ALBERT

Notes: We have investigated the effect of substitution and nitrogenation on the hyperfine parameters in Gd2Fe17−x(Ga,Al)x (x=1–10) and Gd(Fe,Co)12−xMox(Ny) (x=1.5–3.0 and y≈1) by means of 155Gd Mössbauer spectroscopy. We have observed marked changes in the values of the electric field gradient Vzz, the effective Gd-hyperfine field, and isomer shift. Al substitution in Gd2Fe17 leads to a sign reversal of the electric field gradient. No sign reversal of Vzz has been observed in the Gd2Fe17−xGax compounds. The effective hyperfine fields of the Gd2Fe17−x(Ga,Al)x compounds are decreasing with increasing x, due to the reduction of the 6s electron polarization resulting from the decreasing Fe moments. The ultimate value of the isomer shift based on a simple model on chemical effects that would be reached for x=17 is 0.52 mm s−1 for Al and 0.64 mm s−1 for Ga substitution in Gd2Fe17, respectively. The electric field gradient of the GdFe12−xMox compounds were obtained for x=1.5 and x=3.0. Nitrogenation is shown to lead to a sign reversal of the 4f and Fe sublattice anisotropy. Mo substitution in GdCo12−xMox shows a decreasing negative value of Vzz with increasing Mo concentration x. The values ||Heff|| in GdCo12−xMox increase with increasing Mo concentration. This can be attributed to a different near neighbor Gd and 3d-metal contribution to the effective hyperfine field compared to those of GdFe12−xMox. © 1996 American Institute of Physics.

Notes: High-pressure 155Gd Mössbauer measurements in the range 0–20 GPa have been performed in a diamond anvil cell (DAC) at low temperatures. Due to the high Mössbauer transition energy of 86.54 keV, problems with pressure calibration, shielding, and gasketting arise, which are not present in DAC measurements on lower-energy Mössbauer isotopes. A strong point source has been produced using hot neutrons. Consequences for measuring times are discussed, together with a description of the optical system used for the pressure calibration.

Notes: Groynes are the dominant river engineering structures along the lowland section of large European rivers such as the Rhine, Danube and the Elbe. More than 6000 groynes structure the 400 km stretch of the potamal of the Elbe River. After 1945, destruction of the groynes increased through ice and flood events in the eastern part of Germany. In the past ten years, groyne reconstruction was accompanied by a controversial discussion in the context of the ecological integrity of the Elbe River. With the modular habitat model (MHM) a tool was developed to evaluate the suitability and to balance the availability of fish habitats in groyne fields of different conditions. The morphodynamic module produced a digital terrain model and a spatial model of flow velocity for each groyne field separately. Based on point abundance sampling by electro-fishing, models of habitat preference were developed for different life history stages by logistic regression. Statistical models predicting the preference of fish-environment relationships (Leuciscus idus) at different life history stages. The models were discriminated and validated by receiver operating characteristic (ROC) curves. The link between the statistical and the spatial model was realised in the suitability module. The suitability of microhabitats is expressed in classes for each species and life history stage separately. Habitat availability is balanced on the level of mesohabitat, e.g. different types of groyne fields. The temporal dynamics of habitat availability are analysed by considering different levels of discharge. For the stage ‘juvenile A’ and preadult the habitat suitability is better in fields downstream of destroyed groynes. For ‘juvenile B’ and adult stages of the ide, groyne fields in general constitute low habitat suitability. Differences in spatial availability are higher than the differences in temporal habitat availability.

Notes: Fish species densities from 1025 hauls obtained with beam trawl and stow net catches from the southern Tagus and northern Elbe estuaries were analysed for two different periods of time between 1978 and 1997. The data were related with environmental factors to compare species composition and densities, contributions of different ecological guilds, structuring environmental key factors, intra-annual cyclicity and interperiodical stability of ichthyofaunas in large European estuaries at different latitudes. Although the total number of species in both the Tagus and Elbe estuaries (Tagus: 57 species, Elbe: 58 species) was very similar, significant differences were estimated in species compositions, species densities and number of species, and individuals of the different ecological guilds between both estuaries. A total of 15 species occurred in both estuaries but only two of these species, the catadromous Anguilla anguilla and the marine estuarine opportunist Pleuronectes flesus, contributed substantially to the total abundance. Marine species together contributed more than 5% of the total number of species in the Tagus and Elbe estuaries, whereas their contribution in number of individuals amounted to 22% in the Tagus and only 9% in the Elbe. In terms of number of individuals, the ichthyofauna of the Tagus estuary was dominated by estuarine species (especially Pomatoschistus spp.) contributing 68% of the total abundance. In contrast, the fish fauna of the Elbe estuary was dominated by high numbers of anadromous species (88%) individuals, especially Osmerus eperlanus. Canonical corresponding analyses (CCA) indicated that, besides intra-annual variables, salinity and mouth distance were two of the most important environmental factors structuring the fish communities in both estuaries. Moreover, water depth was found to be an important environmental factor in the Tagus estuary, whereas the water temperature substantially affected the ichthyofaunal composition in the Elbe estuary. In both estuaries, contributions by number of species and number of individuals of the different ecological guilds were not statistically different between the two periods considered, 1978–80 and 1995–97 for the Tagus, and 1981–86 and 1989–95 for the Elbe. Nevertheless, from 1981–86 until 1989–95 a remarkable increase of about 4.5 times in the densities of O. eperlanus was observed in the Elbe estuary, probably related to improved water quality, especially of oxygen conditions in the nurseries since 1990. This was also supported by the results of CCA, suggesting important influences of the years 1989 and 1990 on the Elbe estuary fish assemblage. Climate change as a result of global warming may be evident for the observed changes of the densities of P. flesus which decreased in the southern Tagus estuary and, in contrast, increased in the northern Elbe estuary during comparable periods of time. It is recommended to use the ecological guild classification scheme which was applied for the Tagus and Elbe estuaries for further studies of European estuarine fish assemblages.

Notes: From the years 1997 to 2000, fish assemblages from 15 groyne fields and training walls (a current guiding dyke, protecting the groyne field) of the Middle Elbe River (Germany) were examined by electrofishing. The aim of the study was to detect abiotic environmental key variables which determine preadult and adult fish assemblage in typical riverbank structures using multivariate ordination techniques (canonical correspondence analysis) and univariate methods (logistic regression analysis). Habitat preferences of preadult and adult stages of selected fish species were also estimated.Fish sampling was carried out at 190 river stretches; a further 1615 samples were taken by the point-abundance sampling method. In total, 21 732 preadult and adult individuals belonging to 30 fish species were caught; perch, eel, ide, roach, chub, gudgeon, bleak and white bream were the most frequent. Eurytopic species clearly dominated the fish assemblage with 72% total abundance, followed by rheophilic species (26%). Limnophilic fish species (rudd, crucian carp, stickleback and tench) were relatively rare (2% of total abundance), with frequencies of occurrence between 1 and 10%. The highest species diversities were estimated in structurally diverse training walls and groyne fields with defective groynes, whereas low species diversity was found to occur in poorly structured, strongly silted groyne fields with intact groynes. Additionally, typical seasonal and annual changes of the fish assemblage were observed.From the mesoscalic point of view, the structure of the preadult and adult fish assemblage was affected by both spatial (type of groyne field, river stretch) and temporal factors (year, season). Regarding the microhabitat, hydromorphological parameters (slope, current velocity, water level, predominant and secondary substrate) followed by physicochemical water values [dissolved oxygen (DO), temperature and pH-value] are of significance for the structure of the fish community. Furthermore, hiding places (shelter) are of subordinate but significant importance for the fish assemblage in the poorly structured main channel of the Elbe River.Importance of hydromorphological factors on the microhabitat scale was confirmed by multiple logistic regression in 12 of 14 preference models, conducted for different ontogenetic stages of roach, bream, white bream, bleak, ide, chub, gudgeon, asp, perch and eel. Habitat preferences of adult bleak and preadult roach were first determined by physicochemical water values and seasonal influences. Univariate models were generated to describe specific habitat preferences of different species and age-stages regarding the most important environmental variables (depth, velocity, predominant substrate, slope and shelter). Intraspecific differences were noticed between preadult and adult chub regarding preferences for water velocity, or between preadult and adult perch concerning slope of shore. Interspecific differences were observed, e.g. between adult roach and white bream, with regard to the use of shelters.

Notes: Smelt, flounder, eel, three-spined stickleback, twaite shad, ruffe and herring were the major species among 62 forming the fish community of the Elbe estuary. Species richness, species diversity, evenness and total fish biomass decreased in the upstream direction. Total abundance was greatest during summer. Salinity was the most significant physico-chemical factor affecting species richness and total fish biomass, whereas water temperature was the best predictor of total abundance. Marine fish species decreased with decreasing salinity, while the freshwater species roach and ide were absent at salinities 〉 15‰. Bream, blue bream, white bream, perch and pikeperch occurred more frequently in shallow marginal regions with lower current velocities, while herring and other marine species occurred closer to the deep central regions with high current velocities. Annual and seasonal variations of the community structure mainly reflected in population dynamics of smelt, flounder, twaite shad, three-spined stickleback and eel, especially those of 0-age smelt. Populations of smelt and ruffe and their importance in the fish community increased between 1989 and 1992, while those of twaite shad and eel decreased. At oxygen concentrations 〈3 mg I1, smelt and flounder were rare, but eels were at their maximum at 3.5 mg I−1. Changes of species composition and fish abundance were closely related to daytime and tidal cycle.

Notes: Abstract The reflection coefficient in very-low-energy spin polarized electron reflection from magnetized Fe(001) has been investigated in the energy range 0〈E〈23 eV. A peak in the asymmetry of 21% is observed atE=10.6 eV with a mean reflection coefficient ≈0.1. This effect provides a new spin detector with a more than 20 times larger efficiency than with other spin detectors.

Notes: Abstract The application of a long wavelength limit to Debye theory is examined for small particles at low temperatures. The questions of choice and validity of cut-off are examined, as well as sources of error in the general theory of small particles. An approximate expression is given for the specific heat and Mössbauer recoil-free fraction, with curves showing magnitudes of this effect for various functional relations. Several other possible applications are mentioned.

Notes: Abstract The existence of cluster of three-antiferromagnetically coupled spin S=7/2 in the EuMg5 is found to give rise to peculiar behaviour in the thermodynamic as well as the Mössbauer relaxation. The A.C. susceptibility, specific heat and the high field magnetization as well as the ME spectra are reported and interpreted in terms of a model in which the intra-cluster interaction is treated exactly while the inter-cluster interaction is approximated by a molecular field.

Notes: Abstract Co0.5Zn0.5FeCrO4 is typical of concentrated magnetic oxides which show characteristics of the spin glass phenomena in macroscopic measurements. AC susceptibility shows a broad peak at 147 K, spread over the temperature range from 90 to 170 K. Using Mössbauer spectroscopy, we show that the peak in the AC susceptibility is due to gradual conversion of the magnetic lattice into superparamagnetic clusters as the temperature increases, and spin arrangement at 4.2 K is of Yafet Kittel type.