Interaction of oxygen with an AISI 314 stainless steel surface studied by ellipsometry and auger electron spectroscopy in combination with ion bombardment

The influence of pretreatment (ion bombardment) on the initial oxidation of AISI 314 stainless steel has been investigated in ultra high vacuum systems with ellipsometry and Auger electron spectroscopy. The bombardment was carried out with Ar⁺ ions with energies up to 2.2 keV at various doses and angles of incidence. Oxidation was performed at low P_os (< 10^?5 Torr) and sample temperatures (<300°C). The worked layer was removed with low-energy argon ions. High energy bombardment causes an increase in oxidation rate and higher values of the real and imaginary part of the refractive index as compared to the undisturbed sample. The rate and extent of oxidation increases with increasing energy, dose and angle of incidence of the ion beam. The outermost part of the oxide is enriched in iron while the region near the substrate contains more nickel and chromium.     

High Variability of Phytoplankton Photosynthesis in Response to Environmental Forcing in Oligotrophic Lake Malawi/Nyasa

In southern Lake Malawi, seasonal pelagic chlorophyll means were 1.0 ± 0.3 μg L⁻¹ in the deep mixing season (DMS) (May–August), 0.8 ± 0.3 μg L⁻¹ in the dry stratified season (DSS) (September to November) and 0.7 ± 0.3 μg L⁻¹ in the wet stratified season (WSS) (December to April). Despite the low variability in chlorophyll, there was a wide range in chlorophyll specific photosynthetic activity. The photosynthetic parameters, P^b_m (the light saturated rate) and α^b (the light limited slope), varied significantly among seasons and were highly positively correlated, with lowest values in the DSS and highest values in WSS. During deep mixing, P^b_m did not covary with α^b; and the light saturation index, E_k (=P^b_m/α^b), varied in response to changes in α^b rather than in P^b_m. Phytoplankton appeared to be nutrient deficient at all times but less deficient during deep vertical mixing in the DMS. Average daily rates of integrated phytoplankton primary productivity were lowest in the DSS (337 mg C m⁻² d⁻¹) and highest in the WSS (629 mg C m⁻² d⁻¹) despite nearly identical mean chlorophyll concentrations. Along a near shore transect off the Linthipe River, chlorophyll concentrations were higher and more variable (1.4 ± 1.3 μg L⁻¹), phytoplankton were not strongly nutrient deficient and chlorophyll specific photosynthetic activity was as high or higher than at the offshore station. Estimates of phytoplankton productivity in this tropical great lake must account for spatial and temporal variability in photosynthetic parameters imposed by seasonal changes in mixing dynamics.     

Linear and logarithmic speed–accuracy trade-offs in reciprocal aiming result from task-specific parameterization of an invariant underlying dynamics.

The authors examined the origins of linear and logarithmic speed–accuracy trade-offs from a dynamic systems perspective on motor control. In each experiment, participants performed 2 reciprocal aiming tasks: (a) a velocity-constrained task in which movement time was imposed and accuracy had to be maximized, and (b) a distance-constrained task in which accuracy was imposed and movement time had to be minimized. In Experiment 1, accuracy was constant across the 2 tasks; in Experiment 2, movement time was kept constant. Behavior in both tasks could be modeled with a single nonlinear equation of motion. Model coefficients captured the particulars of each task, especially apparent for the slowest or most difficult conditions. The distance-constrained task revealed a strong contribution of nonlinear stiffness with a moderate degree of nonlinear damping, favoring local control of speed. The velocity-constrained task revealed weaker nonlinear stiffness with stronger nonlinear damping, favoring global stabilization of the movement with a more constant rate of phase progression. In this way, the different speed–accuracy trade-offs emerged from the task-specific parameterization of the underlying dynamics. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Behind Fitts’ law: kinematic patterns in goal-directed movements

Half a century ago, Paul Fitts first discovered that the time necessary to complete a pointing movement (MT) linearly increases with the amount of information (ID) necessary to specify the target width (W) relative to the distance (D). The so-called Fitts’ law states that , with ID being a logarithmic function of the D/W ratio. With the rising importance of pointing in human–computer interaction, Fitts’ law is nowadays an important tool for the quantitative evaluation of user interface design. We show that changes in ID give rise to systematic changes in the kinematics patterns that determine MT, and provide evidence that the observed patterns result from the interplay between basic oscillatory motion and visual control processes. We also emphasize the generality and abstract nature of Fitts’ robust model of human psychomotor behavior, and suggest that some adaptations in the design of the (computer-mediated) coupling of perception and production of movement might improve the efficiency of the interaction.     

Biomagnification of DDT through the benthic and pelagic food webs of Lake Malawi, East Africa: importance of trophic level and carbon source

Lake Malawi, an East African Rift Valley lake, is internationally renowned for having the highest diversity of fish species in the world, and these cichlids are highly specialized in their dietary habits. In this lake, tissue stable carbon (delta13C) and nitrogen (delta15N) isotopes can be used over several trophic levels to distinguish those consumers relying upon carbon fixed by either benthic or pelagic primary producers. As such, it was possible to contrast the biomagnification of persistent organochlorines through the benthic and pelagic food webs. In 1996 and 1997, food-web organisms were collected from Lake Malawi and analyzed for organochlorines, delta13C and delta15N to determine the factors that affectthe biomagnification of contaminants in a tropical lake. The pesticide DDT was the most predominant pollutant in the biota from Lake Malawi and was found at the highest concentrations in the largest and fattiest fish species. As observed in temperate systems, log-transformed sigmaDDT concentrations in food-web organisms were significantly predicted by delta15N or log lipid (r2 = 0.32 and 0.40, respectively). In addition, the slope of the regression of log sigmaDDT versus delta15N was significantly higher in the pelagic than the benthic food web. These results indicate that pelagic organisms are at greater risk of accumulating these pollutants than biota relying upon benthic primary production.