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1.
A diatom dataset and diatom-salinity inference model for southeast Australian estuaries and coastal lakes 总被引:1,自引:1,他引:0
To quantify the relationship between diatom species assemblages and the water chemistry of southeast Australian estuaries
and coastal lakes, a new dataset of 81 modern diatom samples and water chemistry data was created. Three hundred and ninety-nine
species from 53 genera were identified in 36 samples from 32 coastal water bodies in eastern Tasmania and 45 samples from
13 coastal water bodies in southern Victoria. Multivariate statistical analyses revealed that the sampling sites were primarily
distributed along salinity and nutrient gradients, and that salinity, nitrate + nitrite, phosphate and turbidity explained
independent portions of variance in the diatom data. Species salinity optima and tolerances were determined and a diatom-salinity
inference model (WAinv r
2 = 0.72, r
2jack = 0.58, RMSEP = 0.09 log ppt) was developed. This new information on diatom species’ salinity preferences provides a
useful tool for quantitatively reconstructing salinity changes over time from diatom microfossils preserved in the sediments
of a range of estuaries and coastal lakes in southeast Australia. This is valuable for studies investigating long-term human
impacts and climate change in the region. 相似文献
2.
Tracking eutrophication in Taihu Lake using the diatom record: potential and problems 总被引:7,自引:0,他引:7
Xuhui Dong Helen Bennion Rick Battarbee Xiangdong Yang Handong Yang Enfeng Liu 《Journal of Paleolimnology》2008,40(1):413-429
Taihu Lake is the third largest freshwater lake in China and has been experiencing eutrophication problems for several decades.
Diatoms in short sediment cores from three bays in northern Taihu Lake were studied in addition to 1-year of seasonal phytoplankton
samples in order to evaluate the rate and magnitude of nutrient enrichment. The dominant species found in the phytoplankton
samples appeared in high percentages in the surface sediment samples, suggesting that the latter faithfully record the modern
diatom flora. The diatom preservation status varied among the three cores, while in all cores the preservation deteriorated
with sediment depth. Due to the superior diatom preservation in the core from Mashan Bay, the fossil diatom record of this
core and an established diatom total phosphorus (TP) transfer function were used to reconstruct the nutrient history of Taihu
Lake. Diatom assemblages changed from Aulacoseira-dominated to other eutrophic planktonic species, such as Stephanodiscus minutulus, Cyclostephanos tholiformis, Cyclotella atomus, C. meneghiniana and S. hantzschii in ca. 1980. Diatom-inferred TP concentrations exhibited little change prior to 1980, with values around 50 μg/l. However,
after 1980 TP concentrations increased significantly and remained in excess of 100 μg/l, reflecting eutrophication of Taihu
Lake. Comparison with TP measurements in the water column from 1988 to 2004, as well as the analogue analysis among fossil
and modern samples, demonstrates that the diatom-TP inference model can reliably hindcast past TP concentrations. Therefore,
the baseline TP value of about 50 μg/l, can be used as a restoration target for Taihu Lake. However, due to the complexity
of this very large, shallow aquatic ecosystem, caution should be exercised when employing the diatom record to track eutrophication.
Further studies on the mechanism of diatom distribution, evolution and preservation are recommended for Taihu Lake. 相似文献
3.
A knowledge of pre-disturbance conditions is important for setting realistic restoration targets for lakes. For European waters
this is now a requirement of the European Council Water Framework Directive where ecological status must be assessed based
on the degree to which present day conditions deviate from reference conditions. Here, we employ palaeolimnological techniques,
principally inferences of total phosphorus from diatom assemblages (DI-TP) and classification of diatom composition data from
the time slice in sediment cores dated to ~1850 AD, to define chemical and ecological reference conditions, respectively,
for a range of UK lake types. The DI-TP results from 169 sites indicate that reference TP values for low alkalinity lakes
are typically <10 μg L−1 and in many cases <5 μg L−1, whilst those for medium and high alkalinity lakes are in the range 10–30 and 20–40 μg L−1, respectively. Within the latter two alkalinity types, the deeper waters (>3 m mean depth) generally had lower reference
TP concentrations than the shallow sites. A small group of shallow marl lakes had concentrations of ~30 μg L−1. Cluster analysis of diatom composition data from 106 lakes where the key pressure of interest was eutrophication identified
three clusters, each associated with particular lake types, suggesting that the typology has ecological relevance, although
poor cross matching of the diatom groups and the lake typology at type boundaries highlights the value of a site-specific
approach to defining reference conditions. Finally the floristic difference between the reference and present day (surface
sample) diatom assemblages of each site was estimated using the squared chord distance dissimilarity coefficient. Only 25
of the 106 lakes experienced insignificant change and the findings indicate that eutrophication has impacted all lake types
with >50% of sites exhibiting significant floristic change. The study illustrates the role of the sediment record in determining
both chemical and ecological reference conditions, and assessing deviation from the latter. Whilst restoration targets may
require modification in the future to account for climate induced alterations, the long temporal perspective offered by palaeolimnology
ensures that such changes are assessed against a sound baseline. 相似文献
4.
Diatom-based total phosphorus (TP) and pH transfer functions for the Irish Ecoregion 总被引:2,自引:0,他引:2
Guangjie Chen Catherine Dalton Manel Leira David Taylor 《Journal of Paleolimnology》2008,40(1):143-163
A 72-lake diatom training set was developed for the Irish Ecoregion to examine the response of surface sediment diatom assemblages
to measured environmental variables. A variety of multivariate data analyses was used to investigate environmental and biological
data structure and their inter-relationships. Of the variables used in determining a typology for lakes in the Irish Ecoregion,
alkalinity was the only one found to have a significant effect on diatom assemblages. A total of 602 diatom taxa were identified,
with 233 recorded at three or more sites with abundances ≥1%. Generally diatom data displayed a high degree of heterogeneity
at the species level and non-linear ecological responses. Both pH and total phosphorus (TP) (in the ranges of 5.1–8.5 and
4.0–142.3 μg l−1 respectively) were shown to be the most significant variables in determining the surface sediment diatom assemblages. The
calibration models for pH and TP were developed using the weighted averaging (WA) method; data manipulation showed strong
influences on model performances. The optima WA models based on 70 lakes produced a jack-knifed coefficient of determination
(r
2
jack) of 0.89 with a root mean squared error (RMSEP) of 0.32 for pH and r
2
jack of 0.74 and RMSEP of 0.21 (log10 μg l−1) for TP. Both models showed strong performances in comparison with existing models for Ireland and elsewhere. Application
of the pH and TP transfer functions developed here will enable the generation of quantitative water quality data from the
expanding number of palaeolimnological records available for the Irish Ecoregion, and thus facilitate the use of palaeolimnological
approaches in the reconstruction of past lake water quality, ecological assessment and restoration. 相似文献
5.
We examined changes at the community and population level of sedimentary diatoms over a wide temporal and spatial gradient
of metal pollution encountered in cores from three lakes in the Abitibi mining region (Québec, Canada). Diatom communities
on the whole appeared to be very tolerant of metal contamination, as shown by diatom cell accumulation rates decreasing only
under the most severe conditions of contamination, which were found from the 1930s to the 1980s in Lac Dufault (cadmium, up
to 94 μg/g dry sediment; Cu, up to 8600 μg/g; Zn, up to 9000 μg/g). Under the moderate conditions of contamination observed
in the other two lakes and in the most recent sediment of Lac Dufault, diatom cell accumulation rates tended to increase over
values typical of the pre-mining period. However, there were increasing rearrangements of the community composition along
the contamination gradient. Under moderate metal enrichment, the diatom community of Lac Vaudray experienced only subtle changes,
with Cyclotella stelligera, albeit decreasing, remaining the dominant taxon. In the intermediately contaminated Lac Caron, several benthic taxa, noticeably
Cymbella silesiaca and several Fragilaria species, rose in taxonomic importance. The most extreme contamination observed in Lac Dufault led to a severely impoverished
community almost entirely represented by Achnanthes minutissima and Brachysira vitrea. With increasing levels of contamination, there was a shift from planktonic to benthic taxa and morphotypes and a consistent
decrease in the siliceous stomatocysts/diatom frustules ratio. These trends suggest that littoral zones may represent an
important refugium under conditions of high contamination. Responses to metal stress were in general more evident at the population
than at the community level. Cyclotella stelligera and B. vitrea had a consistent negative and positive response, respectively, along the gradient and are the most promising indicators of
metal pollution for this region. 相似文献
6.
Holocene diatom flora and stratigraphy from sediment cores of two Antarctic lakes (King George Island) 总被引:6,自引:0,他引:6
The diatom stratigraphy of Holocene sediment cores from two Antarctic lakes (Mondsee, Tiefersee; 62° 10 S/58° 50 W) of King George Island was investigated. The diatom assemblages were dominated by cosmopolitan species. The flora was composed of three main components: 1) taxa of submersed bryophytic habitats, which also occur in the bryophilic diatom flora of southern South America; 2) species of various terrestrial habitats, including some specific subantarctic taxa; 3) species distributed in coastal inland waters influenced by sea-spray.Changes in sediment pattern and diatom species composition seemed to reflect climatic changes. At least three different core sections were distinguished in both lakes: Two sections rich in mosses occur between approx. 7000–4700 B.P. and from 3200 B.P. to present, in general resembling present day conditions. In between is a section of high allochthonous content with generally low diatom concentrations and rising percentages of aerophilic, halophilic and alkaliphilic diatoms. An increase of melt water discharge, possible sea-spray influence, and lake- level fluctuations are discussed.Three new taxa are described: Achnanthes metakryophila nov. spec., Achnanthes renei nov. spec., Navicula australomediocris nov. spec. The taxonomy of selected taxa is discussed. 相似文献
7.
Atmospheric deposition of reactive nitrogen (Nr) has enriched oligotrophic lakes with nitrogen (N) in many regions of the
world and elicited dramatic changes in diatom community structure. The lakewater concentrations of nitrate that cause these
community changes remain unclear, raising interest in the development of diatom-based transfer functions to infer nitrate.
We developed a diatom calibration set using surface sediment samples from 46 high-elevation lakes across the Rocky Mountains
of the western US, a region spanning an N deposition gradient from very low to moderate levels (<1 to 3.2 kg Nr ha−1 year−1 in wet deposition). Out of the fourteen measured environmental variables for these 46 lakes, ordination analysis identified
that nitrate, specific conductance, total phosphorus, and hypolimnetic water temperature were related to diatom distributions.
A transfer function was developed for nitrate and applied to a sedimentary diatom profile from Heart Lake in the central Rockies.
The model coefficient of determination (bootstrapping validation) of 0.61 suggested potential for diatom-inferred reconstructions
of lakewater nitrate concentrations over time, but a comparison of observed versus diatom-inferred nitrate values revealed
the poor performance of this model at low nitrate concentrations. Resource physiology experiments revealed that nitrogen requirements
of two key taxa were opposite to nitrate optima defined in the transfer function. Our data set reveals two underlying ecological
constraints that impede the development of nitrate transfer functions in oligotrophic lakes: (1) even in lakes with nitrate
concentrations below quantification (<1 μg L−1), diatom assemblages were already dominated by species indicative of moderate N enrichment; (2) N-limited oligotrophic lakes
switch to P limitation after receiving only modest inputs of reactive N, shifting the controls on diatom species changes along
the length of the nitrate gradient. These constraints suggest that quantitative inferences of nitrate from diatom assemblages
will likely require experimental approaches. 相似文献
8.
Cladocera as indicators of trophic state in Irish lakes 总被引:1,自引:0,他引:1
We examined the impact of lake trophic state on the taxonomic and functional structure of cladoceran communities and the role
of nutrient loading in structuring both cladoceran and diatom communities. Surface sediment assemblages from 33 Irish lakes
were analysed along a gradient of total phosphorus concentration (TP; 4.0–142.3 μg l−1), using a variety of statistical approaches including ordination, calibration and variance partitioning. Ordination showed
that the taxonomic structure of the cladoceran community displayed the strongest response to changes in lake trophic state,
among 17 measured environmental variables. Trophic state variables chlorophyll-a and TP explained about 20% of the variance in both cladoceran and diatom assemblages from a set of 31 lakes. Procrustes analysis
also showed significant concordance in the structure of cladoceran and diatom communities (P < 0.001). Thus, lake trophic state affects the taxonomic structure of both primary and secondary producers in our study lakes.
We also found a significant decrease in relative abundance of taxa associated with both macrophytes and sediments, or sediments
only, along the TP gradient (r = −0.49, P = 0.006, n = 30), as well as an increase in the proportion of the planktonic group (r = 0.43, P = 0.017, n = 30). This suggests that cladoceran community structure may also be shaped by lake trophic state indirectly, by affecting
habitat properties. We found no relationship between lake trophic state and the relative abundance of each of three cladoceran
groups that display different body size. We compared community structure between bottom and top sediment samples in cores
from six Irish lakes. Results revealed similar trajectories of nutrient enrichment over time, as well as a strong shift in
cladoceran functional structure in most systems. This study confirms that Cladocera remains in lake sediments are reliable
indicators of lake trophic state. This study also highlights the fact that taxonomic and functional structure should both
be considered to account for the multiple factors that shape cladoceran communities. 相似文献
9.
Piero Guilizzoni Aldo Marchetto Andrea Lami Stefano Gerli Simona Musazzi 《Journal of Paleolimnology》2011,45(4):433-445
We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured
sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation
(P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large,
deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer
function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in
sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting
results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP
values higher than 100 μg l−1, the pigment model correctly estimated TP in lakes when TP was <100 μg l−1, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes
and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment
model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related
to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’
re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions. 相似文献
10.
Dynamic adjustment of training sets (‘moving-window’ reconstruction) by using transfer functions in paleolimnology—a new approach 总被引:1,自引:0,他引:1
Thomas Hübener Mirko Dreßler Anja Schwarz Kirsten Langner Sven Adler 《Journal of Paleolimnology》2008,40(1):79-95
This paper presents a new method (moving-windows) that optimizes diatom-based paleolimnological reconstructions of past environmental
conditions from supra-regional training sets. The moving-window method identifies the best number of nearest neighbours (window
size) from a merged supra-regional EDDI and local (MV) training set (n = 429) for each fossil diatom assemblage and the best type of transfer function (ML, WA-PLS) based on the error statistic
of each transfer function (highest cross-validated R
2, lowest cross-validated average bias, maximum bias and RMSEP). At first we evaluated the moving-window approach by comparing
measured TP-values with inferred TP-values using both the moving-window approach and the WA-PLS method. The relative errors
of the moving-window approach were not significantly different for 208 samples that had an error <15 μg/l TP using the WA-PLS
method. However, for the remaining 221 samples with errors >>15 μg/l TP using the WA-PLS method, the moving window approach
significantly reduced the relative error of the inferred TP levels. Secondly, the moving- window approach was used to reconstruct
epilimnetic total phosphorous (TP) for Lake Dudinghausen, Lake Rugensee, Lake Tiefer See and Lake Drewitzer See (Northern
Germany) using both the supra-regional EDDI training set and a local training set from Northern Germany (MV training set).
The moving-window inferred TP-levels of the four study lakes were compared with published reconstructed TP-values and with
inferred TP-values based on the local MV training set. Overall, the moving-window and the published TP-trends agree well with
each other. However, the moving-window reconstructions generally indicated lower TP-levels throughout the past ∼5,000 to 12,000 years,
including past maxima. Thus, the moving-window method seems to generate more realistic absolute TP levels due to the optimized
window size (highest number of modern analogues, best error statistics). The identification of more realistic absolute historic
TP-values is important for the validation of reference conditions for inland waters. This study also demonstrates that a robust
local training set may, similar to moving-window training sets, also lead to reliable reconstructions, if the geological settings
of the local training set lakes and the study lakes are similar. 相似文献
11.
The Ramsar-listed Coorong lagoon lies at the terminus of the Murray-Darling River system in South Australia. Diatom and foraminifera
relationships with water quality were characterised in order to develop diatom- and foraminifera-based models with the potential
to infer water column salinity. Seventy-four samples were collected during 2007, a year of continuing drought in the catchment,
and of no discharges at the Murray Mouth. The sample sites had a salinity gradient of 1.8–190 g l−1 total dissolved solids. The diatom data set comprised 215 taxa, while there were only eight taxa in the foraminiferal data
set. Canonical correspondence analysis of diatom species-environment relationships showed that salinity explained the largest
proportion of diatom variance. Hence, a diatom-based salinity transfer function with reasonable predictive power (measured
vs. diatom-inferred salinity r
jack2 = 0.82; Root Mean Squared Error of Prediction = 16 g l−1) was developed. Application of the transfer function to fossil diatom assemblages from The Coorong suggested that pre-European
salinity values were generally >50 g l−1 and that salinity declined following settlement. These results, however, contradict the recent history of The Coorong where
there have been substantial lagoon-wide salinity increases. The pre-impact diatom flora has no analogue in the modern data
set, highlighting the degree of departure from past conditions. CCA of the foraminiferal data set identified salinity and
total nitrogen as the variables with the greatest explanatory power. However, accurate predictive models could not be developed
using either variable due to low foraminiferal abundance and species richness. These factors may have been a consequence of
diminished foraminiferal recruitment rates over successive years, an artefact of reduced marine water input to The Coorong.
Future attempts to generate predictive models from this region would benefit from the inclusion of data from distant locations,
since suitable analogue sites do not exist in close proximity. The study has generated useful insights to the apparently broad
salinity tolerances for several cosmopolitan diatom and foraminifera species, and has identified a number of diatom and foraminifera
taxa that may prove useful in the qualitative interpretation of down-core trends in The Coorong and the lower Murray River
region. 相似文献
12.
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
13.
Testate amoebae are informative about palaeoecological conditions, but the methods generally used for their analyses in lake
sediments differ from those used for their analyses in peats, making comparisons difficult. This study examines how filter
mesh size and total number of individuals counted affect species richness, Shannon diversity, equitability, density and assemblage
structure. We analysed the complete testate amoeba contents of six sediment samples from Lake Lautrey, France. The abundance
of testate amoebae was high (1,403–10,870 shells cm−3), and species smaller than 63 μm in both length and width represented up to 89% of total abundance and 43% of species richness.
A simulation showed that using 47- or 63-μm mesh-size filters reduced inter-sample differences and changed the patterns of
abundance, species richness and assemblage structure, causing loss of information and leading to potential erroneous palaeoecological
interpretation. Rarefaction analyses suggest that although 170 shells are sufficient to assess the general structure of assemblages,
such small sample sizes can underestimate species richness by overlooking taxa with relative abundances <4%. Total counts
of 400 shells yield better estimates of assemblage structure and recover at least 50% of total species richness, although
species with absolute frequencies below 2% may still be missed. Higher counts are required to obtain reliable estimates of
species richness and assemblage structure in samples that have high testate amoeba densities but are dominated by a few small
taxa. Further studies should determine the bioindicator value and functional roles of small and/or rare species in lakes and
thus to what extent overlooking them affects palaeoecological interpretations. 相似文献
14.
A diatom-based Holocene record of human impact from a coastal environment: Tuckean Swamp, eastern Australia 总被引:1,自引:0,他引:1
Kathryn H. Taffs Luc J. Farago Hendrik Heijnis Geraldine Jacobsen 《Journal of Paleolimnology》2008,39(1):71-82
Diatom-based paleolimnological studies are being increasingly used to track anthropogenic change in estuaries. Little is known,
however, about the direction and nature of long-term environmental changes in Australian estuaries. In this study, shifts
in diatom assemblages preserved in a 210Pb and C14 AMS dated sediment core from Tuckean Swamp were analysed to determine environmental changes that had taken place as a result
of changing land-use practices. Prior to European impact, the diatom assemblage remained relatively stable and was dominated
by Actinocyclus normanii and Diploneis smithii. An increasing dominance of Cyclotella meneghiniana correlates well with changed land use activities in the catchment area and indicates an increase of freshwater influence
in the swamp’s environment. A major shift in species composition began ∼1970, Eunotia flexuosa becoming dominant. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered
by human activities such as vegetation clearance, drainage and the construction of a barrage. This study demonstrates the
use of paleolimnoology in an estuarine environment to provide pre-impact data necessary for management of the aquatic environment. 相似文献
15.
Trace metals and polycyclic aromatic hydrocarbons (PAH) were analyzed in sediment cores from three central Alberta lakes to
determine the contributions of local coal-fired power plants to contaminant loadings. In Wabamun Lake, with four power plants
built since 1950 within a 35-km radius, sediment concentrations of mercury, copper, lead, arsenic and selenium have increased
by 1.2- to 4-fold. Trace metal enrichments were less pronounced in Lac Ste. Anne and Pigeon Lake, situated 20 km north and
70 km south of Wabamun Lake, respectively. Total Hg flux to Wabamun Lake sediments (21–32μg m−2 yr−1) has increased 6-fold since 1950, compared to 2- and 1.5-fold increases in Lac Ste. Anne and Pigeon Lake, respectively, since
circa 1900. Total PAH flux to surface sediments was 730–1100μg m−2 yr−1 in Wabamun Lake, 290–420μg m−2 yr−1 in Lac Ste. Anne, and 140–240μg m−2 yr−1 in Pigeon Lake. Without adoption of pollution-abatement technology that compensates for increases in generating capacity,
continued expansion of coal-burning industry in Alberta will result in increased contaminant deposition, primarily from local
sources. 相似文献
16.
Pierre Giresse Makaya Mvoubou Jean Maley Alfred Ngomanda 《Journal of Paleolimnology》2009,41(2):369-392
Seven vibro-cores were collected from three shallow lakes of the Gabon (Kamalété, Nguène, Maridor) along a 300-km west–east
transect close to the Equator. These lakes are located in very distinct landscapes: coastal forest-savanna mosaic, rain forest
and savanna with colonising forest, respectively. Core chronologies were established by radiocarbon dating. Study of these
lacustrine archives (textural variables, clay minerals, organic matter components, δ13C, pollen) allowed comparison of late Holocene environmental changes recorded at each site and with results from other studies.
Lake Kamalété indicates minor climatic deterioration (increased drying and greater seasonality) between 1,410 and 500 cal.
years BP, which is also recognised in southern Cameroon and east-central Africa. Lake Nguène was surrounded by dense moist
forest throughout the last 4,110 years, but shows significant deterioration from ~2,800 cal. years BP, a phenomenon seen at
nearby sites. Lake Maridor shows a decline of forest initiated a little after 3,800 cal. years BP, which indicates timing
that is distinct from the two other sites. This was probably a response to local conditions (i.e. outlet damming). Although
the three lakes display generally parallel climatic trends perhaps linked to SST oscillations, there is not perfect coherence
between these three sites. Differences among the three basins may be attributable to local factors like groundwater hydrology
and slope instabilities of such shallow lake systems in this equatorial region. 相似文献
17.
Sieving samples for chironomid analysis with a 150 μm mesh was shown to greatly reduce sample preparation time, and use of
only larger specimens did not affect chironomid-inferred salinities in African lakes. Here, we tested if this method is suitable
for temperature reconstruction in colder lakes at higher latitudes. Removal of specimens <150 μm in two training sets, one
from Canada and one from Sweden, had little impact on the performance statistics of the calibration models. Chironomid abundance,
however, decreased greatly because more than half of the head capsules in assemblages were <150 μm. This had major impacts
on the temperature reconstructions. Inferences were on average 2°C warmer with the modified models (all specimens >150 μm)
than those obtained with the full model (all specimens >100 μm). General patterns of temperature change were also altered.
For Lake 7 on Southampton Island, Canada, a cooling trend was reconstructed with the full Canadian model while the modified
Canadian model yielded a warming trend. When only specimens >150 μm were used, two to three times more wet sediment was needed
to obtain a sufficient number of head capsules. These results indicate that, in cold lakes (mean July/August air temperature
≤11°C), large proportions of head capsules are <150 μm, and sieving the samples in a 150 μm mesh leads to altered temperature
reconstructions. 相似文献
18.
Elizabeth J. Barnett 《Journal of Paleolimnology》1994,12(3):259-268
An investigation of Holocene sediments within Lake Alexandrina, a shallow coastal lake at the mouth of the Murray River, South Australia, is presented based on a multidisciplinary approach.14C and210Pb radiometric dating methods are used to establish a geochronological framework for the last 7000 yr BP, and diatoms, sand-siltclay ratios, organic carbon, phosphorus and copper concentrations are used to infer paleoenvironmental changes.The diatom assemblages indicate a change from marine-brackish to oligosaline-freshwater conditions between 7000 and 6000 yr BP, with sea-level stabilisation and continuous barrier formation across the Murray mouth. Sand pulses after 2300 yr BP document sand spit formation in the lake and the commencement of extensive lacustrine sedimentation.In the past 100 years which include the advent of European settlement in the region, the short-term210Pb-based mass accumulation rate of 0.063 g cm–2 yr–1 is greater than that of the longer-term mean14C-based rate (0.023 g cm–2 yr–1), and high concentrations of phosphorus and copper correspond to historical records of blue-green algal blooms.This is the fifth in a series of papers published in this issue on the paleolimnology of aric regions. These papers were presented at the Sixth International Paleolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers. 相似文献
19.
The contemporary distribution of benthic diatoms and their use as ecological indicators were examined in a coastal wetland, the Ebro Delta, as a representative of environmental conditions in Mediterranean coastal wetlands. A total of 424 diatom taxa were identified across 24 sites encompassing a wide range of wetland habitat types (coastal lagoons, salt and brackish marshes, shallow bays, microbial mats and nearshore marine waters) and conductivities. Canonical correspondence analysis showed that water conductivity and water depth were the main factors structuring the diatom assemblages. Cluster analysis identified five habitat types according to the similarity in diatom species composition: salt marshes, brackish marshes, brackish coastal lagoons and bays, coastal lagoons with fresher conditions, and nearshore open sea. For each wetland habitat, diatom indicator species were identified. Partial canonical correspondence analysis showed that water conductivity, a proxy for salinity, was the most statistically significant and independent variable for explaining the distribution of benthic diatoms in the study area. A transfer function, using a weighted average regression model, was developed for conductivity and displayed reasonable performance (r 2 = 0.64; RMSEP = 0.302 log10 mS/cm). Our study in the Ebro Delta provides a basis for using diatom assemblages to make quantitative conductivity inferences, and for using diatom indicator species to identify wetland habitats. These approaches are complementary and may be valuable for paleoenvironmental studies of (1) effects of large-scale, natural changes in the Delta (e.g. sea-level fluctuations), and (2) impacts of short-term anthropogenic changes, such as the introduction and development of rice agriculture. 相似文献
20.
We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central
Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the
Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands.
The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves
decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified,
suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate
paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records
from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences
in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in
paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity
throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the
1970s far exceeds any recorded during the Holocene. 相似文献