The response of fishes to submerged aquatic vegetation complexity in two ecoregions of the mid-Atlantic bight: Buzzards Bay and Chesapeake Bay

Estuarine seagrass ecosystems provide important habitat for fish and invertebrates and changes in these systems may alter their ability to support fish. The response of fish assemblages to alteration of eelgrass (Zostera marina) ecosystems in two ecoregions of the Mid-Atlantic Bight (Buzzards Bay and Chesapeake Bay) was evaluated by sampling historical eelgrass sites that currently span a broad range of stress and habitat quality. In two widely separated ecoregions with very different fish faunas, degradation and loss of submerged aquatic vegetation (SAV) habitat has lead to declines in fish standing stock and species richness. The abundance, biomass, and species richness of the fish assemblage were significantly higher at sites that have high levels of eelgrass habitat complexity (biomass >100 wet g m^?2; density <100 shotts m^?2) compared to sites that have reduced eelgrass (biomass <100 wet g m^?2; density <100 shoots m^?2) or that have completely lost eelgrass. Abundance, biomass, and species richness at reduced eelgrass complexity sites also were more variable than at high eelgrass complexity habitats. Low SAV complexity sites had higher proportions of pelagic species that are not dependent on benthic habitat structure for feeding or refuge. Most species had greater abundance and were found more frequently at sites that have eelgrass. The replacement of SAV habitats by benthic macroalgae, which occurred in Buzzards Bay but not Chesapeake Bay, did not provide an equivalent habitat to seagrass. Nutrient enrichment-related degradation of eelgrass habitat has diminished the overall capacity of estuaries to support fish populations.     

Nucleosynthesis and Mixing in Cassiopeia A

We present results from the first light observations of the Cassiopeia A supernova remnant (SNR) by the Chandra X-Ray Observatory. Based on representative spectra from four selected regions, we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon burning in a young Galactic SNR. Elsewhere in the remnant, we see silicon-rich ejecta from explosive oxygen burning. The Fe-rich ejecta lie outside the Si-rich material, indicating that bulk motions were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core. It is likely that this inversion was caused by "Fe"-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer during the supernova explosion. In addition, the radioactive decay energy from 56Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly sites of cosmic-ray acceleration in Cas A.     

多波束测量中换能器瞬时精密高程的确定

分析了GPS高程信号和Heave信号的频段特征，利用数字信号处理技术，提取了GPS高程和Heave中的有效频段信号，合成了一个全新的信号。该合成信号能够全面反映多波束换能器的实际垂直运动，并能够克服传统多波束测量和数据处理方法的缺陷，大大提高了多波束在垂直方向的成果精度。实验证明了该方法的正确性和可行性。     

Particle fabric in a small, type-2 ignimbrite flow unit (Laacher See, Germany) and implications for emplacement dynamics

 Ignimbrites of the 13-ka Upper Laacher See Tephra were deposited from small, highly concentrated, moderately fluidized pyroclastic flows. Their unconsolidated nature, and the prominence of accidental Devonian slate fragments, make these ignimbrites ideal for clast fabric studies. The upper flow unit of ignimbrite M14 has characteristics typical of a type-2 ignimbrite. Layer 2a and the lower part of layer 2b of the flow unit have strong, upstream-inclined a[p] fabrics (a[p] means long particle axes parallel to flow direction). Only clasts with a/b axial ratios of 2.5 or greater preserve good a[p] fabrics, whereas the a–b planes of flat fragments dip upstream irrespective of axial ratio. The a-axis fabric becomes weaker, flatter, and more girdle-like in the upper half of layer 2b. At one locality the a-axis fabric appears to rotate 40° up through the flow unit, suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base. The existence of similarly strong a[p] fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non-shearing material (layer 2b) on a thin shear layer (layer 2a), and that the entire flow freezes en masse to form the deposit. The data suggest that, if the flow froze en masse, it was shearing pervasively through at least half its thickness. Another possibility is that the flow unit aggraded progressively from the base up, and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition. Received: 13 February 1997 / Accepted: 4 April 1998     

Dynamics of the turbidity maximum zone in a micro-tidal estuary: Hawkesbury River, Australia

Bed sediment, velocity and turbidity data are presented from a large (145 km long), generally well-mixed, micro-tidal estuary in south-eastern Australia. The percentage of mud in the bed sediments reaches a maximum in a relatively narrow zone centred ≈30–40 km from the estuary mouth. Regular tidal resuspension of these bed sediments produces a turbidity maximum (TM) zone in the same location. The maximum recorded depth-averaged turbidity was 90 FTU and the maximum near-bed turbidity was 228 FTU. These values correspond to suspended particulate matter (SPM) concentrations of roughly 86 and 219 mg l^?1, respectively. Neither of the two existing theories that describe the development and location of the TM zone in the extensively studied meso- and macro-tidal estuaries of northern Europe (namely, gravitational circulation and tidal asymmetry) provide a complete explanation for the location of the TM zone in the Hawkesbury River. Two important factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water discharge rate and supply of sediment to the estuary head is very low for most of the time, and (2) suspension concentrations derived from tidal stirring of the bed sediments are comparatively low. The first factor means that sediment delivery to the estuary is largely restricted to short-lived, large-magnitude, fluvial flood events. During these events the estuary becomes partially mixed and it is hypothesized that the resulting gravitational circulation focuses mud deposition at the flood-determined salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The second factor means that easily entrained high concentration suspensions (or fluid muds), typical of meso- and macro-tidal estuaries, are absent. Maintenance of the TM zone during low-flow periods is due to an erosion-lag process, together with a local divergence in tidal velocity residuals, which prevent the TM zone from becoming diffused along the estuary axis.     

The radio properties of radio-quiet quasars

Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν^−α), and high brightness temperatures ( T _B ≥ 10⁵ K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.     

Collocation,dissipation and [overshoot] for time integration schemes in structural dynamics

The concept of collocation, originally used by Wilson in the development of dissipative algorithms for structural dynamics, is systematically generalized and analysed. Optimal schemes within this class are developed and compared with a recently proposed family of dissipative algorithms, called a methods. The α methods are found to be superior on the basis of standard measures of dissipation and dispersion. It is pointed out that the tendency to overshoot is an important and independent factor which should be considered in an evaluation of an implicit scheme. The basis for studying overshoot is discussed and the optimal collocation and α methods are compared. It is found that pathological overshooting is an inherent property of collocation schemes, whereas the overshooting characteristics of the α methods are good.     

Distribution surfaces in solvent extraction systems

In the solvent extraction of metals, the distribution of the metal is usually dependent on the concentrations of several different species in the two phases. Despite this fact, distribution isotherms are usually presented as plots of C_o as a function of C_aq and these are used for a McCabe-Thiele type evaluation of a number of stages and flow rates, etc., each individual isotherm being valid only for one set of initial conditions; this treatment limits the possibilities of optimizing the system. In the present paper an extension of three-dimensional plots is suggested with special reference to the Cu/LIX64N extraction system. The distribution surface is defined and the general properties of the plots are outlined.A procedure is outlined whereby the surface can be represented on a two dimensional plot and a theoretical model for the surface can be superimposed.The paper also shows how distribution ratios (D values) are related to the surface.