Effect of Sediment Nutrient Enrichment and Grazing on Turtle Grass <Emphasis Type="Italic">Thalassia testudinum</Emphasis> in Jobos Bay,Puerto Rico     

Ylva S. Olsen  Ivan Valiela 《Estuaries and Coasts》2010,33(3):769-783

We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5 m plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls.  相似文献   

δ15N and δ13C in the Mondego estuary food web: Seasonal variation in producers and consumers     

Alexandra Baeta  Rute Pinto  Ivan Valiela  Pierre Richard  Nathalie Niquil  João C. Marques 《Marine environmental research》2009,67(3):109-116

Assessments of temporal variation in stable carbon and nitrogen ratios were used to examine seasonal trends of the water column and benthic food webs in the Mondego estuary (Portugal). There was a marked seasonality in weather and water column conditions, including nutrient supply and chlorophyll concentrations. In spite of the pronounced environmental changes, we found little evidence of seasonal variation in δ¹³C and δ¹⁵N of producers and consumers in the Mondego estuary, with a few notable exceptions. Nitrogen isotope ratios in macrophytes (Zostera noltii, Ulva sp., Enteromorpha sp., and Gracilaria sp.), and in two grazers (Idotea chelipes and Lekanesphaera levii) increased during late summer, with the highest δ¹⁵N values being measured in July, during a period of elevated temperatures and drought, which may have favored high rates of denitrification and heavier δ¹⁵N values. The results suggest that stable-isotope values from macrophytes and selected grazers are useful as tracers of seasonal changes in nitrogen inputs into estuaries, and that those of consumers reflect other factors beyond seasonal variations in N and C sources.  相似文献   

Attenuation of Peak Horizontal and Vertical Acceleration in the Dinarides Area     

Herak  Marijan  Markušić  Snježana  Ivančić  Ines 《Studia Geophysica et Geodaetica》2001,45(4):383-394

Peak acceleration attenuation relations for horizontal and vertical components are presented for the Dinarides region, based on 145 3-component accelerograms related to 46 earthquakes with local magnitudes of 4.5 or greater and with epicentral distances of less than 200 km as recorded on 39 recording sites in the greater Dinarides region. The attenuation functions were obtained by two-stage stratified regression on the local magnitude and epicentral distance as independent variables. The predicted peak acceleration values within the distance range covered by the data are comparable to the ones obtained for stiff-soil or rock sites when selected reference relations are used. The rather large average residuals are caused mostly by the lack of information on local site conditions and by the use of epicentral distance instead of fault distance.  相似文献   

Bestimmung der durchgangszeiten von sternen durch die methode der unterbrechung der gesichtsfeldbelichtung     

Josef Kabeláč  Ivan Synek 《Studia Geophysica et Geodaetica》1964,8(2):120-126

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Quasitrapped protons (E p =1–11 MeV) nearL=4 registered by intercosmos-13 satellite     

Ivan Kimák  Karel Kudela  P. V. Vakulov  Reviewer S. Fischer 《Studia Geophysica et Geodaetica》1982,26(2):207-211

Summary Data obtained from apparatus detecting energetic particles on low altitude polar orbiting satellite Intercosmos-13 show enhancement of quiasitrapped proton flux (E_p=1–11 MeV) near L=4 in comparison with both and lower L. This phenomenon persists during intervals with low level of geomagnetic activity. Possibility of influence of false pulses on the result is discussed.  相似文献   

Concepts for reference frames in geodesy and geodynamics: The reference directions     

Erik W. Grafarend  Ivan I. Mueller  Haim B. Papo  Burghard Richter 《Journal of Geodesy》1979,53(3):195-213

Modern high accuracy measurements of the non-rigid earth are to be referred to four-dimensional, i.e., time- and space-dependent reference frames. Geodynamic phenomena derived from these measurements are to be described in a terrestrial reference frame in which both space- and time-like variations can be monitored. Existing conventional terrestrial reference frames (e.g. CIO, BIH) are no longer suitable for such purposes. The ultimate goal of this study is the establishment of a reference frame, moving with the earth in some average sense, in which the geometric and dynamic behavior of the earth can be monitored, and whose motion with respect to inertial space can also be determined. The study is conducted in several parts. In the first part problems related to reference directions are investigated, while subsequent parts deal with positions, i.e., with reference origins and scale. Only the first part is treated in this paper. The approach is based on the fact that reference directions at an observation point on the earth surface are defined by fundamental vectors (gravity, earth rotation, etc.), both space and time variant. These reference directions are interrelated by angular parameters, also derived from the fundamental vectors. The interrelationships between these space- and time-variant angular parameters are illustrated in a commutative diagram, tower of triads, which makes the derivation of the various relationships convenient. In order to determine the above parameters from observations using least squares techniques, a model tower of triads is also presented to allow the formation of linear observation equations. Although the model tower is also space and time variant, its variations are described by adopted parameters representing our current knowledge of the earth.  相似文献   

Computation of diffracted rays in inhomogeneous media with curved interfaces     

Vlastislav Červený  Ivan Pšenčík  Reviewer J. Vaněk 《Studia Geophysica et Geodaetica》1972,16(4):356-366

Summary A method for computing certain types of diffracted rays and corresponding travel-time curves in laterally inhomogeneous media with slightly curved interfaces is suggested. Numerical examples are presented.Dedicated to Academician Alois Zátopek on His 65th Birthday  相似文献   

The effect of earth orientation errors in baseline determination     

Erricos C. Pavlis  Ivan I. Mueller 《Journal of Geodesy》1983,57(1-4):273-282

The determination of baseline lengths from certain space techniques is based on the derived coordinates of the terminal stations. As such, the estimated baselines are susceptible to systematic errors that affect the relative coordinates. One source of error is in the set of parameters which describes the continuously changing relative orientation of the Conventional Terrestrial (CTS) and Inertial (CIS) Reference frames. Due to these errors, the coordinates of each terminal station may in fact refer to a slightly different coordinate svstem, and, therefore, when used for computing the length between the stations, errors will result. The expected magnitudes of such errors and their possible presence in current solutions are investigated. In conclusion, we find that the present level of accuracy and stability of the available parameters connecting the CTS and CIS (e.g., the ERP series) is unsatisfactory for centimeter level baseline length determinations. The available options are either the use of strictly simultaneous SLR data sets (similar to the VLBI data sets) or the improvement of the parameters connecting the CTS and the CIS. The first long-range step in the latter direction is the support of the IAG/IAU Joint Working Group COTES proposal [CSTG Bulletin, 1982], endorsed by both the IAG and IAU in various resolutions [IAU, 1983; IAG, 1982]. Presented at the Annual Fall Meeting of the American Geophysical Union, San Francisco, December 7–15, 1982, and at the Fifth Annual NASA Geodynamics Program Conference and Crustal Dynamics Project Review, Washington, D.C., January 24–28, 1983.  相似文献   

A numerical system to study the transport properties in the Ria de Aveiro lagoon     

Jo?o Miguel?DiasEmail author  José Fortes?Lopes  Ivan?Dekeyser 《Ocean Dynamics》2003,53(3):220-231

This work presents results from numerical modelling studies on the hydrodynamics and sediments and passive particles transport properties in Ria de Aveiro, a shallow lagoon located on the Portuguese Northwest Atlantic coast. The hydrodynamic of the lagoon was systematically studied, from both the Eulerian and Lagragian point of view, in order to understand the overall circulation in the lagoon, characterize the dynamics of its different channels and assess the transport of suspended cohesive sediments.Responsible Editor: Hans Burchard  相似文献   

Moho depth and three-dimensional P and S structure of the crust and uppermost mantle in the Eastern Mediterranean and Middle East derived from tomographic inversion of local ISC data   总被引：1，自引：0，他引：1  

Ivan Koulakov  Stephan V. Sobolev 《Geophysical Journal International》2006,164(1):218-235

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