Spatial and temporal variation of fish assemblage associated with aquatic macrophyte patches in the littoral zone of the Ayapel Swamp Complex, Colombia

ABSTRACT: Aim: The purpose of the present study was to examine spatial and temporal variation in fish assemblage structure associated with aquatic macrophytes in the littoral zone of the ASC. Methods: Specimens were caught between January 2008 and February 2009, over four limnimetric moments, using both cast net and seine net. Data on the temperature, electrical conductivity, pH and dissolved oxygen was recorded for the characterization of the water mass in the sites. Results: A total of 34,151 specimens from 44 species were collected. The most abundant species were Eigenmannia virescens, Astyanax caucanus, Astyanax fasciatus, Roeboides dayi and Cyphocharax magdalenae, which together accounted for more than 75% of the sample. Temporal and spatial comparisons showed variation in the environmental conditions and highlighted the existence of heterogeneous abiotic conditions (p0.05) regarding the fish assemblage structure. The multivariate analysis showed no significant relationship between existing environmental conditions and the fish assemblage (p=0.04). The analysis also showed the absence of a relationship between the fish assemblage and environmental variables with respect to the flood pulse and sampling sites (p>0.05). Conclusion: The uniformity of the fish communities that inhabit aquatic macrophyte patches in the littoral region of the ASC may be related to the availability of suitable habitat in structural terms, that probably supports a more abundant and varied wildlife

RESONANCE STUDIES OF H ATOMS ADSORBED ON FROZEN H2 SURFACES

Nous avons observé la résonance hyperfine dans l'état fondamental d'atomes d'hydrogène contenus dans une cellule de 5 cm de diamètre intérieurement recouverte en surface d'hydrogène moléculaire solide. Le déphasage de la résonance hyperfine pendant le temps où les atomes sont adsorbés produit des déplacements de fréquence qui varient d'un facteur 2 dans la gamme de température allant de 3,7 à 4,6 K, ainsi que des taux de décroissance radiative qui varient d'un facteur 5 dans cette même zone de température. Ces déplacements de fréquence et ces taux de décroissance ont une grandeur et une dépendance en température qui s'expliquent bien en supposant une distribution non uniforme des énergies d'adsorption sur la surface, caractérisée par une valeur moyenne de 38(8) K en accord avec les estimations théoriques qu'on peut faire pour une surface régulière. Si l'on extrapole la valeur de 30 nanosec. mesurée à 4,2 K comme durée moyenne d'adsorption, on peut prédire des durées d'adsorption très longues pour H sur H2 au-dessous de 1 K. L'étude des taux de retour à l'équilibre des populations des niveaux montre l'existence des collisions d'échange de spin électronique en surface entre atomes adsorbés, la durée de ces collisions étant longue devant la période hyperfine ; ceci suggère que les atomes sont partiellement mobiles sur la surface. Les taux les plus bas observés pour le retour à l'équilibre des populations fixent une limite inférieure de l'ordre de 500 au nombre des collisions atome-surface non suivies de recombinaison à 4,2 K.Observations are reported of the ground state hyperfine resonance of hydrogen atoms stored in a 5 cm. diameter bottle coated with frozen molecular hydrogen. Dephasing of the hyperfine resonance while the atoms are adsorbed produces frequency shifts which vary by a factor of two over the temperature range 3.7 K to 4.6 K and radiative decay rates which vary by a factor of five over this range. The magnitudes and temperature dependences of the frequency shifts and decay rates are consistent with a non-uniform distribution of surface adsorption energies with mean about 38(8) K, in agreement with theoretical estimates for a smooth surface. Extrapolation of the 30 nanosec. mean adsorption times at 4.2 K predicts very long adsorption times for H on H2 below 1 K. Studies of level population recovery rates provide evidence for surface electron spin exchange collisions between adsorbed atoms with collision duration long compared to the hyperfine period, suggesting that the atoms are partially mobile on the surface. The lowest rates observed for level population recovery set a lower limit of about 500 atom-surface collisions at 4.2 K without recombination

Hydrogen atom adsorption on molecular solid surfaces

Magnetic resonance techniques similar to those developed in Paris to study adsorption of 3He on molecular solids have been used to measure the energy of adsorption of H atoms bound to solid atomic neon and molecular hydrogen surfaces. Information about sticking probabilities and polarization of the H atoms while adsorbed is also obtained

PROGRESS OF THE STATE-SELECTED BEAM LOW TEMPERATURE HYDROGEN MASER

We describe the source of a beam of liquid helium temperature state-selected hydrogen atoms to be used in the development of a very low temperature atomic hydrogen maser frequency standard. Recent experimental results which affect the design of such a beam are presented, and future experimental plans are outlined

Collisional frequency shifts and line broadening in the cryogenic deuterium maser

We study the spin-exchange frequency shifts and line broadening of the deuterium maser in the temperature range from 0 to 10 K. We consider both a version operating on the β-ε transition at B=3.0 μT and one operating on the β-δ transition at B=3.9 mT. For the first transition, a comparison is made with our earlier results based on the degenerate-internal-states approximation and its first-order corrections [E. Tiesinga, H. T. C. Stoof, B. J. Verhaar, and S. B. Crampton, Physica B 165&166, 19 (1990)]. We find cusp structures due to channel thresholds and also resonance structures, both of which disappear almost completely after thermal averaging in the temperature range considered

Spin-exchange frequency shift of the cryogenic deuterium

We derive expressions for the spin-exchange frequency shift and line broadening of the deuterium maser for low temperatures and present the results of a preliminary calculation based on the Degenerate-Internal-States approximation and its first-order correction

Spin rotation effects and spin waves in gaseous 3He↑

Spin polarized gases such as H↑ or 3He↑ can propagate damped spin waves at low temperatures; their origin is the so-called « identical spin rotation effect » in collisions between identical particles. We report here the observation of such effects in 3He↑ in the temperature range 2 < T < 6 K, and preliminary measurements of the coefficient μ which characterizes the quality factor of the spin oscillations. The measurements are in agreement with the numerical calculations of C. Lhuillier.Les gaz polarisés comme H↑ ou 3He↑ possèdent des modes oscillants et amortis basse température, les ondes de spin, dont l'origine est l'effet de rotation des spins identiques se produisant lors de collisions entre particules indiscernables. Cette lettre décrit l'observation de ces effets dans 3He↑ dans le domaine de température 2 < T < 6 K, et des mesures préliminaires du coefficient μ qui donne la surtension de ces ondes. Ces mesures sont en accord avec les calculs numériques de C. Lhuillier

Hyperfine contribution to spin-exchange frequency shifts in cryogenic hydrogen masers

We have rigorously included hyperfine interactions during electron-spin-exchange collisions between ground state hydrogen atoms. We predict additional frequency shifts which are not compensated for by the usual methods of tuning maser cavities. These shifts are large compared to the potential thermal instabilities of cryogenic masers and are very sensitive to details of the interactions, especially the treatment of non-adiabatic effects

Hyperfine contributions to spin-exchange frequency shifts in the hydrogen maser

We have rigorously included hyperfine interactions during electron-spin-exchange collisions between ground state hydrogen atoms. We predict additional frequency shifts which are not compensated for by the usual methods of tuning maser cavities. These shifts are large compared to the potential thermal instabilities of cryogenic masers and are very sensitive to details of the interactions, especially the treatment of non-adiabatic effects