Omnivory by planktivores stabilizes plankton dynamics, but may either promote or reduce algal biomass

Classical models of phytoplankton–zooplankton interaction show that with nutrient enrichment such systems may abruptly shift from limit cycles to stable phytoplankton domination due to zooplankton predation by planktivorous fish. Such models assume that planktivorous fish eat only zooplankton, but there are various species of filter-feeding fish that may also feed on phytoplankton. Here, we extend these classical models to systematically explore the effects of omnivory by planktivorous fish. Our analysis indicates that if fish forage on phytoplankton in addition to zooplankton, the alternative attractors predicted by the classical models disappear for all realistic parameter settings, even if omnivorous fish have a strong preference for zooplankton. Our model also shows that the level of fish biomass above which zooplankton collapse should be higher when fish are omnivorous than when fish are zooplanktivorous. We also used the model to explore the potential effects of the now increasingly common practice of stocking lakes with filter-feeding fish to control cyanobacteria. Because omnivorous filter-feeding fish forage on phytoplankton as well as on the main grazers of phytoplankton, the net effect of such fish on the phytoplankton biomass is not obvious. Our model suggests that there may be a unimodal relationship between the biomass of omnivorous filter-feeding fish and the biomass of phytoplankton. This implies that to manage for reductions in phytoplankton biomass, heavy stocking or strong reduction of such fish is bes

Effect of bacterial inoculation, plant genotype and developmental stage on root-associated and endophytic bacterial communities in potato (Solanum tuberosum)

Beneficial bacteria interact with plants by colonizing the rhizosphere and roots followed by further spread through the inner tissues, resulting in endophytic colonization. The major factors contributing to these interactions are not always well understood for most bacterial and plant species. It is believed that specific bacterial functions are required for plant colonization, but also from the plant side specific features are needed, such as plant genotype (cultivar) and developmental stage. Via multivariate analysis we present a quantification of the roles of these components on the composition of root-associated and endophytic bacterial communities in potato plants, by weighing the effects of bacterial inoculation, plant genotype and developmental stage. Spontaneous rifampicin resistant mutants of two bacterial endophytes, Paenibacillus sp. strain E119 and Methylobacterium mesophilicum strain SR1.6/6, were introduced into potato plants of three different cultivars (Eersteling, Robijn and Karnico). Densities of both strains in, or attached to potato plants were measured by selective plating, while the effects of bacterial inoculation, plant genotype and developmental stage on the composition of bacterial, Alphaproteobacterial and Paenibacillus species were determined by PCR-denaturing gradient gel-electrophoresis (DGGE). Multivariate analyses revealed that the composition of bacterial communities was mainly driven by cultivar type and plant developmental stage, while Alphaproteobacterial and Paenibacillus communities were mainly influenced by bacterial inoculation. These results are important for better understanding the effects of bacterial inoculations to plants and their possible effects on the indigenous bacterial communities in relation with other plant factors such as genotype and growth stage

Velocidades radiales de galaxias australes

Se informa sobre el estado de avance del programa de velocidades radiales de galaxias con el Espectrógrafo Nebular de Page.Asociación Argentina de Astronomí

Probing Yukawian gravitational potential by numerical simulations. I. Changing N-body codes

In the weak field limit general relativity reduces, as is well known, to the Newtonian gravitation. Alternative theories of gravity, however, do not necessarily reduce to Newtonian gravitation; some of them, for example, reduce to Yukawa-like potentials instead of the Newtonian potential. Since the Newtonian gravitation is largely used to model with success the structures of the universe, such as for example galaxies and clusters of galaxies, a way to probe and constrain alternative theories, in the weak field limit, is to apply them to model the structures of the universe. In the present study, we consider how to probe Yukawa-like potentials using N-body numerical simulations.Comment: 17 pages, 11 figures. To appear in General Relativity and Gravitatio

Producción de forraje verde hidropónico de cebada (Hordeum vulgare) usando efluente de piscigranja de truchas

The objective was to determine the effect of trout farm effluent on the production of green hydroponic forage of barley (GHF). The activities were: oxidation of ammonia (N-NH4+) in the trout farm effluent to nitrate of nitrogen (N-NO3-) in a biofilter (nitrification), and the use of this effluent in the production of GHF. Design was completely randomized with three treatments: 100% of water from water supply channel (T0), 50% of water from biofilter effluent, plus 50% of water from water supply channel (T1), and 100% of water from biofilter effluent (T2); with 10 replications. Variables assessed were: development time of biofilter; NNO3- content in fully developed biofilter; percentage of protein and plant height on 4th, 8th, 12th, and 16th day production; and biomass production of GHF on 16th day. By the end of the 90th day, the biofilter installed to generate N-NO3- was developed at a medium pH of 8.23 and at a medium temperature of 14.9°C. N-NO3- content (mg/L) in biofilter effluent was 2.2 while trout farm effluent was 1.2. No significant difference was found in protein percentage of GHF (P<0.05). However, there was a highly significant effect (P<0.01) on plant height the 4th, 8th, 12th, and 16th days, and biomass production on the 16th day. For both variables, treatment T2 was placed in the A range, treatment T1 in B range and treatment T0 in C range.El objetivo fue determinar el efecto del efluente de pozas de trucha sobre la producción de forraje verde hidropónico de cebada (FVH). Las actividades fueron: La oxidación del amoniaco (N-NH4+) – en el efluente de las pozas de trucha – hasta nitrato (N-NO3-) en un biofiltro (nitrificación) y el uso del efluente en la producción de FVH. Se utilizó el diseño completamente al azar con tres tratamientos: 100% de agua del canal de abastecimiento (T0), 50% de efluente del biofiltro más 50% de agua del canal de abastecimiento (T1) y 100% de efluente del biofiltro (T2); con 10 repeticiones. Las variables evaluadas fueron: tiempo de desarrollo del biofiltro, contenido de N-NO3- en el biofiltro completamente desarrollado, porcentaje de proteína y altura de planta los días 4, 8, 12 y 16 de producción; y producción de biomasa del FVH el día 16. El biofiltro instalado para la generación de N-NO3- se desarrolló a los 90 días, a un pH medio de 8.23 y una temperatura media de 14.9 °C. El contenido de NNO3-(mg/L) en el efluente del biofiltro fue de 2.2 y en el de las pozas de trucha, 1.2. No se encontraron diferencias significativas (P<0.05) en porcentaje de proteína del FVH, pero se observó un efecto altamente significativo (P<0.01) en la altura de planta los días 4, 8, 12 y 16, asimismo en la producción de biomasa del día 16. Para ambas variables el tratamiento T2 se ubicó en el rango A, el tratamiento T1 en el rango B y el tratamiento T0 en el rango C

Curvas de rotación en galaxias australes

Se dan los resultados del análisis de las curvas de rotación de 9 galaxias obtenidas con el Espectrógrafo Rápido Page y tubo de imágenes en Bosque Alegre. Se estiman masas y densidades.Asociación Argentina de Astronomí

A solution for galactic disks with Yukawian gravitational potential

We present a new solution for the rotation curves of galactic disks with gravitational potential of the Yukawa type. We follow the technique employed by Toomre in 1963 in the study of galactic disks in the Newtonian theory. This new solution allows an easy comparison between the Newtonian solution and the Yukawian one. Therefore, constraints on the parameters of theories of gravitation can be imposed, which in the weak field limit reduce to Yukawian potentials. We then apply our formulae to the study of rotation curves for a zero-thickness exponential disk and compare it with the Newtonian case studied by Freeman in 1970. As an application of the mathematical tool developed here, we show that in any theory of gravity with a massive graviton (this means a gravitational potential of the Yukawa type), a strong limit can be imposed on the mass (m_g) of this particle. For example, in order to obtain a galactic disk with a scale length of b ~ 10 kpc, we should have a massive graviton of m_g << 10^{-59} g. This result is much more restrictive than those inferred from solar system observations.Comment: 7 pages; 1 eps figure; to appear in General Relativity and Gravitatio

Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10

The dynamics of the magnetic properties of polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K. The exact nature of Ru spins magnetic ordering is still debated and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac susceptibility vs. T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with magnetically interacting clusters. Such interpretation is supported by themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations are well described by a stretched exponential relation, and present significant ageing effects.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

Comparison of advanced gravitational-wave detectors

We compare two advanced designs for gravitational-wave antennas in terms of their ability to detect two possible gravitational wave sources. Spherical, resonant mass antennas and interferometers incorporating resonant sideband extraction (RSE) were modeled using experimentally measurable parameters. The signal-to-noise ratio of each detector for a binary neutron star system and a rapidly rotating stellar core were calculated. For a range of plausible parameters we found that the advanced LIGO interferometer incorporating RSE gave higher signal-to-noise ratios than a spherical detector resonant at the same frequency for both sources. Spheres were found to be sensitive to these sources at distances beyond our galaxy. Interferometers were sensitive to these sources at far enough distances that several events per year would be expected

Reaction Front in an A+B -> C Reaction-Subdiffusion Process

We study the reaction front for the process A+B -> C in which the reagents move subdiffusively. Our theoretical description is based on a fractional reaction-subdiffusion equation in which both the motion and the reaction terms are affected by the subdiffusive character of the process. We design numerical simulations to check our theoretical results, describing the simulations in some detail because the rules necessarily differ in important respects from those used in diffusive processes. Comparisons between theory and simulations are on the whole favorable, with the most difficult quantities to capture being those that involve very small numbers of particles. In particular, we analyze the total number of product particles, the width of the depletion zone, the production profile of product and its width, as well as the reactant concentrations at the center of the reaction zone, all as a function of time. We also analyze the shape of the product profile as a function of time, in particular its unusual behavior at the center of the reaction zone