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611.
Assessing aquatic macrophyte community change through the integration of palaeolimnological and historical data at Loch Leven,Scotland 总被引:1,自引:0,他引:1
Jorge Salgado Carl Sayer Laurence Carvalho Thomas Davidson Iain Gunn 《Journal of Paleolimnology》2010,43(1):191-204
Submerged macrophytes have a critical role in lake ecosystems affecting nutrient cycling, sediment stability, and community
composition across multiple trophic levels. Consequently temporal changes in the composition of submerged plant populations
can have profound ecological implications and key significance from the perspective of lake conservation. By focusing on macro-remains
of aquatic macrophytes and extensive historical plant records spanning the last approximately 180 years, this study seeks
to evaluate a combined historical-palaeolimnological approach for establishing pre-disturbance macrophytes communities in
Loch Leven, Kinross, Scotland and to provide new information regarding temporal trends in its macrophyte vegetation as well
as potential drivers of change. Some 81% of the species historically recorded for the core site (east side of St. Serf’s Island)
were found as macro-remains. Potamogeton taxa were underrepresented, whereas remains of Elatine hexandra, a small species never recorded historically were found. The core sequence showed good agreement with known floristic changes
including an early (pre- ca. 1850) loss of Isoetes lacustris and Lobelia
dortmanna and a more recent (post-1910) shift to dominance by Potamogeton and Chara taxa associated with eutrophic conditions. A clear pattern in the relationship between macrofossil principal component analysis
(PCA) and loss on ignition suggested a key control of sediment conditions on macrophyte community structure. In particular
the major macrophyte community change of the mid-nineteenth century was concurrent with a substantial increase in organic
matter, likely linked to a historic lake lowering (early 1830s) which would have beached the former gravel-sand shoreline
leading to a much siltier lake littoral. Although recent monitoring data show signs of ecological recovery our study illustrates
that Loch Leven remains a long way from its reference state as a lake with characteristic soft-water macrophytes. To achieve
a full recovery, sediment properties would need to change in addition to nutrient reduction. Consequently restoration strategies
will need to compromise between the desirability of achieving the pre-disturbance state and what is feasible and practicable.
Our study shows the clear potential role of a combined palaeolimnological-historical approach for informing lake management
decisions. 相似文献
612.
The influence of asymmetries in overlying stratification on near-bed turbulence and sediment suspension in a partially mixed estuary 总被引:3,自引:1,他引:2
Data collected from the York River estuary demonstrate the importance of asymmetries in stratification to the suspension and transport of fine sediment. Observations collected during two 24-h deployments reveal greater concentrations of total suspended solids during the flood phase of the tide despite nearly symmetric near-bed tidal current magnitude. In both cases, tidally averaged net up-estuary sediment transport near the bed was clearly observed despite the fact that tidally averaged residual near-bed currents were near zero. Tidal straining of the along-channel salinity gradient resulted in a stronger pycnocline lower in the water column during the ebb phase of the tide and appeared to limit sediment suspension. Indirect measurements suggest that the lower, more intense, pycnocline on the ebb acted as a barrier, limiting turbulent length scales and reducing eddy diffusivity well below the pycnocline, even though the lower water column was locally well mixed. In order to more conclusively link changes in stratification to properties of near-bed eddy viscosity and diffusivity, longer duration tripod and mooring data from an additional experiment are examined, that included direct measurement of turbulent velocities. These additional data demonstrate how slight increases in stratification can limit vertical mixing near the bed and impact the structure of the eddy viscosity below the pycnocline. We present evidence that the overlying pycnocline can remotely constrain the vertical turbulent length scale of the underlying flow, limiting sediment resuspension. As a result, the relatively small changes in stratification caused by tidal straining of the pycnocline allow sediment to be resuspended higher in the water column during the flood phase of the tide, resulting in preferential up-estuary transport of sediment.Responsible Editor: Iris Grabemann 相似文献
613.
The authors have developed a model to predict the radiation stresses in the coastal zone and to estimate currents and set-up/set-down
of mean sea level. The values of radiation stress are calculated from velocity potential, which can be obtained by analytical
means or from a finite element model of the elliptic extended mild slope equation depending on the complexity of the situation
in question. The values of radiation stress are then input into a hydrodynamic model which gives the resulting set-up/set-down
and currents caused by these stresses. The developed model includes convective acceleration and bottom friction. The radiation
stress results of the model have been compared with analytical results and published values. Results for set-up/set-down and
currents have been compared with published results for seven other similar models. The model has been compared with published
results for set-up/set-down and currents created in the vicinity of a detached breakwater and also around a conical island.
The results of the authors’ model compare well with the analytical results, and published results for similar models.
An erratum to this article can be found at 相似文献
614.
Gerhard?JentzschEmail author Adelheid?Weise Carlos?Rey Carl?Gerstenecker 《Pure and Applied Geophysics》2004,161(7):1415-1431
Temporal gravity changes provide information about mass and/or density variations within and below the volcano edifice. Three active volcanoes have been under investigation; each of them related to a plate boundary: Mayon/Luzon/Philippines, Merapi/Java/Indonesia, and Galeras/Colombia. The observed gravity changes are smaller than previously expected but significant. For the three volcanoes under investigation, and within the observation period, mainly the increase of gravity is observed, ranging from 1,000 nm–2 to 1,600 nms–2. Unexpectedly, the gravity increase is confined to a rather small area with radii of 5 to 8 km around the summit. At Mayon and Merapi the parallel GPS measurements yield no significant elevation changes. This is crucial for the interpretation, as the internal pressure variations do not lead to significant deformation at the surface. Thus the classical Mogi-model for a shallow extending magma reservoir cannot apply. To confine the possible models, the attraction due to changes of groundwater level or soil moisture is estimated along the slope of Merapi exemplarily by 2-D modelling. Mass redistribution or density changes were evaluated within the vent as well as deeper fluid processes to explain the gravity variations; the results are compared to the model incorporating the additional effect of elastic deformation. 相似文献
615.
616.
Summary. Estimates of the shape of the sea surface as determined from satellite altiméters have become of increasing importance in geophysics, geodesy and physical oceanography. The most effective error reduction procedures used in producing these surfaces have been some version of a crossing-arc analysis. The problem with this procedure is the absence of an easy means of evaluating the noise structure of the final result. By recasting the problem as one of objective mapping (optimal estimation) we both optimize the error reduction and obtain an explicit estimate of the wave-number structure of the final altimétric surface. Examples, suitable to a small computer, have been generated to show that the error reduction along the sub-satellite tracks is very great, with most of the residual lying in long wavelengths. 相似文献
617.
B.N. Khare Carl Sagan John E. Zumberge Deborah S. Sklarew Bartholomew Nagy 《Icarus》1981,48(2):290-297
The complex dark brown solid of a class called tholins, produced on passage of an electrical discharge through a roughly equimolar mixture of methane and ammonia with 2.6% water vapor, is analyzed by vacuum pyrolysis followed by gas chromatography and mass spectrometry. Pyrolyzates include a wide range of aliphatic and aromatic nitriles, alkanes, aromatic hydrocarbons, pyrrole, and pyridine. This tholin is remarkably stable to 950°C. It and its degradation products are candidate constituents of planetary aerosols in the outer solar system and of the grains in the interstellar medium. 相似文献
618.
619.
We examine a non-linear mechanism for a solar surge in which plasma regions of high electrical conductivity and macroscopic dimension can be rapidly accelerated without diffusion of magnetic field. The mechanism is suggested by Rust's observations, which show that surges occur near sunspots in regions of reversed magnetic polarity. For the purposes of numerical calculation, we replace the magnetic field near a polarity reversal in a sunspot by magnetic fields of current loops. The relaxation of the magnetic field generated by two antiparallel coaxial current loops in an incompressible plasma is traced by computer. The results suggest that plasma in the form of a vortex ring can be expelled at the Alfvén velocity from active solar regions. 相似文献
620.
The recently discovered coorbital satellites of Saturn, 1980S1 and 1980S3, are shown to be librating in horseshoe orbits. By considering the effects of tangential forces on the semimajor axes of the satellite orbits, we derive an accurate relation between the sum of the satellite masses and (a) their minimum angular separation, (b) the variation of their angular separation with time and (c) the libration period. Observations of (b) and (c) are the most practical methods of determining the satellite masses. The orbits of the coorbital satellites of Dione and Tethys are discussed. We demonstrate the possibility of calculating a new value for the mass of Dione and we show that one of the coorbital satellites of Tethys could be moving in a horseshoe orbit even though another satellite is librating in a tadpole orbit about the leading Lagrangian equilibrium point L4. The origin of coorbital satellites and the stability of their orbits are discussed. 相似文献