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341.
342.
Thomas C. Malone Daniel J. Conley Thomas R. Fisher Patricia M. Glibert Lawrence W. Harding Kevin G. Sellner 《Estuaries and Coasts》1996,19(2):371-385
The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m?2 yr?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay. 相似文献
343.
344.
Surface hydrology is recognised as an important component of general circulation climate models. The global and regional climates simulated by such models are demonstrably sensitive to the parameterization of terrestrial hydrologic processes. There exists, therefore, a clear requirement to evaluate different parameterization approaches in terms of the representation of the terrestrial phase of the hydrologic cycle. One potential means of meeting this requirement is by using available continental water-balance summaries. In this study three versions of a GCM, the National Center for Atmospheric Research (NCAR) Community Climate Model Version l (CCM1), differing mainly in spatial resolution and the representation of the surface hydrology, are compared against existing water-balance studies. Additional streamflow data are incorporated as a means of further validating both the water-balance approach and the GCM surface hydrologic parameterization in capturing the gross features of continental-scale hydrology. 相似文献
345.
Thomas Rosswall 《Environmental Geology》1992,20(2):77-78
346.
347.
Preliminary study of crust-upper mantle structure of the Tibetan Plateau by using broadband teleseismic body waveforms 总被引:2,自引:0,他引:2
Lu-Pei Zhu Rong-Sheng Zeng Francis T. Wu Thomas J. Owens George E. Randall 《地震学报(英文版)》1993,6(2):305-316
As part of a joint Sino-U.S. research project to study the deep structure of the Tibetan Plateau, 11 broadband digital seismic
recorders were deployed on the Plateau for one year of passive seismic recording. In this report we use teleseimic P waveforms
to study the seismic velocity structure of crust and upper mantle under three stations by receiver function inversion. The
receiver function is obtained by first rotating two horizontal components of seismic records into radial and tangential components
and then deconvolving the vertical component from them. The receiver function depends only on the structure near the station
because the source and path effects have been removed by the deconvolution. To suppress noise, receiver functions calculated
from events clustered in a small range of back-azimuths and epicentral distances are stacked. Using a matrix formalism describing
the propagation of elastic waves in laterally homogeneous stratified medium, a synthetic receiver function and differential
receiver functions for the parameters in each layer can be calculated to establish a linearized inversion for one-dimensional
velocity structure.
Preliminary results of three stations, Wen-quan, Golmud and Xigatze (Coded as WNDO, TUNL and XIGA), located in central, northern
and southern Plateau are given in this paper. The receiver functions of all three stations show clear P-S converted phases.
The time delays of these converted phases relative to direct P arrivals are: WNDO 7.9s (for NE direction) and 8.3s (for SE
direction), TUNL 8.2s, XIGA 9.0s. Such long time delays indicate the great thickness of crust under the Plateau. The differences
between receiver function of these three station shows the tectonic difference between southern and north-central Plateau.
The waveforms of the receiver functions for WNDO and TUNL are very simple, while the receiver function of XIGA has an additional
midcrustal converted phase. The S wave velocity structures at these three stations are estimated from inversions of the receiver
function. The crustal shear wave velocities at WNDO and TUNL are vertically homogeneous, with value between 3.5–3.6 km/s down
to Moho. This value in the lower crust is lower than the normal value for the lower crust of continents, which is consistent
with the observed strong Sn attenuation in this region. The velocity structure at XIGA shows a velocity discontinuity at depth
of 20 km and high velocity value of 4.0 km/s in the midcrust between 20–30 km depth. Similar results are obtained from a DSS
profile in southern Tibet. The velocity under XIGA decreases below a depth of 30 km, reaching the lowest value of 3.2 km/s
between 50–55 km. depth. This may imply that the Indian crust underthrusts the low part of Tibetan crust in the southern Plateau,
forming a “double crust”. The crustal thickness at each of these sites is: WNDO, 68 km; TUNL, 70 km; XI-GA, 80 km.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, Supp., 581–592, 1992. 相似文献
348.
349.
Thomas Murphy 《Pure and Applied Geophysics》1958,41(1):194-200
Summary In order to obtain an automatic recording nucleus counter with greater accuracy than that ofVerzár, a precision photoelectric nucleus counter (Pollak type) with ceramic lining and internal diameter 2.5 cm has been substituted for the horizontal fog tube inVerzár's automat. Certain small alterations, have been made to the electric circuit of the recorder. The new arrangement can be easily calibrated and shows an accuracy approaching that of the precision counter operated under optimum conditions.
Zusammenfassung Um einen automatischen Kondensationskernzähler von grösserer Genauigkeit als es der vonVerzár ist, zur Verfügung zu haben, wurde das horizontale Nebelrohr im Automaten vonVerzár durch einen photo-elektrischer Präzisions-Kernzähler (BauartPollak) mit unglasiertem Porzellanfutter und einem inneren Durchmesser von 2.5 cm ersetzt. An der elektrischen Schaltung der Registriereinrichtung wurden gewisse kleine Änderungen vorgenommen. Die neue Anordnung kann leicht geeicht werden und besitzt eine Genauigkeit, welche jene des Präzisions-Kernzählers unter optimalen Betriebsbedingungen erreicht.相似文献
350.
Thomas J.M. Schopf 《Earth》1973,9(1):68-69