Cross-taxon responses to elevated nutrients in European streams and lakes

Few studies have compared the response of different taxonomic groups to environmental stress across aquatic ecosystems. We regressed assemblage structure of fish, invertebrates, macrophytes, phytoplankton and benthic diatoms to total phosphorus concentration, after removing the effect of ecosystem size (stream order, lake surface area), using data from 66 streams and 45 lakes across Europe. In streams, the structure of benthic diatom assemblages, measured by nonmetric multidimensional scaling, showed the strongest correlation to elevated nutrient concentrations (adj. R² = 0.495), followed by benthic invertebrates (0.376), fish (0.181) and macrophytes (0.153). For lakes, the patterns were less clear: fish (0.155), macrophytes (0.146) and phytoplankton (0.132). Cross-system comparison showed that stream assemblages were responding more strongly to nutrient concentrations than lake assemblages. Moreover, our results lend some support to the conjecture that response signatures are related to trophic level, with primary producers (benthic diatoms) responding more strongly than consumers (invertebrates, fish). Knowledge of differences in responses among taxonomic groups and between habitats to disturbance can be used to design more cost-effective monitoring programs.     

On the integration of intensity measurements of the solar center and limb near 300 nanometers

Two methods of integrating limb-darkening data to calculate the spectral irradiance are compared. Near 300 nm Moe and Milone's (1978) quadratic approximation to the limb darkening yields an integral 3% larger on average than the two linear segments of Kohl et al. (1980) when the same data are inserted in both formulas. As one would expect, the difference between the two calculations is directly proportional to Moe and Milone's quadratic coefficient. When Moe and Milone's data are inserted in the formula of Kohl et al., the calculated irradiance is 4% higher than when the data of Kohl et al. are used in the same formula. Possible reasons for this difference are suggested.     

A generalized theory for line formation in a homogeneous magnetic field

The formation of spectral lines in a homogeneous magnetic field has been studied. A new method for solving the transfer equations for polarized light has been found. Using this method, the Stokes parameters may be derived without any special assumptions regarding the model atmosphere. With the line formed by pure absorption, the expressions for the Stokes parameters may easily be adapted to numerical calculations. In order to illustrate the method, the line profile for the Zeeman triplet 5250 has been calculated using a photospheric model atmosphere.     

Models for different sunspot umbrae

An empirical approach to interpret the time evolution of the high spatial resolution Ca ii K line is presented. We specify the physical parameters, such as electron temperature, hydrogen density, and velocity (microturbulent and systematic) as functions of height. The electron density is obtained from scaled non-LTE solutions for hydrogen ionization. The population indices, and thus the Ca ii source functions, for a 5-level Ca ii atom are computed by using the generalized Newton-Raphson method. K line profiles are then synthesized for different evolutionary stages and are compared with the observed ones. The explanation of the ‘peculiar’ type profile is also attempted.     

Avalanche Run-Out on Counter-Slopes

Counter-slopes in avalanche paths influence avalanche flow in similarways as defence structures. Measurements and calculations of run-outon such slopes will improve our understanding of retaining damdesign. We have studied three different, well described avalancheevents; for each case we have calculated the run-out distance usingthe Noren, Irgens and Schieldrops continuum model (NIS), adjustingthe parameters to fit the simulated avalancheto the observed one. Using the same parameters, the run-out onidentical terrain, but without counter-slope, is computed. Comparisonbetween the energy dissipation in these different cases tells us whichtopography most efficiently retards an avalanche. With a smoothly curved valley bottom, the energy dissipation at agiven point on the opposite slope is less than on a flatrun-out. The explanation to this is that, when gravity retards anavalanche, a smaller portion of the energy loss is due to dissipationfrom viscous (velocity dependent) friction. When the avalanche travelsover a retaining dam, with much smaller curvature radii, a significantportion of the energy is dissipated. The consequence of this is that defence structures are not sodependent on the counter-slope inclination as on the curvature radiusin the transition to up-gradient. Further, introduction of acounter-slope in an avalanche path may conserve energy rather thancausing dissipation – if the inclination is not altered abruptlyenough. If the counter-slope is smooth, and not large enough tocompletely stop the avalanche, it will not reduce destructive energyon the downstream side. In fact, the destructive energy may bepartially conserved until the run-out.     

Exchange dynamics of a shallow contaminated wetland

Flux pathways are investigated for a wetland system comprised of a river flowing through two shallow forebays and then entering the main basin of a lake. The hydrodynamic exchanges between these three components influence the fate and transport of heavy metals in the system. During non-storm summer flows the river plume, comprised of river water and entrained forebay water, was cooler than the lake surface water and so plunged as it entered the main basin and inserted near the seasonal thermocline. Because the river plume plunged, only a fraction of its metals flux was available to the epilimnion. A return flow into the forebay was always observed immediately above the river inflow, ensuring a predominantly two-layer exchange system. However during days of negative heat flux (surface heating), an additional exchange mechanism existed when the surface waters from the forebay formed a buoyant plume, flowing out into the main basin. This heated outflow produced a significant, but short-lived, arsenic flux from the forebay into the surface waters of the lake.