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151.
Suspended sediment (SS) is an important pollutant in freshwater ecosystems and can be detrimental to fish communities. Although macrophytes mediate sediment deposition, little effort has been put into determining how their removal affects sediment resuspension. The present study examined the immediate and long-term impacts of mechanical macrophyte removal on SS concentrations in streams. The results of this study suggest that bed disturbance during mechanical excavation of macrophytes significantly increases SS in the short term, and concentrations were found to increase by as much as 15,687 mg L–1 immediately after macrophyte removal. Significant long-term (77 day) increases in SS were also observed, indicating that without macrophytes, disturbed material is continually resuspended after excavation by fluvial processes. These results demonstrate that macrophyte removal can result in SS levels that have previously been shown to harm fish, and indicate that this activity may be more detrimental to fish than previously thought. 相似文献
152.
We report on our observing programme to obtain integrated spectrophotometry, intermediate- and high-resolution major-axis
spectra, and UBR surface photometry of a representative sample of ∼ 200galaxies in the nearby field. The main goal of this programme is to
provide a comparison sample for high-redshift studies and to study the variation in star formation rates (SFRs), star formation
history (SFH),excitation, metallicity, and internal kinematics over a wide range of galaxy luminosities and morphological
types. In particular, we extend the work of Kennicutt (1992) to lower-luminosity systems. We present the main results of our
analysis so far. In these proceedings, we condense the two atlases presented of 1) images and radial surface brightness profiles
and colour profiles, and 2) images and integrated spectra into several example images, profiles and spectra, showing the general
trends observed. For the original atlases we refer the reader to the electronic distribution on CDROM or as available on the
Web at http://www.astro.rug.nl/∼nfgs/.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
153.
154.
Constantin Pontikis Elizabeth Hicks Nathalie Michalon 《Comptes Rendus Geoscience》2004,336(15):1409-1412
The arguments and conclusions of the commented Williams and Stanfill article in relation to the validity of the thermal and aerosol hypotheses advanced in order to explain the land–ocean contrast in lightning activity are critically re-examined. This re-examination reveals that the analysis presented by these authors is incomplete, thus weakening the corresponding conclusions. To cite this article: C. Pontikis et al., C. R. Geoscience 336 (2004). 相似文献
155.
We investigated the source and composition of free and protein-bound amino acids during the decomposition ofSpartina alterniflora Loisel in laboratory percolators and in a field experiment in the Great Sippewissett Marsh (Falmouth, Massachusetts). In the percolator experiment, 50% of the nitrogen (N) could be extracted fromS. alterniflora litter in 16 d. This extract consisted of dissolved free amino acid N (28%), suspended protein amino acid N (16%), inorganic N (12%), and nitrogen from unidentified compounds (44%). Much of the free amino acid nitrogen was utilized by detrital microorganisms, resulting in a greater loss of suspended protein amino acid (SPAA) nitrogen from the biologically active percolator due to microbials biomass. Suspended microbial mass accounted for at least 50% of the SPAA washed out of the biologically active percolator. In the field study, 38% of the original litter nitrogen was leached fromS. alterniflora litter in litterbags during the first 13 d. After this initial leaching period, the concentration (41% to 69% of total nitrogen) and composition of most amino acids bound in the litter did not change over the 23-month period of the experiment. Increases in microbial protein did not account for increases in total nitrogen which occurred during the decomposition of the litter. Similarly, adsorbed ammonium did not appear to be responsible for this increase. 相似文献
156.
Bruce Hicks 《Boundary-Layer Meteorology》1993,64(4):421-422
157.
Observations of surface characteristics, meteorological conditions and energy balance components from Vancouver, B.C. are used to test the validity of the output from three one-dimensional surface energy balance models. The results show that whereas all of the models provide good simulations of net radiation, none can consistently predict the turbulent fluxes of sensible and latent heat using easily available input data. Inability to handle the role of water availability and its impact on evapotranspiration is identified as the principal problem. 相似文献
158.
Sheila J. Bennell 《Marine environmental research》1981,5(3):227-240
Fluctuations in the littoral barnacle populations of North Wales from 1974 to 1979 are described. Sites with high barnacle cover had regular seasonal fluctuations in cover. A detailed study of Balanus balanoides in 1979 showed that settlement was heaviest when the wind was offshore and the sea calm, and least along the north coast of Anglesey, where littoral larvae must be more dispersed. Site to site variations in timing differed from year to year. It was suggested that both wind and diffusion patterns in the sea could affect timing and distribution of settlement. 相似文献
159.
Pollen accumulation rates (PARs) provide a potential proxy for quantitative tree volume (m3 ha?1) reconstruction with reliable absolute pollen productivity estimates (APPEs). We obtained APPEs for pine, spruce and birch at their range limits in northern Finland under two temperature periods (‘warm’ and ‘cold’) based on long‐term pollen trap and tree volume records within a 14‐km radius of each trap. APPEs (mean ± SE; × 108 grains m?3 a?1) tend to be higher for the ‘warm’ periods (pine 123.8 ± 24.4, birch 528.0 ± 398.4, spruce 434.3 ± 113.7) compared with the ‘cold’ periods (pine 95.5 ± 37.3, birch 317.3 ± 282.6, spruce 119.6 ± 37.6), although the difference is only significant for spruce. Using an independent temperature record and the APPEs obtained, we reconstruct a low‐frequency record of pine volume changes over the last 1000 years at Palomaa mire, where a high‐resolution record of Pinus PARs is available. Five phases are distinguished in the reconstruction: moderate pine volume, AD 1080–1170; high volume, AD 1170–1340; low volume, AD 1340–1630; very low volume, AD 1630–1810; and rising pine volume, AD 1810–1950. These phases do not coincide with periods of high or low June–July–August temperatures, and thus appear to reflect regional variations in tree volume, while high‐frequency changes within each time‐period block show variations in PARs in response to temperature. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
160.
The influence of heterogeneity of undrained shear strength on the performance of a long slope cut in clay is investigated. Random field theory is used to model the heterogeneity and finite elements are used to compute the slope response. These have been used within a Monte Carlo simulation to compute reliability as a function of global factor of safety. It is shown that three failure modes are possible, depending on the ratio of the horizontal scale of fluctuation to the slope size, as characterised by its height and length. For very small values of this ratio, the slope fails along its entire length and the result is similar to a conventional 2D analysis based on the mean strength. For intermediate values, discrete failures are likely and reliability is a function of slope length. For large values of the ratio, the variability takes on a layered appearance and the result is equivalent to a 2D stochastic analysis. The validity of the boundary conditions is examined by analysing slopes of different length. It is shown that simple probability theory may be linked with stochastic analysis and finite elements to give efficient solutions for some large 3D problems. 相似文献