Relative importance of grazing and nutrient controls of macroalgal biomass in three temperate shallow estuaries     

Jennifer Hauxwell  James McClelland  Peter J. Behr  Ivan Valiela 《Estuaries and Coasts》1998,21(2):347-360

Macroalgal biomass and competitive interactions among primary producers in coastal ecosystems may be controlled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of bottom-up and top-down processes under different nutrient loading conditions, we estimated potential amphipod and isopod grazer impact on a dominant macroalgal species in three estuaries in Waquoit Bay, Cape Cod, Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary based on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa andCymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory grazing rates for the remaining species, and in situ macroalgal growth rates. As nitrogen loading rates increased, macroalgal biomass increased (3×), eelgrass (Zostera marina) was lost, and herbivore abundance decreased (1/4×). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gryllotalpa) and, for two of the three species investigated, were faster on algae from the high-nitrogen estuary in comparison to the low-nitrogen estuary, paralleting the increased macroalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2×) with increasing nitrogen loading rate. The comparison between estimated growth increases versus losses ofC. vagabunda biomass to grazing suggested first, that grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower nitrogen loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates and biomass, plus the diminished abundance of grazers. This study suggests the relative impact of top-down and bottom-up controls on primary producers varies depending on rate of nitrogen loading, and specifically, that the impact of herbivory on macroalgal biomass decreases with increasing nitrogen load to estuaries.  相似文献   

Does mean sea level slope up or down toward North?     

M. G. Arur  Ivan I. Mueller 《Journal of Geodesy》1975,49(3):289-297

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Macroseismic observations of earthquake in Yugoslavia (April 15, 1979, 06h21m gmt), on the territory of Czechoslovakia     

Dana Procházková  Vít Kárník  Ivan Brouček  Reviewer A. Zátopek 《Studia Geophysica et Geodaetica》1980,24(1):102-103

an¶rt;u am mu 15- an 1979. a auu a mumuu auu.  相似文献   

Asymmetry in a nearly symmetric dynamo     

Ivan Cupal  Reviewer G. Siráň 《Studia Geophysica et Geodaetica》1980,24(1):28-38

mam u¶rt;m uu aumuu a m n uamu ¶rt;u nmu umu ¶rt;ua a maum ¶rt;a amu u. a um naa nu nu naama umuu , au a um aumu m n aau umuu mau aum n. aam, m mum au ¶rt;-amu a ma mu aum u u n a aumu m n.  相似文献   

Ergebnisse der Fazialanalyse und sedimentologisghen Studien im Paläogen des westlichen Teils der Magura-Flysch-Gruppe (Westkarpaten)     

Dr. Václav Pesl  Doc. Dr. Ivan Krystek 《International Journal of Earth Sciences》1966,55(3):643-665

Zusammenfassung Die Arbeit beschreibt die stratigraphische Einteilung der Paläogen-Serie des Magura-Flysches in Ost-Mähren und der West-Slowakei. Die lithologischen Änderungen der stratigraphischen Glieder des Magura-Flysches werden charakterisiert. Auf Grund der qualitativen und quantitativen Analysen wurden die lithofaziellen Zonen in der unteren und oberen Abteilung des Paläogens festgestellt (V. Pesl).Die sedimentologische Untersuchung befaßte sich mit den Texturen der Sandsteine, mit Messungen von Strömungsmarken, mit Schwermineral-Assoziationen, um die Entstehung der Flyschsedimente dieses Gebietes zu erklären (I. Krystek).

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The paper gives survey and the results of the lithological and facial division and of the sedimentological studies in the paleogene of the Magura flysch group in Eastern Moravia and Western Slovakia.

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Résumé Cet essai donne les resultats de la division stratigraphique et des recherches sédimentaires au paléogène du groupe Magura flysch à l'Est de la Moravie et à l'ouest de la Slovaquie.

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Earth rotation and network geometry optimization for very long baseline interferometers     

A. Dermanis  Ivan I. Mueller 《Journal of Geodesy》1978,52(2):131-158

Very Long Baseline Interferometry (VLBI) is one of the new techniques which will probably dominate geodesy and geophysics in the near future. Its main advantage lies in the fact that it brings the accuracy of direction measurements to a level previously possible only for range measurements. This closes the gap between powerful range determination techniques such as laser ranging and the much less accurate determination of directions through photographic tracking of artificial earth satellites. The technique is geometric in the sense that the relevant observations are independent of the gravity field of the earth. However, the “orbits” of the observed extragalactic radio sources with respect to an earth-fixed system are dominated and perturbed by the rotation of the earth with respect to inertial frame. This allows the determination of polar motion, precession-nutation and length-of-the-day variations, and the technique becomes also “dynamic” in this respect. The capability of determining the geometry of a network of stations within a short time interval and with a centimeter level accuracy also allows the study of the variation of network geometry with time caused by earth tides and other periodic or secular station drifts. The main objective of the present work is the exploration of the capabilities of VLBI for the recovery of earth rotation and network geometry parameters. For this purpose, a number of characteristic experimental designs based on present and candidate for the near future station locations is chosen. The results from the analysis of simulated observations for each particular design are presented in the paper. Presented at IAG International Symposium on “Optimization of Design and Computation of Control Networks”, Sopron, Hungary, 4–10 July 1977.  相似文献   

Detection of crustal motion using spaceborne laser ranging systems     

Muneendra Kumar  Ivan I. Mueller 《Journal of Geodesy》1978,52(2):115-130

The spaceborne laser ranging (or lasering) system provides a method of precise positioning of a large number of points on the earth's surface in a short period of time. That is, a measure of the relative location of geodetic markers from a space platform can maintain horizontal and vertical control to 2 to 5 cm. At this level of control, small earth surface crustal motions should be detectable. Development of a model for the strain field can be constructed. Furthermore, the spaceborne lasering system can survey an area in a very short period of time (one to two weeks) and resurvey the area as required. System design parameters are now being established by NASA for a possible test flight aboard the Shuttle in 1982. These include design specifications of economical corner cubes for ground retroreflectors coupled with the evolution of engineering model to flight model development. If the experiment of the Shuttle proves to be successful, it is hoped to put the laser in a free flight satellite. This paper presents the results of a simulated analysis for this latter case. The system is conceived as an orbiting ranging device with a ground base grid of reflectors or transponders (spacing 10 to 30 km), which are projected to be of low cost (maintenance-free and unattended) and which will permit the saturation of a local area to obtain data useful to monitor crustal movements. The test network includes 75 stations with roughly half of them on either side of the San Andreas fault zone. Critical study comparatively evaluates various observational schemes and statistically analyzed crustal motion recovery. The study considers laser radar as the main ranging system pending final selection from many possible candidates. The satellite orbit is inclined at 110° and slightly eccentric (e=0.04) with orbital altitudes varying from 370 km to 930 km. The results indicate that the geometric mode (simultaneous ranging) with a minimum of five grid and three distant (fundamental) stations and mixed ranging to satellite and airplane seems to be most promising. The fundamental stations are distinguished from the grid station in their location and this location should be “distant” enough from the area of crustal movement so that they can be considered stationary over the time span of the motion involved. Presented at the 1977 I.A.G. International Symposia on Satellite Geodesy, Budapest, Hungary, June 28–July 1, and on Recent Crustal Movements, Palo Alto, California, USA, July 25–30.  相似文献   

在6厘米波长对类星体3C454.3的九站VLBI观测     

吴盛殷  I.Pauliny-Toth  R.Porcas  A.Zensus 《中国天文和天体物理学报》1986,(2)

3C454.3是近几年引起广泛注意的一个强活动射电类星体,它有毫角秒尺度的核——喷流结构,其核在1981年的一次流量密度大爆发之后,显示出“超光速增亮”现象,在6厘米波长,用包括欧洲、美国及南非的9个射电望远镜,于1983年10月对3C454.3进行了甚长基线干涉网观测。观测结果除了证实其核——喷流结构,显示了从毫角秒到角秒尺度上该类星体结构的系统弯曲外,还清楚地揭示了“超光速增亮”所预期的,在1981年8月到1983年1月期间,核的超光速膨胀现象。  相似文献   

Volcanic ash clouds affecting Northern Europe: the long view     

Graeme T. Swindles  Ivan P. Savov  Charles B. Connor  Jonathan Carrivick  Elizabeth Watson  Ian T. Lawson 《Geology Today》2013,29(6):214-217

The volcanic ash or ‘tephra’ cloud resulting from the relatively small (volume and VEI) eruption of the Icelandic volcano Eyjafjallajökull in 2010 caused major air travel disruption, at substantial global economic cost. On several occasions in the past few centuries, Icelandic eruptions have created ash and/or sulphur dioxide clouds which were detected over Europe (e.g. Hekla in 1947, Askja in 1875, and Laki in 1783). However, these historical observations do not represent a complete record of events serious enough to disrupt aviation in Europe. The only feasible evidence for this is within the geological tephra record. Ash layers are preserved in bogs and lakes where tephra deposited from the atmosphere is incorporated in the peat/mud. In this article we: 1, introduce the analysis of the Northern European sedimentary tephra record; 2, discuss our findings and modelling results; 3, highlight how these were misinterpreted by the popular media; and 4, use this experience to outline several existing problems with current tephra studies and suggest agendas for future research.  相似文献   

The continental record of Ediacaran volcano‐sedimentary successions in southern Brazil and their global implications     

Liliane Janikian  Renato Paes De Almeida  Ricardo Ivan Ferreira Da Trindade  Antonio Romalino Santos Fragoso‐Cesar  Manoel Souza D′Agrella‐Filho  Elton Luis Dantas  Eric Tohver 《地学学报》2008,20(4):259-266

The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft‐bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaquã Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar‐Ar and U‐Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic‐type glacial context during the last Neoproterozoic glacial event and presents the Picada das Graças Formation (580 ± 3.6 Ma) as the first dated non‐glacial unit coeval to the Gaskiers Formation.  相似文献