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11.
We investigated the modern distribution of fossil midges within a dimictic lake and explored downcore patterns of inferred lake depths over the last 2000 years from previously published proxies. Modern midge distribution within Gall Lake showed a consistent and predictable pattern related to the lake-depth gradient with recognizable assemblages characteristic of shallow-water, mid-depth and profundal environments. Interpretations of downcore changes in midge assemblages, in conjunction with quantitative lake-depth inferences across a priori defined (based on diatom data) ~ 500-yr wet and dry periods, demonstrated that both invertebrate and algal assemblages exhibited similar timing and nature of ecological responses. Midges were quantified by their relative abundance, concentrations and an index of Chaoborus to chironomids, and all showed the greatest differences between the wet and dry periods. During the low lake-level period of the Medieval Climate Anomaly (MCA: AD 900 to 1400), profundal chironomids declined, shallow-water and mid-depth chironomids increased, chironomid-inferred lake level declined and the Chaoborus-to-chironomid index decreased. The coherence between multiple trophic levels provides strong evidence of lower lake levels in Gall Lake during the MCA. 相似文献
12.
Gaute Velle Richard J. Telford Oliver Heiri Joshua Kurek H. John B. Birks 《Journal of Paleolimnology》2012,48(3):545-558
Most calibration data sets used to infer past environmental conditions from biological proxies are derived from many sites. An alternative strategy is to derive the calibration data set from within a single site. Transfer functions derived from such intra-site calibration data sets are usually applied to fossil assemblages from the focal lake, but a recent development has been to apply these transfer functions to other sites. Transfer functions derived from intra-site calibration data sets can have impressive cross-validation performance, but that gives little indication of their performance when applied to other sites. Here, we develop transfer functions for lake depth from intra-lake chironomid calibration data sets in Norway and Alaska and test the resulting models by cross-validation and against known depth in external lakes. Lake depth is a statistically significant predictor of chironomid assemblages at all these lakes, and most intra-lake transfer functions perform reasonably well under cross-validation, but their performance against external data is erratic. Downcore reconstructions from transfer functions developed on different lakes are dissimilar. Ignoring the poorly performing transfer functions, only 3 of 14 downcore reconstructions are statistically significant. Few assemblages downcore had good modern analogues in the calibration data set, even when the core was from the same lake as the calibration data set. We conclude that intra-site calibration data sets can find site-specific rather than general relationships between species and the environment and thus should be applied with care and to external sites only after careful and critical validation. 相似文献
13.
The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ18O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between 17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at 16.5 ka. Total pollen accumulation rates ranged between 180 and 1200 grains cm− 2 yr− 1. At 15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At 14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from 15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from 4.5 to 2 ka. 相似文献
14.
Wlodzimierz Kurek Dr. 《GeoJournal》1996,38(2):191-196
The Polish Carpathians is one of the regions with the best rural demographic and economic situation in Poland, as indicated by the growth of population, the very advantageous sex and age structure, the low unemployment rate and good housing standards. However, the situation in agriculture is complicated due to the unfavourable natural conditions of the mountainous terrain, the fragmentation of holdings and the low level of efficiency. On the other hand, tourism is a positive factor in the socio-economic transformation of the Carpathian countryside. Tourism was a significant activity even before the Second World War when lodgings were sought in private houses in many villages of the Sub-Tatra, Podhale Basin and the Silesian and Sacz Beskides. The villages currently active as tourist centres show the most dynamic development and the highest level of the socio-economic infrastructure. 相似文献
15.
Restructuring the Polish countryside is a complex and urgent task. Adapting agriculture for EU entry will mean a considerable
reduction in employment and rural diversification is needed because other sectors of the national economy will not be able
to absorb all the displaced people if they arrive in the towns. However, providing non-agricultural employment in rural areas
depends to a large extent on the technical infrastructure available: running water, sewerage systems, gas supply and telecommunications.
The paper deals with voivodeships in southern and southwestern Poland (formerly Galicia) and shows that while much progress
has been made since the end of the communist period, the inherited inequalities are being perpetuated and in some cases increased.
However, even where the infrastructure is satisfactory, private business does not develop at a uniform rate and action is
needed to stimulate communities in remoter rural areas, particularly in the eastern part of the study region.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
Erin M. Barley Ian R. Walker Joshua Kurek Les C. Cwynar Rolf W. Mathewes Konrad Gajewski Bruce P. Finney 《Journal of Paleolimnology》2006,36(3):295-314
Freshwater midges, consisting of Chironomidae, Chaoboridae and Ceratopogonidae, were assessed as a biological proxy for palaeoclimate in eastern Beringia. The northwest North American training set consists of midge assemblages and data for 17 environmental variables collected from 145 lakes in Alaska, British Columbia, Yukon, Northwest Territories, and the Canadian Arctic Islands. Canonical correspondence analyses (CCA) revealed that mean July air temperature, lake depth, arctic tundra vegetation, alpine tundra vegetation, pH, dissolved organic carbon, lichen woodland vegetation and surface area contributed significantly to explaining midge distribution. Weighted averaging partial least squares (WA-PLS) was used to develop midge inference models for mean July air temperature (r
boot2 = 0.818, RMSEP = 1.46°C), and transformed depth (ln (x+1); r
boot2 = 0.38, and RMSEP = 0.58). 相似文献
17.
Joshua Kurek Jennifer B. Korosi Adam Jeziorski John P. Smol 《Journal of Paleolimnology》2010,44(2):603-612
The effects of low counts on assemblage inferences in paleolimnological investigations have been examined for many biological
proxies, but not yet for Cladocera. Established guidelines leading to the determination of an adequate, minimum count are
absent with respect to sampling cladoceran remains from lake sediments. Using simulated subsamples derived from observed assemblages
of considerably higher counts, we investigated the effect of counting effort on three principal characteristics of an assemblage:
richness, number of new taxa encountered, and the absolute differences in relative abundances of dominant taxa. Results from
six lakes located within diverse ecological regions (i.e. Subarctic, Canadian Shield, Acadian Forest) suggest that a minimum
of between 70 and 100 individuals is satisfactory to characterize most assemblages. Doubling counting effort from 100 to 200
individuals leads to only modest gains in subsample relatedness to the observed assemblage. Greater counting effort may be
necessary when the primary interest is in presence-absence or distributional data, or when abundances of ecologically relevant
taxa are exceptionally low. 相似文献
18.
One of the critical technological parameter in operation of aerated submerged fixed‐bed biofilm reactor (ASFBBR) is the control over process of biofilm detachment. Both, an excessive growth of biomass with its accumulation in the bioreactor and an exceeded biofilm detachment rate, cause serious operational and technological problems. The studies presented in this paper demonstrate that in an ASFBBR reactor with a PVC bed carrier media, an effective way to maintain a proper shear stress acting on the biofilm and causing its detachment could be an internal recirculation of wastewater instead of backwashing with wastewater or air flushing. In case of polishing of oil‐refinery wastewater with average COD loading rate equal to 9 g COD/(m2 day), the minimum value of the hydraulic loading rate in such reactor is 1.9 m3/(m2 h), at which there was no excessive growth of biofilms. Despite a significant decrease of the biofilms thickness and amount of biomass in the reactor, there was no significant decline in the efficiency of pollutant removal from oil‐refinery wastewater, which made it possible to obtain the quality of effluent at the outlet of the bioreactor significantly below the water permits and standards. 相似文献
19.
The focus of this paper is on the analysis of the influence of particles’ velocity distribution and their concentration on the sediment transport rate in bed-load from the Lagrangian perspective.Such aims are addressed by use of the relevant Lagrangian model of spherical saltating particles in which turbulence plays a significant role.The Monte Carlo approach is employed in the simulations to obtain the velocity and concentration of the saltating particles.Numerical simulations using two saltating particle models:(1) with;and(2) without particle-particle collisions are carried out.Based on the numerical results,we address the hypothesis that instead of averaged characteristics,the distributions of particles’ velocity and sediment concentration should be taken into account in the calculation of sediment transport rate.Moreover,our results also show that the interaction between particles during their collisions is the key for better understanding of the physics of sediment transport.All results are compared with well-known experimental formulae i.e.Meyer-Peter and Muller and Fernandez Luque and van Beek. 相似文献
20.
Matthew J. Wooller Joshua Kurek Benjamin V. Gaglioti Les C. Cwynar Nancy Bigelow Joshua D. Reuther Carol Gelvin-Reymiller John P. Smol 《Journal of Paleolimnology》2012,48(1):83-99
Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS 14C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??13C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??18O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??18OPisidiidae and ??13CPisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??18O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers. 相似文献