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1.
A stratigraphic diatom sequence is presented for the period from 13,870-9,170 cal BP from Kråkenes Lake, western Norway. Changes in species assemblages are discussed with reference to the changing environmental conditions in the Allerød, Younger Dryas, and the early Holocene and to the development of the aquatic ecosystem. The site is sensitive to acidification, and diatom-based transfer functions are applied to estimate the past pH status. The combination of rapid sediment accumulation together with an excellent calibrated radiocarbon chronology means that the rate of inferred pH change and associated increase in [H+] can be assessed and compared with recent, anthropogenically acidified situations.The Allerød diatom assemblages are dominated by benthic taxa particularly Fragilaria species, indicating an unproductive, alkaline, turbid, and immature system. Diatoms are absent in the early part of the Younger Dryas, but subsequently a sparse planktonic flora develops reflecting decreased turbidity and/or increased nutrient supply. A clear sequence of diatom assemblages is seen in the early Holocene. A short-lived peak of Stephanodiscus species indicating a period of increased nutrient availability occurred at ca. 11,500 cal BP. Throughout the early Holocene, acid-tolerant species increasingly replaced less acidophilous, circumneutral taxa.The lake became slightly more acid during the Allerød, but this was statistically insignificant in a trend test involving regression of pH or [H+] in relation to age. Diatom-inferred pH declined rapidly during the early Holocene period investigated (9,175-11,525 cal BP) with a statistically significant overall rate of 0.024 pH units per 100 yrs. This consisted of an older (ca. 11,525-10,255 cal BP) phase, where pH fell more rapidly at up to 0.095 pH units per 100 yrs; and a younger phase after ca. 10,500 cal BP where the pH fall was extremely slow (0.008 pH units per 100 yrs) and was not statistically significant.In the Allerød a combination of low catchment productivity together with disturbance, weathering, and minerogenic inwash ensured that the base-cation status remained relatively high. In the Holocene the catchment soils stabilised and base cations were sequestered by terrestrial vegetation and soil. This resulted in reduced base-cation leaching and this, together with the production of organic acids caused the lake to acidify, reaching an equilibrium by ca. 10,000 cal BP.  相似文献   

2.
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   

3.
Two alpine lakes in the south-central part of Norway have been studied for recent changes in diatom assemblages preserved in their sediments. Both lakes experience a post 1980 AD increase in diatom valve accumulation rates possibly reflecting an increase in lake productivity. In addition there is an overall increase in diatom-inferred pH at both sites. Recovery from lake acidification can be disregarded as a possible cause of increased pH as the lakes are situated in catchments with high buffering capacities in areas that have received low amounts of acid deposition. We suggest that recent climate warming has influenced both sites with the important effect of increasing mineralization in the catchments, resulting in greater fluxes of nutrients and base cations to the lakes, leading to an increase in diatom-inferred pH. Taxa that have increased in abundance include Achnanthes minutissima, Achnanthes nodosa, Cyclotella spp., Navicula schmassmannii, Staurosirella lapponica, and S. pinnata. In one of the lakes, the maximum diatom-inferred pH values reached at the top of the core are as high as pH values reconstructed from the diatom assemblages deposited at the end of the Mid-Holocene thermal maximum c. 4000 cal. BP.  相似文献   

4.
In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.  相似文献   

5.
Atmosphere-borne acidic pollutants constitute an important, potentially critical, risk to aquatic ecosystems. Long-time series datasets are a requirement in the assessment of the biological impacts of both increased and decreased loadings of acidic pollutants and allow for the anticipation of future pollution loadings and their effects that can be acted upon with some confidence. Little is known concerning levels of deposition of atmospheric pollutants and their effects on acid-sensitive aquatic ecosystems in Ireland. Moreover, unequivocal evidence of acidification by atmospheric deposition is limited. The objective of this study was to provide a preliminary assessment of pH response in three lake sites that are situated in acid-sensitive areas of Ireland, and to determine whether pH response tends to be uniform or differs due to variations in atmospheric loading and catchment properties. Levels of spheroidal carbonaceous particle (SCP) deposition have been used as a general indicator of the timing and extent of atmospheric pollutant loading and ecosystem response has been assessed by estimating diatom-inferred pH with a weighted-averaging calibration model (72 lakes) developed specifically for the Irish ecoregion. Levels of diatom-inferred pH (DI-pH) appear to respond to increased and decreased levels of atmospheric pollution loads—as represented by SCPs—at two of the three sites examined in the east and west of Ireland. In contrast, the site situated in the southwest of Ireland showed a varying and less directional DI-pH response, possibly due to lower deposition pressures, large lake size, and more buffered edaphic geology.  相似文献   

6.
Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.  相似文献   

7.
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   

8.
The Holocene environmental history and climate are reconstructed for Råtåsjøen, a low-alpine lake in south-central Norway. The reconstructions are based on chironomids, diatoms, pollen, plant macrofossils, and sediment characteristics. From plant macrofossil evidence, birch trees (Betula pubescens) immigrated ca. 10,000 cal BP. The chironomid-inferred mean July air temperature was high, but may be unreliable during the early stages of the lakes history due to the high abundance of Chironomus anthracinus type, a taxon that may include several species. From ca. 9000 cal BP the inferred mean July temperature was lower (ca. 9 °C). Temperatures increased towards 8000 cal BP and pine (Pinus sylvestris) reached its upper limit near the lake. July temperature may have become a significant factor controlling long-term pH in the lake, starting shortly after 8000 cal BP. High pH values were associated with periods of warm summers and lower pH values occurred during periods of colder summers. Alkalinity processes within the lake and/or the catchment are possible factors controlling this relationship. A temperature decline at ca. 5400 cal BP separated two 10.6 °C temperature maxima around 6400 and 4500 cal BP. The 1.5 °C decline in July air temperatures from ca. 4400 cal BP was paralleled by a decrease of pH from 7.2 to 6.8. Following the temperature drop, first pine and then birch trees declined and disappeared from the catchment and organic accumulation in the lake increased. The increased organic accumulation rate had a positive effect on diatom production. At ca. 2700 cal BP the temperature reached a minimum (ca. 9.2 °C) and correspondingly a second pH minimum was reached. Temperature decreased again slightly at ca. 400 cal BP during the Little Ice Age, before increasing by about 0.5 °C towards the present. Percentage organic carbon as estimated by loss-on-ignition appears to be better correlated with chironomid-inferred July temperatures than organic accumulation rates, at least for the last 9000 years. Accumulation rates of organic sediments are more coupled with catchment-related processes, such as erosion and major changes in vegetation, than is percentage organic carbon.  相似文献   

9.
An 11.6 m long continuous succession of annually laminated sediments from Lake Korttajärvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake Päijänne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ13C and another HCl-HF-digested organic fraction was analyzed for both δ13C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttajärvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ13CHCl-HF values of organic matter from ?31.6 to $-29.2\permilleAn 11.6 m long continuous succession of annually laminated sediments from Lake Korttaj?rvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake P?ij?nne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ13C and another HCl-HF-digested organic fraction was analyzed for both δ13C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttaj?rvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ13CHCl-HF values of organic matter from −31.6 to , followed by a subtle decrease to . The changes in pH and δ13CHCl-HF are closely related (r = − 0.91, P < 0.01), and apparently reflect changing environmental conditions in the lake and in its catchment area. δD values show a marked shift to higher values during the early Holocene, which may be partly related to a climatic amelioration leading to the Holocene Climatic Optimum in 6000–2500 BC. The Medieval Warm Period in AD 980–1250 is associated with a local maximum in δD, lending support for a significant warming during that time.  相似文献   

10.
A late Holocene diatom stratigraphy of Bottom Pond (37°05′S, 12°17′W) was studied and interpreted to infer paleolimnological and climate changes on Tristan da Cunha, South Atlantic. The diatom flora of Bottom Pond is dominated by meroplanktonic Aulacoseira alpigena and A. distans and a benthic/epiphytic flora including Achnanthes spp., Naviculadicta spp., Eunotia taxa and Frustulia rhomboides. The flora also includes an important terrestrial and aerophytic component, consisting of several Diadesmis spp., Navicula lapidosa, Nitzschia terrestris and some Pinnularia spp., at times dominating the flora. Peaks of high terrestrial diatom abundances and low abundances and concentrations of Aulacoseira spp., correspond to periods of increased magnetic susceptibility and are interpreted to be the result of increased catchment erosion and/or precipitation leading to enhanced in-wash of species living in the soils around the lake. The diatom record of Bottom Pond most likely registers both local responses to increased erosion in the catchment as well as climatic change of a more regional character. Comparisons with the geochemical record in 2nd Pond on Nightingale Island show a correspondence between the two data sets at 2,000–1,750 cal yrs BP, most likely evidence of increased precipitation in the Tristan da Cunha region. The Tristan da Cunha archipelago is situated at the northern limit of the Southern hemisphere west wind belt and stronger and/or a northward shift of the westerlies, are probably the main causes of the precipitation changes.  相似文献   

11.
Diatom analyses from the longest continuous record of lacustrine sedimentation in the eastern Canadian Arctic indicate four phases of lake development during the past 20000 years. PlanktonicAulacoseira taxa are dominant between 20 and 10 ka and during the Neoglacial. The earliest Holocene is characterized byFragilaria, whereas benthic acidophils (e.g.Frustulia, Brachysira & Eunotia spp.) dominate sediments of early to mid Holocene age. Ordination by correspondence analysis illustrates stratigraphic changes in diatom life form and pH tolerances, and these are related to both regional paleoclimatic conditions and local edaphic factors. The occurrences of planktonic floras during the cold Late Foxe and Neoglacial periods suggests that, even at these times, the lake became ice free during summer. The interplay of increased runoff as a mechanism of ice disintegration and enhanced silicon supply by erosional processes (corroborated by the more clastic nature of the sediments during these periods) likely enabledAulacoseira to flourish. Early and mid Holocene limnological regimes were more strongly controlled by lake authigenic processes.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution.  相似文献   

12.
The post-glacial history of Lake Pieni-Kuuppalanlampi, western Lake Ladoga region, was studied by means of stratigraphic pollen and diatom analyses. Diatoms were analysed to track the isolation history of the basin and the limnological effects of the early land-use phases indicated by pollen analysis. Chrysophycean stomatocysts and Isoëtes spores were also employed in the limnological reconstructions. Sediment dating was provided by six conventional radiocarbon dates.The lower part of the 370-cm long sediment sequence represents early Holocene, large lake conditions: the (freshwater) Yoldia and Ancylus stages of the Baltic basin, with a short-term lagoonal or isolation phase at the end of Yoldia. The basin was isolated due to Ancylus lake regression at 9785 cal B.P. For the small-lake sequence (0–250 cm) we used diatom inferences (WA-method) for hindcasting water chemistry. The post-isolation limnology of Pieni-Kuuppalanlampi reflects the development of vegetation on its small hill-top catchment. In its early development, the lake was mesotrophic, but became more acidic (pH about 6) and oligotrophic after the decline of temperate hardwood trees and the spread of spruce to the area after around 5000 B.P. The lake ecosystem appears to have reacted sensitively to agricultural land-use in the catchment from AD 400–800 onwards: inferred total phosphorus levels and pH both rise during these periods.  相似文献   

13.
The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.  相似文献   

14.
Transects of surface sediment samples were taken in 4 lakes from the Sylvania Wilderness Area, Upper Peninsula of Michigan. These surface samples were compared with diatom samples from a core taken in the Northwest basin of Crooked Lake, also from the Sylvania Wilderness Area. Weighted Averaging calibration was used to reconstruct lake depths in Crooked Lake using the diatom microfossils from the core and the surface samples to infer past lake depth. During the early Holocene the lake was dominated by planktonic species and diatom-inferred water depth was large – approx. 13 m. At about 6700 BP inferred water depth was 2 m and samples were dominated by Fragilaria construens var. venter – a species characteristic of shallow parts of the surface sample transects. From 6700 to 5000 BP reconstructed water level was at its shallowest. From 5000 to 3000 BP it increased. This rise in water level was marked by increasing abundances of Aulacoseira ambigua and occurred at the same time increasing percentages of hemlock pollen indicate increasing available moisture. Modern water depth was reached about 3000 BP. The water level changes at Crooked Lake are consistent with regional climate changes in the Upper Midwest during the Holocene. The lake was shallowest during the mid-Holocene warm period documented by other investigators. It deepened as the Midwestern climate became cooler and wetter during the late Holocene.  相似文献   

15.
A small lake, Kaksoislammi (60° 3830N, 24° 4550E), in southern Finland was studied for Cladocera, diatoms and pollen from a core which covers the entire Holocene. The diatom remains indicate a steady development from alkaliphilous taxa towards the dominance of acidophilous forms and lowering pH in the late Holocene. About 1800–1700 BP, dramatic changes took place in the microfauna, mainly the planktonic Cladocera. Bosmina longirostris, the dominant species, suddenly disappeared, and Daphnia, Chydorus sphaericus and Chaoborus increased. The change is simultaneous with a decline of the diatom-inferred pH to 4.8. It is probable that there was a sudden, profound change in predator-prey relationships. The acidity of the lake water probably increased to such a low level that it led to the disappearance of even the most acid-tolerant fish. Consequently invertebrate predators increased and quickly altered the species composition in the lake. There is also pollen evidence of the onset of Iron Age cultivation and grazing almost simultaneously with the faunal change. Therefore, it cannot be ruled out that the sudden lowering of pH was indirectly caused by prehistoric human activity; possibly the acidic peatland surrounding the lake was disturbed.  相似文献   

16.
Diatom species counts were conducted on 171 sediment samples from the 13-m-long core PG1351 from Lake El’gygytgyn, northeast Siberia. The planktonic Cyclotella ocellata-complex dominates the diatom assemblage through most of the core record, persisting through a variety of climate conditions. Periphytic diatoms, although less abundant, have greater diversity and greater down-core assemblage variation. During warm climate modes, longer summer ice-free conditions may have allowed more complex diatom communities to develop in shallow-water habitats, and enhanced circulation may have increased transport of these diatoms to deeper parts of the lake. Zones of low overall diatom abundance further support inferred intervals of low lake productivity during times of extended lake ice and snow cover. More data on the modern spatial and temporal distribution of diatom species in the Lake El’gygytgyn system will improve inferences from core records. This is the last in a series of eleven papers published in this␣special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献   

17.
As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km2) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.  相似文献   

18.
The development of Soppensee (Central Switzerland, 596 m a.s.l.) has been reconstructed using algal remains such as diatoms, chlorophytes and fossil pigments, as well as the pollen and spores of macrophytes. Sediment accumulation in Soppensee began at the end of the last glacial period, approximately 15,000 yrs ago. During the Oldest Dryas biozone (> 12,700 radiocarbon yrs B.P.) the lake had low primary productivity. After reforestation with birch and later pine, around 12,700 B.P., phases of summer anoxia occurred in the lake. These anoxic conditions were most probably caused by additional carbon input from the catchment, as well as longer phases of stratification due to reduced wind exposure caused by the sheltering effect of increased tree cover. From the Younger Dryas biozone (10,800 to 10,000 radiocarbon yrs B.P.) onwards, Soppensee became meromictic for several millennia.The fossil diatom assemblages are dominated by planktonic alkaliphilous to alkalibiontic species with mainly meso- to eutrophic preferences. Diatom-inferred total phosphorus reconstructions suggest meso- to eutrophic conditions throughout the Holocene. Eutrophic conditions are also suggested by the presence of pigments of cyanobacteria, including Oscillatoria species. First human activity in the catchment is evidenced ca. 5000 radiocarbon yrs B.P. by the occurrence of cereal pollen. Diatom-inferred total phosphorus concentrations also increased slightly during the Neolithic period.According to the fossil pigment record, meromictic conditions ended during the Iron Age. Deep-water anoxia, however, persisted at least during the stratification period. During the Middle Ages massive deforestation in the catchment and around the lake changed the limnological conditions drastically. The lack of forest increased the wind fetch and, therefore, also the mixing of the lake, while soil erosion and retting of hemp supplied additional nutrients. Because of intensive agriculture in its catchment, Soppensee has become hypertrophic and diatom assemblages have consequently changed completely in the last 50 yrs.  相似文献   

19.
We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.  相似文献   

20.
During the middle and late Holocene, the Iberian Peninsula underwent large climatic and hydrologic changes, but the temporal resolution and regional distribution of available palaeoenvironmental records is still insufficient for a comprehensive assessment of the regional variability. The high sedimentation rate in karstic, meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a detailed reconstruction of the regional palaeoecology over the last 5,340 years using diatom analysis, aquatic pollen, sedimentological data, and historic documentary records. Results show marked fluctuations in diatom species assemblage composition, mainly between dominant Cyclotella taxa and small Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta reflects changes in trophic state, while the succession of centric and pennate species most likely reflects changes in the hydrology of the lake. The diatom assemblages were used to identify six main phases: (1) high productivity and likely lower lake levels before 2350 BC, (2) lower lake levels and a strong arid phase between 2350 and 1850 BC, (3) lake level increase between 1850 and 850 BC, (4) relatively high lake level with fluctuating conditions during the Iberian and Roman Epochs (650 BC–350 AD), (5) lower lake levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by increased human activities in the watershed during the Medieval Climate Anomaly (900–1300 AD), and (6) relatively higher lake levels during the LIA (1380–1850 AD) and afterwards. The combined study of diatoms, algae and pollen provides a detailed reconstruction of past climate, which refines understanding of regional environmental variability and interactions between climate and socio-economic conditions in the Pyrenees.  相似文献   

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