Wetlands in the Prairie Pothole Region of the North American Northern Great Plains perform multiple ecosystem services and are biodiversity hotspots. However, climatological changes can result in sudden shifts in these important ecosystems. For example, marked increases in precipitation in the last few decades have resulted in a widespread shift in wetlands across the Prairie Pothole Region to a new ecohydrological state. We used multiproxy analyses (diatom community composition and invertebrate stable isotopes) of Pb-210-dated sediment cores from two adjacent, but morphologically and hydrologically different, prairie-pothole wetlands to assess the effects of hydroclimatic variability on these wetland ecosystems. Our results provide evidence that the recent ecohydrological shift in the region's wetlands is unprecedented over the past ca. 178 yr. Oxygen stable isotopes in chironomid head capsules provide a record of paleohydrology changes. The most recent sediments (i.e., those deposited after the state shift) from both wetlands revealed novel changes in diatom communities that differed greatly from earlier community compositions. In addition, a depleted signal in deuterium and C-13 carbon stable isotopes observed in chironomid head capsules and Daphnia ephippia, respectively, after 1993 is likely related to an increase in methane production in these wetlands. Our study highlights the importance of considering basin morphometry including whether a wetland has an overflow point, and multiple biological indicators to study climate-change influences on freshwater ecosystems. Research using these techniques can lead to an improved understanding of recent ecosystem shifts, an understanding that will be essential for future climate-change adaptation and mitigation in this ecologically important region.

In this multi-proxy palaeo study, the response of primary producers and consumers to anthropogenic disturbances in a large shallow lake was investigated. Overall, eutrophication and water-level changes (partly inferred by bisaccate pollen grains) are well-reflected in changes in sedimentation rates of pigments, diatom and cladoceran species composition and other proxies (e.g. Rhabdocoela oocytes). In particular, the stepwise decrease in relative abundance of Pediastrum simplex compared to P. boryanum/P. duplex proved to be a good indicator for eutrophication intensity. Furthermore, reductions in the body size (i.e. coenobia diameter) of Pediastrum simplex were clearly related to phases of low visibility that were caused by lake damming in the 1950 s and by accelerating eutrophication in the 1990 s. During the latter period, strong algal growth led to turbid conditions, which are indicated by the co-occurrence of the diatoms Fragilaria crotonensis, Aulacoseira ambigua and A. granulata, the green algae Staurastrum sp. as well as the cladoceran Anthalona cf. milleri. This study highlights that subfossil remains, which are often found in, for instance, standard cladoceran analysis (green algae such as Pediastrum spp. and Staurastrum spp. or bisaccate pollen grains), can add valuable information to the reconstruction of past environmental conditions and biotic response. Such better and more holistic understanding of freshwater ecosystems is crucial to meet future challenges in a rapidly changing world.

In the context of global warming, extreme drought events have occurred more frequently and this is projected to continue in the future in many parts of the world. However, their ecological consequences on shallow lakes are not well studied. Here we compare the succession of diatom assemblages of two medium-sized shallow lakes in southwest China during the past similar to 70 years, comprising an extreme drought event between similar to 2009 and similar to 2013. The diatom community of Lake Chahei showed a clear response to eutrophication from similar to 1980. The pioneer species Achnanthes minutissima increased strongly with elevated sedimentary total nitrogen content and at the expense of Epithemia sorex. In contrast, the diatom community of Lake Yuxian was stable and constantly dominated by the Fragilaria construens/pinnata complex. For both lakes, the most striking change occurred with the extreme drought event when environmental conditions and diatom compositions were homogenised. With strongly shrinking lake areas, peaks in the proportion of silt were observed in the sediments, suggesting increased particle burial rates and decreased light availability. These conditions selected for motile and presumably heterotrophic species mainly of the genera Navicula and Nitzschia which exhibited abrupt increases in their relative abundances at both lakes. Meanwhile, the degree of community homogeneity (inferred by Jaccard's similarity index) of the formerly distinct diatom assemblages increased strongly between both lakes. Hence, the extreme drought event has overridden and homogenised previous abiotic conditions and led to a decrease in beta diversity between the two study sites. Navicula and Nitzschia species may be reliable (paleo) indicators for extreme drought events in comparable shallow lake systems.

Lake nutrient budgets and hydrology are being altered by human activities and climate change, yet little is known of how fertilization and hydrologic mechanisms interact to structure assemblages of primary producers. Here, we present sediment records from a large shallow lake in Southwest China to separate the relative importance of nutrients and hydrological fluctuation in regulating the abundance and composition of primary producers during the twentieth century, with a focus on their differential effects on diatoms and cyanobacteria. Shifts in sedimentary particle-size distribution were consistent with the documented events of hydrological regulation during ~ 1953–1971 and subsequent changes in water level associated with severe droughts. Nutrient enrichment since ~ 1965 resulted in a significant increase in the abundance of total phototrophs (pheophytin a, β-carotene) as inferred from pigment analyses, with stronger responses of cyanobacteria (echinenone, zeaxanthin) over siliceous algae (diatoxanthin). Fossil diatom assemblages revealed a pronounced replacement of benthic taxa by eutrophic and planktonic species (e.g., Fragilaria crotonensis) since ~ 1973, but we observed a significant increase of small benthic Fragilaria sensu lato taxa following ~ 2005, which generally corresponded with a moderate increase in fossil pigments. Although eutrophication was the paramount predictor of changes in phototrophs during the last century, variation in lake hydrology due to climate and water management also modulated phototroph abundances and, more recently, diatom assemblages. Specifically, our sediment evidence suggests that hydrological fluctuation has overridden fertilization by nutrients in structuring diatom composition, leading to a heterogeneous response of cyanobacteria and diatoms to external forcing of this shallow lake.

Functional group classification based on the morphological, physiological and ecological characteristics of diatoms is advanced in describing the tolerance and sensibility of diatoms to environmental stressors. In this study, we examined the spatial distribution of diatom functional groups in surface sediments of Haixi Lake and explored the relationships between diatom distributions and environmental variables using multivariate analyses. Principal components analysis (PCA) and redundancy analysis (RDA) results showed that water depth and total nitrogen (TN) were the key drivers for the heterogeneous distribution of diatom functional groups. Water depth of ca. 8 m was the threshold depth in influencing diatom functional group structures linking with thermal stratification. These results provided a basis for fossil record interpretation of the short core. Diatom functional groups fluctuated over the last century, in keeping with mean annual temperature, sediment TN, and median grain size. Climate warming, accelerated nutrient enrichment and intensified hydrological changes led to the dominance of functional group D, MP and P. Additionally, climate warming and nutrient enrichment led to increased diatom functional group diversity, while increased water depth (led by damming and reservoir reinforcement) contributed to diversity loss around 1957 and 1990 AD, respectively.

随着云南社会经济的持续发展与极端气候事件的频发,高原大中型湖泊面临着水质恶化、生态与环境功能退化的长期胁迫.为识别亚热带大型湖泊面临的主要环境压力,以杞麓湖为研究对象,在对沉积物钻孔进行物理（粒度、烧失量）、生物（色素、硅藻）等指标分析的基础上,结合现代监测和湖泊调查数据,重建了近两百年来湖泊水文条件、富营养化和环境变化的历史,并对硅藻群落结构的演化进行了驱动过程识别.沉积物粒度在1958年之前变化总体较为稳定且有较高的黏土含量;随着围湖造田等流域开发的增强,沉积物粒度组成自1960s开始频繁波动且粗颗粒组分快速增加.1981—2000年期间,随着落水洞泄水工程的修建杞麓湖的水位控制与水文调控得到加强,沉积物砂质含量降低且粒度组成变化较小;2000—2013年期间,湖泊疏浚工程的开展和区域降水的持续减少都导致了沉积物粒度组成波动较大、粗颗粒组成较高.沉积物色素记录了湖泊初级生产力的缓慢上升出现于19世纪中后期,并自1960s开始总叶绿素与蓝藻色素含量总体出现了较大幅度的增加趋势并持续至今.而在2000—2005年期间,湖泊浅水区的疏浚清淤导致了内源营养盐输入量的降低与藻类生物量的明显下降;沉积物蓝藻色素含量在1998、2008和2012—2013年左右出现明显的峰值,指示杞麓湖可能出现了较大范围的蓝藻暴发事件.统计分析结果显示,湖泊硅藻群落结构出现了多次明显转变且呈现底栖硅藻百分比长期降低的特征,水体富营养化的持续是驱动硅藻群落结构演替的主要因子,而水生植物退化、水文条件与气候变化也对硅藻群落的构建产生了重要的叠加影响.本文的沉积物分析结果表明,亚热带大型湖泊的生态治理与环境保护需要重点围绕营养盐负荷控制、水文调控优化与底栖生境恢复,并需应对全球变暖与极端气候事件产生的叠加影响.

Hydrological control of lakes has been increasingly practiced in many parts of the world, however, the long-term ecological impact of hydrological regulation and their dependence on lake impoundment intensity has been rarely examined. We combined a spatial survey of surface sediments with sediment core analyses to quantify the limnological changes over the last two centuries for an oligo-mesotrophic lake, which was dammed in 1957 and reinforced during 1987-1990, respectively. A water depth inference model constructed from surface sediment clay components was applied to a well-dated sediment core for water level reconstruction. The inferred water depth increased from 6.2 ± 0.9 m to 8.7 ± 1.7 m after dam construction and further to 13.6 ± 2.6 m after dam enforcement, resulting in an increase in the magnitude of water level fluctuation (WLF). Accordingly, bulk sediment C/N ratio and median grain size spiked in ∼1957 and ∼1990, respectively, reflecting a large input of terrestrial sources due to impoundment. With a consistent loss of littoral zone and benthic diatoms over time, a significant decrease in C/N ratio and an abrupt depletion of carbon isotopic signal suggested a shift of carbon transfer towards a pelagic pathway after ∼1990. While there was a significant increase in algal production since ∼1990, the accumulation rate of carbon and nitrogen burial displayed an accelerating drop since ∼1957, reflecting a diluting effect derived from expanding water storage. Furthermore, there was a significant increase in both the ratio between inorganic and organic carbon fluxes and sediment burial of inorganic carbon, reflecting enhanced degradation and low storage of aquatic organic carbon in stratified deep waters since ∼1990. Hydro-morphological variables were found to exert strong impact on diatom communities, with an increasing interplay with nutrient and climate variables over time. While there existed a significant shift of diatom composition in ∼1960, species richness and community dissimilarity showed a significant decrease when water depth was raised to above ∼10 m or the magnitude of WFL was above ∼2 m. Thus, our sediment surveys provide evidence on the significant impact of lake regulation on hydro-morphology, carbon burial and ecological shift over time, as well as its stronger interaction with other forcing with increased impoundment intensity.

Regional warming and atmospheric nitrogen deposition have been widely recorded to impact remote catchments and alpine lakes; however, their independent roles and interactions have rarely been identified. Here, we combined down-core analyses of sedimentary mercury (Hg) and aluminum (Al) with multiple proxies (i.e. nitrogen stable isotope, chlorophyll a pigments, diatoms) for a radiometrically-dated sediment core of an alpine lake in southeast Tibet to track the atmospheric deposition of pollutants, and to examine possible effects of climate and catchment forcing over the past three centuries. The sediment data revealed that airborne deposition of Hg was recorded from the ~1860s, with an accelerating increase in anthropogenic Hg flux since the ~1960s. A synchronous decrease in reconstructed lake-water TOC indicated that acid deposition may have affected lake-water carbon concentrations and impaired catchment export of decomposed organic matter (OM). A moderate depletion of bulk sediment δ15N started from the ~1820s, but was followed by an enriching trend after the ~1970s. This positive shift of δ15N was associated with elevated sediment OM and decreased catchment runoff of clastic materials (as inferred by Al). Sediment OM content displayed an accelerating increase from the ~1960s, with an increased input of autochthonous sources (i.e. lower bulk sediment C:N ratios), such as algae (as inferred by sedimentary chlorophyll a pigments). Meanwhile, climate warming and decreased lake-water TOC enhanced the production of algae, which was characterized by a more enriched δ15N signal than that of allochthonous OM. Furthermore, atmospheric acid deposition was significantly related to diatom assemblage changes, with an increase in acidophilous taxa. Our sediment evidence revealed the dominating impact of climate and catchment processes on lake-water chemistry and algal shifts in the context of atmospheric nitrogen deposition, and highlighted an increasing link of external forcing with in-lake processes in enriching sediment δ15N signal over the last few decades.

The synergistic influence of multiple environmental stressors on lake ecosystems has typically been evaluated paleolimnologically, through examination of a single biological indicator. Aquatic organisms, however, may display heterogeneous responses because of differences in their ecological sensitivity, and/or because of ecological consequences caused by strong interactions among multiple stressors. We applied paleolimnological methods to compare patterns of algal and invertebrate response to multiple stressors in a large, shallow lake in southwest China over the last two centuries. Our multi-proxy records show a clear trajectory of lake eutrophication (greater total nitrogen) and increasing lake productivity (greater sediment Chlorophyll- a concentration) during the last century. Nutrient enrichment and lake productivity played significant, but different roles, in structuring diatom and cladoceran assemblages, accounting for 31.4 and 77.3% of the total variance in the communities, respectively. Furthermore, there was a pronounced influx of the endemic diatom species Cyclotella rhomboideo - elliptica Skuja, which was strongly associated with the second axis of a diatom PCA (Principal Component Analysis). This occurred synchronously with a documented reversal of hydrological connectivity with a downstream, nutrient-poor lake during the early twentieth century, suggesting a role for species dispersal in modulating community reorganization. There are also strong differences between the two organism groups in their sensitivity to hydrological fluctuations, as hydrodynamics, indicated by sand content, was a significant driver for cladocerans (>25%), but showed only minor influence on diatom assemblages in our selection of minimum adequate models. The proportion of the total variance explained by our measured variables (<30%) was much lower for diatom assemblages than for both cladoceran assemblage and accumulation data (~77%), reflecting differences in community reorganization between the two biological indicators. The sediment-based evaluation of community responses revealed the differential impact of eutrophication and hydrological fluctuations on the biota of this large, shallow lake. Therefore, multiple biological indicators should be evaluated in limnological surveys to assess the full scope of ecological changes in highly stressed lake systems targeted for conservation or restoration.

Limnological studies have long focused on the nine largest plateau lakes and ecological assessment of lake eutrophication in Yunnan. However, comprehensive surveys of small and me-dium-sized lakes and their response to multiple stressors are largely absent. Here, we combined multi-proxy sediment analyses and documentary data, aiming to identify the long-term pattern of diatom community changes under the influence of climate change and anthropogenic disturbances, and to quantify their driving strengths. The results showed that Xihu Lake was in a pre-impact state with little ecological variability before the 1950s. Thereafter, expansion of land use increased the nutrient input and altered the lake hydrology, resulting in a significant shift of dominant diatoms from Cocconeis placentula to Fragilaria spp. From 1997, accelerating nutrient enrichment and hydrological changes led to the replacement of benthic diatoms by planktonic ones, resulting in a consistent loss of macrophytes and a decrease of ecological stability. Therefore, the types and strength of catchment development should be fully evaluated for effective protection of small and medium-sized alpine lakes in the context of global warming.

The increased intensity and frequency of extreme droughts due to global warming have led to the dedine of water levels and significant shortage of water resources in many lakes and reservoirs, which could severely threaten the socio-economic development and ecological security. To date the ecological evaluation of extreme droughts have been mainly focused on terrestrial ecosystems in China. This study aim to assess the impact of extreme droughts on diatom assemblages and diversity patterns through multi-proxy sediment analyses in Yuxian Lake, which has experienced sig-nificant water level change during the period of 2009 to 2013. The grain size records showed an increase in sediment particles starting from about 2011, which corresponded to the occurrence of the most recent drought event. The isoto-pic and elementary records indicated that the trophic state remained oligotrophic over the past century. Diatom as-semblages were dominated by benthic and limnetic diatoms (i.e.Fragilaria construens/pinnata), but these taxa were there after replaced by fluvial and riverine taxa (i.e.Nitzschia and Navicula) with the most recent decline of lake levels. Our results demonstrated that the diatom PCA axis was strongly associated with extreme climate, which accounted for 55.8% of the total variance for diatom assemblages. Our study provided sediment evidence for the role of extreme droughts in regulating freshwater environment and biota and highlighted the urgency of climate mitigation actions in maintaining water security and ecosystem functioning for this climate-sensitive region.

To investigate the distribution pattern and significance of grain size and provide accurate analogues for grain size records from lake cores, we analyzed surface sediments from 15 lakes of Yunnan Province, China for grain size.Meanwhile, the grain size components of the surface sediments of the 15 lakes were partitioned using the log-normal distribution function method.The results showed that frequency curves of samples indicated polymodal and consisted of two to five grain-size components with the modal size varying within ranges of 0.6~423.0 μm.These components are specified from fine to coarse modes as clay (C1), fine silt (C2), silt (C3), fine to medium sand (C4) and coarse sand (C5).The grain size analysis reveals that the C1 components were the long-term superfine dust and aerosol in the atmosphere which entered the lake water body through the process of gravity and precipitation, and may have represented the background level of atmospheric dust and aerosols in southwest China;C2 and C3 were the main components of surface sediments, the modal size of which reflected the precipitation intensity of the watershed;C4 and C5 components only appeared in minority surface sediments of lakes with the contents were extremely low, and were significantly related to the lake hydrodynamic force.For the large lakes, such as Dianchi and Fuxian Lakes, it requires two processes for the sand components to reach the central part of lakes: process of from drain basin to lakes and from lakeshore to the middle of lakes, and the second one plays a main role, affected by lake current, water depth, gradient of lake basin and so on.Under the background of the drought in southwest China, some small lakes in Yunnan shrinked and even disappeared.However, because of the decrease of precipitation and weakening of general circulation, surface runoff and hydrodynamics of lakes recede, thus the material which transmit to the center of the lakes were primarily fine grain.Small lakes located in the mountainous region, with the limited catchment area and conspicuous terrain slope, can be affected by the surface runoff directly and strongly.Therefore, the increase of coarse grain in the sediments of this kind of lakes reflected the enhancement of precipitation or the precipitation intensity.

在人类活动持续干扰的背景下,云南部分湖泊面临着污染物输入增加的环境压力,特别是营养盐富集和重金属污染.以云南地区遭受过严重工业污染的阳宗海为研究对象,通过沉积物硅藻群落、砷浓度、营养元素与稳定同位素、粒度等多指标分析,结合文献记录和湖泊调查结果,揭示了阳宗海硅藻群落对湖泊富营养化和砷污染的长期响应特征,并识别了不同时期的主要环境压力与其驱动强度.结果表明:长期的营养盐累积使得浮游硅藻逐渐占据优势地位,且耐污染的底栖硅藻种的快速增加与砷污染出现的时段一致.在阳宗海长期富营养化的背景下,当水体砷污染物浓度达到一定阈值水平后,硅藻群落结构的改变和多样性的降低都指示了湖泊生态系统发生了灾难性的转变.同时1965年开始的湖泊引水工程导致了贫营养种的突然增加.因此,水体富营养化、重金属污染与湖泊水文调控是导致阳宗海硅藻群落长期变化的主控因子,对阳宗海的生态修复与综合治理需要综合考虑不同胁迫因子的长期影响与驱动作用.

Subtropical plateau lakes in Southeast Yunnan are known to have experienced multiple environmental stressors (lake pollution, eutrophication, land reclamation and droughts) during the past decades. Here, we applied multi-proxy analyses (i.e., diatom, grain size, C and N isotopic and chronology) of lake sediments, aiming to track the history of lake environment change and the characteristics of diatom community responses in Datun Lake for the past century. The results proved that the sedimentary diatom community records showed significant changes with the replacement of Fragilaria construens by Achnanthes minutissima. Combining with multiple-proxy records (i.e., isotopic data) and modern monitoring records, the ordination analyses and variation partitioning further suggested that industrial pollution and eutrophication were the major driving factors that led to the long-term shift of diatom community. In addition, the grain size results and meteorological data revealed that the reductions in hydrodynamic and water exchange intensity associated with damming and the occurrence of extreme droughts, thus led to the corresponding diatom community change.