Effects of Cropping on Growth in the Mediterranean Seagrass Posidonia oceanica (L.) Delile*     

Karl J. Wittmann  Jorg A. Ott 《Marine Ecology》1982,3(2):151-159

Abstract. In view of proposed human use of seagrass production the influence of cropping on the growth of Posidonia oceanica (L.) DELILE was investigated. Removal of photosynthetic tissue reduces productivity throughout most of the growth season. This is consistent with the observed low natural grazing rates in aquatic macrophytes. A high mortality of shoots points to possible irreversible damage to the meadow. Harvesting of Posidonia should therefore be restricted to the end of the leaf growth period in early fall.  相似文献   

Adaptive Strategies at the Ecosystem Level: Examples from Two Benthic Marine Systems     

Jörg A.  Ott 《Marine Ecology》1981,2(2):113-158

Abstract. Adaptive strategies on the ecosystem level are demonstrated using the properties of systems (community biomass, trophic level energy transfer). Development of stabilizing storages versus rapid turnover and energy transfer through the grazing food chain versus detritus food chain are shown to be adaptive processes leading towards an optimal performance of the system. A model of incorporation and processing of information in ecosystems is proposed which contains the essential features of evolution. The persistence of "interspecific altruism" is discussed.  相似文献   

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs     

Hans-Michael Seitz  Gerhard P. Brey  Jutta Zipfel  Ulrich Ott  Stefan Weyer  Soodabeh Durali  Stephan Weinbruch 《Earth and Planetary Science Letters》2007,260(3-4):582-596

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.  相似文献   

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components     

V.K. Rai  S.V.S. Murty  U. Ott 《Geochimica et cosmochimica acta》2003,67(22):4435-4456

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio ³⁸Ar/³⁶Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of ⁴⁰Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported ³⁶Ar excesses due to possible decay of extinct ³⁶Cl in the Efremovka meteorite.Trapped ²²Ne in the range of (1.4-2.5) × 10⁻⁸ cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. ²¹Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (¹³²Xe/³⁶Ar)₀, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (¹³²Xe/³⁶Ar)₀ is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ¹⁷O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio ¹²⁹Xe/¹³²Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of ¹²⁹I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.  相似文献   

NGC 3741: the dark halo profile from the most extended rotation curve   总被引：1，自引：0，他引：1  

Gianfranco Gentile  Paolo Salucci  Uli Klein  Gian Luigi Granato 《Monthly notices of the Royal Astronomical Society》2007,375(1):199-212

We present new H  i observations of the nearby dwarf galaxy NGC 3741. This galaxy has an extremely extended H  i disc, which allows us to trace the rotation curve out to unprecedented distances in terms of the optical disc: we reach 42 B -band exponential scalelengths or about 7 kpc. The H  i disc is strongly warped, but the warp is very symmetric. The distribution and kinematics are accurately derived by building model data cubes, which closely reproduce the observations. In order to account for the observed features in the data cube, radial motions of the order of 5–13 km s⁻¹ are needed. They are consistent with an inner bar of several hundreds of pc and accretion of material in the outer regions.
The observed rotation curve was decomposed into its stellar, gaseous and dark components. The Burkert dark halo (with a central constant density core) provides very good fits. The dark halo density distribution predicted by the Λ cold dark matter (CDM) theory fails to fit the data, unless NGC 3741 is a 2.5σ exception to the predicted relation between concentration parameter and virial mass and at the same time a high value of the virial mass (though poorly constrained) of  10¹¹ M_⊙  . Noticeably, modified Newtonian dynamics (MOND) seems to be consistent with the observed rotation curve. Scaling up the contribution of the gaseous disc also gives a good fit.  相似文献   

Noble gases in two shergottites and one nakhlite from Antarctica: Y000027, Y000097, and Y000593     

S.P. Schwenzer  S. Herrmann  U. Ott 《Polar Science》2009,3(2):83-99

We have investigated secondary influences on the noble gas budget in rim and interior pairs of three Martian meteorites from Antarctica: the lherzolitic shergottites Y000027 and Y000097, and the nakhlite Y000593. Three factors have been found to influence the original Martian noble gas budget: shock metamorphic overprint, cosmic irradiation, and terrestrial weathering. The ³He/⁴He ratio of the shergottites is between 0.189 and 0.217, which indicates almost complete loss of radiogenic ⁴He. This is expected from the high shock pressure observed in the shergottite samples. The concentration of ⁴He in these shergottite samples ranges from 33.8 to 39.4 × 10⁻⁸ ccSTP/g. ²²Ne in the shergottites is on the order of 14 × 10⁻⁹ ccSTP/g. The nakhlite has 800 × 10⁻⁸ ccSTP/g ⁴He and 26 × 10⁻⁹ ccSTP/g ²²Ne. An indication for solar cosmic ray contribution to the neon budget can be found in the shergottites. As Y000027 and Y000097 are reported to be paired we conclude the cosmic ray exposure (CRE) age T₍₃₊₂₁₎ of this shergottite to be 4.41 ± 0.54 Ma. For the nakhlite Y000593 T₍₃₊₂₁₎ is 11.8 ± 0.3 Ma. Heavy noble gas concentrations show large differences between rim and interior samples with the rim samples having 1.3–2.9, 1.7–38, and 1.4–20 times as much ³⁶Ar, ⁸⁴Kr, and ¹³²Xe, respectively. The enrichment of heavy noble gases in the rim samples indicates severe terrestrial contamination. The relation between ¹²⁹Xe/¹³²Xe and ⁸⁴Kr/¹³²Xe in the rim samples shows that the incorporation mechanism caused elemental fractionation of Kr and Xe to the extent that in the Y000027 shergottite samples any Martian signature is completely masked by terrestrial contamination, if the total is taken. Only the 1400 °C steps show clear evidence for Martian atmosphere. The Y000593 nakhlite interior sample, on the other hand, shows low ⁸⁴Kr/¹³²Xe in relation to ¹²⁹Xe/¹³²Xe, which is characteristic for fractionated Martian atmosphere observed in nakhlites.  相似文献   

Intercomparison and Evaluation of Precipitation Forecasts for MAP Seasons 1995 and 1996     

R. Mladek  J. Barckicke  P. Binder  P. Bougeault  N. Brzovic  C. Frei  J. F. Geleyn  J. Hoffman  W. Ott  T. Paccagnella  P. Patruno  P. Pottier  A. Rossa 《Meteorology and Atmospheric Physics》2000,72(2-4):111-129

Summary  The precipitation forecasts of four operational numerical weather prediction models over the Alpine region are evaluated and intercompared for two periods of interest to the Mesoscale Alpine Programme (MAP). The new analysis of Alpine rainfall of Frei and Scha?r (1998) is used to validate the models. It is found that the models have a tendency to overestimate the total precipitation and the frequency of intense rain events over high orography. The skill scores show good consistency between models, except for the ability to forecast light rain or heavy rain events. The partition between convective and stratiform rainfall is rather variable between the models. Received March 15, 1999/Revised July 12, 1999  相似文献   

The ALFA Laser: Beam Relay and Control System     

T. Ott  W. Hackenberg  S. Rabien  A. Eckart  S. Hippler 《Experimental Astronomy》2000,10(1):89-101

The ALFA laser subsystem uses a 4 watt continuous wave laser beam to produce an artificial guide star in the mesospheric sodium layer as a reference for wavefront sensing. In this article we describe the system design, focusing on the layout of the beam relay system. It consists of seven mirrors, four of which are motor-controlled in closed loop operation accounting for turbulences inside the dome and flexure of the main telescope.The control system features several computers which are located close to analysis and control units. The distribution of the tasks and their interaction is presented, as well as the graphical user interface used to operate the complete system. This is followed by a discussion of the aircraft detection system ALIENS. This system shuts off the laser beam when an object passes close to the outgoing laser.  相似文献   

Spectral imaging of the Sagittarius B2 region in multiple 3-mm molecular lines with the Mopra telescope     

P. A. Jones  M. G. Burton  M. R. Cunningham  K. M. Menten  P. Schilke  A. Belloche  S. Leurini  J. Ott  A. J. Walsh 《Monthly notices of the Royal Astronomical Society》2008,386(1):117-137

  相似文献   

The CO content of the Local Group dwarf irregular galaxies IC 5152, UGCA 438 and the Phoenix dwarf     

P. Buyle †  D. Michielsen  S. De Rijcke †  J. Ott ‡  H. Dejonghe 《Monthly notices of the Royal Astronomical Society》2006,373(2):793-798

We present a search for  CO(1 → 0)  emission in three Local Group dwarf irregular galaxies: IC 5152, the Phoenix dwarf and UGCA 438, using the ATNF Mopra radio telescope. Our scans largely cover the optical extent of the galaxies and the stripped H  i cloud west of the Phoenix dwarf. Apart from a tentative but non-significant emission peak at one position in the Phoenix dwarf, no significant emission was detected in the CO spectra of these galaxies. For a velocity width of 6 km s⁻¹, we derive 4σ upper limits of 0.03, 0.04 and 0.06 K km s⁻¹ for IC 5152, the Phoenix dwarf and UGCA 438, respectively. This is an improvement of over a factor of 10 compared with previous observations of IC 5152; the other two galaxies had not yet been observed at millimetre wavelengths. Assuming a Galactic CO-to-H₂ conversion factor, we derive upper limits on the molecular gas mass of  6.2 × 10⁴, 3.7 × 10³  and  1.4 × 10⁵ M_⊙  for IC 5152, the Phoenix dwarf and UGCA 438, respectively. We investigate two possible causes for the lack of CO emission in these galaxies. On the one hand, there may be a genuine lack of molecular gas in these systems, in spite of the presence of large amounts of neutral gas. However, in the case of IC 5152 which is actively forming stars, molecular gas is at least expected to be present in the star-forming regions. On the other hand, there may be a large increase in the CO-to-H₂ conversion factor in very low-metallicity dwarfs  (−2 ≤[Fe/H]≤−1)  , making CO a poor tracer of the molecular gas content in dwarf galaxies.  相似文献