Recent environmental change in an upland reservoir catchment: a palaeoecological perspective

Reservoirs provide important water resources and require careful management, through ecological monitoring, to identify and mitigate changes in water quality. Long-term data on vegetation changes and the impacts of human activities on reservoir water chemistry, however, are often limited. Setting restoration targets can therefore be problematic. Palaeoenvironmental research has made little use of reservoir sediments and there is great potential for palaeoecological data to be incorporated into management planning. Diatoms and pollen were analysed in sediment cores from Venford Reservoir, southwest England, to infer pH and land-use changes, respectively, over the last century. Diatom-inferred (DI) pH indicates that reservoir pH declined from ~pH 6.0 in the early part of the record and reached a low between AD 1920 and 1940 (~pH 5.6), which was likely associated with fossil fuel combustion and acid deposition. DI-pH then increased, but values remained relatively low, even in the most recent sediments (~pH 5.7), and the magnitude of inferred pH change over time was small. Land-use changes, such as increased grazing intensity and erosion, and establishment of pine plantations, also likely influenced reservoir water chemistry changes over time. Understanding the impacts of such factors on water chemistry has implications for future catchment land-use planning, which is essential for managing water resources. The pollen record indicates a shift from heather-dominated to grass-dominated vegetation since ~AD 1935–1950, which could be related to increased grazing intensity. The palaeoecological dataset is valuable as a long-term record against which short-term monitoring datasets and future changes can be assessed.     

阳宗海硅藻群落对水体污染和水文调控的长期响应模式

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。     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: IX. Summary of paleorecords of environmental change and catchment deforestation at Lake Tanganyika and impacts on the Lake Tanganyika ecosystem

Paleorecords from multiple indicators of environmental change provide evidence for the interactions between climate, human alteration of watersheds and lake ecosystem processes at Lake Tanganyika, Africa, a lake renowned for its extraordinary biodiversity, endemism and fisheries. This paper synthesizes geochronology, sedimentology, paleoecology, geochemistry and hydrology studies comparing the history of deltaic deposits from watersheds of various sizes and deforestation disturbance levels along the eastern coast of the lake in Tanzania and Burundi. Intersite differences are related to climate change, differences in the histories of forested vs. deforested watersheds, differences related to regional patterns of deforestation, and differences related to interactions of deforestation and climate effects. Climate change is linked to variations in sediment accumulation rates, charcoal accumulation, lake level and water chemistry, especially during the arid-humid fluctuations of the latter part of the Little Ice Age. Differences between forested and deforested watersheds are manifested by major increases in sediment accumulation rates in the latter (outside the range of climatically driven variability and for the last 40 years unprecedented in comparison with other records from the lake in the late Holocene), differences in eroded sediment and watershed stream composition, and compositional or diversity trends in lake faunal communities related to sediment inundation. Variability in regional patterns of deforestation is illustrated by the timing of transitions from numerous sedimentologic, paleoecologic and geochemical indicators. These data suggest that extensive watershed deforestation occurred as early as the late-18th to the early-19th centuries in the northern part of the Lake Tanganyika catchment, in the late-19th to early-20th centuries in the northern parts of modern-day Tanzania, and in the mid-20th century in central Tanzania. Rapid increases in sediment and charcoal accumulation rates, palynological and lake faunal changes occurred in the early-1960s. We interpret this to be the result of greatly enhanced flushing of sediments in previously deforested watersheds triggered by extraordinary rainfall in 1961/62. Regional differences in deforestation histories can be understood in light of the very different cultural and demographic histories of the northern and central parts of the lake shoreline. Incursion of slaving and ivory caravans from the Indian Ocean to the central coast of Lake Tanganyika by the early-19th century, with their attendant diseases, reduced human and elephant populations and therefore maintained forest cover in this region through the late-19th to early-20th centuries. In contrast, the northeastern portion of the lakeshore did not experience the effects of the caravan trades and consequently experienced high human population densities and widespread deforestation much earlier. These studies demonstrate the importance of paleolimnological data for making informed risk assessments of the potential effects of watershed deforestation on long-term lake ecosystem response in the Lake Tanganyika catchment. Differences in sediment yield and lake floor distribution of that yield, linked to factors such as watershed size, slope, and sediment retention, must be accounted for in management plans for both human occupation of currently forested watersheds and the development of future underwater reserves.     

Lake evolution in a semi-arid montane environment: response to catchment change and hydroclimatic variation

Pollen, geochemical and sedimentological data from Sidi Ali, a montane Moroccan lake, provide a 7000 yr record of changes in climate, catchment vegetation and soil erosion intensity. Diatoms, non-silicious algae, macrophyte fossils and ostracods from the same core record the dynamics of the lake ecosystem. Oxygen isotope and trace-element ratios of benthic ostracods appear to be relatively insensitive to climatic variation in this open lake with low water-residence time, but diatom plankton / periphyton (P/L) ratios show lake-level variations that are probably climate controlled. At least two superimposed processes are recorded, but at different timescales: catchment vegetation and soils show long-term changes due to climate and human impact, whereas P/L ratios suggest century-scale oscillations in lake depth. The timing of changes in algal and macrophyte productivity and carbon cycling within the lake broadly corresponds to changes in terrestrial vegetation, suggesting either that lake nutrient status is linked to catchment vegetation and soils, or that both were influenced by climate. The lack of a sensitive and independent (non-biological) climate proxy makes it more difficult to assess the lake's ecological response to short-term climate variation. Overall, the lake's evolution has been influenced both by catchment-mediated nutrient flux and by changes in water balance, thus having characteristics in common with both temperate and arid zone lakes.     

Baptiste Lake,Alberta — A late Holocene history of changes in a lake and its catchment in the southern Boreal forest

Lead-210 assay and dating are subject to several sources of error, including natural variation, the statistical nature of measuring radioactivity, and estimation of the supported fraction. These measurable errors are considered in calculating confidence intervals for ²¹⁰Pb dates. Several sources of error, including the effect of blunders or misapplication of the mathematical model, are not included in the quantitative analysis. First-order error analysis and Monte Carlo simulation (of cores from Florida PIRLA lakes) are used as independent estimates of dating uncertainty. CRS-model dates average less than 1% older than Monte Carlo median dates, but the difference increases non-linearly with age to a maximum of 11% at 160 years. First-order errors increase exponentially with calculated CRS-model dates, with the largest 95% confidence interval in the bottommost datable section being 155±90 years, and the smallest being 128±8 years. Monte Carlo intervals also increase exponentially with age, but the largest 95% occurrence interval is 152±44 years. Confidence intervals calculated by first-order methods and ranges of Monte Carlo dates agree fairly well until the ²¹⁰Pb date is about 130 years old. Older dates are unreliable because of this divergence. Ninety-five per cent confidence intervals range from about 1–2 years at 10 years of age, 10–20 at 100 years, and 8–90 at 150 years old.This is the third of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

Lake and catchment response to Holocene environmental change: spatial variability along a climate gradient in southwest Greenland

The Kangerlussuaq area of southwest Greenland is a lake-rich landscape that covers a climate gradient: a more maritime, cooler and wetter coastal zone contrasts with a dry, continental interior. Radiocarbon-dated sediment sequences (covering ~11,200?C8,300?cal?year) from paired lakes at the coast and the head of the fjord were analysed for lithostratigraphic variables (organic-matter content, bulk density, Ti, Ca). Minerogenic and carbon accumulation rates from the four lakes were compared to determine catchment and lake response to Holocene climatic variability. Catchment erosion at the coast was dominated by cryonival processes, with considerable sediment production due to the limited vegetation cover and exposed rock faces. Input of minerogenic sediment at one site (AT4) was high (>1?gDW?cm^?2?year^?1) during the period 5,800?C4,000?cal?year BP, perhaps reflecting intensification of cryogenic processes on northeast-facing slopes and rapid delivery to the lake. This period of erosional activity was not observed at the nearby, higher elevation site (AT1) due to the lower catchment relief; instead, there was an abrupt decline in carbon and minerogenic accumulation rates at ~5,800?cal?year BP. Sediment accumulation rates at the inland sites were much lower (<0.005?gDW?cm^?2?year^?1) reflecting greater catchment stability (more extensive vegetation cover), lower relief and substantially lower precipitation, but synchronous increases in mineral accumulation rates from ~1,200 to 1,000?cal?year BP may reflect wind erosion associated with regional cooling and local aridity. Carbon-accumulation-rate profiles were similar at the two inland sites, with higher-than-average accumulation (~6?C8?g?C?m^?2?year^?1) during the early Holocene and a subsequent decline after ~6,000?cal?year BP. At the inland lakes, both mineral and carbon accumulation rates exhibited a stronger link to climate, driven by trends in effective precipitation and regional aeolian activity. Catchment differences (relief, altitude) lead to more individualistic records in both erosion history and lake productivity at the coast.     

Sediment-chemistry response to land-use change and pollutant loading in a hypertrophic lake, southern Sweden

Responses to recent land-use changes and pollutant loading in the sediment of a hypertrophic lake in southern Sweden were studied by comparison of geochemical, pollen and magnetic records with historical land-use data. A chronology was constructed for the last two centuries by correlating changes in the pollen diagram to major events in the land-use history. Sediment accumulation was low (mean c. 0.2 g cm^-2 yr^-1) prior to 1800 AD, when less than 25% of the catchment was arable land. Reorganization of the agrarian system during the 19th century increased the annually tilled area by 300%, which accelerated soil erosion and substantially increased the accumulation of allochtonous matter in the lake. Since the turn of the century 90% of the catchment has been ploughed every year. The deposition of clastic matter in the lake has, however, decreased due to a gradual rerouting of the drainage system, which has reduced the effective catchment area by c. 85%.Authigenic vivianite (Fe3(PO4)2.8H2O) is a major P phase in the preindustrial non-sulphidic sediments, which suggests that the sediments at that time served as a fairly efficient sink for P. The arable expansion, increased manuring and, eventually, the introduction of artificial fertilizers during the 19th century led to a massive influx of nutrients, which elevated primary production in the lake. Subsequent development of bottom water anoxia around 1900, in combination with an additional pollutant burden of sulphate within the lake basin, led to major alterations of the biogeochemical cycles. The most critical change in the post-1900 sediments involved the cycling of Fe and P. The linkage between the lacustrine P and Fe cycles can explain that FeS formation was paralleled by a release of P from the sedimentary pool. This supply of P to the lake basin must have supplemented the nutrient supply by modern agriculture and contributed to recent hypertrophication. The bacterial sulphate reduction also affected the generation of alkalinity which supported a significant calcite precipitation in the post-1900 sediments.S is enriched 10-fold in the post-1900 sediments compared to preindustrial values. Along with the rise in S, soot particles derived from fossil fuel combustion appear in the sediments for the first time. Therefore, Bussj¨osj¨on is thought to be a good example of how a well-buffered, highly productive lake may respond to the pollution by sulphur from acid rain.     

Trophic interactions in an arid ecosystem: From decomposers to top-predators

In this review we compile results on interactions above and below ground and food web functioning in an arid environment at the Baza Basin, in the Iberian southeast. Our data reveal that herbivory is difficult to estimate in our system. Some herbivores (rabbits) and granivores (Messor ants) create nutrient and detritus-rich patches, with important effects on the diversity and abundance of species both above and below ground. Fluctuations of prey availability, especially those caused by changes in their circadian rhythms, provoke seasonal migrations and omnivory in predators. We also present experimental evidence of the effects of interactions between aboveground and belowground macroarthropods on litter degradation; belowground detritivores are responsible for a high percentage of ground surface litter decomposition. We demonstrate that belowground herbivores and decomposers can dramatically change aboveground multitrophic interactions. Finally, we identify the biotic and abiotic factors controlling aboveground and belowground macrofaunal distribution at a broad scale. We conclude that trophic interactions in this semi-arid area are numerous and complex with many of the interactions involving more than two or three organisms. The interactions between above- and belowground organisms added complexity to this system. These habitats in which organisms deal with extreme abiotic conditions promote odd interactions resulting in an increase of biodiversity. Finally, despite the large research effort devoted to understanding food web structure and dynamics in this area recently, our state of knowledge is still far from providing a complete picture of interactions and their implications in the regulation and functioning of the system.     

Climate-driven changes in water level: a decadal scale multi-proxy study recording the 8.2-ka event and ecosystem responses in Lake Sarup (Denmark)

A two-stage change in lake level during the 8.2-ka event was identified in Lake Sarup, Denmark (55°N), using a multiproxy approach on precise radiocarbon wiggle-matched annually laminated sediments deposited 8740–8060 cal. yr BP. Changes in δ¹³C and δ¹⁸O indicated closed lake hydrology driven by precipitation. The isotopic, sedimentary and plant macrofossil records suggested that the lake level started to decrease around 8400 cal. yr BP, the decrease accelerating during 8350–8260 before an abrupt increase during 8260–8210. This pattern shows that the climate anomaly started ~150 years before the onset of the 8.2-ka cooling event registered in Greenland ice cores, but was synchronous with hydrologic change in the North American Lake Agassiz drainage. The lake level decrease was accompanied by a higher accumulation rate of inorganic matter and lower accumulation rates of cladoceran subfossils and algal pigments, possibly due to increased turbidity and reduced nutrient input during this drier period. Pigment analysis also showed added importance of diatoms and cryptophytes during this climate anomaly, while cyanobacteria became more important when the water level rose. Moreover, Nymphaeaceae trichosclereids were abundant during the period of algal enrichment. Cladoceran taxa associated with floating leaved plants or benthic habitats responded in a complex way to changes in water level, but the cladoceran assemblages generally reflected deep lake conditions throughout the period. The lake did not return to its pre-8.2-ka event status during the period of analysis, but remained more productive for centuries after the climatic anomaly as judged from the pigment accumulation and assemblage composition. The change to more eutrophic conditions may have been triggered by erosion of marginal deposits. Together, these data confirm the chronology of hydrologic changes and suggest, for the first time, that lake levels exhibited both a decline and an increase in rapid succession in response to the 8.2-ka event in southern Scandinavia.     

Biological responses to recent eutrophication and hydrologic changes in Xingyun Lake,southwest China

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.     

Diatom succession related to land use during the last 6000 years: a study of a small eutrophic lake in southern Sweden

Diatoms, pollen, physical and magnetic analyses of the sediments have been used to reconstruct the development over the last 6000 years of Lake Bussjösjön, a small lake in southern Sweden. Stratigraphic variations in a core of more than 15 m reveal changes in diatom assemblages, which correspond closely to changes in pollen, loss-on-ignition, and magnetic measurements that are related to land use and vegetation changes in the catchment. From ca 6000 BP to 2700 BP, a forest surrounded what was then a slightly eutrophic lake. The sudden appearance of Cyclostephanos dubius (Fricke) Round and several epiphytic/epipsammic diatoms at 2700 BP coincides with deforestation of the catchment (2700 BP to 2500 BP). A change in land use from predominantly pasture to arable land from 1300 BP to 1100 BP caused a high level of soil erosion with a decrease of C. dubius and the increase of Stephanodiscus species. An increase of epiphytic/epipsammic species coincides with increased arable farming and the change from a field-rotation to a crop-rotation system, and shows not only an increase in eutrophication but also changes in water depth. The influence of the catchment through time resulted in a smaller, shallower and eutrophic to hypertrophic lake.     

Magnetic susceptibility in sediment records of Lake Ådran,eastern Sweden: correlation among cores and interpretation

Magnetic susceptibility was used to test the representativity of the lithostratigraphy of a master sediment sequence from Lake Ådran, eastern Sweden. Five further sediment cores from the same lake were correlated and compared with the master sequence using magnetic susceptibility records. Mineral magnetic correlations are generally based on matching prominent susceptibility features but may be significantly improved by using slot sequence analyses. The result of these analyses show that the sediment in the Lake Ådran master sequence can be considered representative for the basin. The variations in the magnetic susceptibility in relation to pollen and diatom analyses also seem to reflect water level changes and shore displacement in the Baltic between 9900 ¹⁴C years B.P. and the isolation at 5900 ¹⁴C years B.P. The first emergence of scattered non-vegetated islands is characterised by high susceptibility values. Along with a continuous water lowering and the development of the vegetation during the Ancylus fresh water stage, susceptibility values gradually decrease. A rapid phase of the Ancylus regression between 9200 and 9000 ¹⁴C years B.P. is seen as an increase in magnetic susceptibility. During the following Baltic brackish water stage, the Litorina stage, susceptibility values are at their lowest, followed by higher values at the isolation from the Baltic with a maximum around 5000 ¹⁴C years B.P.Shore displacement between 9900 and 5900 ¹⁴C years B.P. is discussed based on the magnetic susceptibility measurements and their relationship to the pollen and diatom analyses.     

Sediment dynamics in an upland temperate catchment: changing sediment sources,rates and deposition

We examine sediment dynamics in an upland, temperate lake system, Lake Bassenthwaite (NW England), in the context of changing climate and land use, using magnetic and physical core properties. Dating and analysis of the sedimentary records of nine recovered cores identify spatially variable sedimentation rates across the deep lake basin. Mineral magnetic techniques, supported by independent geochemical analyses, identify significant variations both in sediment source and flux over the last ∼2100 years. Between ∼100 years BC and ∼1700 AD, sediment fluxes to the lake were low and dominated by material sourced from within the River Derwent sub-catchment (providing 80% of the hydraulic load at the present day). Post-1700 AD, the lake sediments became dominantly sourced from Newlands Beck (presently providing ∼10% of the lake’s hydraulic load). Three successive, major pulses of erosion and increased sediment flux appear linked to specific activities within the catchment, specifically: mining activities and associated deforestation in the mid-late nineteenth century; agricultural intensification in the mid-twentieth century and, within the last decade, the additional possible impact of climate change. These results are important for all upland areas as modifications in climate become progressively superimposed upon the effects of previous and/or ongoing anthropogenic catchment disturbance.     

Pulp and paper mill pollution and subsequent ecosystem recovery of a large boreal lake in Finland: a paleolimnological analysis

Sedimentary diatom frustules and chironomid remains, in addition to the chemical stratigraphy of 32 elements and resin acids, were studied from short-core samples taken from two basins and a sheltered bay of Lake Päijänne in southern Finland (about 61 °10–62 °15 N, 25 °30 E). The lake was formerly oligotrophic but has been subject to effluent loading from pulp and paper mills during the past century since the opening of the first pulp mill in the 1880s. Four developmental phases were distinguished and named mainly according to the effects found in the basin close to the pollution source: (1) the pre-industrial phase (from the 1800s to ca. 1920), including the early years of industrialization, with low impacts; (2) the phase of increasing pollution (ca. 1920–1969); (3) the phase of severe pollution (1970–1981); and (4) the water protection phase (1982 onwards).Distinct differences were observed between pelagic (diatoms) and benthic (chironomid) ecosystems in their response to pollution. Effluent loading from the pulp and paper mills had a pronounced impact on chironomid assemblages, but only a slight influence on diatoms in the basin close to the pollution source. In the southern basin, approximately 60 km downstream, an increase in acidophilous species was observed in the diatom assemblages during the early years of pollution, but changes in chironomids in this basin were negligible. The water protection work of the past 30 years, which has led to an improvement in water quality and a marked recovery of the profundal biota, has also resulted in a slight increase in primary production of the lake. The decrease in brown-coloured effluent water has led to an increase in thickness of the lake's photic productive layer, while the toxicity of the water has simultaneously diminished. Rapid changes in chironomid assemblages and sedimentation observed in the sheltered bay highlight the importance of local land-use activities as causes of environmental change in this area, and especially their effects on sedimentation and benthic life.     

Acidification and metal contamination in Whitepine Lake (Sudbury,Canada): a paleolimnological perspective

Diatom and chrysophyte assemblages from a sediment core from Whitepine Lake were examined to infer changes in lakewater pH, nickel and aluminum concentrations since pre-industrial times, and to help determine the cause of the virtual extirpation of the lake trout population from the lake during the 1960s and 1970s. Our study indicates that acidification started in the 1920s, and that the maximum inferred pH decline (from 6.2 to 5.8) occurred between 1960 and 1970, coincident with the peak in metal mining and smelting activity in the Sudbury basin. Lakewater [Al] and [Ni], as inferred from our diatom transfer functions, increased. It appears that in addition to the pH decline, elevated [Al] may have played an important role in the decline of lake trout from Whitepine Lake in the 1960s and 1970s. Diatom-inferred lakewater pH and [Ni] have recovered slightly in the recent sediments, which coincides with reductions in emissions that have occurred since the mid-1970s.     

Phytoplankton dynamics in Lake Biwa during the 20th century: complex responses to climate variation and changes in nutrient status

We examined algal remains and fossil pigments in ²¹⁰Pb-dated sediment cores from Lake Biwa to explore historical changes in the phytoplankton community of the lake over the past 100 years and to identify environmental factors that caused those changes. Fluxes of fossil pigments and algal remains were very low before the 1960s, but increased through the 1960s and 1970s, indicating that the lake had eutrophied in the 20 years since 1960. After 1980, however, fluxes of all fossil pigments and algal remains decreased or stabilized. Redundancy analysis with meteorological and limnological variables explained more than 70% of the variation of these fluxes and showed that the decrease in fluxes of most algal taxa that occurred in the 1980s was related to changes in meteorological variables such as wind velocity, rather than changes in the lake’s trophic state. Sedimentary records of algal remains also revealed that Aulacoseira nipponica, an endemic diatom species that grows in winter, decreased dramatically after 1980, while Fragilaria crotonensis, a cosmopolitan spring diatom species, became dominant. Replacement of one dominant diatom species by another could not be explained simply by changes in the lake trophic state, but was reasonably strongly related with an increase in winter water temperature. These results suggest that the phytoplankton community in Lake Biwa was influenced by changes in local environmental conditions (nutrient loading) through the 1960s and 1970s, but more so by regional (meteorological) and global (climate warming) factors since 1980.     

长江中游网湖近代沉积环境演变及其对人类活动的响应

采用放射性核素²¹⁰Pb和¹³⁷Cs计年方法,建立了长江中游网湖近代沉积物的年代序列,并对网湖沉积物的粒度组成、磁性参数、重金属元素、花粉组合以及营养盐含量等指标进行了分析。根据沉积指标垂直分布的综合特征,结合湖区史料,重建了网湖100年来沉积环境的演变过程。通过对沉积指标与湖区人口变化、工农业产值等数据的比对,发现中华人民共和国成立以后沉积物中TOC和TN浓度与人口变化存在较高的相关性,相关系数分别达到0.8892和0.8928(P<0.001);沉积物中重金属元素富集序列、营养盐富集序列与孢粉含量的变化序列与湖区人类活动具有较好的耦合性。表明网湖沉积指标可以敏感地反映湖区的人类活动。     

The influence of Holocene tree-line advance and retreat on an arctic lake ecosystem: a multi-proxy study from Kharinei Lake,North Eastern European Russia

A consequence of predicted climate warming will be tree-line advance over large areas of the Russian tundra. Palaeolimnological techniques can be used to provide analogues of how such changes in tree-line advance and subsequent retreat affected lake ecosystems in the past. A Holocene sediment core taken from Kharinei Lake (Russia) was dated radiometrically and used for multi-proxy analyses with the aim of determining how climate and tree-line dynamics affected the productivity, community structure, carbon cycling and light regime in the lake. Pollen and macrofossil analyses were used to determine the dates of the arrival and retreat of birch and spruce forest. C:N ratios and percent loss-on-ignition were used to infer past changes in sediment organic matter. Visible-near-infrared spectroscopy and diatom analysis were used to infer past changes in lake-water carbon. Algal pigments and aquatic macrophytes were used to determine changes in lake productivity and light. Chironomids together with remains of the aquatic flora and fauna were used to provide information on past July temperature and continentality. Lake sedimentation was initiated shortly before 11,000 cal. years BP, when both chironomid- and pollen-inferred temperature reconstructions suggest higher summer temperatures than present, between 1 and 2°C warmer, and lake productivity was relatively high. A few trees were already present at this time. The spruce forest expanded at 8,000 cal. year BP remaining in the vicinity of the lake until 3,500 cal. year BP. This period coincided with a high concentration of organic material in the water column, and relatively high benthic productivity, as indicated by a high benthic: planktonic diatom ratio. After tree-line retreat, the optical transparency of the lake increased, and it became more open and exposed, and was thus subject to greater water-column mixing resulting in a higher abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.     

Sciaridae in lake sediments: indicators of catchment and stream contribution to fossil insect assemblages

The larval head capsules of Sciaridae (black fungus gnats) are transported into lakes from terrestrial habitats, in most cases via streams or rivers, and preserve well in lake sediments. The abundance of sciarid remains can provide information on the importance of terrestrial and running-water input into fossil insect assemblages in lake sediments if examined in relation to the number of fossils of aquatic insects such as the Chironomidae, Thaumaleidae, Ceratopogonidae, or Simuliidae. Here we describe fossil head capsules of sciarid larvae and provide an example of how these remains can be used to constrain past changes in taphonomic processes that influence fossil chironomid records.