首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
The natural eutrophication of lakes is still an accepted concept in limnology, arising as it does from the earliest efforts to classify lakes and place them in an evolutionary sequence. Recent studies of newly formed lakes at Glacier Bay, Alaska, only partially support this idea, and suggest more variable trends in lake trophic development which are under local (catchment-level) control. Here we use sediment cores from several lakes in Glacier Bay National Park to examine the relationship between successional changes in catchment vegetation and trends in water-column nitrogen (a limiting nutrient) and lake primary production. Terrestrial succession at Glacier Bay follows several different pathways, with older sites in the lower bay being colonized directly by spruce (Picea) and by-passing a prolonged alder (Alnus) stage that characterizes younger upper-bay sites. Sediment cores from three sites spanning this successional gradient demonstrate that the variability in nitrogen trends among lakes is a consequence of the establishment and duration of N-fixing alder in the lake catchment. In the lower-bay lakes, diatom-inferred nitrogen concentrations rise and then fall in concert with the transient appearance of alder in the catchment, while in the upper bay, high nitrogen concentrations are sustained by the continuous dominance of alder. Diatom accumulation, a proxy for whole-lake biological productivity, increases steadily at all three sites during the first century following lake formation, but declines in more recent times at the lower-bay sites in apparent response to the disappearance of alder and decreasing lake-water nitrogen. These results demonstrate a tight biogeochemical coupling between terrestrial succession and lake trophic change during the early developmental history of Glacier Bay lakes.  相似文献   

2.
Sediment diatom and chemical analyses of cores from three poorly buffered extra-glacial lakes on the northeastern margin of the Canadian Shield (Cumberland Peninsula, Baffin Island) record interactions between aquatic and terrestrial spheres that were influenced by late Quaternary climatic conditions. Although differences exist between each of the lakes, notably with regards to the intensity of pre-Holocene catchment erosion and the timing of the onset of organic sedimentation, an underlying pattern of lake ontogeny, common to all three lakes, is identified. Although intensified watershed erosion characterized the Late Wisconsinan and Neoglacial cold periods, the lakes nonetheless remained viable ecosystems at these times. Sudden catchment stabilization during the late-glacial to earliest Holocene is associated with incipient organic sedimentation. Lake-water pH increased at this time, likely in response both longer base cation residence times as lake flushing rates decreased, and enhanced alkalinity production from sediment biogeochemical reactions. Subsequently, as the catchments remained stable during the productive early Holocene (c.9–7 ka BP), then gradually received a renewed increase of minerogenic sedimentation, the breakdown of sources of lake alkalinity resulted in natural acidification. Burial of cation-rich mineral sediments and the loss of permanent sedimentary sinks for the products of microbial reduction likely impeded within-lake alkalinity production, and catchment-derived base cations appeared ineffective in curtailing pH declines. The general nature of the Holocene development of these lakes is similar to that observed elsewhere on crystalline terrains, following deglaciation. Our data therefore suggest that catchment glaciation is not a necessary precursor for models of lake development characterized by initial base cation enrichment and subsequent gradual acidification.  相似文献   

3.
Analysis of the distributions of chironomid (midge) and other dipteran subfossils from two high elevation lake sediment cores in the Cascade Mountains reveals changes in midge communities and inferred climate since the late-glacial. Cabin Lake and 3M Pond are located near treeline in the subalpine Engelmann Spruce/Subalpine Fir biogeoclimatic zone of British Columbia. In Cabin Lake, chironomid head capsule assemblages depict a typical late-glacial community, and three distinct Holocene communities. In Cabin Lake, the late-glacial community is composed of cold-stenothermous taxa dominated by Stictochironomus, Mesocricotopus, Heterotrissocladius, Parakiefferiella nigra, Protanypus and Paracladius, whereas warm water midges are absent or rare, indicating cold conditions. A late-glacial chironomid community was not found in 3M Pond. In both lakes the early Holocene is dominated by a diverse warm-adapted assemblage, corresponding to the warm climatic conditions of the xerothermic period. Cabin Lake's mid-Holocene zone records a decrease in relative abundance of the warm water types and is accompanied by an increase in cold-stenotherms. At 3M Pond this period shows a dramatic loss in diversity of warm-adapted taxa, as the temperate genus Dicrotendipes dominates. This zone corresponds to Hebda's (1995) mesothermic period. Further cooling in the late Holocene (to modern conditions) is inferred from continued reduction of warm water midges and persistence (at Cabin Lake) or appearance (at 3M Pond) of a cold-stenothermal community. This late Holocene cooling is similar in timing to Neoglacial advances in the Coast, Cascade, and Rocky Mountains of southern British Columbia. Similarities in the timing of chironomid and vegetation community changes at these high elevation sites, along with the more rapid response time of the Chironomidae, support the sensitivity of midges to postglacial climatic change at high elevation sites.  相似文献   

4.
Based on extensive data from a long-term investigation, a new genetic classification of lake basins is proposed for Estonia. Eight lake groups are distinguished, tectonic-denudation, glacial, chemical, fluvial, coastal (neotectonic), telmatogenic, cosmogenic and artificial, containing 13 subgroups and 19 basin types. Also proposed is a new lithological classification of Estonia's organic and calcareous lake sediments, based on analyses of more than 2000 sediment samples from 90 contemporary and 50 late-glacial (extinct) lakes. Of the ca. 1150 Estonian lake basins that formed on mineral substrate, the two largest basins are of preglacial, tectonic-denudation origin, later modified by glaciers. Eight hundred lakes are of glacial origin, and 300 of other origins in the Holocene. In addition, ca 20 000 bog pools formed on peat in the Holocene. Only minerogenous sedimentation occurred in the lakes in the late-glacial period. After that, organic (gyttjas) and/or calcareous sediments have formed. Azonal factors have been largely responsible for the wide variation in Estonia's lacustrine deposits.This article belongs to a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series.  相似文献   

5.
Chironomids typical of cold, well-oxygenated, oligotrophic environments are common in late-Pleistocene deposits, but these taxa are rare in Holocene sediments of most small temperate lakes. Hypotheses to explain the demise of these taxa include variations in climate, lake trophic state, lake levels, terrestrial vegetation, and/or sediment composition. In southwestern British Columbia, this demise correlates with palynological evidence for a lodgepole pine decline, and for rapid climatic amelioration, at about 10 000 yr B.P. Faunal changes are poorly correlated with lithological boundaries. The similar timing of the declines among lakes suggests that a regional influence, climate, has possibly been the principal determinant of early chironomid faunal succession.This is the eleventh of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R.B. Davis for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series.  相似文献   

6.
A surface-sediment survey of pigments in 100 lakes in the Scandes Mountains, northern Sweden, was combined with a reconstruction of Holocene sedimentary pigments from Lake Seukokjaure to assess the major factors regulating phototrophic communities, and how these controls may have changed during the period from the deglaciation (~9700 cal. years BP) to the present. The study area covers a pronounced gradient of temperature and precipitation, and encompasses the subarctic tree line, an important ecotonal boundary in this region. Lake Seukokjaure is located in a presently treeless basin close to the modern tree line. The spatial survey of sedimentary pigments was analyzed using principle components analysis (PCA) and redundancy analysis (RDA). PCA explained 73–83% of variance in pigment abundance and composition, whereas RDA explained 22–32% of variation in fossil assemblages. Dissolved organic carbon (DOC) content of lake water, sediment δ13C, maximum lake depth, elevation and lake-water conductivity were all identified as environmental variables with significant association with pigment abundances in the spatial survey, although phototrophic communities of lakes situated in different vegetation zones (alpine, birch, conifer/birch) were incompletely distinguished by the ordinations. In the RDAs, the primary pigment variability occurred along a production gradient that was correlated negatively to water-column DOC content and δ13C signature of sediments. This pattern suggested that the important controls of primary production were light regime and terrestrial supplies of 13C-depleted carbon. In contrast, depth, elevation and conductivity were found to be more important for the differentiation of the phototrophic community composition. Application of these spatial survey results to the Holocene sediment record of Lake Seukokjaure demonstrated the importance of DOC for the temporal development of the lake, from an early state of high production to a period of slight oligotrophication. In general, the algal changes were regulated by the interaction of DOC and conductivity, although transitions in the phototrophic community during the late Holocene were less easily interpreted. Terrestrial vegetation development thus appears to be of utmost importance for the regulation of primary production in oligotrophic alpine and subarctic lakes and climate impacts on lakes, whereas other basin-specific factors may control the ontogeny of algal community composition.  相似文献   

7.
The Holocence paleolimnology of Lake Sämbosjön is described using geochemical and diatom analyses. The objective of this study is the reconstruction of major changes in trophic state and productivity, and to interpret the major causative processes. The accumulation of organic matter indicates a relatively high productivity in early Holocene, and the diatom analysis indicates a relatively high trophic state and pH. A succeeding decrease in productivity and trophic state and lowering in pH is recorded from about 8000 BP. If lake development had been primarily edaphically conditioned, viz. determined by nutrient supply from catchment soils, such a progressive oligotrophication would represent the common development of temperate lakes. Between about 6000 BP and 4000 BP Lake Sämbosjön was characterized by relatively stable productivity and pH. From about 4000 BP the analyses reveal an increase again in trophic state, productivity, and in pH. This eutrophication, which continued throughout the late Holocene, was caused by an exceptionally strong human influence on the catchment of Lake Sämbosjön. The increased supply of nutrients from cleared and deforested catchment soils changed the trophic state and provided the basis for increased lake productivity.  相似文献   

8.
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.  相似文献   

9.
Chrysophyte cysts preserved in recent and pre-industrial lake sediment samples from 54 Muskoka-Haliburton (Ontario) lakes were used in a paleolimnological study to determine the impact of acidic precipitation and cottage development on water quality. A total of 246 cyst morphotypes were identified. Ecological preferences of cyst morphotypes were determined using multivariate statistical analysis, cluster analysis, and species-environment correlations. Recent cyst assemblages were related to water chemistry and lake morphometric variables using Redundancy Analysis (RDA). The distribution of morphotypes was related to a gradient of acid neutralising capacity (ANC), expressed through the association of variables related to buffering (i.e. longitude, watershed area, and ionic concentration) with the first axis (1 = 0.29). Cyst assemblages were also defined, to a lesser extent (2 = 0.06), by a trophic status gradient, created through the combination of total nitrogen (TN), total phosphorus (TP), volume-weighted cottage density, and lake depth variables. The identification of lakewater pH and trophic status as important determinants of cyst assemblage structure allowed for the reconstruction of acidification and eutrophication related water chemistry changes using fossil cyst assemblages. The reconstruction of pre-industrial (pre-1850) water quality conditions with fossil cyst assemblages indicated that pH significantly decreased in 24.1% of the study lakes and increased in 16.7% of the lakes. Increases in pH in more alkaline drainage basins are attributed to alkalinity generation processes induced by acidic precipitation as has been shown in other studies. Total phosphorus (TP) concentrations significantly declined in 12.9% of the lakes and increased in 16.6% of lakes. Increases in [TP] were linked to cottage development. Decreases in trophic status may be due to landuse changes, the result of the acidification occurring in the area, or warmer and drier climates. A comparison of chrysophyte cyst and diatom water quality inferences show similar trends in pH changes. There is a good agreement between diatom and chrysophyte bioindicators with respect to [TP] changes in oligotrophic lakes (< 10 g/L); however, diatom inferences suggest that lakes with current [TP] values greater than 10 g/L have decreased in trophic status over time, while chrysophyte reconstructions suggest that these same lakes have become more productive systems.  相似文献   

10.
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 14C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 14C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 14C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 14C yrs BP and remained low until 3000 14C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.  相似文献   

11.
Medicine Lake is a highly saline meromictic lake in eastern South Dakota. A lithostratigraphic and biostratigraphic study of the late-glacial and early post-glacial sediments shows that it was a relatively deep dilute freshwater lake during the period of the Picea pollen zone. When spruce forest changed to a Betula and then to a Quercus/Ulmus woodland in the early post-glacial period, the lake water became more concentrated but remained fresh. However, during the subsequent rapid transition to prairie in the early Holocene, when Gramineae, Ambrosia, and Artemisia dominated dry-land vegetation, the freshwater diatom flora was progressively replaced by a saline assemblage characterized by Cyclotella quillensis, Chaetoceros, and eventually Cyclotella caspia. The lake became permanently saline at about 9000 yr BP. A comparison of the fossil diatom assemblages with surface-sediment samples from a range of lakes in the Dakotas indicates that the change involved an increase in conductivity from about 500 S cm–1 in the late-glacial period to > 10 000 S cm–1 in the early Holocene. This rapid change is also marked by an abrupt increase in sulphate concentration in the sediment, the first appearance of bands of gypsum crystals, and the beginning of a well-laminated core sequence that continues through the remaining sediment record. Conditions of high salinity have prevailed to the present day.Contribution 299, Limnological Research Center.  相似文献   

12.
To investigate the impact of forest harvesting on lake ecosystems, six lakes (four impact, two reference) from central British Columbia, Canada were assessed using cladoceran remains preserved in the lake sediment cores. Two temporal resolutions were analyzed: a decadal scale for the past 70 years, and a high-resolution scale (2 year intervals for ten years before and after harvesting). Three lakes which experienced forestry activity in their watersheds in the early 1960s, and one lake which experienced forestry activityin the mid 1970s, showed subtle but statistically significant changes in cladoceran species composition following forest harvesting (analysis of similarity tests) at the decadal scale, whereas only two of these lakes showed significant changes at the higher resolution. These changes may be due to increased nutrient levels that might be associated with forest harvesting. The two reference lakes, which had not experienced any known large-scale anthropogenic watershed disturbances in the past century, showed no significant shifts in the cladoceran species assemblages at either temporal scale.  相似文献   

13.
Paleoenvironmental studies have documented the late Pleistocene to Holocene evolution of the lakes in the central and southern parts of the basin of Mexico (Texcoco and Chalco). No information was available, however, for the lakes in the north-eastern part of this basin. The north-eastern and the central and southern areas represent, at present, different environmental conditions: an important gradient exists between the dry north and the moister south. To investigate the late Pleistocene to Holocene characteristics of the north-eastern lakes in the basin of Mexico two parallel cores (TA and TB) were drilled at the SE shore of Lake Tecocomulco. Stratigraphy, magnetic properties, granulometry, diatom and pollen analyses performed on these sediments indicate that the lake experienced a series of changes between ca. > 42,000 yr BP and present. Chronological control is given by five radiocarbon determinations. The base of the record is represented by a thick, rhyolitic air-fall tephra that could be older than ca. 50,000 yr BP. After this Plininan event, and until ca. 42,000 yr BP, Lake Tecocomulco was a moderately deep, freshwater lake surrounded by extended pine forests that suggest the presence of cooler and moister conditions than present. Between ca. 42,000 and 37,000 yr BP, the lake became shallower but with important fluctuations and pollen suggests slightly warmer conditions. Between ca. 37,000 and 30,000 yr BP the lake experienced two relatively deep phases separated by a dry interval. A second Plinian eruption, represented in the sequence by a dacitic an air-fall tephra layer dated at 31,000 yr BP, occurred in the area by the end of this dry episode. Between ca. 30,000 and 25,7000 yr BP Tecocomulco was a fresh to slightly alkaline lake with a trend towards lower level. After ca. 25,700 yr BP very low lake levels are inferred, and after ca. 16,000 yr BP the data indicate the presence of a very dry environment that was persistent until the middle Holocene. After 3,500 yr BP lacustrine conditions were re-established and the vegetation cover shows a change towards higher percentages of herbaceous taxa.  相似文献   

14.
Alpine lakes near Mount Rainier are dammed by late-glacial moraines. Multiple cores show that sedimentation started less than 350 years after deglaciation. Numerous tephra layers provide excellent marker horizons that allow comparison of the stratigraphy without the errors associated with radiocarbon dating. Cores from different locations within a lake display the same stratigraphy and basal sediments. Core location therefore is not crucial to obtaining the oldest sediments. Continuous sedimentation in all cores is interpreted as evidence for stable lake levels throughout the Holocene. The cores are valuable tools for reconstruction and dating of glacier advances and volcanic activity.  相似文献   

15.
The Kråkenes late-glacial palaeoenvironmental project   总被引：1，自引：0，他引：1  
Kråkenes is the site of a small lake on the west coast of Norway that contains a long sequence of late-glacial sediments. The Younger Dryas is well represented, as a cirque glacier developed in the catchment at this time. This site offers unique opportunities to reconstruct late-glacial environments from independent sources of evidence; physical evidence (glacial geomorphology, sedimentology, palaeomagnetism, radiocarbon dating), and biological evidence from the remains of animals and plants derived from both the terrestrial and aquatic ecosystems. This report describes the background to the site, and the international multidisciplinary project to reconstruct late-glacial and early Holocene environmental and climatic changes at Kråkenes.  相似文献   

16.
The 18 papers in this Special Issue of the Journal of Paleolimnology report new records of Holocene environmental and climate change from Arctic lake sediment. At least 15 distinct physical, chemical, and biological properties were analyzed at lakes located across the North American Arctic and subarctic, and northwestern Europe. The studies are notable for their multi-proxy approach (eight present data for at least five different proxies), and for the high quality of their geochronological control. Three of the studies analyzed sediment from more than one lake to test the influence of contrasting physiographic settings on the response of proxies to the same climate forcing. The sedimentary sequences analyzed in seven studies extend beyond 11.5?cal?ka, providing evidence for pronounced climate shifts that took place during the late-glacial period. Two-thirds extend beyond 8?cal?ka; many of these records were interpreted in terms of the shift in temperature and moisture that occurred during the transition from the warm early to middle Holocene to the cooler late Holocene. These records contribute to the growing network of sites that is needed to reconstruct the spatial pattern of this pronounced paleoclimate transition, and to address how ocean-atmospheric circulation changed with the mean?state of climate.  相似文献   

17.
Thirty-six Coleoptera (beetle) taxa and other insects were identified from the late-glacial and early-Holocene sediments at Kråkenes Lake. Compared with other Scandinavian late-glacial sites, this is a rather sparse record. The water beetles found in the Allerod are characteristic of a poorly vegetated clear-water lake. The terrestrial fauna is indicative of dwarf-shrub and moss vegetation. A marked decline in the number of species at the start of the Younger Dryas was rather rapid, probably over less than 80 calendar yrs. No obligate tundra species replaced the Allerod fauna. Most of the Younger Dryas is virtually devoid of beetles. The increase in numbers and diversity of both aquatic and terrestrial species at the Younger Dryas/Holocene transition is very rapid. After an initial pioneer stage, beetles associated with dwarf-shrub heath and willow scrub appeared, but no obligate tree or forest taxa were recorded.Mutual Climatic Range (MCR) temperature reconstructions suggest that the Allerod was colder and more continental than present. The near absence of beetles in the Younger Dryas probably reflects very cold conditions. A rapid temperature rise at the start of the Holocene resulted in a warmer and more continental climate than present.  相似文献   

18.
Analyses of lithology, organic-matter content, magnetic susceptibility, and pollen in a sediment core from Okpilak Lake, located in the northeastern Brooks Range, provide new insights into the history of climate, landscape processes, and vegetation in northern Alaska since 14,500?cal?year BP. The late-glacial interval (>11,600?cal?year BP) featured sparse vegetation cover and the erosion of minerogenic sediment into the lake from nearby hillslopes, as evidenced by Cyperaceae-dominated pollen assemblages and high magnetic susceptibility (MS) values. Betula expanded in the early Holocene (11,600?C8,500?cal?year BP), reducing mass wasting on the landscape, as reflected by lower MS. Holocene sediments contain a series of silt- and clay-dominated layers, and given their physical characteristics and the topographic setting of the lake on the braided outwash plain of the Okpilak River, the inorganic layers are interpreted as rapidly deposited fluvial sediments, likely associated with intervals of river aggradation, changes in channel planform, and periodic overbank flow via a channel that connects the river and lake. The episodes of fluvial dynamics and aggradation appear to have been related to regional environmental variability, including a period of glacial retreat during the early Holocene, as well as glacial advances in the middle Holocene (5,500?C5,200?cal?year BP) and during the Little Ice Age (500?C400?cal?year BP). The rapid deposition of multiple inorganic layers during the early Holocene, including thick layers at 10,900?C10,000 and 9,400?C9,200?cal?year BP, suggests that it was a particularly dynamic interval of fluvial activity and landscape change.  相似文献   

19.
The sediment record from a 5.3-m core from Sargent Mountain Pond, Maine USA indicates strong co-evolutionary relationships among climate, vegetation, soil development, runoff chemistry, lake processes, diatom community, and water and sediment chemistry. Early post-glacial time (16,600–12,500 Cal Yr BP) was dominated by deposition of mineral-rich sediment, low in organic matter and secondary hydroxides of Al and Fe; pollen indicate tundra conditions; diatom taxa indicate pH between 7.5 and 8, and total P concentrations of about 25 μg L−1, favoring higher productivity. Chemical weathering was rapid, with high alkalinity, pH, Ca, and P in runoff. As climate ameliorated, about 12,500 Cal Yr BP, forest vegetation became established; soils would have developed vertical zonation, including organic matter accumulation, and incipient podzolic horizons, with accumulating secondary hydroxides of Al and Fe that sequestered P in the soils. Labile minerals (primarily apatite, Ca5(PO4)3(OH,F,Cl)) became depleted in the soil, further reducing the supply of P to the lake. Dissolved organic carbon (DOC) from soil organic matter mobilized Al and Fe to the lake where Al(OH)3 (primarily) and Fe(OH)3 (minor) were precipitated. The sedimenting hydroxides adsorbed P from the water column, further reducing bioavailable P. These long-term trends of moderating climate, and changing terrestrial biology, soils, and aquatic chemistry and phytoplankton were interrupted by the 1,000-year long Younger Dryas cooling, which led to a temporary reversal of these processes, a period that ended with the major onset of Holocene warming. The sequestration of P by soils would have strengthened because of long-term soil acidification and pedogenesis. The lake was transformed from a more productive, high P, high pH, low DOC system into an oligotrophic, relatively low P, acidic, humic lake over a period of 16,600 years, a natural trend that continues. In contrast to many human-affected lakes that become increasingly eutrophic, many lakes become more oligotrophic during their history. The precursors for that are: (1) absence of human land-use in watersheds, (2) bedrock lithology and soil with a paucity of soluble Ca-rich minerals, and (3) vegetation that promotes the accumulation of soil organic matter, podzolization, and increased export of metal-DOC complexes, particularly Al.  相似文献   

20.
Three cores from two connected lakes in Central Ireland (Lough Kinale and Derragh Lough) were investigated using diatom analysis to establish the Holocene development of the lacustrine system, any local variations within the lakes and any anthropogenic influences. The study area was situated in a lowland location and the lakes were shallow, unstratified and interconnected. Litho-and bio-stratigraphical analyses of the lake cores and deposits beneath a mire separating the two lakes showed the changing spatial configuration of the lake system in the early Holocene and the separation of the initial lake into three basins (cf. lacustrine cells) and finally into two interlinked lakes. The evolution of the lake system is conceptualised as the development of distinct lacustrine cells, and its sediments have recorded changes in the physical (geography, depth and sedimentation) and chemical (water chemistry) properties of the lakes inferred through diatom analyses. The longest sequence, from the early Holocene, records fluctuating lake levels and these are correlated with geomorphological mapping and surveying of palaeoshorelines. The diatom assemblages of the upper 2 m of the three cores, covering approximately the last 2000–3000 radiocarbon years show considerable difference in trophic status and life-form categories. This is related to the location of the cores in the lake and also the distance from human settlement with particular reference to proximity to crannog (artificial island) construction and use. The most central core from the deepest part of Lough Kinale has the least representation of the human settlement and agricultural activity in the catchment and on the fringes of the lake, whereas the core taken from the edge of a crannog is able to identify when construction and use of the crannog occurred. The local nature of the palaeoecological response to human activity due to incomplete water mixing has the advantage of allowing the lake sediment cores to be used to determine spatially discrete settlement patterns.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号