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21.
22.
J. A. Elliott 《Boundary-Layer Meteorology》1972,2(4):476-495
An instrument is described which is capable of making Eulerian measurements of microscale fluctuations in the static pressure when it is placed within the turbulent flow of the lower atmospheric boundary layer. The sampling ports are located on an accurately shaped streamlined circular disc; dynamic pressure changes at these ports, due to the flow fluctuations, are small when compared to the static pressure changes. Wind tunnel tests and anin situ test show that the instrument can sample the amplitudes and phases of fluctuations in the static pressure to better than ± 20% and to about ± 5 °, respectively. 相似文献
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24.
The zooplankton community of Mission Bay, San Diego, California, was monitored over two years, to study spatial and temporal
patterns and the response of zooplankton species composition to environmental variation. Data were collected every two weeks
from six stations and included hydrographic parameters, dissolved nutrient concentrations, and phytoplankton and zooplankton
species composition. Hydrography varied seasonally, along a spatial gradient from the mouth to the back of the bay, and between
the two years around the influence of rainfall. Spatially, Mission Bay could be divided into three regions during this study
based on hydrography and zooplankton species composition. Zooplankton species composition followed a predictable seasonal
progression, with different groups of species being characteristic to particular times of the year. Variability in zooplankton
species composition was also evident between years, as certain species were more common in one or the other year of the study.
Spatial patterns were more consistent than temporal ones, and related to distance from the mouth of the bay during much of
the year and distance from freshwater inlets during the relatively short rainy season. Multivariate analysis revealed that
variation in zooplankton species composition was best related to measured abiotic factors (temperature, salinity, rainfall,
and tidal velocity). 相似文献
25.
A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO2 (log fO2 ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites. 相似文献
26.
Use of annular flumes to determine the influence of current velocity and bivalves on material flux at the sediment-water interface 总被引:3,自引:0,他引:3
John Widdows Mary D. Brinsley Peter N. Salkeld Mike Elliott 《Estuaries and Coasts》1998,21(4):552-559
A benthic annular flume for both laboratory and in situ deployment on intertidal mudflats is described. The flume provides a means of quantifying material flux (i.e., biodeposition of suspended particulates, sediment resuspension, nutrients, oxygen, and contaminants) across the sediment-water interface in relation to changes in current velocity and benthic community structure and/or population density of key macrofauna species. Flume experiments have investigated the impact of the infaunal bivalveMacoma balthica and the epifaunal bivalveMytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbatorMacoma was found to increase the sediment resuspension and/or erodability by 4-fold, at densities similar to those recorded at the Skeffling mudflat (Humber estuary) (i.e., >1000 individuals m?2). There was a significant correlation between sediment resuspension andMacoma density (r=0.99; p<0.001), which supported previous in situ field observations indicating bioturbation byMacoma enhanced sediment erodability. Biodeposition rates (g m?2 h1) ofMytilus edulis andCerastoderma edule were quantified and related to changes in population density in a mussel bed (Cleethorpes, Humber estuary). Biodeposition rates were up to 40-times the natural sedimentation rates. At the highest mussel bed densities (i.e., 50–100% cover or >1400 mussels m?2) the physical presence of this epifaunal bivalve on the sediment surface reduced erosion by 10-fold. The shift from net biodeposition to net erosion occurred at current velocities of 20–25 cm s?1. These results demonstrate that infaunal and epifaunal bivalves can have a significant impact on seston flux or sediment deposition and on sediment resuspension or erodability in estuaries where there are extensive mudflats. 相似文献
27.
28.
Entrapped biogenic gas in peat can greatly affect peatland biogeochemical and hydrological processes by altering volumetric water content, peat buoyancy, and ‘saturated’ hydraulic conductivity, and by generating over‐pressure zones. These over‐pressure zones further affect hydraulic gradients which influence water and nutrient flow direction and rate. The dynamics of entrapped gas are of global interest because the loss of this gas to the atmosphere via ebullition (bubbling) is likely the dominant transport mechanism of methane (CH4) to the atmosphere from peatlands, which are the largest natural terrestrial source per annum of atmospheric CH4. We investigated the relationship between atmospheric pressure and temperature on volumetric gas content (VGC) and CH4 ebullition using a laboratory peat core incubation experiment. Peat cores were incubated at three temperatures (one core at 4 °C, three cores at 11 °C, and one core at 20 °C) in sealed PVC cylinders, instrumented to measure VGC, pore‐water CH4 concentrations, and ebullition (volume and CH4 concentrations). Ebullition events primarily occurred (71% of the time) during periods of falling atmospheric pressure. The duration of the drop in atmospheric pressure had a larger control on ebullition volume than the magnitude of the drop. VGC in the 20 °C core increased from the onset of the experiment and reached a fluctuating but time‐averaged constant level between experiment day 30 and 115. The change in VGC was low for the 11 °C cores for the initial period of the experiment but showed large increases when the growth chamber temperature increased to 20 °C due to a malfunction. The core maintained at 4 °C showed only a small increase in entrapped gas content throughout the experiment. The 20 °C core showed the largest increase in VGC. The increases in VGC occurred despite pore‐water concentrations of CH4 being below the equilibrium solubility level. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
29.
Robert W. Slawson Zoran Ninkov Elliott P. Horch 《Astrophysics and Space Science》2007,312(3-4):171-187
We have obtained and analyzed UBVRI
CCD frames of the young, 4–10 Myr, open cluster NGC 3293 and the surrounding field in order to study its stellar content and determine the cluster’s IMF. We found significantly fewer lower mass stars, M≤2.5M
⊙, than expected. This is particularly so if a single age for the cluster of 4.6 Myr is adopted as derived from fitting evolutionary
models to the upper main sequence. Some intermediate-mass stars near the main sequence in the HR diagram imply an age for the cluster of about 10 Myr. When compared with the Scalo (The stellar initial mass function. ASP
conference series, vol. 24, p. 201, 1998) IMF scaled to the cluster IMF in the intermediate mass range, 2.5≤M/M
⊙≤8.0 where there is good agreement, the high mass stars have a distinctly flatter IMF, indicating an over abundance of these stars, and there is a sharp turnover in the distribution at lower masses. The radial
density distribution of cluster stars in the massive and intermediate mass regimes indicate that these stars are more concentrated
to the cluster core whereas the lower-mass stars show little concentration. We suggest that this is evidence supporting the
formation of massive stars through accretion and/or coagulation processes in denser cluster cores at the expense of the lower
mass proto-stars.
R.W. Slawson and E.P. Horch are guest investigators at the University of Toronto Southern Observatory, Las Campanas, Chile. 相似文献
30.
Medium- and Long-term Recovery of Estuarine and Coastal Ecosystems: Patterns,Rates and Restoration Effectiveness 总被引:1,自引:0,他引:1
Ángel Borja Daniel M. Dauer Michael Elliott Charles A. Simenstad 《Estuaries and Coasts》2010,33(6):1249-1260
Many estuarine and coastal marine ecosystems have increasingly experienced degradation caused by multiple stressors. Anthropogenic
pressures alter natural ecosystems and the ecosystems are not considered to have recovered unless secondary succession has
returned the ecosystem to the pre-existing condition or state. However, depending upon the scales of time, space and intensity
of anthropogenic disturbance, return along the historic trajectory of the ecosystem may: (1) follow natural restoration though
secondary succession; (2) be re-directed through ecological restoration, or (3) be unattainable. In order to address the gaps
in knowledge about restoration and recovery of estuarine and coastal ecosystems, this special feature includes the present
overview and other contributions to provide a synthesis of our knowledge about recovery patterns, rates and restoration effectiveness.
From the 51 examples collated in this contribution, we refine the recovery from the list of stressors into six recovery mechanisms:
(1) recovery from sediment modification, which includes all aspects of dredging and disposal; (2) recovery by complete removal
of stressors limiting natural ecosystem processes, which includes tidal marsh and inundation restoration; (3) recovery by
speed of organic degradation, which includes oil discharge, fish farm wastes, sewage disposal, and paper mill waste; (4) recovery
from persistent pollutants, which includes chemical discharges, such as TBT; (5) recovery from excessive biological removal,
related to fisheries and (6) recovery from hydrological and morphological modification. Drawing upon experience both from
these many examples and from an example of one comprehensive study, we show that although in some cases recovery can take
<5 years, especially for the short-lived and high-turnover biological components, full recovery of coastal marine and estuarine
ecosystems from over a century of degradation can take a minimum of 15–25 years for attainment of the original biotic composition
and diversity may lag far beyond that period. 相似文献