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  • 1
    Electronic Resource
    Electronic Resource
    Response of tundra ecosystem in southwestern Alaska to Younger-Dryas climatic oscillation (2002)
    Oxford, UK : Blackwell Science Ltd
    Global change biology 8 (2002), S. 0 
    Details
    ISSN: 1365-2486
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Notes: Climatic warming during the last glacial–interglacial transition (LGIT) was punctuated by reversals to glacial-like conditions. Palaeorecords of ecosystem change can help document the geographical extent of these events and improve our understanding of biotic sensitivity to climatic forcing. To reconstruct ecosystem and climatic variations during the LGIT, we analyzed lake sediments from southwestern Alaska for fossil pollen assemblages, biogenic-silica content (BSiO2%), and organic-carbon content (OC%). Betula shrub tundra replaced herb tundra as the dominant vegetation of the region around 13 600 cal BP (cal BP: 14C calibrated calendar years before present), as inferred from an increase of Betula pollen percentages from 〈〈 5% to 〉〉 20% with associated decreases in Cyperaceae, Poaceae, and Artemisia. At c. 13 000 cal BP, a decrease of Betula pollen from 28 to 〈〈 5% suggests that shrub tundra reverted to herb tundra. Shrub tundra replaced herb tundra to resume as the dominant vegetation at 11 600 cal BP. Higher OC% and BSiO2% values suggest more stable soils and higher aquatic productivity during shrub-tundra periods than during herb-tundra periods, although pollen changes lagged behind changes in the biogeochemical indicators before c. 13 000 cal BP. Comparison of our palaeoecological data with the ice-core dδ18O record from Greenland reveals strikingly similar patterns from the onset through the termination of the Younger Dryas (YD). This similarity supports the hypothesis that, as in the North Atlantic region, pronounced YD climatic oscillations occurred in the North Pacific region. The rapidity and magnitude of ecological changes at the termination of the YD are consistent with greenhouse experiments and historic photographs demonstrating tundra sensitivity to climatic forcing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2486.2002.00550.x
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  • 2
    Electronic Resource
    Electronic Resource
    Oxidation state of plutonium in the Irish Sea (1978)
    [s.l.] : Nature Publishing Group
    Nature 276 (1978), S. 599-601 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Based on comparisons with natural chemical analogues, the different oxidation states of plutonium are expected to show markedly different biogeochemical properties. Uranium and thorium, for example, are very similar in basic chemical properties to Pu(VI) and Pu(IV) respectively. Although the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/276599a0
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  • 3
    Electronic Resource
    Electronic Resource
    Morphogenesis of opal teeth in calanoid copepods (1990)
    Springer
    Marine biology 106 (1990), S. 91-101 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Opal teeth of calanoid copepods develop early in the premolt phase of the molt cycle. They form in the apolysis space beneath the old tooth row on the mandibular gnathobase. We examined stages of tooth formation inNeocalanus spp. andCalanus pacificus. Apolysis occurs early in the distal gnathobase, then several epidermal cell types participate sequentially in formation of new teeth. Fibrous molds in the shapes of the new teeth are extruded onto the epidermal surface, then additional materials, probably proteinaceous, are secreted into them. Tooth molds next are linked to a gland in the proximal part of the gnathobase by ducts of an unusual type, “lamellar-walled ducts.” Silicification follows, apparently using highly osmiophilic material supplied by the proximal gland. Opal is laid down at the outer periphery of the mold then thickens toward the attachment of the mold to newly formed chitin at its base. During apolysis the epidermal cells move proximally without breaking ducts that connect small dermal glands with pores in the sides of the opal teeth. The molds for the new teeth form surrounding these ducts, and new pores result. The glands are like the ciliary exocrine glands seen in other Crustacea, with the duct deriving from fusion of the axonemes of a cilium. Presumably the glands secrete a substance into food newly broken by the teeth. This could be a toxin or a digestive enzyme.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02114678
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  • 4
    Electronic Resource
    Electronic Resource
    Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. I. Nutrient effects (1979)
    Springer
    Marine biology 50 (1979), S. 305-312 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Two marine diatoms, Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal), were grown both separately and together in batch cultures on a mixture of waste water and seawater enriched with different components of f medium. At 17°C, the maximum division rates of the two species were statistically indistinguishable. The waste water-seawater mixture used proved to have insufficient Si, relative to N and P, for the growth of T. pseudonana, which requires approximately 5x10-14 g-at Si cell-1 to divide at a maximum rate. P. tricornutum, on the other hand, although capable of taking up nearly 9x10-15 g-at Si cell-1, could sustain maximum rates of division with 4.3x10-18 g-at Si cell-1 or less. No allelopathic interaction between the two species could be detected. We conclude that P. tricornutum enjoys a considerable competitive advantage over T. pseudonana in a waste water-seawater-based mariculture system that is not supplemented with Si. Although Si proved necessary for T. pseudonana to complete more successfully with the other diatom, the presence of excess amounts of Si is not necessarily sufficient for the maintenance of T. pseudonana in mixed continuous culture with P. tricornutum: other factors, such as light-related or photoperiod-related growth response, are believed to determine the ultimate outcome of competition between these algae in light-limited continuous culture.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00387007
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  • 5
    Electronic Resource
    Electronic Resource
    Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. II. Light limitation (1979)
    Springer
    Marine biology 50 (1979), S. 313-318 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The marine diatoms Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal) were grown under both continous illumination and a 14 h light: 10 h dark cycle at light intensities ranging from 1.53×10-4 to 2.95×10-1 ly min-1. Under both photoperiods, T. pseudonana exhibited higher division rates than P. tricornutum at high light intensities, but the reverse was true at all light intensities 〈3×10-3 ly min-1. Comparison of these results with available data on light-limited growth of other planktonic algae suggests that P. tricornutum may be unusually efficient at maintaining its cell division rate at low light intensity. This efficiency may contribute substantially to its success in turbid, nutrient-enriched mass algal culture systems, the only environments in which it is known to attain great numbers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00387008
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