Initial ionization rates in shock-heated Argon, Krypton, and Xenon

The rate of ionization behind strong shock waves in argon, krypton, and xenon, is observed by a transverse microwave probe, over a range of electron densities low enough that atom-atom inelastic collisions are the rate-determining mechanism. Shocks of Mach number 7.0 to 10.0 propagate down a 2-in. sq. aluminum shock tube into ambient gases at pressures of 3.0 to 17.0 mm. Hg., heating them abruptly to atomic temperatures of 5500°K to 9600°K. The subsequent relaxation toward ionization equilibrium is examined in its early stages by the reflection, transmission, and phase shifts of a 24.0 Gc/sec (1.25 cm) transverse microwave beam propagating between two rectangular horns abreast a glass test section. The data yield effective activation energies of 11.9 ± 0.5 eV for argon, 10.4 ± 0.5 eV for krypton, and 8.6 ± 0.5 eV for xenon. These coincide, within experimental error, with the first excitation potentials, rather than the ionization potentials of the gases, indicating that in this range ionization proceeds via a two-step process involving the first excited electronic states of which the excitation step is rate controlling

A Conceptual Model of Natural and Anthropogenic Drivers and Their Influence on the Prince William Sound, Alaska, Ecosystem

Prince William Sound (PWS) is a semi-enclosed fjord estuary on the coast of Alaska adjoining the northern Gulf of Alaska (GOA). PWS is highly productive and diverse, with primary productivity strongly coupled to nutrient dynamics driven by variability in the climate and oceanography of the GOA and North Pacific Ocean. The pelagic and nearshore primary productivity supports a complex and diverse trophic structure, including large populations of forage and large fish that support many species of marine birds and mammals. High intra-annual, inter-annual, and interdecadal variability in climatic and oceanographic processes as drives high variability in the biological populations. A risk-based conceptual ecosystem model (CEM) is presented describing the natural processes, anthropogenic drivers, and resultant stressors that affect PWS, including stressors caused by the Great Alaska Earthquake of 1964 and the Exxon Valdez oil spill of 1989. A trophodynamic model incorporating PWS valued ecosystem components is integrated into the CEM. By representing the relative strengths of driver/stressors/effects, the CEM graphically demonstrates the fundamental dynamics of the PWS ecosystem, the natural forces that control the ecological condition of the Sound, and the relative contribution of natural processes and human activities to the health of the ecosystem. The CEM illustrates the dominance of natural processes in shaping the structure and functioning of the GOA and PWS ecosystems

Seed Reserve Hot Spots for the Sub-Tropical Seagrass Halodule wrightii (Shoal Grass) in the Northern Gulf of Mexico

© 2020, Coastal and Estuarine Research Federation. Halodule wrightii (shoal grass) is a dioecious seagrass with a widespread tropical and subtropical distribution. Like all seagrass species, H. wrightii has the ability to expand asexually through rhizome elongation and to reproduce sexually through seed. To better understand H. wrightii sexual recruitment dynamics in the northern Gulf of Mexico, we investigated seed bank densities at 815 sites from south Texas to the Florida Panhandle. H. wrightii seed reserves were spatially variable across the region, with seed densities ranging from 0 to 5290 seeds m−2. Spatial analysis revealed clusters of high seed densities (“hot spots”) in Upper Laguna Madre, TX, and Santa Rosa Sound, FL, and clusters of low seed densities (“cold spots”) in Lower Laguna Madre and Aransas Bay, TX. Hot spots were dominated by H. wrightii, whereas cold spots were dominated by Thalassia testudinum (turtle grass). We frequently found intact seed coat halves, suggesting germination; however, we also encountered broken seed coat pieces, characteristic of seed predation. Genotypic surveys within and adjacent to seed hot spots revealed genetically diverse adult populations 6 years post seed bank sampling. Our data show that H. wrightii seed reserves are heterogeneous across the northern Gulf of Mexico and that the factors driving variation in seed bank density, viability, and germination remain poorly understood. Information on the spatial heterogeneity of H. wrightii seed densities has relevance for seagrass management, including targeting meadows with high levels of reproductive effort for protection or designation as marine reserves