Climate change causing phase transitions of walleye pollock (Theragra chalcogramma) recruitment dynamics

In 1976 the North Pacific climate shifted, resulting in an average increase of the water temperature. In the Gulf of Alaska the climate shift was followed (i.e. early 1980s) by a gradual but dramatic increase in the abundance of groundfish species that typically prey on pre-recruitment stages of walleye pollock. In the present study we used a previously parameterized model to investigate the effect of these climate and biological changes on the recruitment dynamics of walleye pollock in the Gulf of Alaska. Simulations covered the 1970–2000 time frame and emphasized the medium-to-long temporal scale (i.e. about 5–10 years) of environmental variability. Results showed that during periods characterized by high sea surface temperature and high predation on juvenile pollock stages, recruitment variability and magnitude were below average, and recruitment control was delayed to stages older than the 0-group. Opposite dynamics (i.e. high abundance and variability, and early recruitment control) occurred during periods characterized by low temperature and predation. These results are in general agreement with empirical observations, and allowed us to formulate causal explanations for their occurrence. We interpreted the delay of recruitment control and the reduction of variability as an effect of increased constraint on the abundance of post age-0 stages, in turn imposed by high density dependence and predation mortality. On the other hand, low density-dependence and predation favoured post age-0 survival, and allowed for an unconstrained link between larval and recruitment abundance. Our findings demonstrate that the dominant mechanisms of pollock survival change over contrasting climate regimes. Such changes may in turn cause a phase transition of recruitment dynamics with profound implications for the management of the entire stock

Optical-model potential for electron and positron elastic scattering by atoms

An optical-model potential for systematic calculations of elastic scattering of electrons and positrons by atoms and positive ions is proposed. The electrostatic interaction is determined from the Dirac-Hartree-Fock self-consistent atomic electron density. In the case of electron projectiles, the exchange interaction is described by means of the local-approximation of Furness and McCarthy. The correlation-polarization potential is obtained by combining the correlation potential derived from the local density approximation with a long-range polarization interaction, which is represented by means of a Buckingham potential with an empirical energy-dependent cutoff parameter. The absorption potential is obtained from the local-density approximation, using the Born-Ochkur approximation and the Lindhard dielectric function to describe the binary collisions with a free-electron gas. The strength of the absorption potential is adjusted by means of an empirical parameter, which has been determined by fitting available absolute elastic differential cross-section data for noble gases and mercury. The Dirac partial-wave analysis with this optical-model potential provides a realistic description of elastic scattering of electrons and positrons with energies in the range from ~100 eV up to ~5 keV. At higher energies, correlation-polarization and absorption corrections are small and the usual static-exchange approximation is sufficiently accurate for most practical purposes

Measurements of atmospheric trace gas concentrations during the field campaign in Juelich 1988

Our current understanding of the gas phase chemistry of the lower troposphere is based on measurements of trace gas concentrations in the atmosphere, laboratory studies of gas phase reactions and model calculations of the relevant chemical processes. The ultimate validation of a reaction scheme can only be derived from a comparison of model results with experimental data from measurements in the atmosphere. In order to establish an extended and reliable experimental data set on which such a comparison could be based the Institut fuer Atmosphaerische Chemie of the Forschungszentrum Juelich (KFA) has been conducting field campaigns for a number of years. One such campaign took place in Juelich from May 9 - June 10 in 1988. Many of the relevant trace species were measured on a continuous basis together with photolysis frequencies and meteorological parameters. All measurements were ground based and included OH, O_3, NO, and NO_2, NO_y, HCHO, SO_2, CH_4, CO, non methane hydrocarbons up to C_5, peroxyacetylnitrate, and aldehydes. The photolysis frequencies of O_3 and NO_2 were also monitored, as were atmospheric temperature, relative humidity, wind speed, and wind direction. All quantities were measured in-situ except OH and SO_2, which were exclusively measured by a long path differential optical absorption technique. HCHO, NO_2, and O_3 were determined by in-situ and also by long-path-absorption methods for intercomparison. (orig./EF)SIGLEAvailable from TIB Hannover: RA 831(2731) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

A Comparison of Measured OH Concentrations with Model Calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the regional acid deposition model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NO x concentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Jülich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Jülich with average NO x concentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi‐steady state model under the spatially inhomogeneous conditions in Jülich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants