Distribution of whitemouth croaker (Micropogonias furnieri, Desmarest 1823) larvae in the Río de la Plata estuarine front

Whitemouth croaker (Micropogonias furnieri) larvae obtained and hydrographic data collected in the Rio de la Plata estuary (35°S–56°W) between 1987 and 2000 were used to explore the early life stages spatial and temporal distribution patterns and their relation to oceanographic features. The spatial distribution, restricted to a band in the inner part of the estuary, coincided with the bottom salinity front and the maximum turbidity zone (MTZ, turbidity front). Larvae were present during the warmest months (October through May) within a range of 14–24.5 °C temperature and 0.9–33 salinity. A vertically stratified sampling performed in the region where the largest abundance was found (December 2005 and March 2006) was used to test the hypothesis that larvae retention occurs in the bottom salinity front.The vertically stratified sampling showed larvae throughout the water column with high predominance in the river–estuary transition zone. A positive correlation between abundance and the bottom salinity horizontal gradient was found. The size analysis showed that the largest individuals (>10 mm SL), probably undergoing the settlement process, inhabited near the bottom and that the smallest (<10 mm SL) were present in the whole water column. Length distribution along the front showed no trend.Results support the estuarine retention hypothesis of previous studies on whitemouth croaker gravid females, eggs distribution and outcomes from a numerical simulation model. Retention in the salinity front/MTZ would allow larvae to benefit from food accumulation in the region, the high turbidity level provide shelter against predators and retention in the estuary secure closeness to the main nursery ground.     

A point hailfall classification based on hailpad measurements: The ANELFA scale

The ANELFA scale for hailfall intensity is proposed on the model of the 6-class Fujita scale for tornadoes. It is based on more than three thousand point hailfalls measured by hailpads over a 16-year period in France. The class number of a hailfall is determined by the integer value of the largest measured hailstone diameter in cm, or by equivalence with current objects: A0 to A5 for pea, grape, pigeon's egg, walnut, hen's egg, orange. The class number is followed by a plus or minus sign if the ground is significantly more or less than half-covered by hailstones respectively. When the scale is applied to the ANELFA data, a log-normal distribution is found for the class distribution, allowing the frequency determination of the upper class ever observed until now at the hailpad stations.     

Behavior of the fresh- and saline-water phases in an urban area in Western Buenos Aires Province, Argentina

Given the particular hydrogeological setting at Salliqueló County, Western Buenos Aires Province, Argentina, this paper places attention on the lens that supplies fresh water to its capital, Salliqueló. This lens constitutes the most important fresh-water reservoir in the county. It overlies saline waters, and is recharged through high-permeability sandy sediments at a rate that exceeds by far the actual water demand. Restrictions in exploitation are not limited by the total requirement but rather by the flows extracted from individual wells that may generate up-coning of saline waters, with a resulting overall deterioration in quality of the pumped water. The variations in the position of the interface between saline and fresh waters, as related to the change in extraction rates, were obtained by means of a simulation of the flow of two fluids of different densities. Finally, based on the general hydrogeological characterization and on the simulation results, various methods for the management of these fresh-water resources are proposed, such as the deactivation of urban supply wells, the monitoring of the saline interface position, and the establishment of a protection perimeter for the water-supply wells.

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Resumen Enmarcado dentro de las características hidrogeológicas generales del partido de Salliqueló, oeste de la provincia de Buenos Aires, Argentina, este trabajo se centra en la lente que abastece de agua dulce a Salliqueló, la capital del partido. Esta lente constituye la reserva de agua dulce más importante del partido, se ubica sobre aguas salinas y es recargada a través de sedimentos arenosos de alta permeabilidad a una tasa que excede grandemente los requerimientos de la población. Sin embargo, las restricciones en la explotación no están dadas por las demandas totales sino por las extracciones en perforaciones individuales, que pueden generar ascensos de agua salina con el consiguiente deterioro en la calidad del agua extraída. Las variaciones de la posición de la interfase respecto de los cambios en la explotación fueron obtenidas por medio de la simulación del flujo de dos fluidos de diferente densidad. Finalmente, con base en la caracterización hidrogeológica general y en los resultados de la simulación, se proponen varias medidas para la protección del recurso hídrico, tales como: la desactivación de los pozos de abastecimiento en el ámbito urbano, el control de la posición de la interfase salina y la definición de perímetros de protección de los pozos de abastecimiento, entre otras.

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Résumé Etant donné les particularités hydrogéologiques de la région de Salliqueló, ouest de la province de Buenos Aires, Argentine, cet article traite de la lentille deau douce alimentant en eau potable la capitale, Salliqueló. Cette lentille constitue la réserve la plus importante deau douce de la région. Elle se superpose aux eaux salées et est rechargée via les sédiments sableux et perméables, à un taux dépassant largement la demande actuelle en eau. Les restrictions dans lexploitation ne sont pas limitées par la demande totale mais par les débits extraits des forages privés, qui pourraient générer des rabattements induisant des arrivées deaux salées, arrivées qui peuvent dégrader la ressource pompée. Les variations de la position de linterface eau douce–eau salée, en relation avec les taux dexploitation, ont été obtenues en simulant lécoulement de deux fluides de différentes densités. Finalement, sur base dune caractérisation hydrogéologique et des résultats de la simulation, des méthodes variées pour la gestion de la ressource en eau douce sont proposées, entre autre, la désactivation de pompages urbains, suivi de linterface eau douce–eau salée, et la mise en place de zones de protections des captages deau souterraine.

     

An analytical study of the effect of penetration rate on piezocone tests in clay

Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post‐penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi‐rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd.     

Attitude stability of a gravity-stabilized gyrostat satellite

This paper treats analytically the problem of the stability of the attitude motions of a gravity-stabilized gyrostat satellite that is in a circular orbit around a spherical planet. The vehicle considered consists of a body with no special symmetries that has any number of rotors attached to it. The internal angular momentum vector due to these rotors is parallel to one of the principal axes of the entire satellite; this axis is aligned with (or close to) the normal to the orbit plane. Both the cases in which each rotor is driven by a motor at a constant spin rate relative to the main body of the vehicle and the one in which each rotor is rotating freely, without any friction, are treated. Stability (both infinitesimal and in the sense of Liapunov) of the attitude motions of the vehicle can be quickly predicted by using the results derived here, which are summarized in the form of a continuous, three-dimensional, stability diagram.National Research Council Post-Doctoral Research Associate at NASA-Ames Research Center, Moffett Field, Calif. Parts of this research were included in [12].     

Chemical abundances and ionizing clusters of H ii regions in the LINER galaxy NGC 4258

We present long-slit observations in the optical and near-infrared of eight H  ii regions in the spiral galaxy NGC 4258. Six of the observed regions are located in the south-east inner spiral arms, and the other two are isolated in the northern outer arms. A detailed analysis of the physical conditions of the gas has been performed. For two of the regions, an electron temperature has been derived from the [S  iii ] λ 6312 line. For the rest, an empirical calibration based on the red and near-infrared sulphur lines has been used. The oxygen abundances derived by both methods are found to be significantly lower (by a factor of 2) than previously derived by using empirical calibrations based on the optical oxygen lines.
In the brightest region, 74C, the observation of a prominent feature caused by Wolf–Rayet (WR) stars provides an excellent constraint over some properties of the ionizing clusters. In the light of the current evolutionary synthesis models, no consistent solution is found to explain at the same time both the WR feature characteristics and the emission-line spectrum of this region. In principle, the presence of WR stars could lead to large temperature fluctuations and also to a hardening of the ionizing radiation. None of these effects is found in region 74C, for which the electron temperatures found from the [S  iii ] λ 6312 line and the Paschen discontinuity at 8200 Å are equal within the errors, and the effective temperature of the ionizing radiation is estimated at around 35 300 K.
Both more observations of confirmed high-metallicity regions and a finer metallicity grid for the evolutionary synthesis models are needed in order to understand the ionizing populations of H  ii regions.     

Electrochemical gold precipitation to explain extensive vertical and lateral mineralization in the world‐class Poderosa‐Pataz district,Peru

The world‐class Poderosa‐Pataz district is famous for its gold endowment in vertically and laterally extensive quartz‐sulphide veins. Precipitating mechanisms are investigated to determine why gold is so laterally and vertically distributed. Micro‐XRF and LA‐ICP‐MS element mapping of pyrites surrounding gold grains reveals systematic enrichment of As around or near visible gold accumulations. These As‐enriched zones define discordant rims and corridors overprinting pyrites. LA‐ICP‐MS spot analyses performed within and outside the As enrichment zones indicate that As is enriched on average by two orders of magnitude in association with gold. Secondary pyrite transformation by hydrothermal fluids with elevated As‐Au induced a change in the semiconducting properties of pyrite grains, resulting in the precipitation of visible gold particles at the interfaces of As‐enriched zones. The electrochemical precipitation mechanism acted as a filter to extract gold in solution regardless of variations in pressure and temperature, hence explaining the vertically and laterally extensive gold mineralization.     

Seasonal variability of primary production in a fjord ecosystem of the Chilean Patagonia: Implications for the transfer of carbon within pelagic food webs

We characterized the seasonal cycle of productivity in Reloncaví Fjord (41°30′S), Chilean Patagonia. Seasonal surveys that included measurements of gross primary production, community respiration, bacterioplankton secondary production, and sedimentation rates along the fjord were combined with continuous records of water-column temperature variability and wind forcing, as well as satellite-derived data on regional patterns of wind stress, sea surface temperatures, and surface chlorophyll concentrations. The hydrography and perhaps fjord productivity respond to the timing and intensity of wind forcing over a larger region. Seasonal changes in the direction and intensity of winds, along with a late-winter improvement in light conditions, may determine the timing of phytoplankton blooms and potentially modulate productivity cycles in the region.Depth-integrated gross primary production estimates were higher (0.4–3.8 g C m^?2 d^?1) in the productive season (October, February, and May), and lower (0.1–0.2 g C m^?2 d^?1) in the non-productive season (August). These seasonal changes were also reflected in community respiration and bacterioplankton production rates, which ranged, respectively, from 0.3 to 4.8 g C m^?2 d^?1 and 0.05 to 0.4 g C m^?2 d^?1 during the productive and non-productive seasons and from 0.05 to 0.6 g C m^?2 d^?1 and 0.05 to 0.2 g C m^?2 d^?1 during the same two periods. We found a strong, significant correlation between gross primary production and community respiration (Spearman, r=0.95; p<0.001; n=12), which suggests a high degree of coupling between the synthesis of organic matter and its usage by the planktonic community. Similarly, strong correlations were found between bacterioplankton secondary production and both gross primary production (Spearman, r=0.7, p<0.05, n=9) and community respiration (Spearman, r=0.8, p<0.05, n=9), indicating that bacterioplankton may be processing an important fraction (8–59%) of the organic matter produced by phytoplankton in Reloncaví Fjord. In winter, bacterial carbon utilization as a percentage of gross primary production was >100%, suggesting the use of allochthonous carbon sources by bacterioplankton when the levels of gross primary production are low. Low primary production rates were associated with a greater contribution of small cells to autotrophic biomass, highlighting the importance of small-sized plankton and bacteria for carbon cycling and fluxes during the less productive winter months. Fecal pellet sedimentation was minimal during this period, also suggesting that most of the locally produced organic carbon is recycled within the microbial loop. During the productive season, on the other hand, the area exhibited a great potential to export organic matter, be it to higher trophic levels or vertically towards the bottom.