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1.
Upper Klamath Lake, in south-central Oregon, contains long sediment records with well-preserved diatoms and lithological variations that reflect climate-induced limnological changes. These sediment archives complement and extend high resolution terrestrial records along a north–south transect that includes areas influenced by the Aleutian Low and Subtropical High, which control both marine and continental climates in the western United States. The longest and oldest core collected in this study came from the southwest margin of the lake at Caledonia Marsh, and was dated by radiocarbon and tephrochronology to an age of about 45 ka. Paleolimnological interpretations of this core, based upon geochemical and diatom analyses, have been augmented by data from a short core collected from open water environments at nearby Howards Bay and from a 9-m core extending to 15 ka raised from the center of the northwestern part of Upper Klamath Lake. Pre- and full-glacial intervals of the Caledonia Marsh core are characterized and dominated by lithic detrital material. Planktic diatom taxa characteristic of cold-water habitats (Aulacoseira subarctica and A. islandica) alternate with warm-water planktic diatoms (A. ambigua) between 45 and 23 ka, documenting climate changes at millennial scales during oxygen isotope stage (OIS) 3. The full-glacial interval contains mostly cold-water planktic, benthic, and reworked Pliocene lacustrine diatoms (from the surrounding Yonna Formation) that document shallow water conditions in a cold, windy environment. After 15 ka, diatom productivity increased. Organic carbon and biogenic silica became significant sediment components and diatoms that live in the lake today, indicative of warm, eutrophic water, became prominent. Lake levels fell during the mid-Holocene and marsh environments extended over the core site. This interval is characterized by high levels of organic carbon from emergent aquatic vegetation (Scirpus) and by the Mazama ash (7.55 ka), generated by the eruption that created nearby Crater Lake. For a brief time the ash increased the salinity of Upper Klamath Lake. High concentrations of molybdenum, arsenic, and vanadium indicate that Caledonia Marsh was anoxic from about 7 to 5 ka. After the mid-Holocene, shallow, but open-water environments returned to the core site. The sediments became dominated (>80%) by biogenic silica. The open-water cores show analogous but less extreme limnological and climatic changes more typical of mid-lake environments. Millennial-scale lake and climate changes during OIS 3 at Upper Klamath Lake contrast with a similar record of variation at Owens Lake, about 750 km south. When Upper Klamath Lake experienced cold-climate episodes during OIS 3, Owens Lake had warm but wet episodes; the reverse occurred during warmer intervals at Upper Klamath Lake. Such climatic alternations apparently reflect the variable position and strength of the Aleutian Low during the mid-Wisconsin.  相似文献   

2.
The subsiding Upper Klamath Lake Basin contains sediments that were continuously deposited in a shallow, freshwater lake for more than 40 000 years. Well dated by radiometric methods and containing volcanic ashes of known age, these sediments constitute a valuable paleoclimate record. Sediment constituents and properties that reflect past climatic conditions in the area include pollen, diatoms, sediment geochemistry, and sediment magnetic properties. Many of these proxy measurements are also useful for comparing natural conditions in the lake to conditions following human settlement. Because of its location, the paleoclimate record from Upper Klamath Lake is valuable for comparisons to offshore marine records and as part of latitudinal transects of paleoclimate records along the west coast of the Americas.  相似文献   

3.
Diatom responses to 20th century climate-related environmental change were assessed from three high-elevation lakes in the northern Canadian Cordillera. Dominance of small benthic Fragilaria diatoms reflect the generally cold conditions with long periods of ice cover that have characterized these mountain lakes over at least the last ~300 years until the period of recent warming. At the turn of the 20th century, salient shifts in the diatom assemblages reveal individualistic limnological responses with the onset of climate warming trends in northwest Canada. At YK3 Lake, an oligotrophic, chemically dilute, alpine lake, increased representation of the planktonic Cyclotella pseudostelligera may reflect longer ice-free conditions and/or more stable thermal stratification. By contrast, in the more productive, alkaline lakes (BC2 and Deadspruce lakes), changes to more diverse assemblages of periphytic diatoms suggest greater benthic habitat availability, most likely associated with the enhanced growth of aquatic plants with lengthening of the growing seasons. In addition, diatom assemblages from these lakes suggest less alkaline conditions following the onset of 20th century climate warming. Continued alkalinity reduction throughout the 20th century is qualitatively inferred at the lower elevation, treeline lake (Deadspruce Lake), while greater representation of alkaliphilous Fragilaria diatoms after ~1950 suggested increased alkalinity at the alpine BC2 Lake. Our results confirm the sensitivity of diatoms from high-elevation mountain lakes to regional climate change in northwest Canada. Individualistic limnological responses to 20th century warming are potentially attributed to differences in their physical setting (e.g., bedrock geology, elevation, catchment vegetation) in this complex mountain environment.  相似文献   

4.
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter.  相似文献   

5.
A paleolimnological investigation of post-European sediments in a Lake Michigan coastal lake was used to examine the response of Lower Herring Lake to anthropogenic impacts and its role as a processor of watershed inputs. We also compare the timing of this response with that of Lake Michigan to examine the role of marginal lakes as early warning indicators of potential changes in the larger connected system and their role in buffering Lake Michigan against anthropogenic changes through biotic interactions and material trapping. Sediment geochemistry, siliceous microfossils and nutrient-related morphological changes in diatoms, identified three major trophic periods in the recent history of the lake. During deforestation and early settlement (pre-1845–1920), lake response to catchment disturbances results in localized increases in diatom abundances with minor changes in existing communities. In this early phase of disturbance, Lower Herring Lake acts as a sediment sink and a biological processor of nutrient inputs. During low-lake levels of the 1930s, the lake goes through a transitional period characterized by increased primary productivity and a major shift in diatom communities. Post-World War II (late 1940s–1989) anthropogenic disturbances push Lower Herring Lake to a new state and a permanent change in diatom community structure dominated by Cyclotella comensis. The dominance of planktonic summer diatom species associated with the deep chlorophyll maximum (DCM) is attributed to epilimnetic nutrient depletion. Declining Si:P ratios are inferred from increased sediment storage of biogenic silica and morphological changes in the silica content of Aulacoseira ambigua and Stephanodiscus niagarae. Beginning in the late 1940s, Lower Herring Lake functions as a biogeochemical processor of catchment inputs and a carbon, nutrient and silica sink. Microfossil response to increased nutrients and increased storage of biogenic silica in Lower Herring Lake and other regional embayments occur approximately 20–25 years earlier than in a nearby Lake Michigan site. Results from this study provide evidence for the role of marginal lakes and bays as nutrient buffering systems, delaying the impact of anthropogenic activities on the larger Lake Michigan system.  相似文献   

6.
Fossil diatom assemblages in a sediment core from a small lake in Central Kamchatka (Russia) were used to reconstruct palaeoenvironmental conditions of the late Holocene. The waterbody may be a kettle lake that formed on a moraine of the Two-Yurts Lake Valley, located on the eastern slope of the Central Kamchatka Mountain Chain. At present, it is a seepage lake with no surficial outflow. Fossil diatom assemblages show an almost constant ratio between planktonic and periphytic forms throughout the record. Downcore variations in the relative abundances of diatom species enabled division of the core into four diatom assemblage zones, mainly related to changes in abundances of Aulacoseira subarctica, Stephanodiscus minutulus, and Discostella pseudostelligera and several benthic species. Associated variations in the composition and content of organic matter are consistent with the diatom stratigraphy. The oldest recovered sediments date to about 3220 BC. They lie below a sedimentation hiatus and likely include reworked deposits from nearby Two-Yurts Lake. The initial lake stage between 870 and 400 BC was characterized by acidic shallow-water conditions. Between 400 BC and AD 1400, lacustrine conditions were established, with highest contributions from planktonic diatoms. The interval between AD 1400 and 1900 might reflect summer cooling during the Little Ice Age, indicated by diatoms that prefer strong turbulence, nutrient recycling and cooler summer conditions. The timing of palaeolimnological changes generally fits the pattern of neoglacial cooling during the late Holocene on Kamchatka and in the neighbouring Sea of Okhotsk, mainly driven by the prevailing modes of regional atmospheric circulation.  相似文献   

7.
Pollen and sediment from Grass Lake, California provide a history of vegetation and climate in the southern Cascade Range from 36 to 19 cal ka, revealing climate changes that led to the glacial advances recorded at Upper Klamath Lake (Rosenbaum and Reynolds 2004a – this issue). Variations in the percentages of conifer and Artemisia (sagebrush) pollen at Grass Lake recorded shifts in vegetation that reflect changes in precipitation. Between 36 and 34 cal ka, a progression from steppe to open pine forest to dense pine forest indicates that precipitation increased. After 32 cal ka, the forest became more open and by 30 cal ka sagebrush steppe surrounded the lake, implying that precipitation decreased. The area was arid for most of the interval between 30 and 19 cal ka. Increases in conifer pollen recorded increases in precipitation from 21 through 19 cal ka, when open pine forest colonized the lake area. Throughout the period from 36 to 19 cal ka, centennial- to millennial-scale intervals with increased conifer pollen imply that the arid interval was interrupted by periods of increased precipitation. Pollen data also provide evidence that the major fluctuations in sand concentration in the Grass Lake core reflect temperature shifts. Changes in sediment particle size are closely related to variations in pollen concentration and accumulation rate, which in turn reflect changes in plant cover, implying that sand was deposited in the lake due to deflation of clay- and silt-sized particles from sparsely-vegetated alpine areas of the watershed. Sand deposition increased as climatic cooling led to reductions in the elevation of upper treeline and alpine conditions affected a larger part of the watershed. There is no evidence of glaciation in the basin, but pollen data show the area was above upper treeline during Cold Period III (34–32 cal ka), one of several very cold intervals. Vegetation decreased at about 28 cal ka and remained sparse for at least 9000 years, implying that the climate became cooler and remained cool until after 19 cal ka. Cold Period II developed at about 25 cal ka and terminated by 23 cal ka. The Grass Lake watershed was again above upper treeline with the onset of Cold Period I, soon after 19 cal ka. Comparison of the Grass Lake record with those from Upper Klamath Lake, Oregon and Tulelake, California suggests a persistent pattern of environmental changes in this time interval throughout the Modoc Plateau region.  相似文献   

8.
Studies of magnetic properties enable reconstruction of environmental conditions that affected magnetic minerals incorporated in sediments from Upper Klamath Lake. Analyses of stream sediment samples from throughout the catchment of Upper Klamath Lake show that alteration of Fe-oxide minerals during subaerial chemical weathering of basic volcanic rocks has significantly changed magnetic properties of surficial deposits. Titanomagnetite, which is abundant both as phenocrysts and as microcrystals in fresh volcanic rocks, is progressively destroyed during weathering. Because fine-grained magnetite is readily altered due to large surface-to-volume ratios, weathering causes an increase in average magnetic grain size as well as reduction in the quantity of titanomagnetite both absolutely and relative to hematite. Hydrodynamic mineralogical sorting also produces differences in magnetic properties among rock and mineral grains of differing sizes. Importantly, removal of coarse silicate and Fe-oxide grains by sorting concentrated extremely fine-grained magnetite in the resulting sediment. The effects of weathering and sorting of minerals cannot be completely separated. These processes combine to produce the magnetic properties of a non-glacial lithic component of Upper Klamath Lake sediments, which is characterized by relatively low magnetite content and coarse magnetic grain size. Hydrodynamic sorting alone causes significant differences between the magnetic properties of glacial flour in lake sediments and of fresh volcanic rocks in the catchment. In comparison to source volcanic rocks, glacial flour in the lake sediment is highly enriched in extremely fine-grained magnetite.  相似文献   

9.
A combination of tephrochronology and 14C, 210Pb, and 137Cs measurements provides a robust chronology for sedimentation in Upper Klamath Lake during the last 45 000 years. Mixing of surficial sediments and possible mobility of the radio-isotopes limit the usefulness of the 137Cs and 210Pb data, but 210Pb profiles provide reasonable average sediment accumulation rates for the last 100–150 years. Radiocarbon ages near the top of the core are somewhat erratic and are too old, probably as a result of detrital organic carbon, which may have become a more common component in recent times as surrounding marshes were drained. Below the tops of the cores, radiocarbon ages in the center of the basin appear to be about 400 years too old, while those on the margin appear to be accurate, based on comparisons with tephra layers of known age.Taken together, the data can be combined into reasonable age models for each site. Sediments have accumulated at site K1, near the center of the basin, about 2 times faster than at site CM2, on the margin of the lake. The rates are about 0.10 and 0.05 cm/yr, respectively. The chronological data also indicate that accumulation rates were slower during the early to middle Holocene than during the late Holocene, consistent with increasing wetness in the late Holocene.  相似文献   

10.
Water chemistry and surface sediments were analyzed from 41 shallow lakes representing three previously-defined hydrological categories in the Slave River Delta, Northwest Territories, Canada, in order to identify relationships between hydrological and limnological conditions and their associations with recently deposited diatom assemblages. Evaporation-dominated lakes are physically removed from the influence of the Slave River, and are characterized by high alkalinity and high concentrations of nutrients and ions. In contrast, flood-dominated lakes tend to receive a pulse of floodwater from the Slave River during the spring thaw and have low alkalinity and low concentrations of most nutrients and ions. Exchange-dominated lakes are variably influenced by floodwaters from the Slave River and seiche events from Great Slave Lake throughout the spring thaw and open-water season, and are characterized by a broad array of limnological conditions that are largely dependent on the strength of the connection to these sources of floodwater. Specific diatom ‘indicator’ taxa have been identified that can discriminate these three hydrological lake categories. Evaporation-dominated lakes are associated with high relative abundance of common epiphytic diatom taxa, while diatoms indicative of flood- and exchange-dominated lakes span a wide range of habitat types (epiphytic, benthic) but also include unique planktonic diatoms (Stephanodiscus and Cyclostephanos taxa) that were not found in surface sediments of evaporation-dominated lakes. The planktonic diatom taxa originate from the Slave River, and thus are indicative of river influence. In complex, remote, freshwater ecosystems like the Slave River Delta, integration of results from hydrological and limnological approaches provides a necessary foundation to assess present, past and future hydroecological responses to changes in river discharge and climate.  相似文献   

11.
Paleolimnological analyses were used to infer limnological changes during the past ~ 300 yrs in the west basin of Peninsula Lake, a small (853 ha) Precambrian Shield lake in Ontario, Canada, that has been subjected to moderate cultural disturbances (forest clearance, cottage and resort development). This study represents a pioneering attempt to use sedimentary chironomid assemblages and weighted-averaging models to quantify past hypolimnetic anoxia (expressed as the anoxic factor, AF). Impacts of forest clearance and human land-use on deepwater oxygen availability and surface water quality were assessed by comparing chironomid-inferred AF and diatom-inferred total phosphorus concentration ([TP]) to changes in terrestrial pollen and historical data. This study also discusses the ability of chironomids to quantitatively infer changes in AF.Pre-disturbance chironomid assemblages were stable and dominated by taxa indicative of oxygen-rich hypolimnetic conditions (e.g., Protanypus, Heterotrissocladius, Micropsectra type), while diatoms indicated oligotrophic lake status (diatom inferred [TP] = 5-7 g·l-1). Chironomids characteristic of lower oxygen availability (e.g., Chironomus, Procladius) increased following land-clearance, road construction, establishment of a grist mill and lakeshore development beginning ca. 1870. Increased abundances of Tanytarsus s. lat., a multigeneric group of mainly littoral chironomids, since 1900, indicated that littoral chironomids may have comprised a greater proportion of fossil assemblages during periods of eutrophication and prolonged anoxia. Abundances of meso-eutrophic diatom taxa (e.g., Fragilaria crotonensis, Asterionella formosa, Aulacoseira ambigua, A. subarctica) increased concurrent with European settlement (ca. 1870) and diatom-inferred [TP] doubled (~ 6-12 g·l-1), further indicating that naturally-oligotrophic Precambrian Shield lakes were extremely sensitive to initial land-clearance activities.Recent increases in oligotrophic diatom taxa (e.g., Cyclotella stelligera) indicate a shift to more oligotrophic conditions since ca. mid-1960s, with greatest changes since ca. 1980. The chironomids Heterotrissocladius and Micropsectra type also increased at this time suggesting greater deepwater oxygen availability. These recent water-quality improvements, possibly in response to enhanced nutrient removal from detergents and sewage, climate-related reductions in external phosphorus loads, and catchment (but not lake) acidification and reforestation, suggest that habitat for commercially-valuable cold-water fishes has improved in recent decades despite greater recreational lake-use.Paleolimnological assessment of trophic status changes in Peninsula Lake using fossil diatom and chironomid assemblages were in good agreement. Diatom inferences of [TP] and chironomid inferences of AF both suggest that Peninsula Lake was historically oligotrophic, became oligo-mesotrophic after European settlement, and returned to oligotrophy in recent yrs. Chironomid inferences of [TP] consistently underestimated the trophic status of Peninsula Lake, possibly due to its relatively large hypolimnion. These results suggest that AF represents a useful tool for quantitatively reconstructing the past trophic status of deeper, stratified lakes.  相似文献   

12.
Postglacial sediments from Lake Jues, located at the SW margin of the Harz Mountains, were investigated for pollen, diatoms and sediment characteristics. The paper focuses on the time period between 7600 and 1200 y cal. BP (calibrated years before present), during which human impact began to influence the environnment of the lake. Climate variability was mainly inferred from sediment characteristics and changes in algal assemblages. The observed climatic changes coincide well with those recorded from other sites. Neolithic settlement started during the warm and dry Atlantic period. Intensive land use occurred under favourable climatic conditions during the Bronze and the Iron Ages, while human activities declined during cool and wet periods around 4000 and 2700 y cal. BP and after 2300 y cal. BP. The study shows that climate strongly influenced human settlement at remote locations like Lake Jues.  相似文献   

13.
Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Zmax 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to 137Cs and 210Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year−1 during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water  相似文献   

14.
Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l−1 in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km2. Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.  相似文献   

15.
Frozen sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high resolution decadal to annual scale record of changing lake paleoenvironments and geochemistry that reflects changing landuse and hydrology in the catchment over the past c. 190 years. A reliable chronology is available from AMS 14C and 210Pb dating of the sediments, with the timing of the older part of the record supported by the first appearance of pollen of introduced Pinus and Plantago lanceolata associated with European settlement of Auckland in the early 1840s. Diatom stratigraphy, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions commensurate with European development of the Lake Pupuke catchment, in particular enhanced algal productivity controlled by the influx of nutrients after c. 1920 AD. Attempts to prevent nuisance algal blooms in 1933, 1934 and 1939 using CuSO4 addition produced Cu spikes in the sediment that allowed confirmation of the accuracy of the 210Pb chronology. Hence, the elemental and isotopic composition of the Lake Pupuke sediments reflect the timing of significant anthropogenic activities, rather than climatic variations, that have occurred within the watershed over the past c. 190 years. The comparison of records of land use change in the catchment with the multi-proxy record of changes in the sediments usually allowed unambiguous identification of the signatures of change and their causes.  相似文献   

16.
The floodplain of the Upper Paraná River, Brazil, is strongly influenced by hydrology, which in turn affects geomorphological and environmental conditions, and controls the form of islands in the river. Such islands develop by deposition of river-borne sediment that creates small lateral sediment bars. Geomorphological processes can produce a variety of aquatic environments on such islands, e.g. channels, backwaters, lakes, transitional areas, and swamps. Our objective was to test whether subfossil diatoms preserved in the sediment on an island in the Upper Paraná River floodplain responded to changes in limnological conditions brought about by such geomorphological modifications. We hypothesized that the composition of diatom assemblages in the sediment shifted in response to past geomorphic, and hence limnological conditions. We analyzed diatom subfossils in a 2-m-long sediment core with a calibrated date near the base of 1047–1224 cal yr AD. Absence of diatoms at the bottom of the sequence was associated with the channel phase, followed by appearance of diatoms 1229–1381 cal yr AD that were adapted to flow, in the backwater phase. After another 100–200 years, presence of Eunotia species in the lake phase suggests a decrease in pH, phosphorus and nitrogen. Replacement of Eunotia spp. by Diadesmis species, following a transition phase, suggests different environmental conditions, with reduced water depth. Diatoms in surface deposits are distinct from assemblages in the other phases in the core and contain taxa that suggest a disturbed environment, with variations in water depth and flow. The data illustrate the importance of physical and hydrological factors in shaping diatom communities and show the utility of diatoms as bioindicators in this floodplain environment.  相似文献   

17.
Lake Mattamuskeet, North Carolina, USA is a large (162 km2) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D. 360 and A.D. 1435 (calibrated 14C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased, and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can respond rapidly to climate change and human development.  相似文献   

18.
Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-1 throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.  相似文献   

19.
选用不同年代的地形图、以及遥感图象数据等信息源,用地理信息系统以及景观指数分析方法对射阳湖湖沼环境的动态变化进行了初步研究。结果表明,射阳湖的面积逐年减少,从1958年到1974年,射阳湖的面积减少了153.245km2,年均递减9.578km2,这两个时期的年均面积递减率是最大的。景观指数分析表明射阳湖湖沼湿地的破碎化程度和空间异质性越来越高。大量水利工程的修建导致上游来水减少是射阳湖面积减小的主要原因,滩地围垦、水产养殖业以及居民地扩展等是其变化的主要因素。  相似文献   

20.
Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.  相似文献   

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