Determination of the fate of polynuclear aromatic hydrocarbons in natural water systems

The polynuclear aromatic hydrocarbons, or PAH, are of current concern as water pollutants and potential health hazards. The presence of PAH in natural water systems was evaluated and an analytical technique for specific PAH was developed. It was found that the PAH are not soluble in water but they either are present as particulate material or as material adsorbed on solid surfaces in natural water systems. The photodecomposition of two PAH, 1,2 benzanthracene, or BA, and 3,4 benzpyrene, or BP, was examined. Both compounds decompose under ultraviolet light to form their quinones, which then further decompose. Both BP and BA decompose following first order kinetics in true solution in 20 percent acetone in water. Particulate BA also decomposes following first order reaction kinetics, a1 though particulate BP will decompose only to a depth of 0.2 pm before decomposition stops. This decomposition is relatively unaffected by water chemistry and will occur under solar radiation and in turbid waters.U.S. Department of the InteriorU.S. Geological SurveyOpe

Improved Implementation of Point Location in General Two-Dimensional Subdivisions

We present a major revamp of the point-location data structure for general two-dimensional subdivisions via randomized incremental construction, implemented in CGAL, the Computational Geometry Algorithms Library. We can now guarantee that the constructed directed acyclic graph G is of linear size and provides logarithmic query time. Via the construction of the Voronoi diagram for a given point set S of size n, this also enables nearest-neighbor queries in guaranteed O(log n) time. Another major innovation is the support of general unbounded subdivisions as well as subdivisions of two-dimensional parametric surfaces such as spheres, tori, cylinders. The implementation is exact, complete, and general, i.e., it can also handle non-linear subdivisions. Like the previous version, the data structure supports modifications of the subdivision, such as insertions and deletions of edges, after the initial preprocessing. A major challenge is to retain the expected O(n log n) preprocessing time while providing the above (deterministic) space and query-time guarantees. We describe an efficient preprocessing algorithm, which explicitly verifies the length L of the longest query path in O(n log n) time. However, instead of using L, our implementation is based on the depth D of G. Although we prove that the worst case ratio of D and L is Theta(n/log n), we conjecture, based on our experimental results, that this solution achieves expected O(n log n) preprocessing time.Comment: 21 page

Expanding student teachers’ implicit theories about explanations for the science classrooms

This study explored student teachers’ implicit theories about explaining for the science classroom in three courses at diverse universities. Based on microteaching situations, the participants simulated explanations and discussed the elements they considered relevant for giving peer feedback. This led to the design of rubrics for peer assessment, which expressed their implicit theories about what a good explanation for the science classroom would look like. The three rubrics are presented and discussed in the light of the connections between teachers’ thinking and practice. Shulman’s ideas about professional teaching knowledge development, as well as negotiation of meaning, provide theoretical under-pinning for understanding and expanding student teachers’ thinking about explanations for the science classrooms.</p

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO2 and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO2 related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO2. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO2. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 ?atm) and future (1,000 ?atm) CO2 levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high?nitrogen batch cultures and lower?nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO2 independent of the culture regime. We also find that, despite species?specific variation in growth response to high CO2, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO2. The CO2?induced growth response is a good predictor of CO2?induced changes in competition (R2 > .93) and community composition (R2 > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO2 levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction