Trophic Links in the Plankton in the Low Salinity Zone of a Large Temperate Estuary: Top-down Effects of Introduced Copepods     

Joanna K. York  George B. McManus  Wim J. Kimmerer  Anne M. Slaughter  Toni R. Ignoffo 《Estuaries and Coasts》2014,37(3):576-588

We investigated trophic relationships involving microzooplankton in the low salinity zone of the San Francisco Estuary (SFE) as part of a larger effort aimed at understanding the dynamics of the food web supporting the endangered delta smelt, Hypomesus transpacificus. We performed 14 cascade experiments in which we manipulated the biomass of a copepod (Limnoithona tetraspina, Pseudodiaptomus forbesi, or Acartiella sinensis) and quantified responses of lower trophic levels including bacterioplankton, phytoplankton, and microzooplankton. Microzooplankton comprised a major food source for copepods; 9 out of 14 experiments showed removal of at least one group of microzooplankton by copepods. In contrast, the impact of copepods on phytoplankton was indirect; increased copepod biomass led to greater growth of phytoplankton in 3 of 14 experiments. Estimated clearance rates on microzooplankton were 4 mL day^?1 for L. tetraspina and 2–6 mL day^?1 for P. forbesi, whereas A. sinensis consumed mainly copepod nauplii. Complex trophic interactions, including omnivory, among copepods, microzooplankton, and different components of the phytoplankton likely obscured clear trends. The food web of the SFE is probably less efficient than previously thought, providing poor support to higher trophic levels; this inefficient food web is almost certainly implicated in the continuing low abundance of fishes, including the delta smelt that use the low salinity zone of the San Francisco Estuary.  相似文献   

Corrigendum: Pleistocene Rhine–Thames landscapes: geological background for hominin occupation of the southern North Sea region     

Marc P. Hijma  Kim M. Cohen  Wil Roebroeks  Wim E. Westerhoff  Freek S. Busschers 《第四纪科学杂志》2012,27(3):336-336

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Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models   总被引：1，自引：0，他引：1       下载免费PDF全文

Andrés Vargas‐Luna  Alessandra Crosato  Wim S.J. Uijttewaal 《地球表面变化过程与地形》2015,40(2):157-176

The presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed‐shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed‐shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Amorphous silica mobilization by inter‐rill erosion: insights from rainfall experiments          下载免费PDF全文

Wim Clymans  Eric Struyf  An Van den Putte  Christoph Langhans  Zhengang Wang  Gerard Govers 《地球表面变化过程与地形》2015,40(9):1171-1181

Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Influence of Obstacles on the Aerodynamic Roughness of the Netherlands     

Joost J. M. De Jong  Arjen C. De Vries  Wim Klaasen 《Boundary-Layer Meteorology》1999,91(1):51-64

The aim of this study was to analyse the influence of large- and small-scale obstacles (orography, tree lines, and dikes) on the effective aerodynamic roughness of the Netherlands, a relatively flat, small-scale landscape. The roughness averaging approach was based on drag coefficients. The effective roughness was locally dominated by small-scale obstacles such as tree lines and dikes. Even at a regional scale (40,000 km2), the small-scale obstacle drag was of the same order of magnitude as the shear stress due to landuse. The neglect of those obstacles on a regional scale would result in approximately 10% overestimated averaged windspeed at 10~m above the surface. It was concluded that small-scale obstacles need to be taken into account to calculate the aerodynamic roughness of flat landscapes. Orography was of minor importance in this lowland country.  相似文献   

The impact of surface‐wave attenuation on 3‐D seismic survey design          下载免费PDF全文

Tomohide Ishiyama  Gerrit Blacquière  Wim A. Mulder 《Geophysical Prospecting》2017,65(1):86-96

Three‐dimensional seismic survey design should provide an acquisition geometry that enables imaging and amplitude‐versus‐offset applications of target reflectors with sufficient data quality under given economical and operational constraints. However, in land or shallow‐water environments, surface waves are often dominant in the seismic data. The effectiveness of surface‐wave separation or attenuation significantly affects the quality of the final result. Therefore, the need for surface‐wave attenuation imposes additional constraints on the acquisition geometry. Recently, we have proposed a method for surface‐wave attenuation that can better deal with aliased seismic data than classic methods such as slowness/velocity‐based filtering. Here, we investigate how surface‐wave attenuation affects the selection of survey parameters and the resulting data quality. To quantify the latter, we introduce a measure that represents the estimated signal‐to‐noise ratio between the desired subsurface signal and the surface waves that are deemed to be noise. In a case study, we applied surface‐wave attenuation and signal‐to‐noise ratio estimation to several data sets with different survey parameters. The spatial sampling intervals of the basic subset are the survey parameters that affect the performance of surface‐wave attenuation methods the most. Finer spatial sampling will reduce aliasing and make surface‐wave attenuation easier, resulting in better data quality until no further improvement is obtained. We observed this behaviour as a main trend that levels off at increasingly denser sampling. With our method, this trend curve lies at a considerably higher signal‐to‐noise ratio than with a classic filtering method. This means that we can obtain a much better data quality for given survey effort or the same data quality as with a conventional method at a lower cost.  相似文献   

Power law or power flaw?     

Frank Pattyn  Wim Van Huele 《地球表面变化过程与地形》1998,23(8):761-767

Since its introduction by Svensson in 1959, the power law curve y = ax^b (where x and y are horizontal and vertical direction, respectively) has been widely used in morphological analysis of glacial trough cross-profiles. The numerical constants a and b are obtained by a linear regression analysis of the logarithmic form of the power law curve (ln y = ln a + b ln x). The value b then gives a measure for the form of the cross-section. However, over the years this form of the power law has endured a lot of criticism. This criticism is well founded, since this particular form of the power law is not suitable for curve fitting in morphological analyses. In this paper a general power law is proposed, of the form y − y₀ = a|x − x₀|^b (where x₀, y₀ are the coordinates of the origin of the cross-profile). A unique and unbiased solution for this equation is obtained with a general least-squares method, thereby minimizing the error between the cross-profile data and the curve, and not between the logarithmic transform of the data and its regression line. This provides a robust way to characterize trough cross-section forms. © 1998 John Wiley & Sons, Ltd.  相似文献   

Two-dimensional equilibrium of a neutral sheet in a gravitational field     

Wim J. Weber 《Solar physics》1979,61(2):345-362

We solve the two-dimensional MHD-equations to find the equilibrium structure of a neutral sheet having its axis parallel to the gravitational field in an atmosphere. The evolution of an initially plane sheet which is a self-consistent, non-equilibrium solution of the MHD-equations, is followed until static equilibrium of forces is obtained. The effect of field line anchoring in a cool dense layer at the bottom of the sheet is taken into account.Presently at The Auroral Observatory, Boks 953, 9001 Tromsø, Norway.  相似文献   

Late Quaternary deglaciation and climate history of the Larsemann Hills (East Antarctica)     

Elie Verleyen  Dominic A. Hodgson  Koen Sabbe  Wim Vyverman 《第四纪科学杂志》2004,19(4):361-375

The Late Quaternary climate history of the Larsemann Hills has been reconstructed using siliceous microfossils (diatoms, chrysophytes and silicoflagellates) in sediment cores extracted from three isolation lakes. Results show that the western peninsula, Stornes, and offshore islands were ice‐covered between 30 000 yr BP and 13 500 cal. yr BP. From 13 500 cal. yr BP (shortly after the Antarctic Cold Reversal) the coastal lakes of the Larsemann Hills were deglaciated and biogenic sedimentation commenced. Between 13 500 and 11 500 cal. yr BP conditions were warmer and wetter than during the preceding glacial period, but still colder than today. From 11 500 to 9500 cal. yr BP there is evidence for wet and warm conditions, which probably is related to the early Holocene climate optimum, recorded in Antarctic ice cores. Between 9500 and 7400 cal. yr BP dry and cold conditions are inferred from high lake‐water salinities, and low water levels and an extended duration of nearshore sea‐ice. A second climate optimum occurred between 7400 and 5230 cal. yr BP when stratified, open water conditions during spring and summer characterised the marine coast of Prydz Bay. From 5230 until 2750 cal. yr BP sea‐ice duration in Prydz Bay increased, with conditions similar to the present day. A short return to stratified, open water conditions and a reduction in nearshore winter sea‐ice extent is evident between 2750 and 2200 cal. yr BP. Simultaneously, reconstructions of lake water depth and salinity suggests relatively humid and warm conditions on land between 3000 and 2000 cal. yr BP, which corresponds to a Holocene Hypsithermal reported elsewhere in Antarctica. Finally, dry conditions are recorded around 2000, between 760 and 690, and between 280 and 140 cal. yr BP. These data are consistent with ice‐core records from Antarctica and support the hypothesis that lacustrine and marine sediments on land can be used to evaluate the effect of long‐term climate change on the terrestrial environment. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Average fluxes from heterogeneous vegetated regions     

Wim Klaassen 《Boundary-Layer Meteorology》1992,58(4):329-354

Using a surface-layer model, fluxes of heat and momentum have been calculated for flat regions with regularly spaced step changes in surface roughness and stomatal resistance. The distance between successive step changes is limited to 10 km in order to fill the gap between micro-meteorological measurements and meso-scale models. A single-layer big leaf model of the vegetation is compared with a multi-layer model to assess the performance of the former in the determination of surface fluxes in heterogeneous terrain.The sub-models of vegetation and atmosphere are mainly based on well-known theory. However, a modification of the mixing-length closure of atmospheric exchange is included to achieve a more realistic calculation of fluxes near step changes at the surface. Measurements, presented in the literature, are used to determine the mixing-length parameters and to validate the calculated fluxes downwind of a change in vegetation cover.The single-layer model, well validated for homogeneous surfaces, underestimates the effects of local advection upon the surface fluxes as this model neglects air flow across the edges of tall vegetation. Using the multi-layer model, local advection results in an increase of up to 50% in regional momentum flux and smaller changes in regional evaporation. Even widely spaced heterogeneities appear to influence regional fluxes.  相似文献