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David A. Caron Mark R. Dennett Darcy J. Lonsdale Dawn M. Moran Ludmilla Shalapyonok 《Deep Sea Research Part II: Topical Studies in Oceanography》2000,47(15-16)
Microplankton abundances and phytoplankton mortality rates were determined at six stations during four cruises spanning three seasons in the Ross Sea polynya, Antarctica (early spring, Oct.–Nov. 1996; mid-late summer, Jan.–Feb. 1997; fall, Apr. 1997; mid-late spring, Nov.–Dec. 1997). Rates of microzooplankton herbivory were measured using a modified dilution technique, as well as by examining the rate of disappearance of phytoplankton (chlorophyll) in samples incubated in the dark (i.e. grazing in the absence of phytoplankton growth). Strong seasonal cycles of phytoplankton and microzooplankton abundance were observed during the study. Microzooplankton abundance varied by more than three orders of magnitude during the four cruises, and was positively correlated with phytoplankton biomass over the entire data set. Nevertheless, microzooplankton grazing was insufficient to impact significantly phytoplankton standing stocks during most of the experiments performed in this perenially cold environment. Only thirteen out of a total of 51 experiments yielded phytoplankton mortality rates that were significantly different from zero. The highest mortality rate observed in this study (0.26 d−1) was modest compared with maximal rates that have been observed in temperate and tropical ecosystems. Results from twenty experiments examining the rate of decrease of phytoplankton biomass during incubations in the dark agreed quite well with the results of the dilution experiments performed at the same time. The range of mortality rates for the dark incubations was −0.09–0.06 d−1, and the average was essentially zero (−0.01 d−1). That is, chlorophyll concentration was virtually unchanged in samples incubated in the dark for 3 d. A number of factors appeared to contribute to the very low rates of microbial herbivory observed, including low water temperature, and the size and taxonomic composition of the phytoplankton assemblage. Based on our results we conclude that the seasonal, massive phytoplankton blooms observed in the Ross Sea are due, in part, to low rates of removal by microbial herbivores. 相似文献
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François Chabaux Mathieu Granet Philippe Larqué Jean Riotte Eugeni V. Skliarov Olga Skliarova Ludmilla Alexeieva François Risacher 《Comptes Rendus Geoscience》2011,343(7):462-470
The geochemical study of springs and lake waters from the Ol’khon Region, Siberia, confirms the strong chemical variability of these water samples, more particularly regarding their salinity. Such variability does not result from a simple mixing process between surface freshwaters and deeper saline waters. The variability, observed at the scale of a few square kilometers, would preferentially result from a secondary concentration processes (evaporation and/or cryogenesis) of lake waters of variable intensity from one lake to another. The U-disequilibria data suggest that the duration of the secondary process is certainly an important parameter to be considered to account for the variable salinity of these lakes. The data indicate that the lakes, however modest in size, could be as old as several ky, confirming therefore that the lake sedimentary deposits could represent relevant paleoenvironmental recorders for the last thousands years. 相似文献
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Comet C/1999 S4 (LINEAR) was exceptional in many respects. Its nucleus underwent multiple fragmentations culminating in the complete disruption around July 20, 2000. We present circular polarization measurements along the cuts through the coma and nucleus of the comet during three separate observing runs, in June 28-July 2, July 8-9, and July 21-22, 2000. The circular polarization was detected at a rather high level, up to 0.8%. The left-handed as well as right-handed polarization was observed over the coma with the left circularly polarized light systematically observed in the sunward part of the coma. During our observations the phase angle of the comet varied from 61 up to 122°, which allowed us to reveal variations of circular polarization with the phase angle. Correlation between the degree of circular polarization, visual magnitude, water production rate, and linear polarization of Comet C/1999 S4 (LINEAR) during its final fragmentation in July 2000 was found. The mechanisms that may produce circular polarization in comets and specifically in Comet C/1999 S4 (LINEAR) are discussed and some tentative interpretation is presented. 相似文献
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