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21.
Robert J. Orth Michael R. Williams Scott R. Marion David J. Wilcox Tim J. B. Carruthers Kenneth A. Moore W. Michael Kemp William C. Dennison Nancy Rybicki Peter Bergstrom Richard A. Batiuk 《Estuaries and Coasts》2010,33(5):1144-1163
Chesapeake Bay supports a diverse assemblage of marine and freshwater species of submersed aquatic vegetation (SAV) whose broad distributions are generally constrained by salinity. An annual aerial SAV monitoring program and a bi-monthly to monthly water quality monitoring program have been conducted throughout Chesapeake Bay since 1984. We performed an analysis of SAV abundance and up to 22 environmental variables potentially influencing SAV growth and abundance (1984–2006). Historically, SAV abundance has changed dramatically in Chesapeake Bay, and since 1984, when SAV abundance was at historic low levels, SAV has exhibited complex changes including long-term (decadal) increases and decreases, as well as some large, single-year changes. Chesapeake Bay SAV was grouped into three broad-scale community-types based on salinity regime, each with their own distinct group of species, and detailed analyses were conducted on these three community-types as well as on seven distinct case-study areas spanning the three salinity regimes. Different trends in SAV abundance were evident in the different salinity regimes. SAV abundance has (a) continually increased in the low-salinity region; (b) increased initially in the medium-salinity region, followed by fluctuating abundances; and (c) increased initially in the high-salinity region, followed by a subsequent decline. In all areas, consistent negative correlations between measures of SAV abundance and nitrogen loads or concentrations suggest that meadows are responsive to changes in inputs of nitrogen. For smaller case-study areas, different trends in SAV abundance were also noted including correlations to water clarity in high-salinity case-study areas, but nitrogen was highly correlated in all areas. Current maximum SAV coverage for almost all areas remain below restoration targets, indicating that SAV abundance and associated ecosystem services are currently limited by continued poor water quality, and specifically high nutrient concentrations, within Chesapeake Bay. The nutrient reductions noted in some tributaries, which were highly correlated to increases in SAV abundance, suggest management activities have already contributed to SAV increases in some areas, but the strong negative correlation throughout the Chesapeake Bay between nitrogen and SAV abundance also suggests that further nutrient reductions will be necessary for SAV to attain or exceed restoration targets throughout the bay. 相似文献
22.
A 1978–81 survey of submersed aquatic macrophytes in the tidal Potomac River showed that there were virtually no plants in the freshwater tidal river between Chain Bridge and Quantico, Virginia, decades after the disappearance of plants in the late 1930’s. Plant populations were monitored in subsequent years (1983–85) using qualitative shoreline surveys and quantitative resampling of the original 1978–81 transects. In 1983, 12 species of submersed aquatic macrophytes were found in the tidal river. Population increases were dramatic; by fall 1985, plants had colonized all shallow areas between Alexandria and Gunston Cove, Virginia.Hydrilla verticillata dominated in Dyke Marsh-Hunting Creek and Swan Creek. Most other areas contained a variable mixture ofHeteranthera dubia, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria americana, Najas guadalupensis andHydrilla verticillata. No plants were found along the main river or in tidal embayments in the reach between Gunston Cove and Quantico, Virginia. Total dry weight collected in the upper tidal river in fall 1985 was 14.5 times that of spring 1985, and four times that of fall 1984. 相似文献
23.
Kacper R. Rybicki 《Pure and Applied Geophysics》1990,134(2):283-301
Faulting in a medium with an inhomogeneity is analysed applying two-dimensional models consisting of a shear crack in a presence of a circular inclusion. The stress drop and the stress intensity factor are calculated for mode II and III cracks of various positions in relation to the inclusion. The results demonstrate that the effect of an inhomogeneity on a shear zone strongly depends on the location of a zone for either mode II or mode III shear zone. This effect is mostly due to the spatial distribution of external effective shear stress around an inhomogeneity. Depending on the position, an inhomogeneity may have either a destabilizing effect (the stress intensity factor becomes greater) or a stabilizing influence (the stress intensity factor is decreased or faulting is prohibited so the inhomogeneity acts as an asperity or a barrier). There is a substantial difference, however, between mode II and mode III shear zones approaching an inhomogeneity centrally. Namely, the effect of inhomogeneity on the mode III shear zone located in the immediate vicinity of the inhomogeneity is in this case considerably more pronounced than that for mode II shear zone and depends to a far greater extent on the rigidity contrast between the inhomogeneity and the surrounding medium. Another important conclusion is that the quantitative effect of an inhomogeneity on faulting depends essentially on the initial value of the stress drop of a shear zone approaching an inhomogeneity, being decidedly higher for a shear zone of small stress drop. It means that in specified areas in the proximity of medium inhomogeneities one should expect substantially greater faulting activity in which weak events prevail than in other regions surrounding inhomogeneities where such activity should be distinctly reduced. Such conclusions apply to both high rigidity inhomogeneities, which, in particular, may be associated with intrusions from the upper mantle, and to low rigidity inhomogeneities such as volcanos. The present model sets forth the plausible explanation regarding why earthquakes from the same region are occasionally characterized by various values of the stress drop. The model also presents the quantitative insight concerning how heterogeneity of the medium, in the sense of spatial variation of elastic constants, affects faulting. 相似文献
24.
Changing light availability may be responsible for the discontinuous distribution of submersed aquatic macrophytes in the freshwater tidal Potomac River. During the 1985–1986 growing seasons, light attenuation and chlorophylla and suspended particulate material concentrations were measured in an unvegetated reach (B) and in two adjacent vegetated reaches (A and C). Light attenuation in reach B (the lower, fresh to oligohaline tidal river) was greater than that in reach A (the recently revegetated, upper, freshwater tidal river) in both years. Reach B light attenuation was greater than that in reach C (the vegetated, oligohaline to mesohaline transition zone of the Potomac Estuary) in 1985 and similar to that in reach C in 1986. In reach B, 5% of total below-surface light penetrated only an average of 1.3 m in 1985 and 1.0m in 1986, compared with 1.9 m and 1.4 m in reach A in 1985 and 1986, respectively. Water column chlorophylla concentration controlled light availability in reaches A and B in 1985, whereas both chlorophylla and suspended particulate material concentrations were highly correlated with attenuation in both reaches in 1986. Reach C light attenuation was correlated with suspended particulate material in 1986. The relationship between attenuation coefficient and Secchi depth was KPAR=1.38/Secchi depth. The spectral distribution of light at 1 m was shifted toward the red portion of the visible spectrum compared to surface light. Blue light was virtually absent at 1.0 m in reach B during July and August 1986. Tidal range is probably an important factor in determining light availability for submersed macrophyte propagule survival at the sediment-water interface in this shallow turbid system. 相似文献
25.
David Gubbins Emile A. Okal Roberto Scarpa R. E. Anderson Leszek Czechowski Rodney W. Page Bernhard Spörli Magdalena Kądziałko-Hofmokl J. A. Majorowicz Gregory Houseman Donald L. Turcotte David F. McTigue Teng-fong Wong David Lockner David V. Wiltschko David M. Fountain G. P. Glasby R. J. Angel S. J. Gibowicz Jonathan Franzen Kacper R. Rybicki Brian Farrell Anil Suri Zbigniew W. Kundzewicz Witold G. Strupczewski 《Pure and Applied Geophysics》1988,127(4):695-752
26.
Role of weather and water quality in population dynamics of submersed macrophytes in the tidal Potomac River 总被引：1，自引：0，他引：1
Virginia Carter Nancy B. Rybicki Jurate M. Landwehr Michael Turtora 《Estuaries and Coasts》1994,17(2):417-426
Weather and water-quality data from 1980 to 1989 were correlated with fluctuations in submersed macrophyte populations in the tidal Potomac River near Washington, D.C., to elucidate causal relationships and explain population dynamics. Both reaches were unvegetated in 1980 when mean growing-season Secchi depths were <0.60 m. Macrophyte resurgence in the upper tidal river in 1983 was associated with a growing-season Secchi depth of 0.86 m, total suspended solids (TSS) of 17.7 mg l?1, chlorophyll a concentrations of 15.2 μg l?1, significantly higher than average percent available sunshine, and significantly lower than average wind speed. From 1983 to 1989, mean seasonal Secchi depths <0.65 m were associated with decrease in plant coverage and mean seasonal Secchi depths >0.65 were associated with increases in plant coverage. Changes in mean seasonal Secchi depth were related to changes in mean seasonal TSS and chlorophyll a concentration; mean Secchi depths >0.65 generally occur when seasonal mean TSS is <19 mg l?1 and seasonal mean chlorophyll a concentration is ≤15 μg l?1. Secchi depth is highly correlated with plant growth in the upper tidal river and chlorophyll a and TSS with plant growth in the lower tidal river. Wind speed is an important influence on plant growth in both reaches. 相似文献
27.
Kacper Kotulak Adam Froń Andrzej Krankowski German Olivares Pulido Manuel Henrandez-Pajares 《Acta Geophysica》2017,65(1):13-28
In radioastronomy the interferometric measurement between radiotelescopes located relatively close to each other helps removing ionospheric effects. Unfortunately, in case of networks such as LOw Frequency ARray (LOFAR), due to long baselines (currently up to 1500 km), interferometric methods fail to provide sufficiently accurate ionosphere delay corrections. Practically it means that systems such as LOFAR need external ionosphere information, coming from Global or Regional Ionospheric Maps (GIMs or RIMs, respectively). Thanks to the technology based on Global Navigation Satellite Systems (GNSS), the scientific community is provided with ionosphere sounding virtually worldwide. In this paper we compare several interpolation methods for RIMs computation based on scattered Vertical Total Electron Content measurements located on one thin ionospheric layer (Ionospheric Pierce Points—IPPs). The results of this work show that methods that take into account the topology of the data distribution (e.g., natural neighbour interpolation) perform better than those based on geometric computation only (e.g., distance-weighted methods). 相似文献
28.
A method has been developed for computing the gravitational force field of an axially symmetric flat galaxy from its surface mass density. The method is simple to program, fast, and accurate. An inversion formula is derived that allows computation of surface density from rotation curves by use of any method that converts density to force. The method is compared with a method of Clutton-Brock that utilizes Hankel transforms of Laguerre functions. 相似文献
29.
George B. Rybicki 《Astrophysics and Space Science》1971,14(1):15-19
It is shown that the time of relaxation by particle encounters of self-gravitating systems in the plane interacting by 1/r
2 forces is of the same order of magnitude as the mean orbit time. There-fore such a system does not have a Vlasov limit for large numbers of particles, unless appeal is made to some non-zero thickness of the disk. The relevance of this results to numerical experiments on galactic structure is discussed. 相似文献
30.
Kacper Rybicki 《Pure and Applied Geophysics》1973,110(1):1955-1966
Summary The formulas for the elastic residual field of a very long strike-slip fault in a multilayered elastic half-space are given. It is assumed that the medium is made up ofn parallel, isotropic and homogeneous layers lying over an isotropic, homogeneous half-space and being in welded contact. The formulas which allow the calculation of the displacement and stress fields due to an arbitrary very long strike-slip fault are derived. The explicit expressions of coefficients which appear in these formulas are listed for the casesn=1 andn=2. 相似文献