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The late‐glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000–13 000 14C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 14C yr BP, and thus clearly pre‐dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 14C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre‐Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in‐wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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Cladoceran microfossil remains were analysed from a sediment core taken from a lake basin at Kråkenes, western Norway. The sequence included immediate post-glacial conditions (ca. 12,300 14C BP), the Allerod, Younger Dryas, and early Holocene to approximately 8,500 14C BP. The interpretation of changes in the cladoceran assemblages is based on the known ecology of the taxa, the documented environmental history of the study sequence, the variations in the organic content of the sediment, the radiocarbon dates, and the results of analyses of other biotic groups, including diatoms, macrophytes, and chironomids. In addition, a quantitative reconstruction of changes in air temperature is presented for the study period. This reconstruction is based on transfer functions developed from a separate Swiss surface-sediment cladoceran data set.The cladoceran assemblages throughout the sequence are dominated by littoral chydorid taxa. Bosmina, Daphnia, and Simocephalus represent the open-water component of the zooplankton. Chydorus piger and Daphnia were the only immediate post-glacial pioneer taxa. A rapid proliferation of the open-water and littoral cladoceran taxa began with the onset of the Allerod and persisted for approximately 1,000 yrs. At the start of the Younger Dryas a local glacier formed and drained into the lake, causing a sudden decline in chydorid diversity, with only Chydorus sphaericus and Acroperus harpae persisting throughout this period. Chydorid diversity started to recover in the upper Younger Dryas and continued in the early Holocene. Progressive acidification and oligotrophication are also discernible from the cladoceran assemblages present in the Holocene.The reconstructed mean summer air temperature was from 8-21 °C, with prediction errors of 1.8-2.5 °C. The Allerod was only slightly warmer than the Younger Dryas period, but a progressive increase in temperature is apparent during the early Holocene. In conclusion, the results of this study provide a further demonstration of the value of cladocera as indicators of a variety of palaeoenvironmental parameters, including temperature. 相似文献
14.
Palaeolimnological Development of Lake Njargajavri, Northern Finnish Lapland, in a Changing Holocene Climate and Environment 总被引：1，自引：2，他引：1
Kaarina Sarmaja-Korjonen Marjut Nyman Seija Kultti Minna Väliranta 《Journal of Paleolimnology》2006,35(1):65-81
This study used palaeolimnological approaches to determine how Holocene climatic and environmental changes affected aquatic
assemblages in a subarctic lake. Sediments of the small Lake Njargajavri, in northern Finnish Lapland above the present treeline,
were studied using multi-proxy methods. The palaeolimnological development of the lake was assessed by analyses of chironomids,
Cladocera and diatoms. The lake was formed in the early Holocene and was characterized by prominent erosion and leaching from
poorly developed soils before the establishment of birch forests, resulting in a high pH and trophic state. The lake level
started to lower as early as ca. 10,200 cal. BP. In the resulting shallow basin, rich in aquatic mosses, pH decreased and
a diverse cladoceran and chironomid assemblage developed. It is likely that there was a slight rise in the water level ca.
8000 cal. BP. Later, during the mid-Holocene characterized by low effective moisture detected elsewhere in Fennoscandia, the
lake probably completely dried out; this is manifest as a hiatus in the stratigraphy. The sediment record continues from ca.
5000 cal. BP onwards as the lake formed again due to increased effective moisture. The new lake was characterized by very
low pH. The possible spread of pine to the catchment and the development of heath community may have contributed to the unusually
steep (for northern Fennoscandia) decline in pH via change in soils, together with the natural decrease in leaching of base cations. Furthermore, the change in pH may have been
driven by cooling climate, affecting the balance of dissolved inorganic carbon in the lake. 相似文献
15.
Peter R. Leavitt Patricia R. Sanford Stephen R. Carpenter James F. Kitchell 《Journal of Paleolimnology》1994,11(2):133-149
Annual and subannual paleolimnological records of pigments and zooplankton were used to analyze three whole-lake manipulations. The relative abundance of cladoceran remains in recent seasonal laminae was significantly correlated with the relative abundance of species in the plankton (r
2=0.59,P<0.001). Comparison of food-web structure after piscivore introduction showed that there are multiple outcomes of predation and that final food-web structure depended on the strength of interaction between piscivorous and planktivorous fish. Intense predation on cyprinids (Phoxinus eos, P. neogaeus, Umbra limi) by largemouth bass (Micropterus salmoides) allowed large herbivores (Daphnia pulex) and invertebrate predators (Chaoborus punctipennis) to dominate. Analysis of fossil invertebrate morphology suggested that small grazers (Bosmina longirostris, Diaphanosoma birgei) were eliminated by invertebrate predators. Under moderate predation by rainbow trout (Oncorhynchus mykiss), cyprinids remained, only intermediate-size herbivores (D. rosea) increased in abundance, andBosmina persisted. In contrast to food-web manipulations, increased algal abundance resulting from watershed disturbance (road construction) did not alter the species composition or size-structure of fossil Cladocera. 相似文献
16.
The introduced collecting method is based on a common entomologist exhauster, which is specified for the fieldwork in aquatic systems with a sieve-inset and a bottom-lid. The living organisms are collected visually from the sampled debris and no further extraction is needed. The apparatus is designed for qualitative sampling of microcrustaceans (e.g. Cladocera, Copepoda, Ostracoda) and minimises disturbance at the sampling site. 相似文献
17.
N. Emir 《洁净——土壤、空气、水》1994,22(6):280-288
This research has been undertaken to determine the zooplanktonic fauna (Rotatoria, Crustacea) and their seasonal distribution in two lakes with different characters. Çavu?cu lake has a surface area of 1000 ha with a maximum depth of 4 m and is classified as an oligo-mesotrophic lake. Eber lake has a surface area of 5200…17000 ha with a maximum depth of 2.5 m and is classified as an eutrophic lake. Zooplankton samples have been collected seasonally between the period of 1990 to 1993 from three stations with a zooplankton net of 44 μm mesh size. From Çavu?cu lake totally 50 Rotifera, 2 Copepoda, and 7 Cladocera species; 49 phytoplankton genera and 5 macrophytes have been identified while, 37 Rotifera, 3 Copepoda, 5 Cladocera; 45 phytoplankton genera and 6 macrophytes have been identified from Eber lake. 13 new rotifer records for the Turkey were found, which were Brachionus diversicornis, Macrochaetus collinsi, Lecane stichaeta, Cephalodella sterea, Cephalodella auriculata, Trichocerca iernis, Encentrum saundersiae. Pompholyx complanata, Floscularia ringens, Conochilus natans, Filinia pejleri, Rotaria rotatoria, and Philodina megalotrocha. 相似文献
18.
We studied Holocene lake-level fluctuations from a small lake, Iso Lehmälampi, southern Finland, utilizing cladoceran and diatom analyses. We report data from a sediment core (A) taken from the deepest part of the lake (8.1 m) where two layers of moss, mixed with gyttja, were found. These layers were formed in situ during the early Holocene (1. ca. 8100-7900, 2. ca. 7300 BP). Lake-level fluctuations were inferred also from another core C, which did not have moss layers. According to the ratio of planktonic/littoral Cladocera, the water level was high around 9000 BP and started to fall before 8000 BP. The lowering continued until 7000 BP and the moss layers were formed during this lowering. Water level was high again ca. 6000 BP and lowered towards ca. 4000 BP. The late Holocene is characterized by several rapid fluctuations of lake-level. The ratio of planktonic/littoral Cladocera and the diatom species composition in core A showed drastic changes between the moss layers and the non-moss gyttja sections of the core. We suggest that they reflect changes in sedimentary facies between the local moss environment and the pelagic bottom. Thus, cores which contain moss layers may lead to erroneous interpretations of lake-level fluctuations. 相似文献
19.
Adam Jeziorski Andrew M. Paterson Norman D. Yan John P. Smol 《Journal of Paleolimnology》2008,39(3):421-425
Despite reductions in atmospheric sulphur emissions and the resulting decline in acidic deposition, many lakes on the Canadian
Shield that have experienced acidification are either recovering at a rate slower than expected or not recovering at all.
This lack of recovery is believed to be partly the consequence of the depletion of exchangeable base cations (principally
calcium, Ca) from watershed soils. Although the implications of reduced Ca availability for biological communities remain
poorly understood, the effects on crustacean zooplankton populations may be severe, as Ca is the primary structural component
of the crustacean zooplankton exoskeleton. Because the daphniid resting egg is protected by a modified portion of the exoskeleton
(ephippia) and because these ephippia are well preserved in lake sediments, we investigated whether inter-specific differences
in Ca content are recoverable from Daphnia ephippia. However, using two methods, we did not find a recoverable Ca signal in the ephippia, indicating that future efforts
should be focused upon sedimentary taxonomic assemblage differences, not differences in ephippial Ca levels. 相似文献
20.
We quantified the seasonal changes in the zooplankton abundances collected from the Huetzalin Lake (Mexico City, Mexico) for two years (February 2003–January 2004 and then March 2005–February 2006). Selected physicochemical variables (Secchi depth, temperature, pH, conductivity, dissolved oxygen, phosphorus, nitrogen, carbon and chlorophyll a concentration) were also measured at the time of zooplankton collection. The data on zooplankton abundances and the physicochemical variables were subjected to multiple correlation analysis and we also derived Shannon–Wiener species diversity index. Secchi depth ranged from 9 to 65 cm. Generally the lake was alkaline (pH 7–12). The conductivity ranged from 500–1000 mS cm−1, while the mean water temperature was 20.5 °C. Dissolved oxygen levels were generally >3 mg L−1 and were higher in the winter than warmer months. Nitrates (90–95 μg L−1) and phosphates (.2–.5 mg L−1) indicated that the water was eutrophic. Chlorophyll a levels ranged from 143 to 696 μg L−1 during the study period. The zooplankton community was dominated by rotifers (46 species), followed by cladocerans (9 species) and there were only two copepod species. The dominant rotifer species were Brachionus angularis, Brachionus calyciflorus, Brachionus havanaensis, Brachionus quadridentatus, Lecane bulla and Polyarthra vulgaris. Rare rotifer species in Lake Huetzalin were Lecane ohioensis, Dicranophorus forcipatus, Lecane pyriformis, Lindia torulosa, Pleurotrocha petromyzon and Brachionus durgae. Highest densities (occasional peaks of 400 ind L−1) of B. quadridentatus occurred between April and December, while B. havanaensis reached peak densities, during June to October. B. calyciflorus reached densities higher than 1240 ind L−1 during May–September. Cladoceran and copepod densities in Lake Huetzalin were much lower than that of the rotifers. This study confirmed the earlier findings that Xochimilco system of canals is dominated by rotifers and the crustacean zooplankton have much lower abundances possibly due to predation from fish. 相似文献