructions stimulated the growth of macrophytes, which may be important in buffering against an ecosystem state change towards a phytoplankton-dominated and turbid state with ongoing eutrophication. However, a recent increase in Zn, TP and the emergence of eutrophic diatom and chironomid species indicate initial signs of water quality deterioration which may be related to the combined effects of hydrological stabilization and aquaculture. Over all, the sediment record from Futou Lake emphasizes the importance of interactions between hydrological change and pollutant loads in determining floodplain lake ecosystem state.

Peatlands are important ecosystems for biodiversity conservation, global carbon cycling and water storage. Hydrological changes due to climate variability have accelerated the degradation of global and regional ecosystem services of peatlands.Diatoms are important producers and bioindicators in wetlands, but comprehensive diatom-based inference models for palaeoenvironmental reconstruction in peatlands are scarce. To explore the use of diatoms for investigating peatland hydrological change, this study established a training set consisting of diatom composition and twelve environmental factors from 105 surface samples collected from five Sphagnum peatlands in northeastern China. Diatom communities were dominated by Eunotia species. Ordination analyses showed that depth to the water table (DWT) was the most important factor influencing diatom distribution, independently accounting for 4.99% of total variance in diatom data. Accordingly, a diatom-based DWT transfer function was developed and thoroughly tested. The results revealed that the best-performing model was based on weighted averaging with inverse deshrinking (R 2 = 0.66, RMSEP = 8.8 cm with leave-one-out cross validation). Quantitative reconstruction of DWT on a short peat core collected from the Aershan Peatland (Inner Mongolia) recorded climate-mediated hydrological changes over the last two centuries. This study presents the first diatom-water table transfer function in Sphagnum peatlands, and highlights the potential of diatoms as a powerful tool to assess the magnitude of past hydrological changes in peatlands of northeastern China, as well as similar peat environments worldwide.

No abstract

Lakes in Yunnan Province, SW China, have been subjected to increased environmental stress over the last century. In order to assess the impacts of these stresses on the biota, a sediment core was collected from Lugu Lake covering the past 120 years, and detailed chemical, biological (subfossil chironomids), and physical analyses of the lake sediments were undertaken. The analyses indicated consistent trends of increased soil erosion since the early 1970s associated with significant changes in chironomid assemblages, which occurred simultaneously with the sedimentological proxies. A redundancy analysis (RDA) using a range of sedimentary proxies indicated that the shifts in the chironomid communities were mainly attributable to soil erosion. Constant soil erosion caused dramatic reductions in the available organic materials and large increases in fine sediments, leading to changes in the chironomid fauna and reduced chironomid abundance. The chironomid succession revealed that Procladius, the likely top predator in the chironomid community food chain, decreased in abundance under the impact of soil erosion, whereas the proportion of small forms of filter feeders, represented by Tanytarsus mendax‐type, increased rapidly. Due to the loss of suitable habitats, the population of some bottom collector‐gatherers (e.g., Polypedilum nubeculosum‐type) decreased sharply. The results of this study suggest that increased catchment soil erosion may seriously impact benthic communities and potentially alter ecosystem functioning.