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.

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Accurate chronologies for recent sediments of shallow lakes in the Yangtze floodplain are critical to calibrate proxy records for reconstructing environmental changes during the past century. This study presents the results of detailed 210 Pb analysis from eight lake sediment cores collected from the middle Yangtze reaches, southeast China. Unsupported 210 Pb activities generally declined exponentially with mass depth in the eight cores. The chronologies and sedimentation rates for the sediment cores were calculated using different 210 Pb-based mathematical models. The 137 Cs chronomarker (i.e. the 1963 fallout peak) and the spheroidal carbonaceous particle (SCP) chronomarker (i.e. the start of the rapid increase in 1970 AD) were selected to validate the 210 Pb dating. Sedimentation rates derived from different models were validated using historical data including lake area, arable land area, sediment discharge and reservoir volume in the Yangtze basin. The SCP-corrected CRS (constant rate of supply) model performs better than other models, based on validation using historical documents in the Yangtze basin. The 137 Cs chronomarker might be erroneous due to catchment-driven 137 Cs inputs from soil erosion and post-depositional diffusion. Both SCPs and 137 Cs are susceptible to inputs from catchment soil erosion, but SCPs showno apparent degradation and post-depositional changes in lake sediments. The SCP profile provides a relatively reliable chronomarker, which can be used for validating 210 Pb chronologies in these floodplain lakes. Generally, sedimentation rates in the eight lakes were less than 0.2 g cm -2 yr -1 before the 1930s, and then increased to a peak in the 1960s. Afterwards, sedimentation rates decreased and remained low after the 1980s.

Dongting Lake, a wetland of international importance under the Ramsar Convention, is the hydrological lynchpin of the Yangtze floodplain. It is subjected to eutrophication and heavy metal pollution, but little is known of recent rapid degradation of its aquatic environment and potential causes. Here, we use sedimentary records of particle size, elements and pigments, combined with historical hydrological and limnological data, to assess aquatic environmental changes since ca 1960. Coarse particles in the sediments increased after 2003, while concentrations of K, Li, Al and Mg declined. These changes are probably linked to strong erosion of the downstream riverbed and declining supply of ion-rich material from the upper Yangtze reaches after the Three Gorges Dam (TGD) began to operate. Furthermore, enrichment of sedimentary heavy metals and total phosphorus since the 1990s can be attributed to the influx of anthropogenic pollutants. Biotic responses are manifested in higher concentrations of fossil algal pigments, especially those from chlorophytes and siliceous algae after the TGD impoundment. Multivariate analysis revealed that changes in fossil pigments were significantly correlated with total phosphorus and Pb and three indicators for hydrological conditions (i.e. coarse particles, water exchange ratio and sediment discharge), indicating that influx of anthropogenic pollutants and hydrological regulation by the TGD are main drivers of algal community change. Our results provide reference conditions for restoration of Dongting Lake and highlight the importance of hydrological connection with the mainstream and pollution control for continuing efforts to restore Dongting Lake, as well as other similar floodplain lakes worldwide.

This study aimed to understand changes in the biogeochemical processing of organic matter (OM) in response to multiple stressors (e.g., littoral area expansion, wastewater input, and hydrological regulation) in East Dongting Lake (Central China) over the past 60 years, using analyses of total organic carbon (TOC), total nitrogen (TN), C/N ratios, δ 13 C, δ 15 N, and diatoms from 2 sediment cores collected from the littoral and central parts of the lake. OM mainly originated from phytoplankton and C 3 plant-derived soil OM based on the ranges of C/N ratios (from 7 to 11) and δ 13 C (between −27‰ and −23‰). Littoral area expansion due to siltation caused an increasing influx of terrestrial soil OM in the 1980s and the 1990s, subsequently lowering δ 13 C values and rising C/N ratios in both sediment cores. Meanwhile, higher δ 15 N was linked to a high influx of isotopically heavy nitrate from urban and agricultural wastewaters. After 2000, slight decreases in TOC and TN in the littoral area were attributable to reducing inputs of external OM, likely linked to declining sediment influx from the upper reaches resulting from the Three Gorges Dam impoundment. Contrasting increases in TOC, TN, and C/N ratios in the central part indicated a high influx of terrestrial soil OM due to the declining distance from the shoreline with littoral area expansion. Declining δ 15 N values after 2000 indicated an increase in N 2 -fixing cyanobacteria with eutrophication. Changes in diatom assemblages in both the littoral and central zones reflected nutrient enrichment and hydrological alterations. These results indicate that littoral expansion, declining riverine influx, and anthropogenic nutrient inputs are potential driving forces for the biogeochemical processing of OM in floodplain lakes. This study provides sedimentary biogeochemical clues for tracking past limnological conditions of floodplain lakes that are subjected to increasing disturbances from hydrological regulation and eutrophication. KEYWORDS East Dongting Lake, eutrophication, floodplain landscape, hydrological changes, lake sediment, stable carbon and nitrogen isotope

To reconstruct the history of environmental degradation in Sanliqi Lake (Daye City, central China), multiple proxies were analyzed in a sedimentary core which was dated using (137)Cs and spheroidal carbonaceous particles (SCPs). The results show that Sanliqi Lake has experienced serious degradation during the past 60 years, resulting from a large influx of metals and nutrients. Expansion of agricultural and industrial activities between 1945 and 1993 enhanced nutrient and metal enrichment, indicated by increases in metals, SCPs, magnetic susceptibility, total phosphorus, total nitrogen and total organic carbon. Further enrichment of Zn, Cd, Ni and Cr after 1993 was linked to a recent intensification of mining activities. Decreases in Cu and Pb after 2006 probably resulted from recent environmental remediation. This study verified the coupling between lake sediment pollution and human activities in Daye City during the past 60 years. The reconstructed history of lake pollution can provide reference information for continued restoration of Sanliqi Lake and other similar heavily polluted lakes in the developing regions.

Abstract:Compared with the extensively surveyed temperate and boreal peatlands, subtropical peatlands have received rather little scientific attention. In this study, spatial variations in water chemistry were investigated in 57 samples collected from three subtropical peatlands (central China). Generally, water pH and major cation concentrations of these samples approached the characteristics of poor fens along the bog-rich fen gradient. Four distinct groups were identified by cluster analysis and principal component analysis. High concentrations of trace elements in Group I were probably linked to airborne dust pollutants from a nearby road under construction. Low pH values and low major cation concentrations in Group II may be attributed to the great cation exchange capacity of Sphagnum. In contrast, samples in Group III, mainly collected from sites suffering from Sphagnum harvest, showed high pH levels and major cation concentrations. Relatively low concentrations of trace elements (except boron) in Group IV can be regarded as reference conditions for restoration efforts in these peatlands. Taken together, spatial variations in the water chemistry of the three peatlands were closely linked to natural processes (e.g., cation exchange of Sphagnum) and anthropogenic pressures (e.g., Sphagnum harvest). Overall, the mean health risk values of the six trace elements were low, indicating good water quality in these peatlands. This study provided baseline water chemistry information for the sustainable management of these subtropical peatlands, as well as other similar ecosystems.