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1.
Using molecular genetic methods and an ancient DNA approach, we studied population and species succession of rotifers of the genus Brachionus in the Kenyan alkaline-saline crater lake Sonachi since the beginning of the 19th century as well as distribution of Brachionus haplotypes in recent and historic sediments of other lakes of the East African Rift System. The sediment core record of Lake Sonachi displays haplotypes of a distinct evolutionary lineage in all increments. Populations were dominated by a single mitochondrial haplotype for a period of 150 years, and two putatively intraspecific turnovers in dominance occurred. Both changes are concordant with major environmental perturbations documented by a profound visible change in sediment composition of the core. The first change was very abrupt and occurred after the deposition of volcanic ash at the beginning of the 19th century. The second change coincides with a major lake level lowstand during the 1940s. It was preceded by a period of successively declining lake level, in which two other haplotypes appeared in the lake. One of these putatively belongs to another species documented in historical and recent Kenyan lake sediments. The analysis of plankton population dynamics through historical time can reveal patterns of population persistence and turnover in relation to environmental changes.  相似文献   

2.
To clarify long-term variations in the resting egg production of Daphnia galeata in Lake Biwa during the 20th century, we examined an abundance of plankton remains and ephippia in a 26-cm sediment core with a time resolution of approximately 2–6 years. Historical changes shown by these plankton remains indicated that the Daphnia population has not produced resting eggs since the 1980s, but it has remained the most abundant zooplankton species in the lake. Plankton monitoring data collected from 1966 to 2000 revealed that the overwintering individuals (January–March) of D. galeata plankters showed a significant increasing trend in recent years, such increase being negatively correlated with ephippial abundance in the sediment samples. Further analyses showed that the dominant phytoplankton in winter has changed from large inedible diatoms to edible flagellated algae, probably due to changes in vertical mixing regimes resulting from winter warming. These changes that occurred in the last several decades suggest that global warming has improved winter food conditions and thus enabled the Daphnia plankton to maintain its population without producing resting eggs in Lake Biwa.  相似文献   

3.
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4.
Cladoceran remains were analysed in a 1344 cm long sediment core from Sägistalsee (Swiss Alps, 1935 m asl) which covered the last 9000 years. Planktonic Cladocera were almost exclusively represented by Daphnia species, which occurred throughout the core. The chydorid fauna consisted of four species: Alona quadrangularis, Alona affinis, Acroperus harpae and Chydorus sphaericus of which the former was by far the most frequent species. The chydorid succession was characterised by disappearance and re-appearance of Acroperus harpae and Chydorus sphaericus at about 8400 and 3340 cal. BP, respectively. As a result, there was a long period of about 5000 years in which only two chydorid species were present with strong predominance (88.9%) of Alona quadrangularis. There was also a long-term trend of an increase of Alona affinis at the expense of Alona quadrangularis throughout the core.  相似文献   

5.
We retrieved a lake sediment record from an oligosaline, meromictic lake in southwest Greenland. The record spans the last 8,200 cal. years and was radiocarbon dated and analysed for macroscopic remains of plants and animals. The record extends the known history of several invertebrate species back in time, and provides minimum ages for their immigration to Greenland after the last deglaciation. Shells of the ostracod Ilyocypris bradyi were found in sediments dated to the time interval c. 7,000–6,500 cal. years BP. Shells of this species were found previously in a nearby oligosaline lake, where its occurrence was dated to about the same short time interval. The species is a thermophilous, non-arctic taxon that is absent from the present day Greenland fauna, and we suggest that its former occurrence in west Greenland marks the peak of the Holocene thermal maximum. This is in agreement with other records from Greenland.  相似文献   

6.
The multidisciplinary study of sediment cores from Laguna Zoñar (37°29′00′′ N, 4°41′22′′ W, 300 m a.s.l., Andalucía, Spain) provides a detailed record of environmental, climatic and anthropogenic changes in a Mediterranean watershed since Medieval times, and an opportunity to evaluate the lake restoration policies during the last decades. The paleohydrological reconstructions show fluctuating lake levels since the end of the Medieval Warm Period (ca. AD 1300) till the late 19th century and a more acute dry period during the late 19th century – early 20th century, after the end of the Little Ice Age. Human activities have played a significant role in Laguna Zoñar hydrological changes since the late 19th century, when the outlet was drained, and particularly in the mid-20th century (till 1982) when the spring waters feeding the lake were diverted for human use. Two main periods of increased human activities in the watershed are recorded in the sediments. The first started with the Christian conquest and colonization of the Guadalquivir River Valley (13th century) particularly after the fall of the Granada Kingdom (15th century). The second one corresponds to the late 19th century when more land was dedicated to olive cultivation. Intensification of soil erosion occurred in the mid-20th century, after the introduction of farm machinery. The lake was declared a protected area in the early 1980s, when some agricultural practices were restricted, and conservation measures implemented. As a consequence, the lake level increased, and some littoral zones were submerged. Pollen indicators reflect this limnological change during the last few decades. Geochemical indicators show a relative decrease in soil erosion, but not changes in the amount of chemical fertilizers reaching the lake. This study provides an opportunity to evaluate the relative significance of human vs. climatic factors in lake hydrology and watershed changes during historical times. Paleolimnological reconstructions should be taken into account by natural resources agencies to better define lake management policies, and to assess the results of restoration policies.  相似文献   

7.
The article presents the results of lithological and geochemical investigations of recent sediments of Lake Pusty Staw. The analysed sediments document about 300 years of the history of this lake. Historical materials indicate that significant changes in the catchment of the lake took place from the beginning of the 18th century (deforestation and afforestation) followed by 19th century tourist development, and 20th century industrialisation. The sediments were dated using the 210Pb and 137Cs method and core lithology. These made it possible to establish a reliable chronology to the year 1730. The calculated sedimentation rates ranged between 0.17 and 0.83 cm year–1. There was a period of intensified erosion caused by land clearance between 1734 and 1810, which resulted in an acceleration in sedimentation rate (0.36 cm year–1) and a change of lithological type of sediment from detritus gyttja to clayey gyttja. On the basis of Cu, Zn, Cd and Pb content in the sediments, it was determined that the beginning of pollution of the lake dates back to the 19th century and was caused probably by the existence of a health resort. A systematic increase in pollution occurred in the 20th century as a result of industrial plants. Normalised with respect to Al, the content of heavy metals in polluted sediments was from several to twenty times higher than in sediments of the preindustrial period, and a comparison of historical materials with changes of sediments in most cases made it possible to identify the direct causes of the increase in pollution.  相似文献   

8.
The hydroseral development of a former small lake in Southern Finland was studied by means of subfossil cladoceran remains, diatoms, pollen, plant macrofossils and other sediment data. The diatom analysis shows the lake to have become markedly acid during the Early Holocene. This is reflected in the cladoceran communities in the form of a fall in the proportions of Bosmina longirostris etc. and a reciprocal rise in Bosmina (Eubosmina) longispina, an increase in the proportions of the chydorid species indicative of acidity, and the appearance of certain new morphotypes. The concentrations of both planktonic and littoral species and the numbers of such species increase with acidification. Advancement in the hydroseral succession is reflected in the disappearance of benthic species from the chydorid communities and a pronounced increase in exclusively phytophile species. Sedimentation and the drop in water levels are seen to have led to a spread of helophytes and floating-leaved plants over the water body around 6500–7000 B.P., and a vegetation-filled swamp was created at the site. The zooplankton was practically exterminated, but the concentrations of littoral cladocerans reached their peak at this point. The basin became overgrown completely at the beginning of the Subboreal chronozone (approx. 4600–4800 B.P.), simultaneously with the low-water phase observed in many lakes. It became covered with a Sphagnum stand, and this in turn led to complete destruction of the cladoceran communities. The planktonic/littoral ratio among the Cladocera closely reflected the relation between open water and the macrophyte zone as a function of time. Climatically induced rises and falls in water level are shown to have played a significant role in promoting the advancement of the hydroseral succession.  相似文献   

9.
The remains of diatoms, cladocerans, and midges are usually the most abundant of freshwater organisms and to now have been most useful in interpreting past conditions in a lake. Each taxocene consists of two separate communities, one in the warm littoral zone and the other offshore. Remains of inshore organisms are moved offshore by wind-generated currents, the amount of transport varying with individual characteristics of the lakes. Nowhere do remains of the two communities become completely integrated numerically, although the remains of the littoral chydorid Cladocera become integrated by species before they are incorporated into the sediments. The taxa of the planktonic Eubosmina and of the offshore midges correspond to levels of productivity in present-day lakes, and hence changes in the fossil record are commonly regarded as indicating eutrophication over time. The deepwater midges respond to the concentration of dissolved oxygen in deep water, which may be controlled more by a decrease in volume of deep water through accumulation of sediments than by any real increase in edaphic productivity. While such changes are going on offshore during the Holocene, the littoral communities of cladocerans and midges are scarcely changing at all, suggesting a different response of the inshore from the offshore communities to longterm changes resulting from increasing productively or from other functions. Thus, considering these different responses of the two communities of organisms and their incomplete mixing, the remains of littoral and offshore taxa recovered from an offshore core of sediments must be tabulated separately and interpreted separately. For any studies involving accumulation rates, there must be an understanding of the integration of inshore and offshore remains, its variation over the lake bottom, and how it may have varied with marked fluctuations in water level.This is the third of 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. Drs. Davis and Löffler are serving as guest editors of this series.  相似文献   

10.
Banks Island (N.W.T.) has become a focal point for climate change studies in the Canadian Arctic. However, long-term climatic and environmental data are very sparse from this large island, as they are for the entire southwestern region of the Canadian Arctic Archipelago. In this paleolimnological study, diatom species assemblage shifts documented in cores collected from a pond and a lake on Banks Island were interpreted to represent a response to climate warming commencing in the nineteenth century. We found that, although the timing and overall nature of the species changes in the two cores were consistent, the signal was muted in the deeper site likely as a result of differences in ice cover extent and duration between lakes and ponds. A high-resolution study was also conducted from a second pond, at sub-decadal resolution, that only spanned the last ∼60 years. In the deeper lake site, Fragilaria construens and F. pinnata dominated the assemblages, similar to those noted in other high Arctic regions where lakes are characterized by extended ice cover. In contrast, Denticula kuetzingii dominated the shallower ponds and, in the case of the pond core representing the longer time period, this taxon increased in the post-1850 sediments, likely coincident with climate warming. In all cores, diatom assemblages became more diverse and Achnanthes species (particularly A. minutissima) increased from ∼1850 to the present, similar to changes documented in other Arctic regions. Beta diversity values calculated for the diatom species changes indicated that assemblage shifts in the Banks Island cores were of similar magnitude to those recorded in other Arctic regions with high species turnover, such as Ellesmere Island. A diatom-based Total Nitrogen (TN) transfer function previously developed for Banks Island was applied to the three 210Pb dated cores as an exploratory tool for inferring past changes in nitrogen concentrations. In both the lake and pond cores, diatom-inferred TN concentrations tended to increase in the more recent sediments, as may be expected with warming; however these trends were not very distinct.  相似文献   

11.
A combined analysis of modern zooplankton and fossil Cladocera assemblages from a Himalayan lake, Lake 40, revealed that the endemic Daphnia tibetana disappeared in the late-1980s, after persisting as the only Daphnia species for almost 3000 years. Daphnia head shields, which are rarely recovered from the sediments, were the most abundant Daphnia remains in the lake. The remains were of the Ctenodaphnia type; the smaller ones had a large central hole. Head shields of the same type were also found in zooplankton water samples, rich in exuviae as well as of intact specimens of D. tibetana, from a nearby lake. Small individuals had a distinct nuchal organ in the dorsal region of the head. We therefore postulated that the head shields with the hole were from young (newborn or individuals in the two first moults). Up to now, the nuchal organ has been described in laboratory populations of D. (Ctenodaphnia) magna, where it disappears quite early in life. It probably functions as an osmoregulatory organ, essential for the survivorship of late embryos and early juveniles. On the other hand, as far as we know, head shields with a hole have never been recorded in plankton or in sediments. In view of the fact that head shields were representative of Daphnia abundance, we used them to reconstruct changes in density and body size during ca. 3000 years. In fishless mountain lakes, mean Daphnia body size tends to increase toward the end of the productive season. The number of moults and maximum body size depend mainly on the duration of the ice-free period, and on summer temperatures. In cold years, when the productive season is short, the number of moults will be low, and the range of Daphnia body size will be narrow. We used Daphnia body size and abundance estimates, in addition to an analysis of changes in the Cladocera assemblage, to reconstruct past environmental conditions.  相似文献   

12.
To evaluate the comparability of neo- and paleo-limnology, we made year-to-year as well as seasonal comparisons of contemporary zooplankton data and cladoceran remains in thick (9–42 mm) annual laminations in sediment of Lake Vesijärvi, southern Finland. We calculated the expected annual exuviae production of nine planktonic taxa in the water column using contemporary zooplankton records, and compared the value to the observed net accumulation of their remains in deep sediment for 7 years. Although all of the species studied occur commonly in the lake pelagic zone, deposition of remains differed significantly among taxa. The observed accumulation of three Bosmina species and Chydorus sphaericus was similar to or exceeded expected values, suggesting good preservation of their remains as well as focusing of sediment into the deepest part of the lake. The accumulation of Limnosida and Leptodora remains exceeded expected values several fold, suggesting under-representation of these species in pelagic plankton samples, as well as efficient transportation and spatial averaging of their remains in the sediment. Daphnia, Diaphanosoma, and Ceriodaphnia were clearly under-represented in the sediment due to poor preservation of their remains and use of a 50-μm sieve in sediment processing. Thus, sieving should be taken into account as a possible source of bias. Correlation between relative abundances of all species in the sediment and in the plankton was weak (r s = 0.54, P < 0.001), but for well-preserved species, the correlation was strong (r s = 0.91, P < 0.001). Inter-annual variation in the deposition of remains suggested that resuspension and sediment focusing may vary between years, thus making it difficult to interpret absolute abundances, even though the deepest part of the basin shows clear varve formation and seasonality is well recorded. Detailed study of the uppermost lamination showed the seasonal succession of a cladoceran community within an annual varve as well as differences in the seasonality and intensity of ephippia production among the species. We recommend that results of cladoceran analyses be expressed in several ways (relative abundances, per unit dry weight, per unit organic matter, and as net accumulation values) before drawing final conclusions, as each approach may reveal a different aspect of the deposition process. In addition, sedimentation differences between epilimnetic and hypolimnetic species should be considered in stratified lakes.  相似文献   

13.
A combination of biotic, sedimentary and biogeochemical proxies was used to investigate the timing and causes of post-18th century changes in the stratigraphic record of a large, deep lake on the Boothia Peninsula, Nunavut, Canada (70°15′ N, 94°30′ W). A varve chronology verified with radioisotopic dating (210Pb and 137Cs) revealed a complex pattern of environmental dynamics since c. AD 1830. An increase in the diatoms Asterionella formosa, Stephanodiscus minutulus and Cyclotella atomus and a decrease in Aulacoseira taxa in the uppermost centimetre of sediment suggested that environmental conditions have favoured the growth of smaller and/or lighter planktonic species since the 1980s. Longer term changes in some benthic species, the chrysophyte cysts to diatom valve ratio, %C, and C/N ratios suggest declined river inflow and a relative reduction in allochthonous inputs during the last century. Higher than average δ15N values in the late 19th to early 20th centuries coincide with changes in bulk carbon and nitrogen profiles, and below average values since approximately 1950 may be associated with increased atmospheric N loading or reduced productivity. Biogenic silica and organic carbon accumulation in the sediments suggest a possible decline in lake production during the 20th century that may be associated with changes in the river discharge regime. The short and long-term ecological and biogeochemical trends were also reflected in the sedimentary structure through declining varve thickness for the duration of the record and an abrupt change in sedimentology in the uppermost 1 cm, coinciding with deposition since ca. AD 1987. Together, these biological and physical changes suggest changes in hydroclimatic conditions in the 20th century, and an increase in planktonic diatom taxa since the 1980s that coincides with a distinct period of climate warming.  相似文献   

14.
藏南沉错地区近1400年来的介形类与环境变化   总被引：7，自引：0，他引：7  
通过分析藏南沉错CC1孔的介形类动物群所提供的古环境信息，探讨近1400年来的湖泊演化。CC1孔介形类共计7属15种。据其属种，数量及生态特征，可划分为7个组合，同时发现1400年来沉错环境变化可分为3个时期：（1）公元6世末－14世纪下半叶沉错主要为较深水湖，其中大约在708－780年和1199－1213年湖泊迅速扩大加深；（2）14世纪下半叶－19世纪末沉错主要为浅水湖，其中大约在1454－1525年，1645－1670年和1803－1891年3个时段湖泊强烈退缩，环境极不稳定，而大约在1731－1803年湖泊发生逆向转化，湖水增多，湖面抬升；（3）19世纪末至今沉错由较深水湖转变为浅水湖，20世纪60年代以前为较深水湖，其中大约在1929－1935年湖泊急剧加深，20世纪70年代以来湖泊退缩，湖水变浅。  相似文献   

15.
Although the phytoplankton and protozoan assemblages of Lake Winnipeg are quite variable both, spatial and temporally, knowledge of their current distribution and ecology enables them to be used as indicators providing unique information concerning past and recent ecosystem conditions. Many of the main taxa have been similar throughout history although there have been dramatic changes in their abundance. Lake Winnipeg is currently very responsive to the climatic conditions in its drainage basin and it appears, historically, to have been a diatom-bluegreen algal lake with Tintinnids and thecate amoeba forming significant components of the protozoan community. A wide variety of microfossil remains have been used to infer past conditions in this large prairie lake. Microfossils reported from the long core 103 (8 m) taken from the north basin of LakeWinnipeg and short core Namao 7a taken from the south basin of the lake in August 1994 indicate that the lake has experienced several changes over its history. Diatom assemblages, low throughout the core 103 below 50 cm, except for a peak around 300–400 cm are virtually absent below 690–800 cm (Lake Agassiz). Stephanodiscus and Aulacoseira are the two major pelagic diatom genera represented throughout the lake's history. Species changes occur near the top of the core indicate increased anthropogenic eutrophication. Shallow water littoral taxa were never abundant at either coring sites. The presence of planktic cyanoprokaryote (cyanophyte, cyanobacteria, bluegreen algae) remains (akinetes) from 600 cm depth and the progressive increase in abundance from 400 cm to through the top 100 cm of the core indicates increasing phosphorus levels, warming summer temperatures and increasing summer nitrogen limitation in the lake. Nitrogen fixing bluegreens (especially Anabaena and Aphanizomenon akinetes) and the diatoms (Aulacoseira ambigua, A. granulata, A. islandica, S. binderanus, and S. niagarae) were abundant in the upper sediments and changes after 100 cm can be interpreted as the effects of human impact. These taxa indicative of increase eutrophication, in addition to S. agassizensis, Melosira varians and Cyclostephanos dubius, are representative of present day plankton. The short core Namao 7a was dated by Pb210 and the fossil remains also show a rise in the Aulacoseira (particularly A. granulata and A. ambigua) and bluegreen akinetes in the last 40 years as was seen in the upper sediments of the 103 core. There was an interesting peak in chrysophyte cysts during the 1930's which corresponds nicely with climatic conditions during this period. The fires accompaning the hot dry period in the mid 70's can be seen in the dramatic rise in charcoal during this period. Multiple types of micro fossils remains provides several useful tools for interpreting past lacustrine conditions.  相似文献   

16.
Pollen and spores with resistant exines are preferentially preserved in soils, and during periods of soil erosion they can become incorporated into lake sediments. As a result, the contemporary vegetation may be poorly represented by the palynomorphs in the lake sediments because of the reworked component of inwashed pollen and spores. We record the proportion of palynomorphs with corroded exines in sediment cores from four lakes in the eastern North Island of New Zealand to document changing sources of palynomorphs over the last 2000 years. During this period, the catchments experienced major vegetation disturbances, both natural (from volcanism and fire) and anthropogenic including deforestation ca. 600 years ago, and the European conversion of fern-scrubland to pasture in the 19th century. Corroded palynomorphs are more abundant in inwashed sediments than authigenic sediments. Catchment soil disturbance was minor during the forested period, and characterised by small, inwashed, sediment pulses after storms, and a relatively low percentage of corroded palynomorphs. Although initial Maori forest clearance by fire led to a temporary increase in erosion in one lake catchment, rapid replacement of forest by a dense bracken fern cover helped to minimise soil erosion and reworking of palynomorphs in this period. European pastoralists replaced the bracken fern with shallow-rooted pasture grasses about 150 years ago. In erosion prone lake catchments, this led to a rapid increase of inwashed eroded soils and littoral sediments, and their component of resistant palynomorphs, reaching the lake sediments. As a result, the palynological records from these catchments during the European period are distorted by reworking. By contrast, over the same period, the palynological record from a lake with no inflowing streams and stable catchment soils more faithfully represented the contemporary vegetation cover. Exine corrosion has been used to help identify periods of reworking in the lake sediments and to allow for a correction of distortion caused by reworking.  相似文献   

17.
Responses to recent climatic changes in the sediment of subarctic Lake Saanajärvi in northwestern Finnish Lapland are studied by comparison of various biological and sedimentological proxies with the 200-year long climate record, specifically reconstructed for the site using a data-set of European-wide meteorological data. The multi-proxy evidence of simultaneously changing diatom, Cladocera, and chrysophyte assemblages along with the increased rates of organic matter accumulation and pigment concentrations suggest that the lake has undergone a distinct typological change starting from the turn of the 20th century. This change, indicating an increase in lake productivity, parallels a pronounced rise in the meteorologically reconstructed mean annual and summer temperatures in the region between ca. 1850 and 1930's. We postulate that, during the Little Ice Age, the lake was not, or was only weakly, thermally stratified during summer, whereas the subsequent increase in air and hence epilimnetic water temperatures resulted in the development of the present summer stratification. The increased thermal stability of the lake created more suitable conditions for the growth of phyto- and zooplankton and changed the overall primary production from benthos to plankton. Mineral magnetic and carbonaceous particle records suggest long-distance pollution, particularly since the 1920's, yet the observed changes in lake biota and productivity can hardly be explained by this very minor background pollution; the 20th century species configurations are typical of neutral waters and do not indicate any response to pollution.  相似文献   

18.
Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.  相似文献   

19.
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.  相似文献   

20.
In this paper, the relationships between paleo-precipitation and the regional influence of El Nino Southern Oscillation (ENSO) in South America are assessed from a high-resolution calendar varve-thickness record. Two short laminated sediment cores (53 and 61 cm length) from Lago Puyehue (40° S) are analysed by continuous varve measurements through the last 600 years. The calendar varve years are determined by the occurrence of graded planktonic-rich layers. The annual sediment accumulation rates are reconstructed by using the standard varve-counting methods on thin sections. The 1980–2000 varve-thickness record is interpreted in terms of climate through correlation with limnological and local monthly instrumental climate databases. The comparison between the standardized varve thickness with the instrumental records reveals a strong correlation (r = 0.75, р = 0.07) between the total varve thickness and the austral autumn/winter precipitation. We argue that strong austral winter winds and precipitation are the forcing factors for the seasonal turn-over and phytoplankton increase in the lake sediments. During strong El Nino events the precipitation and the winds decrease abnormally, hence reducing the thickness of the biogenic sediments deposited after the winter turn-over. Our results show one significant regional maximum peak of winter precipitation (>900 mm) in the mid 20th century and a significant period with lower winter precipitation (<400 mm) before the 15th century, i.e., the late Medieval Warm Period. The first peak in the mid 20th century is confirmed by the regional precipitation database. The influence of ENSO cycles over the last 600 years is assessed by spectral analysis in Fagel et al. (2007). The possible influence of the regional volcanism and/or the seismic activity on the local climate record is also discussed. This is the sixth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M. -F. Loutre and E. Chapron.  相似文献   

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