Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces and silicon AFM tips or glass microspheres of different sizes have been extensively studied as a function of relative humidity (RH) in the range 5-90%, as model systems for the behavior of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R ∼ 20 nm) gave pull-off forces close to values predicted by simple Laplace-Kelvin theory (∼20 nN), but the values with microspheres (R ∼ 20 µm) fell well below predictions for sphere-flat or sphere-sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behavior, with no significant RH dependence. The pull-off force again fell well below predicted values (Johnson-Kendall-Roberts contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20-40%, accompanied by a long-range noncontact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behavior on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially formed water film, giving rise to patch charges and long-range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behavior of cohesive powders with different coatings or those which show a large humidity dependence (e.g., zeolites) or show electrostatic charging effects (e.g., silica aerogels).

Three cyanobacteria ( Microcystis aeruginosa Kütz. emend. Elenkin, Merismopedia tenuissima Lemmermann, and Oscillatoria sp.) and one diatom ( Aulacoseira granulata var. angustissima O. Mull. emend. Simonsen) were isolated from the tidal freshwater Potomac River and maintained at 23 Њ C and 40 mol photons и m Ϫ 2 и s Ϫ 1 on a 16:8 L:D cycle in unialgal culture. Photosynthetic parameters were determined in nutrient-replete cultures growing exponentially at 15, 20, 25, and 30 Њ C by incubation with 14 C at six light levels. P B max was strongly correlated with temperature over the entire range for the cyanobacteria and from 15 to 25 Њ C for Aulacoseira , with Q 10 ranging from 1.79 to 2.67. The ␣ values demonstrated a less consistent temperature pattern. Photosynthetic parameters indicated an advantage for cyanobacteria at warmer temperatures and in light-limited water columns. P B max and I k values were generally lower than comparable literature and field values, whereas ␣ was generally higher, consistent with a somewhat shade acclimated status of our cultures. Specific growth rate ( ), as measured by chlorophyll change, was strongly influenced by temperature in all species. Oscillatoria had the highest at all temperatures, joined at lower temperatures by Aulacoseira and at higher temperatures by Microcystis. Values of for Aulacoseira were near the low end of the literature range for diatoms consistent with the light-limited status of the cultures. The cyanobacteria exhibited growth rates similar to those reported in other studies. Q 10 for growth ranged from 1.71 for Aulacoseira to 4.16 for Microcystis. Growth rate was highly correlated with P B max for each species and the regression slope coefficients were very similar for three of the species. Abbreviations: I , photon flux density (light level); I k ϭ P B max и␣ Ϫ 1 ; I opt , photon flux density at which P B max is reached where photoinhibition is present; P B , photosynthetic rate/Chl a ; P B max , maximum of P B -I curve; ␣ , slope of P B -I curve; , specific growth rate 1

The impact of urbanization on stream insect communities was determined by sampling 22 sites in northern Virginia representing a range of human population densities. Watershed development had little effect on the total insect numbers (no./m2), but shifted the taxonomic composition markedly. Relative abundance of Diptera (mainly chironomids) increased at more highly urbanized sites, while most other insect orders including Ephemeroptera (mayflies), Coleoptera (bettles), Megaloptera (dobsonflies), and Plecoptera (stone‐flies) decreased. Trichoptera (caddisfiles) exhibited a variable response. Genus diversity and richness (number of genera) were significantly higher in less urbanized streams. Two genera of chironomids were positively correlated with increased urbanization, while 14 other genera (scattered through several orders) were negatively related to human population density. Principal components analysis demonstrated a gradient from more urbanized to less urbanized stations based on generic and order level biological data. Results of this study indicate that watershed urbanization has a major impact on benthic insect communities even in the absence of point source discharges.

Adhesion between polystyrene particles in high purity water has been measured for a wide range of particle sizes. The results are compared with predictions based on the JKR model using literature values of the solid-vapor surface energy. When the surface roughness of the particles is accounted for, the JKR adhesion is still too large. An optimum value of the surface energy is calculated for one tip particle size and used for predicting the pull-off force for all of the substrate particle sizes, and it was found that this produces much closer agreement between theory and experiment although the scatter in the data is large. The mean pull-off force for all of the particle-particle measurements studied here is of the order of a few nanonewtons. Because of the large scatter in the pull-off force data, no clear trend in the pull-off force for different particle sizes has been observed. It is suggested that the effect of surface roughness on the pull-off force is significant.